Note: Descriptions are shown in the official language in which they were submitted.
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HEAVY-DUTY PLATE LASER WITH LINEAR MOTORS FOR X AND Y
POSITION CONTROL
Field Of The Invention
The present invention relates to laser-equipped machine tools, and more
particularly to a heavy-duty laser plate cutting machine.
Related Applications
This application is a continuation-in-part of U.S. Application Serial No.
09/396,954, filed September 15, 1999, the teachings and disclosure of which
are
hereby incorporated in their entirety by reference thereto.
Background And Summary Of The Invention
In the following paragraphs, background information, and information
summarizing the invention will be presented together so as to convey a
coherent
view of the significance of the invention.
Laser-equipped machine tools are often used to cut parts from sheet metal
and plate. In such machine tools a laser beam, concentrated by a focusing lens
or
mirror to a small diameter spot, is directed to position the focal point
above, on or
below the surface of the material to be cut. The laser beam is directed by the
focusing optic through a nozzle disposed immediately above the workpiece, with
a pressurized gas being directed through the nozzle, typically coaxially with
the
laser beam, to assist making the cut. The pressurized gas interacts with the
laser
beam and material, facilitating the cutting process, and creates a high
velocity
stream that carries the melted material away from the cut.
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Laser-equipped machine tools are usually Computer Numerically
Controlled, and are manufactured in many configurations and sizes and with
lasers of various types and power. The present invention relates to heavy-duty
plate lasers, such as those which are capable of cutting steel plate on the
order of
one-inch thick or more at production cutting rates on the order of 24 inches
per
minute (ipm). The present invention is directed to a machine having those
capabilities and sufficient adaptability to also efficiently handle lighter
materials,
such as sheet metal. In the most preferred embodiment, a "flying optic"
configuration is utilized. In that configuration the cutting head is adapted
for
movement along one axis, such as the Y-axis which is mounted on a bridge
adapted for movement along an orthogonal, X-axis. The work is supported on a
stationary pallet or table below the bridge. Movement of the cutting head is
coordinated with movement of the bridge to define a precise path on the part.
The cutting head and laser are controlled to pierce, cut and form holes and
shapes
in the material, and then to cut the part from the material.
In a laser cutting machine, the laser beam is produced in a laser generator
and is directed along a beam path via a beam delivery system. A beam delivery
system is a collection of optical elements, such as reflective mirrors and
transmissive optics, which may redirect the beam, alter the propagation
characteristics of the beam or focus the beam. The beam delivery system is
enclosed for safety and for control of the beam path environment within. The
laser beam is concentrated by a focusing lens or mirror to a small diameter
spot,
which is directed to an appropriate position relative to the surface of the
material
to be processed.
In most implementations, the laser beam exits the laser through an output
coupler, a partially transmissive and partially reflective optical element
which
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seals the laser cavity and transmits a portion of the beam out of the laser
cavity or
resonator. The beam is then directed along a beam path to a focusing optic in
a
processing head near the work. In most cutting applications, the beam is
directed
by the focusing optic through a nozzle disposed immediately above the
workpiece to be cut. A pressurized gas is also directed through the nozzle,
typically coaxial to the beam, to assist the cutting process. The pressurized
gas
serves to facilitate and/or shield the cutting process, and creates a gas
stream
which helps remove vaporized and molten material from the cut or kerf. Kerf
refers to the zone of material which is acted upon and removed by a cutting
process. Kerf width refers to the width of the slot created by the cutting
process,
such as the width of the slot cut by a laser beam as it moves along a path.
Key factors in laser processing include the diameter of the focus spot and
the position of the focus spot relative to the material to be processed. The
control
of these focal characteristics is critical to maintaining the quality of the
process.
During processing, unintended deviation in the focus spot size and position
may
produce a deterioration in process quality and may even cause the process to
fail.
The first of two main factors which influence the focus characteristics is
the diameter of the laser beam at the focal optic. Due to diffraction, the
minimum focal spot diameter, for a given focal length optic, is limited.
Diffraction causes light beams to diverge or spread transversely as they
propagate. As the input laser beam diameter increases for a given focal optic,
the
focus spot diameter decreases due to a decrease in diffraction. In addition,
as the
input laser beam diameter increases for a given focal optic, the focus spot
position shifts closer to the focus optic.
The raw laser beam, issuing from the laser resonator, exhibits the
characteristic of divergence. The beam diameter will change as a function of
the
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distance from the output coupler. Typically, as the processing head moves over
the processing area the distance from the output coupler to the focal optic
will
change. When a large processing area is required, some method of maintaining
the proper beam diameter must be employed in order to avoid significant
changes
in focus diameter and position.
Additionally, changes in the output power level of the laser will affect the
divergence of the output beam. The largest effect on beam divergence comes
from the thermal loading of the output coupler which produces thermal tensing.
Thermal tensing is distortion of an optical component caused by heat absorbed
from the input beam. The absorbed portion of the beam causes expansion of the
output coupler such that the curvature of the surface changes. The expansion
causes a change in the divergence of the output beam thereby changing the beam
size at any given distance from the output coupler. The rate and amount of
distortion is dependent upon the power of the beam, optic contamination,
thermal
conductivity of the optic and its cooling system and the length of time the
beam
is applied. Upon reaching thermal equilibrium, when absorbed heat is in
balance
with that removed by the lens cooling system, the shape of the optic surface
remains constant. When the beam is turned off, the optic surface gradually
relaxes and returns to its original shape. When a high output power laser is
required, some method of maintaining the proper beam diameter, in a time
dependent response to output power changes, must be employed if significant
changes in focus diameter and position are to be avoided.
The second of two main factors which influence the focus characteristics
is the distortion of the focus optic due to heat absorption. In a manner
similar to
that described for the laser output coupler, thermal tensing occurs in the
focus
optic. The expansion of the focus optic reduces the effective radius of
curvature
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which causes the focal spot to shift closer to the focus optic. When a high
output
power laser is required, some method of maintaining the proper focal position,
in
a time dependent response to input laser power changes, must be employed if
significant changes in focus position are to be avoided.
5 Proper focal position is very important in cutting heavy plate. In
initiation
of a cut, the plate must be pierced, and a preferable piercing technique
requires
"driving" the beam through the plate. This can be accomplished by altering the
position of the focal spot, by actually moving it into the plate as the
piercing
operation progresses. Furthermore, in cutting different types of materials, it
is
often useful to alter the focal spot position with respect to the surface of
relatively
thick materials so as to optimize the quality of the cut.
Turning now to the divergence issue mentioned above, one method
employed to reduce the divergence of the laser beam is to expand or magnify it
with a collimator. The rate of divergence of a beam is reduced in inverse
proportion to the amount it is magnified. If a beam is magnified by 125
percent
its rate of divergence is reduced 20 percent. If it is magnified by 200
percent its
rate of divergence is reduced by 50 percent.
Collimators are optical devices, also known as beam expanders and
condensers. Such devices also have other characteristics and functions known
to
those skilled in the art. Manufacturers of laser optics publish literature
providing
information on design variations and examples of use. One example of such
literature is the II-IV Incorporated publication, Beam Expander-Condensers,
published 3/92. Collimators can be constructed of transmissive optics such
that
the beam is passed through the optics. Such collimators are commonly used in
laser-equipped machines up to about three kilowatt power levels and sometimes
above.
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Collimators used on low powered lasers are designed or adjusted to
magnify the beam a given amount, and then locked in place. Use of transmissive
collimators with lasers having power levels above three kilowatts becomes
increasingly problematic due to thermal Tensing and due to limits on the
energy
density that transmissive optic materials can withstand. Impurities within
optical
materials, crystal growth conditions, surface contamination and surface
imperfections are primary causes for a portion of a laser beam to be absorbed
and
converted to heat within a transmissive optical element.
The distortion produced by thermal Tensing can influence the divergence
and mode quality of the beam passing through or reflecting off of the optical
delivery and focusing components and thereby cause detrimental shifts of focus
position. Thermal Tensing is a greater problem with transmissive optics. For
example, when a high power beam is directed at the curved surface of a plano-
convex focal lens, which has a curved first surface and a flat second surface,
the
absorbed portion of the beam causes expansion of the lens such that the
curvature
of the surface changes. The expansion reduces the effective radius of
curvature
which causes the focal spot to shift upward or closer to the lens. The rate of
curvature change is greater toward the center of the lens due to the power
distribution of the incident laser beam. Therefore, the heating and the
expansion
is greater toward the center of the lens. Fixed collimators constructed of
transmissive optics are very susceptible to thermal Tensing which reduces
their
effectiveness for use with high power lasers.
Collimators are also constructed of reflective optics, combinations of flat
and shaped mirrors, such that the light beam is reflected from the optical
elements. Reflective optical elements are typically manufactured from
materials,
such as copper, which can withstand greater energy densities without damage.
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Also, thermal Tensing is not as severe in reflective optics as compared to
transmissive optics. Thus reflective collimators are more suitably used in
high
power laser applications. However, a fixed, reflective collimator cannot
compensate for the thermal Tensing of a laser output coupler nor for the
thermal
Tensing of a focal optic.
Certain numerically controlled laser cutting machines have been
configured to cut heavy plate and others have been configured to cut lighter
sheet
metal. While both types of machines use a laser, and use some mechanism for
traversing the laser with respect to the workpiece, the two different
applications
(heavy plate versus light sheet) cause the machines to be configured with
significant differences. For example, a machine configured for cutting heavy
plate must, of course, have a high power laser. The beam path and optical
system
in such machines must be suitable for the application. Typically longer focal
length lenses are used to cut thick material. The mechanism for traversing the
cutting head over the workpiece usually employs conventional rotary servo
motors with ball screws or rack and pinion drives. These tried and true drive
mechanisms have speed capabilities which are more than adequate for the slow
cutting rates (generally 100 inches per minute or less) which characterize the
cutting of thick plate. In addition, piercing of the workpiece is usually a
more
significant problem with heavy duty machines.
A typical arrangement for a laser plate cutting machine is disclosed in
Maruyama U. S. Patent No. 5,237,151. In that configuration the laser is
carried
on the machine structure and travels with the machine. Such machines are
successful in cutting very large thick plates. However, such machines carry
heavy moving loads which reduces their acceleration and deceleration
capabilities. Therefore, these machines lack the nimbleness required for
cutting
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thin materials at high velocities. Moreover, material handling on such
machines
tends to be cumbersome in that the material is loaded over and between the two
rails which carry the machine.
A typical requirement for machines which cut lighter duty material such as
sheet stock is for high production which usually means high cutting speeds and
fast traverse rates. The sheet stock is thinner so the power of the laser need
not
be very high. With lower power lasers the optics systems are often less
complex.
Shorter focal length lenses are used in the cutting head, such that the lens
is
closer to the cutting nozzle. In the context of lighter duty machines it has
been
suggested to utilize linear motors for enhancing the cutting speed and
accuracy.
Linear motors provide travel at speeds up to 6000 ipm and are direct drive
systems resulting in very accurate positioning. Linear motors, however, bring
their own set of problems. Large magnetic attraction forces exist between the
linear motor and its magnet track. Machine ways and structure must be capable
of carrying this load while precisely maintaining a small gap between the
motor
and magnets. At the same time, very precise alignment is required between the
pairs of linear motors driving the bridge, which can be difficult given the
machine's structural requirements of strength and rigidity to support the
linear
motors.
Heretofore efforts to produce a universal machine capable of cutting both
thin and thick material have not been particularly successful, insofar as the
current inventors are aware. A light duty machine "beefed up" to cut heavier
stock will usually not be successful. A machine configured for heavy duty,
thick
material cutting will have inadequate nimbleness to cut thin material quickly
and
economically.
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To provide a versatile machine tool adaptable for cutting both thick plate
as well as thinner sheet, the present invention provides a heavy-duty laser
plate
cutting machine that, in conjunction with other features, provides for the use
of
focal optics with different focal lengths and employs linear motors for the
main
traversing mechanisms. Linear motors are used to traverse the bridge along the
X-axis and the cutting head along the Y-axis. For sheet metal or other thin
materials which can be cut at higher speeds, linear motors provide high
acceleration rates, high velocity and accurate positioning. For thick plate,
linear
motors provide highly accurate positioning at lower speeds and rapid
acceleration, travel and deceleration when traversing between cuts. The
relatively light weight bridge structure allows high cutting speeds and rapid
traverse speeds, and in conjunction with the ability to change the focal
optics,
provides a heavy-duty laser plate cutting machine adaptable for cutting heavy
plate and light sheet at commercially acceptable production rates.
The focal length of the optic contributes to the diameter of the focal spot
and thus the energy density, Watts per unit area, at the focal spot. Shorter
focal
length optics create smaller focal spots having higher energy densities. The
focal
length of the optic also contributes to depth of focus of the focal spot with
longer
focal lengths having greater depth of focus. Shorter focal length optics are
advantageous for cutting thinner materials while longer focal length optics
are
advantageous for cutting thicker material. To provide for the use of focal
optics
with different focal lengths, an optic with one focal length can be used for
cutting
thick plate, and another with a different focal length can be used for cutting
thinner materials. The ability to change focal lengths is an important feature
in a
heavy-duty plate machine adapted to also cut lighter materials. When the
machine is not being operated for thick plate, a new focusing optic may be
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inserted and the machine can cut thinner materials at high speeds, minimizing
the
downtime of the machine.
Preferably, an automatic and real time control system maintains the size of
the laser beam on the focusing lens and compensates for thermal tensing of the
laser output coupler and of the focusing lens. The machine is preferably of a
"flying optic" construction with a separated material loading station outside
of
the cutting area, the material being loaded on pallets which are moved into
the
cutting area for processing, makes loading and unloading material much easier.
The material carrying pallets are readily accessible for manual loading and
10 unloading and are easily adapted for automated material handling systems.
In view of the foregoing, it is a general aim of the present invention to
provide a heavy duty laser cutting machine which is specially configured for
economical cutting of medium and lighter duty materials at reasonably high
production rates.
An objective of the present invention is to provide a laser equipped cutting
machine having a combination of features and functions adapted to cutting a
relatively thick plate at production rates, with sufficient versatility to
function on
thinner materials at commercially acceptable cutting rates.
A further object of the present invention is to provide a laser equipped
cutting machine that is easily adjusted prior to running the machine to
prevent
binding due to misalignment.
In accomplishing the foregoing objectives, the present invention combines
a number of features in a unique and novel way. The heavy duty cutting is
facilitated by utilizing a high power laser, generally 3 kilowatts and above.
The
optical system is provided with real time compensation for thermal changes in
the output coupler and focusing optics occasioned by the high power of the
laser
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beam. The cutting head is provided with the ability to switch focal lengths,
so
that the same cutting head can be provided with different focal length optics
depending on the nature of the material being cut. Finally the machine is
provided with linear motors for the main traversing mechanisms (at the least
the
X and Y mechanisms). In cutting heavy stock, the linear motors provide
increased accuracy and rapid acceleration, deceleration, and traverse between
cuts. In cutting lighter duty material, cutting speed can be substantially
increased
without sacrificing accuracy. Additionally, the linear motors are accommodated
in the machine of the present invention in a way which simplifies alignment
while maintaining a rigidified structure.
Other objects and advantages of the invention will become more apparent
from the following detailed description when taken in conjunction with the
accompanying drawings.
Brief Description Of The Drawings
The accompanying drawings incorporated in and forming a part of the
specification, illustrate several aspects of the present invention, and
together with
the description serve to explain the principles of the invention. In the
drawings:
Figure 1 is a front elevation, partly simplified and partly broken away,
showing a laser-equipped machine tool in which the present invention can be
embodied;
FIG. 2 is a plan view of the machine of FIG. 1;
FIG. 3 is an end elevation of the machine of FIG. 1;
FIGS. 4a - 4d are diagrams illustrating the phenomenon of thermal
lensing;
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FIG. 5 is a diagram illustrating focal position offset as a function of steady
state output power for two typical focusing lenses;
FIG. 6 is a diagram illustrating the rate at which a lens will absorb energy
from an incident laser beam at different power levels, and thus change in size
and
transmissive characteristics;
FIG. 7 is a diagram illustrating the thermal loading of a lens, showing the
signals applied to the laser and relative distortion in the lens;
FIG. 8 is a diagram illustrating an example of a three dimensional
correction curve as used in the practice of the present invention;
FIG. 9 is a block diagram illustrating a control system exemplifying the
present invention.
FIG. 10 is a plan view of the machine, partially cut-away to reveal the
base, ways and trucks shown in FIG. 3.
FIG. l0a is an enlarged view of circle l0a of FIG. 10.
FIG. l Ob is an end view of FIG. 10a.
FIG. 11 is a front view of the bridge of the machine of FIG. 1.
FIG. 12 is a cross-sectional view taken about line 12-12 in FIG. 11.
FIG. 13 is a bottom view of the base of the bridge of FIG. 11.
FIGS. 13a and 13b are partial cross-sectional views of the bridge base of
FIG. 13 taken along the lines 13a-13a and 13b-13b.
FIG. 14 is a plan view of the machine of FIG. 2 having a material
load/unload station.
While the invention will be described in connection with certain preferred
embodiments, there is no intent to limit it to those embodiments. On the
contrary, the intent is to cover all alternatives, modifications and
equivalents as
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included within the spirit and scope of the invention as defined by the
appended
claims.
Detailed Description Of The Preferred Embodiments
Turning now to the drawings, FIGS. 1 and 2 illustrate a laser-equipped
cutting machine capable of cutting heavy plate in accordance with the
invention.
In the illustrated embodiment, a laser cutting machine tool 20 typically
includes a
collimator 22 interposed between a laser source 21 and a cutting head
indicated
generally at 23. Briefly, laser cutting machine 20 consists of a base 30 which
supports a worktable 31 on which rests a workpiece 32. The laser source 21
will
not be described in any detail except to note that in this case it is a high
power
device producing a beam having power of 3 kilowatts or more, preferably 4
kilowatts and most preferably in the order of 6 kilowatts or more.
Laser source 21 delivers a high power laser beam to collimator 22 which
directs a collimated beam 60 to first bending mirror 56, then to second
bending
mirror 56a mounted above cutting head 23, and then to a focusing optic mounted
in a lens holder 36. The laser beam is directed from focusing optic through
nozzle 37 disposed immediately above the workpiece. Pressurized gas is also
directed through nozzle 37, coaxially to the laser beam, to assist the cutting
process. The pressurized gas serves to facilitate and/or shield the cutting
process,
and creates a gas stream which helps remove vaporized and molten material from
the cut.
Cutting head 23 is adapted for movement along one axis, here called the
Y-axis which is mounted to bridge 24. Bridge 24 is adapted for movement along
an orthogonal X-axis. The workpiece or plate 32 is supported on a pallet or
table
31 below bridge 24. Movement of cutting head 23 is coordinated with movement
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of bridge 24 to define a precise path on plate 32. Laser 21 and cutting head
23
are controlled to pierce and cut holes and shapes and then the boundary of a
part
from the plate 32. The X and Y axes include linear motors to provide versatile
locomotion to the bridge 24 and cutting head 23 for cutting heavy plate as
well as
lighter or thinner material, as will be describe in more detail herein.
Cutting head 23 traverses the full length of the worktable 31 (left to right
as shown in FIG. 2) and the full width of the worktable 31 (top to bottom as
shown in FIG. 2 or left to right as shown in FIG. 3). Those boundaries define
the
cutting area and the maximum size workpiece 32 that the machine can process.
With this machine configuration the length of the laser beam path between the
laser output coupler and the focusing optic in cutting head 23 changes as the
cutting head 23 moves over the cutting area.
As best shown in FIG: 2, beam 60 is directed from collimator 22 to
bending mirror 56 which is attached to the end of bridge 24 by way of panel
25.
Mirror 56 moves with bridge 24 such that the distance between collimator 22
and
minor 56 is variable and dependent on the position of bridge 24 within its
length
of travel.
As best shown in FIGS. 2 and 3, beam 60a is beam 60 reflected from
minor 56 to mirror 56a mounted above cutting head 23. Minor 56a moves with
cutting head 23 such that the distance between minor 56 and mirror 56a and
thus
the length of beam path 60a is variable and dependent on the position of
cutting
head 23 within its length of travel.
As best shown in Fig. 3, beam 60b is beam 60a reflected from minor 56a
through a lens (not shown) carried in lens holder 36 and then through nozzle
37
to the workpiece 32. Cutting head 23 is shown in a retracted position above
the
work. Nozzle 37 would typically be positioned within a few mm above the
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surface of the work 32 when cutting. The length of beam 60b is variable and
dependent upon the thickness of material 32 processed, the position of the
focal
optic within its length of travel and the vertical position of cutting head 23
within
its length of travel.
5 In summary, the length of the beam path between the laser output coupler
and the workpiece is variable in a flying optic cutting machine. The range of
variation of the path length is dependent upon the length of travel along the
X, Y,
and Z axes. In practical terms, the amount of variation in this Z-axis is
insignificant and can be ignored. However in some configurations the amount of
10 variation in the Z-axis is significant and should be compensated. In the
configuration shown, the beam path is shortest when cutting head 23 is
positioned to the extreme left end of the work support 31, see Fig l, and to
the
extreme right side of the work support 3 l, see Fig. 3, and when cutting thick
material. It is longest when cutting head 23 is positioned to the extreme
right end
15 of the work support 31, see Fig 1, and to the extreme left side of the work
support
31, see Fig. 3, and when cutting thin material. In the exemplary machine, the
difference in beam path length between those two extremes is a little over 6
meters.
Collimator 22, part of an automatic beam control system, provides means
to vary the divergence of laser beam 60 at the collimator output, and thus to
control the size of the beam at the focusing optic in the cutting head.
Broadly,
the automatic beam control system and the collimator can be considered to
assist
the focusing optic in the cutting head to focus the beam. Preferably, it does
so by
maintaining a controlled and consistent spot size projected onto the focusing
optic. However, it can also be controlled to vary the spot size to produce
desired
effects on the beam projected onto the workpiece. The collimator is preferably
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motorized by means of a servo motor or other precision prime mover, and
positioned under CNC control to correct and compensate for changing beam
characteristics.
Path length changes of the order noted can have a marked impact on the
size of the beam incident on the focusing optic and correspondingly on the
shape
and position of the focused beam aimed at the workpiece. Observable changes
will be noted in the quality of the cut and size of the part as the cutting
head is
traversed across its range of travel.
In accordance with one aspect of the present invention, a control system
operates a beam control device, in the illustrated embodiment the collimator
22,
to control beam divergence in such a way as to maintain a desired beam size at
the focusing optic. In the simplest case, the beam size at the focusing optic
is
controlled to maintain a constant size. However, there are cases where the
system
can introduce controlled variations in beam size to compensate for other
system
variables. Unless the context indicates otherwise, the term "controlled beam
size" is intended to encompass both alternatives. In its most preferred form
the
present invention is utilized with a reflective collimator so as to be able to
operate
in what is considered a high power range, generally three to four kilowatts
and
above. Transmissive collimators or other transmissive beam correction devices
are preferably avoided in high power applications because of the additional
thermal lensing and beam distortion problems they introduce when operated at
high power levels. A preferred form of collimator 22 is described and claimed
in
a commonly owned application in the name of Ira E. Cole III, serial number
09/353,936, filed July 15, 1999, the disclosure of which is hereby
incorporated by
reference. Other continuously adjustable collimators can also be used in the
practice of the present invention. U.S. Patent 5,442,436 shows an adjustable
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collimator having four reflective optical elements. Such a collimator, with
the
addition of a servo motor adapted to drive its adjustment mechanism, could
also
be used in the practice of the present invention.
A controllable drive system, such as a servo motor and drive, is required
to operate a continuously adjustable collimator and thereby correct for beam
divergence changes. Other forms of controllable drive systems such as stepper
motors, servo controlled linear motors, or servo controlled fluid driven
cylinders
could be used. Such systems are characterized by the ability to precisely
position
a driven device such as a collimator. Such servo systems usually include some
form of position feedback. Adaptive optics, which alter the curvature of a
mirror,
may be employed to the same purpose as the collimator.
In addition to path length caused changes of beam characteristics, thermal
Tensing causes another change. Thermal Tensing is the distortion of an optical
component caused by heat absorbed from the input beam. Absorbed heat distorts
the optic causing a change in focus characteristics. The rate and amount of
distortion is dependent upon the power of the beam, thermal conductivity of
the
optic and its cooling system and the length of time the beam is on or off.
Upon
reaching thermal equilibrium, when the absorbed heat is in balance with that
removed by the lens cooling system, the shape of the optic surface remains
constant. When the beam is turned off, the optic relaxes and returns to its
original shape.
Thermal Tensing is more pronounced in transmissive optics such as a laser
output coupler or a focal lens. FIGS. 4a-4d illustrate thermal Tensing. FIG.
4a
illustrates a laser output coupler 80 which partially reflects and partially
transmits
beam 81. As is typical, the inner surface 82 and outer surface 82a are
contoured
such that transmitted beam 83 has a narrower waist 84 positioned "L" distance
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from the output coupler and having a diameter 85. Fig. 4b illustrates the
effects
of thermal Tensing. The output coupler expands as heat is absorbed. The
original
optical surfaces 82a and 82b, shown in dotted lines, are distorted, as shown
in
solid lines and in an exaggerated fashion as 82c and 82d, thus causing a
change in
characteristics of output beam 83'. The beam waist 84' is made smaller and
shifts
O L in position.
The change in beam propagation characteristics of an optic operating at
various power levels can be determined by test and is substantially
repeatable.
From test data, it is possible to derive an equation to predict the magnitude
of the
beam waist shift and divergence change as a function of integrated time and
power. As will be described in greater detail below, the present invention
provides the ability to combine such information with beam path length
information to position a collimator to compensate for and thus correct
changes
in beam characteristics such that intended characteristics are maintained.
Focal optics are also subject to thermal Tensing. FIG. 4c shows a plano-
convex focal lens 86 having focal spot 87. FIG. 4d shows optic 86' distorted
in
exaggerated fashion to illustrate thermal Tensing and shows a shifted focal
spot
87'. Distance d, between focal spots 87and 87' represents the shift in focus
caused by thermal Tensing in exaggerated fashion. In high power devices this
shift can be substantial. For example a piano-convex zinc selenide 10 inch
focal
length lens subjected to a 6 kilowatt beam 35mm in diameter was determined to
have a focus shift exceeding 6mm. In the present invention, thermal Tensing of
the focal optic is compensated separately from thermal Tensing of the output
coupler and is accomplished by adding a correction signal to the drive system
which positions the focal optic.
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FIG. 3 shows the cutting head 23 with nozzle 37 positioned over the
workpiece 32. The Z-axis driving mechanism is schematically illustrated and
identified as D2. That drive moves the cutting head in the vertical, Z-axis
direction, and positions the nozzle at a predetermined distance above
workpiece
32 for cutting. A second drive, identified schematically as D,, translates the
lens
carrier 35 within cutting head 23. The lens carrier drive, as it is sometimes
referred to herein, adjusts the position of the focal spot relative to the
surface of
workpiece 32 without changing the position of the nozzle with respect to the
same surface. It is used to position the focus spot correctly for piercing and
cutting various materials. In some cases the lens is driven downwardly during
part of the piercing cycle. In all cases the position of the focal spot has a
predetermined desired position. However, thermal Tensing will cause an
undesirable shift in focal spot position. In practicing certain aspects of the
invention, drive D, is provided with a Z-axis position command and a Oz
position
offset which corrects for a shift in position of the focus spot caused by
thermal
Tensing.
FIG. 5 illustrates, for two different optics, the amount of focal point shift
as a function of power incident on the optic. Curve A represents the focal
spot
shift produced by various power levels from zero through six kilowatts. Curve
B
represents the same information for a different focal length optic. The
invention
compensates for such shifts by introducing corrective action.
Compensating for thermal Tensing is not a simple steady state problem as
suggested in FIG. 5. A lens can be considered a thermal integrator, which
stores
and releases heat with the rate of change dependent upon the power of the
beam,
the effectiveness of the cooling system, and duration which the beam is
applied.
The resulting changes in focus characteristics occur at a rate which can be
CA 02319293 2000-09-14
described by an exponential curve. Typically a laser output coupler will have
a
time constant on the order of 6 seconds after turn-on before sixty-three
percent of
the full thermal effect is realized. In FIG. 6 exponential curves Pi, P2 and
P;
graph the rate of change of beam characteristics based on different average
power
5 levels applied to an output coupler. P1 is the lowest power level and P3 is
the
highest .
Also it must be considered that the laser will not be continuously on, but
will be switched on and off for fairly brief intervals. When the laser is
switched
off, the lens will cool at a rate also describable by an exponential curve. In
10 summary, the amount of thermal distortion of an output coupler or other
optic is a
variable, dependent upon the power on the optic with the rate of change
describable by an exponential function having a time constant matching that of
the optic system and dependent on the time which has lapsed after the beam is
turned on or off. FIG. 7 shows a power versus time plot for a typical optic,
such
15 as the output coupler of a high power laser. Curve segment 90 extending
from to
to t, shows the rate of thermal buildup in the optic after power is initially
applied.
At time t, the laser is switched off. Curve 91 shows the exponential cooling
rate
of the optic until it reaches t2 at which point the laser is turned on again.
Curve
92 shows the rate of thermal buildup from t2. The curve of FIG. 7 can be
20 considered an integrated power time representation of the amount of thermal
energy stored in an optic. Such information is used in the practice of the
present
invention to determine compensation values to correct for thermally caused
changes in beam characteristics.
In order to compensate for changes in the optical system, a signal is
utilized indicative of the integrated energy level stored in an output
coupler,
operating between its two steady state energy points of off and fully
saturated. In
CA 02319293 2000-09-14
21
real time, the amount of thermal energy stored in the optic is tracked and
determined. That information is used to determine a correction value. The
correction value is introduced in real time to a drive system to adjust a beam
control optic to correct focal characteristics of the beam. In systems, such
as the
S exemplary one, in which the beam path length changes, the amount of thermal
energy stored in the output coupler and the length of the beam path are used
in
combination and in real time to determine the compensation value. In machine
tools having a fixed beam path length only the thermal energy stored in the
output coupler would be used to determine the compensation value.
In a currently preferred practice of the invention, a reflective collimator is
interposed between the laser and the focusing optic, and has an adjustment
mechanism operated to compensate for both thermal tensing changes and path
length changes. The nature of the changes can be conceptualized as introducing
a
correction based on a three-dimensional curve, a form of which is illustrated
in
Fig. 8. Turning to Fig. 8, a three axis grid is shown in which a first axis
100
defines path length changes from a fixed reference, such as zero, at the
ordinate
to the maximum path length change. Thus, the cutting head in the shortest path
length position, corresponds to a point on the axis 100 at the ordinate, and
movement of the cutting head in directions which increase the path length move
the point in the direction of the arrow 100.
Integrated output power in units such as kilowatts, is plotted along the axis
101. The minimum power point is at or near the ordinate, and increasing power
levels are displaced from the ordinate in the direction defined by arrow 101.
The
output power plotted along this axis is the integrated output power at any
given
point in time, such as is shown in Fig. 7.
CA 02319293 2000-09-14
22
The third axis in the three-dimensional plot is the offset for the collimator.
The offset in one example has a zero position at the ordinate and increasing
positive deviation indicated by the arrow 103. The scale can also be arranged
with zero offset at an intermediate position providing both positive and
negative
offsets on respective sides of the zero point.
Fig. 8 has a three-dimensional surface 105 plotted thereon which is the
relationship between output power, path length, and collimator offset for a
particular machine tool. Thus, it is known that for any given amount of
integrated energy in the optic and for any given path length in the machine,
the
collimator will need to be adjusted by the appropriate offset defined by the
surface 105 in order to maintain the beam waist (Fig. 4a and 4b) at the
appropriate size and position, in order to keep the size of the laser beam
incident
on the focusing optic at the desired diameter. As will be clear from Fig. 7
the
integrated output power in the lens will change over time based on whether the
laser is on or off, and thus the input along the axis 101 will be continuously
changing as the laser beam is triggered. Similarly, the path length will be
changing as the cutting head is traversed to cut a particular part, causing
the path
length along the axis 100 to be continuously changing. As a result, the three-
dimensional relationship identified by the surface 105 will cause the
resulting
offset to be continuously changing, and the offset will be coupled to a servo
or
other prime mover in the collimator to continuously and in real time adjust
the
collimator to maintain the beam size on the focusing optic at the desired
size.
Consider for example that at a given instant in time the operating
conditions for the system are defined by a point P, on the surface 105. That
demands a given offset as determined by the three-dimensional relationship. As
the laser remains on, however, thermal loading will increase and the operating
CA 02319293 2000-09-14
23
point will begin to move in the direction indicated by the arrow 107.
Similarly, as
the cutting head is traversed the operating point P, will move in one
direction or
the other as indicated by the double headed arrow 108. The result may, for
example, be a movement of the point from P~ to P2 along the path 109. That
requires a continuous change in offset which is communicated to the collimator
in order to maintain the desired beam size.
To correct for thermal Tensing of the focusing optic integrated power-time
information for that optic is utilized to determine a compensation value which
is
introduced in real time to lens driving system to correct and thereby maintain
the
intended position of the focal spot. In the illustrated embodiment the
correction
signal is added as an offset to the signal which drives the servo which
controls
the position of lens carrier 35 in cutting head 23. In other systems, such as
those
using adaptive optics, the shift signal can be used as an offset in the
controller for
the adaptive optic. In some cases, the shift signal can also be used as an
offset
for the Z-axis control of the cutting head.
When compensations are made for both the output coupler and the focal
optic, the position of the focal spot relative to the workpiece is rendered
substantially consistent regardless of the position of the cutting head,
regardless
of the operating power level, regardless of the steady state conditions of
beam
full on and off, and regardless of the intermediate conditions between those
steady states.
Turning now to Fig. 9, there is shown a block diagram for a control system
adapted to make the aforementioned corrections in accordance with the present
invention. A computer numerical control system 120 is represented by the large
block, with a number of functional blocks within it. The CNC central processor
121 is separately illustrated, although it will be appreciated by those
skilled in the
CA 02319293 2000-09-14
24
art that many of the functions separately illustrated within the block 120 are
performed in whole or in part by the CNC central processor. They are shown
separate in Fig. 9 as an aid in understanding the present invention.
The illustrated numerical control system is a four axis device, with
conventional X, Y and Z axes and a fourth axis parallel to the Z-axis devoted
to
the position of the lens carrier. The invention can be applied with more or
fewer
axes. The CNC system 120 includes a position and velocity control module for
each of the X, Y and Z axes. Thus, an X position and velocity control module
123 responds to signals from the central processor 121 to control an X-axis
servo
drive 124 which in turn controls the X-axis servo motor 125. The position of
the
bridge along the X-axis is represented by the box 126, and it is seen that a
feedback position element 127 has an output 128 connected as a feedback input
to the X position and velocity control module 123. Thus, the CNC is capable of
driving the bridge along the X-axis to any coordinate and at any selected
velocity
in a conventional fashion.
A Y position and velocity control module 133 has associated elements
including a Y-axis servo drive 134 and a Y-axis servo motor 135 which control
the position of the cutting head along the Y-axis 136. Position feedback
element
137 provides feedback to the Y position and velocity control module 133. The Z
position and velocity control module 143 is similar to the X and Y modules in
the
present embodiment, in that is acts through a Z-axis servo drive 144 upon a Z-
axis servo motor 145. In this case, however, the servo motor 145 has feedback
element 146 associated directly with the motor, and that position feedback is
coupled back to the feedback input of the Z position and velocity control
module
143. The Z-axis position is represented by the block 147, and the block 148
indicates that it is the cutting head 148 which is positioned along the Z-
axis, and
CA 02319293 2000-09-14
more particularly the nozzle which is represented by the block 149. Additional
position feedback 150 is provided from the nozzle and coupled back to the Z
position and velocity control module 143. The cutting head can have, for
example, position feedback dependent on the proximity to the workpiece which
5 obviously can vary in thickness. Thus the position feedback element 146 can
respond in absolute coordinates to the position of the drive, whereas the
feedback
position element 1 SO may respond in terms of the position of the nozzle 149
with
respect to the workpiece.
In practice, whenever a cut is to be made, the CNC central processor 121
10 operates through the modules 123, 133, and 143 to position the cutting head
with
the nozzle above a selected point on the workpiece. Piercing is accomplished
and
then the nozzle is traversed across the workpiece in directions coordinated to
the
shape and size of the part to be cut, with the laser beam switched on and off
during the traverse of the cutting head to cut the appropriate part.
15 In practicing the invention the numerical control system 120 includes a
module 151 which operates in conjunction with the modules 123 and 133 to
determine an X, Y path length calculation for purposes of compensating for the
beam path length. Thus, the modules 123 and 133 have precise information on
the position of the cutting head with respect to the workpiece, and thus have
the
20 information necessary to calculate the path length or deviations in the
path length
from a set or home position. The module 1 S 1 performs that computation to
produce the path length correction which is one of the inputs to the three-
dimensional correction of Fig. 8.
The CNC central processor 121 also has the capability of positioning the
25 lens holder along the Z-axis. This is, in effect, a fourth numerically
controlled
axis, sometimes called the U-axis. A focus position control module 153 similar
CA 02319293 2000-09-14
26
to the modules 123, 133, and 143, responds to commands from the central
processor 121 to control the position of the lens carrier within the cutting
head.
An output signal from the module 153 is coupled to a focus servo drive 154 and
which controls a focus servo motor 155. The servo motor 155 has a feedback
module 156 associated therewith which couples a position feedback signal to
the
module 153. The servo motor 155 controls the vertical position of the lens
carrier represented by the block 157 which carries the lens holder represented
by
the block 158 which in turn carries the lens represented by the block 159.
Thus, as so far described, the focus position control operates in a similar
fashion to the Z position control module 143 to control the position of the
lens
within the cutting head. Operation of the focus position control 153 with the
Z
position maintained constant will tend to shift the focus point either toward
or
away the workpiece depending on the direction of controlled movement.
The CNC central processor 121 also controls certain aspects of the laser
operation, including laser beam on/laser beam off, the power level at which
the
laser is commanded to operate, and in cases of pulse width modulated control
of
the laser, the frequency and duty cycle of the pulse width modulated output.
Thus, the CNC central processor 121 is coupled to a beam on/off control module
160 which in turn is coupled to a laser control processor 161 in the laser
control
module generally indicated at 162. The laser processor 161 accepts signals
from
a numerical controller to take such actions as turning the beam on and off.
The
module 160 is adapted to send those signals to the computerized control 161
which then responds by controlling the on or off state of the beam. The
control
module 162 also includes a laser power control unit 163. The CNC processor
121 has associated therewith a power level control module 164 which is an
interface to the laser power control processor 163. Thus, if a particular cut
is to
CA 02319293 2000-09-14
27
be made with the laser beam set at 3000 watts, for example, the processor 121
will output data to the power level control module 164 indicating that the
laser
power control 163 is to be instructed to operate the laser at the 3000 watt
level.
The module 164 thereupon sends a signal to the laser power control unit 163
which causes the processor within the module 163 to operate the laser with a
3kW output.
For operating the laser beam under pulse width modulated control, the
CNC processor 121 has interface elements including a frequency control module
170, a duty cycle control module 171, and a pulse width modulated pulse
control
172 to set the frequency of the on and off periods and the duty cycle within
that
frequency at which the laser beam is to be operated. The frequency is set by
way
of interface module 170 and the duty cycle by way of interface module 171.
Those signals are combined in the pulse width modulated pulse control
interface
element 172 to produce a duty cycle output signal on a line 173 which is
coupled
to the laser power control computer 163 for pulsing the laser on and off for a
given duty cycle at a given frequency as demanded by the CNC central processor
121.
In practicing the invention, a pair of laser power integrators 180, 181 are
provided for monitoring the energy stored within two different optical
elements
in the system. In the typical application, the modules 180 and 181 will have
different time constants and different energy levels, and will be associated
with
particular optical elements in the system to monitor the stored energy
condition
of the those elements. Recalling Fig. 7, it will be seen that the integrators
180
and 181 are capable of monitoring the position along the curve relating time
to
stored energy. Thus, an output signal from the integrator will continuously
indicate the energy stored within the associated optical element in real time.
CA 02319293 2000-09-14
28
In the illustrated embodiment, the integrator 181 is associated with the
output coupler of the laser. It may have, for example, a time constant in the
range of six to ten seconds and be capable of indicating the stored energy in
the
optic associated with the laser output power level. It is recalled that the
curve of
Fig. 7 is intended to illustrate the energy stored in the laser output
coupler, and
thus the integrator 181 monitors the on and off intervals of the laser during
normal and PWM operation. The integrator 181 has stored therein a curve which
represents the energy storage and energy release values and time constants for
the
associated optical elements and thereupon integrates positively to increase
the
stored energy level whenever the laser is on, (such as illustrated at 90 and
92 of
Fig. 7), and integrates negatively to decrease the stored energy level, (such
all
illustrated at 91 of Fig. 7), whenever the laser is off. Thus, curves such as
illustrated in Fig. 7 will be generated over time by the integrator 181 and
will
serve as a measure of the energy stored within the laser output coupler.
The integrator 180 is similar in construction but will typically have a
different time constant associated therewith. In the preferred embodiment the
integrator 180 is associated with the focus optic. As such it may have a time
constant which is much nearer twenty seconds than the six seconds of the laser
output coupler. However, it will have an exponential build-up and exponential
decay, similar to the output coupler. Since focusing optics are typically
indirectly
cooled, the cooling curve may be somewhat shallower than the warming curve.
However, those conditions will be determined by experiment for a particular
set
of hardware, to produce a particular set of parameters which generate a curve
such as that illustrated in Fig. 7, but with values and constants defined by
the
physical response of the particular optic to incident laser energy at given
power
levels. It is also noted that typically a laser will have a shutter box which
will be
CA 02319293 2000-09-14
29
closed at times with the laser beam on. In those conditions, the output
coupler
will have energy incident thereon, but the focus optic will not, so the
integrators
will be operating under different conditions.
In practicing the preferred embodiment of the present invention, the path
length determination made by module 151 is combined with the integrated
energy information collected by integrator 181 to determine from a
characteristic
such as the aforementioned three-dimensional relationship, a position control
correction to be introduced into the laser beam by way of the collimator 22.
Thus, a collimator position control module 190 has a first input coupled to
the
energy integrator 181, and a second input coupled to the X, Y path length
calculation module 151. The collimator position and control module 190 has
data stored therein data corresponding to the three-dimensional relationship
of
Fig. 8 and produces an output representing a correction signal to be sent to
the
collimator. In the present embodiment that output is produced as an offset
signal
on a signal line coupled to the collimator servo drive 191. The drive 191
operates
the collimator servo motor 192 which in turn positions the collimator 194 to
adjust the divergence of the laser beam so as to maintain a beam size of the
desired dimension at the focus optic. The collimator servo motor 192 has a
position feedback element 193 associated therewith which returns a feedback
signal to the collimator position control 190.
The focus optic integrator 180 has an output coupled as an input to the
focus position control 153. It is recalled that the position control 153
operates on
a primary signal from the CNC central processor 121 to control the position of
the lens 159. An offset signal is provided by the integrator 180 which serves
to
modify the output signal of the focus position control in accordance with the
energy stored in the focus optic. Thus, the output signal from the focus
position
CA 02319293 2000-09-14
control 153 is modified to take account of the distortion of the focus optic
caused
by the laser beam of particular power incident thereon for its actual on and
off
times. That offset adjusts the output signal of the focus position control so
that
the actual position of the lens 159 is adjusted in accordance not only with
the
5 machine position commands of the CNC central processor 121, but also to
correct for distortion in the optics created by the laser beam passing through
the
focus optic at particular power levels for particular lengths of time.
In summary, the control system illustrated in Fig. 9 operates in a
conventional fashion to traverse the cutting head over the workpiece and to
10 control the power level and on and off condition of the laser beam to cut
parts
from the workpiece. In addition, the laser power control 163 has a pair of
integrators associated therewith to integrate the stored energy in the two
primary
devices which will distort as a result of high power laser energy being
incident
thereon. In connection with the output coupler, the energy is integrated
15 according to the power level set for the laser and the actual on and off
intervals
for the laser, and that signal is coupled along with a beam path length
correction
to operate through a three-dimensional correction curve to adjust the
collimator
so as to maintain a constant beam size at the focus optic for all positions of
the
cutting head and all possible states of distortion of the output coupler.
Thus, as
20 the CNC controller 121 operates the cutting head to move it across the
workpiece
to cut particular shapes, the path length calculation is continuously made by
the
module 151 and the collimator position control is continuously adjusted to
maintain the desired spot size at the focus optic, all in real time.
Similarly, as the
laser is on for longer periods of time and tends to increasingly distort the
output
25 coupler until equilibrium is reached, a signal is produced by the
integrator 181 to
CA 02319293 2000-09-14
31
also adjust the collimator to take account of the distortion of the output
coupler
occasioned during the cut.
In addition, also in real time, a second integrator with a separate time
constant maintains the integrated energy level of the focus optic and couples
that
signal through a focus position controller 153 to separately adjust the
position of
the lens with respect to the workpiece. Thus, for example, the first
correction
control system maintains a constant spot size or beam diameter on the focus
optic, and the second control system which takes account of distortion in the
focus optic adjusts the focus of that optic to maintain the spot size where
the
CNC central processor 121 commanded it to be.
Having described in detail the operational elements of the laser system,
with particular emphasis on the beam delivery system and its control,
attention
will now be directed to certain aspects which provide adaptability of the high
power laser plate cutting machine for both thick plate and lighter stock such
as
sheet metal.
In order to adapt the machine for cutting a wide range of material
thickness, provision is made for changing the focal length of the focusing
optic.
As best seen in FIG. 11, cutting head 23 includes an optical housing 35 which
carries a focusing optic (not shown). The focusing optic is mounted in a
holder
36 which is mounted in a slot 36a in the optical housing or carrier 35 . While
a
number of interchangeable lens configurations can be used, the illustrated
embodiment employs the preferred implementation in which the carrier 35
provides a plurality of slots 36a (4 slots being shown in FIG. 11 ). Each slot
is
intended to hold an optical element of a different focal length. For example,
one
of the slots is dedicated to 10 inch focal length lenses. When the machine is
desired to operate with that focal length optic, a lens holder, carrying a 10
inch
CA 02319293 2000-09-14
32
focal lens, is inserted in the optical housing 35 in the slot 36a for 10 inch
focal
length optics. All of the other slots 36a will be filled with blank holders,
which
contain no lens, and simply serve to seal the beam path through the cutting
head.
When it is desired to change to a longer or shorter lens, the 10 inch focal
length
optic holder is removed and replaced with a blank holder, whereas a new lens
holder, say for 12.5 inch focal length is put into the slot physically
associated
with that focal length.
The ability to change optics as described above is important in a versatile
machine. It will be most efficient to use relatively long focal lengths (say
10 inch
or more) for thick plate, because of the increased depth of focus and larger
spot
size they provide. However, with thinner materials, it is preferable to use
shorter
focal length lenses. The ability to install different focal length lenses in
the
cutting head without making other adjustments provides means to quickly change
from longer focal length lenses for cutting heavy plate to shorter focal
length
lenses for lighter or thinner material.
In addition to the ability to change optics, the present invention also
employs linear motors to adapt the machine for cutting a wide range of
material
thickness. As previously indicated, when cutting thinner materials the cutting
speed of the machine can and should significantly increase relative to the
cutting
speed of thick plate. Linear motors provide for accurate positioning at high
speeds for thin sheet, while for thick plate linear motors provide highly
accurate
positioning at lower speeds, as well as rapid acceleration, travel and
deceleration
when traversing between cuts. For example, the temperature of a thick plate
workpiece in the area being cut can become unacceptably high due to the high
power laser and relatively slow cutting speed. The linear motors permit fast
and
precise re-positioning of the cutting head to a second area for initiating a
different
CA 02319293 2000-09-14
33
cut, after which the cutting head can return to the first area to finish the
initial cut.
The high traverse velocities provided by the linear motors, preferably above
3000 ipm, makes such an operation feasible, increasing the productivity of the
machine.
Referring briefly to FIG. 3 for orientation, the machine base includes a
pair of machine legs 201, 202 which carry the machine bridge 24 for
translation
along ways generally indicated at 203, 204. The configuration of the machine
base generally comprises a plurality of interlinked plates which form the
supporting structure for the raised legs 201, 202 and slag collection bed 200.
The
structure of the base and bed will not be described in detail herein, but is
described in applications Heavy-Duty Laser Plate Cutting Machine, Serial No.
09/636,205, filed August 10, 2000, and Serial No. 09/302,278, filed April 30,
1999, the disclosures of which are incorporated herein by reference. The legs
201, 202 have upper surfaces 207, 208 respectively which carry the ways 203,
204 which support and guide the machine bridge 24.
FIG. 10 shows a plan view of the machine, partially cut-away to reveal the
machine base, ways and trucks. The ways 203, 204 are supported on the upper
surfaces 207, 208 of the legs 201, 202 respectively, which are separated by
slag
bed 200. Trucks 211, 212 are slidably mounted to the ways 203, 204, preferably
utilizing linear ball slides or bearings 241 (FIG. l0a) corresponding with the
ways 203, 204. The trucks 211, 212 are adapted for translation along the X-
axis,
and support opposing ends of the bridge 24 (not shown) for translating the
bridge
along the X-axis. In practice, bellows 225 (shown cut away) attach to opposite
ends of the trucks 211, 212 to cover and protect the portions of the legs 201,
202,
ways 203, 204 and magnet track 223, 224 that would otherwise be exposed.
CA 02319293 2000-09-14
34
As described above with reference to FIG. 9, the CNC system 120 controls
an X-axis servo motor 125 and a Y-axis servo motor 135. Position feedback
elements 127 and 137 provide feedback of position and velocity in the X and Y
directions respectively. In the embodiment illustrated in FIGS. 10-1 l, The X-
axis servo motor 125 comprises two linear motors, indicated generally at 221
and
222, for driving the trucks 21 l, 212 and hence bridge 24 along ways 203, 204.
The Y-axis servo motor 135 also comprises a linear motor 321 (FIG. 12) for
driving the cutting head 23. These linear motors can translate the laser
cutting
head 23 at speeds corresponding to cutting heavy plate, as well as thinner
plate
such as sheet metal, in an extremely quick and precise manner.
As illustrated in FIG. 10, the position feedback element 127 comprises
two position sensors, indicated generally at 263 and 264. By way of example,
FIG. l0a illustrates position sensor 263, which includes read head 265
attached to
the truck 211 at a position proximate optical strip 261 affixed to leg 201,
for
detecting position and velocity data. Similarly, position sensor 264 includes
a
read head and optical strip 262 for detecting position and velocity data. This
feedback data is coordinated by the CNC system 120 for translation of the
bridge
24 along the X-axis.
As shown in FIG. 10, the trucks 21 l, 212 are driven by linear motors 221,
222. The linear motors 221, 222 each include a powered coil assembly 237, 238
called a primary section that translates over corresponding magnet track 223,
224. The primary sections 237, 238 are attached to respective trucks 211, 212
so
as to be coaxial with the magnet tracks 223, 224. The magnet tracks 223, 224
are
affixed to the upper surfaces 207, 208 of the legs 201, 202 and preferably
disposed between the ways 203, 204. The magnet tracks 223, 224 are comprised
of a plurality of individual permanent magnets stacked end to end to form a
CA 02319293 2000-09-14
continuous track and magnetic field. The primary sections 237, 238 are
energized to repel the magnetic field of the magnet tracks 223, 224 to
generate
motive force to the trucks 211, 212, and hence the bridge 24.
Shock absorbers 250a, 250b, 250c, 250d are attached to the legs 201, 202
S at each end of the ways 203, 204 to stop the traverse of the bridge 24 and
protect
the machine in the event the bridge exceeds the boundaries of the cutting
area.
Each of the shock absorbers 250a, 250b, 250c, 250d preferably comprise a
hydraulic mechanism 252 operatively connected to shaft 253 projecting
therefrom, the shaft having a rubber bumper 254 at its exposed end for
engaging
10 a runaway truck, and a return spring disposed between the hydraulics 252
and
bumper 254 to return the shaft 253 to its extended and ready position.
The trucks 211, 212 and linear motors 221, 222 are substantially the same,
and will be described in more detail with reference to FIGS. l0a and lOb
illustrating enlarged top and end views of truck 211 respectively. Truck 211
15 generally comprises four linear bearings 241 slidably engaging the ways
203.
The bearings 241 support a truck body 213 for translation along the ways 203.
The underside of the truck body 213 is structured to receive the primary
section
237, which is preferably bolted to the truck body 213. The truck body 213 also
includes a series of tapped apertures 295 at each longitudinal end for
attaching an
20 end of the bridge bottom base structure 24a (FIG. 13) of the bridge 24 to
the
truck 211. An aperture 291 is also provided in the truck body 213 for tightly
receiving a dowel pin (not shown) to connect the truck 211 and bridge bottom
base structure 24a. In this way, each end of the bridge 24 is supported by a
truck
211, 212 for translation along the X-axis. It is also of mention that the
truck 211
25 has attached thereto a conduit structure 227 having a clamp 229 for
collecting the
tubes, wires, etc. from the linear motor, position sensor, etc. for protection
and
CA 02319293 2000-09-14
36
easily translation with the truck 211. A flange 226 is connected to an end of
truck 211 for attachment of bellows 225.
Turning to the end view of FIG. l Ob (having an end plate removed for
clarity), it can clearly be seen that the truck body 213 is supported by
bearings
241 for linear translation along the ways 203. The primary section 237 of the
linear motor 221 is attached to the underside of the truck body 213 and
disposed
proximate the magnet track 224. The distance between the coil assembly 237 and
track 224 is kept very small, precisely sized to ensure high motive power and
efficiency for fast translation along the ways, as is well known in the art.
The magnet track 224 can more clearly be seen in FIG. lOb, wherein the
magnet track 224 comprises a mounting plate 232 supported on the upper surface
207 of the leg 201 by a pair of non-compressible strips 230 that serve as
insulators to keep heat away from the mountings surfaces 207, 208. Space is
provided for cooling tubes 231 which may be provided if so desired to draw
heat
from the magnet track 224. The plurality of aligned magnets, one of which is
shown at 234, are attached to the mounting plate, preferably by an adhesive or
glue. A protective cover 236 encloses the magnets and is affixed thereto, also
preferably by glue. The protective cover is of a non-ferromagnetic material,
which is stainless steel in the preferred embodiment. The mounting plate 232,
including the magnets 234 and cover 236 are attached to the leg 201,
preferably
via bolts as shown in FIGS. 10 and 10a.
FIG. lOb also shows the position sensor 263 which detects position and
velocity data regarding the movement of truck 211. The position sensor 263
comprises a read head 265 which determines position and velocity and which is
attached to an outer side of the truck body 213 and disposed adjacent the
upper
surface 207 of leg 201. An optical strip is attached to the leg 201 for use
with
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37
read head 265. Such position sensors are well known in the art, and as also
well
known, there are numerous other position sensors or linear encoders also well
suited for use with the present invention to provide feedback to the CNC
system
120.
By virtue of position sensors 263, 264, linear motors 221, 222 and the
CNC system 120, the bridge 24 is capable of traversing the ways 203, 204 at
speeds suitable for cutting both heavy plate as well as thinner material. The
linear motors 221, 222 provide a rapid and smooth movement of the bridge 24
by virtue of their direct drive from the magnet tracks 223, 224 to linear
motors
221, 222 directly affixed to the trucks 211, 212. As the drive force and the
position sensing are near the load, the use of linear motors eliminates
backlash
and provides high cutting speed and accuracy for cutting thin material at high
speeds or thick material (heavy plate) at slow speeds. The linear motors
themselves have no moving or sliding parts and therefore are very low
maintenance items and are capable of very high traverse velocities, such as
6000
ipm. The CNC system 120 is capable of utilizing data from the sensing elements
127, 137 to accurately control the linear motors 221, 222 for traversing the
bridge
at cutting speeds suitable for both heavy plate and thinner sheet material.
The same benefits of utilizing linear motors to traverse the bridge 24 along
the X axis also applies to traversing the cutting head 23 along the Y axis.
Turning attention to FIG. 11 depicting a front view of the bridge 24, much
like
the structure for providing translation in the X direction, the bridge 24
includes a
pair of ways 304 for directing the translation of the cutting head 23 along
the Y-
axis. The ways 304 span the breadth of the bridge 24, a portion of which has
been
cut away in FIG. 11. The vertical position of the cutting head 23 (Z-axis) and
the
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vertical position of the lens holder 35 (U-axis) are controlled by
conventional
rotary servo motors 302 and 301.
The cutting head 23 includes four linear bearings or slides 341 (shown in
hidden) bolted to a rear surface of the cutting head 23 for translation along
the
ways 304 in the Y direction. Each end of the bridge 24 also includes two shock
absorbers 350a, 350b for stopping the translation of the cutting head 23 in
the
event it excessively exceeds the boundaries of the cutting area. Each shock
absorber 350a, 350b includes a hydraulic mechanism 352 having a shaft 354
projecting therefrom. The exposed end of the shafts 354 include cushioning
bumpers 356 for engaging the cutting head 23, while springs 358 are positioned
between the hydraulic mechanism 352 and bumpers 356 for returning the shafts
354 to their extended positions after absorbing a load. A linear motor 321 is
used
to translate the cutting head 23 along the Y-axis, the linear motor 321
generally
comprising a magnet track 324 and a primary section 337 (shown in hidden in
FIG. 11 and best seen in FIG.12) attached to the rear of the cutting head
supporting truck 355 for cooperation with the magnet track 324. The magnet
track 324 comprises a series of permanent magnet sections stacked end to end
along the Y-axis to generate a magnetic field for repulsion by the primary
section
337. The bridge 24 includes a bottom base structure 24a for attachment of the
bridge 24 to the trucks 221, 222.
Turning to FIG. 12, a cross-sectional view of the bridge 24 and the cutting
head 23 is depicted. The bridge 24 is constructed of a plurality of tubular
members forming a frame including a honeycomb construction to provide a low-
weight bridge that permits high acceleration and deceleration, high cutting
and
traverse speeds, yet also can sustain the forces imposed by linear motor 321.
The
bridge includes a base member 24a bolted to the trucks 211, 212 for
translation in
CA 02319293 2000-09-14
39
the X direction. Focusing attention to the cutting head 23, the cutting head
23 is
slidably attached to the bridge 24 by way of bearings 341 attached to a rear
surface of the cutting head supporting truck 355. The cutting head supporting
truck 355 is disposed to engage the ways 304 for translation of the cutting
head
23 along the Y-axis. The ways 304 are bolted to a forward facing surface of
the
bridge 24 to guide the cutting head 23. The rear surface of the cutting head
supporting truck 355 also includes flange 386 attached to an upper end thereof
and flange 384 attached to a lower end for protecting the linear ways 304 from
dust and debris.
The rear surface of the cutting head supporting truck 355 is also structured
to receive the primary section 337 of linear motor 321. The primary section
337
is preferably bolted to the cutting head supporting truck 355, wherein flanges
382
are disposed proximate the primary section 337 on opposing sides to promote
circulation of air and cooling. The primary section 337 is disposed coaxial
with
and very close to the magnet track 324. Briefly, the magnet track 324 includes
a
mounting plate 332 bolted to the bridge 24 and spaced slightly away therefrom
by non-compressible insulating spacers 330 which keep heat away from the
bridge 24. The plurality of magnets, one of which is shown at 334 are attached
to
the mounting plate 332, preferably by adhesive, to produce a magnetic field
for
repulsion by coil assembly 337. The series of magnets 334 are covered and
protected by shield or cover 336 which is made of a non-ferromagnetic
material,
stainless steel in the preferred embodiment. The magnet track 234 and coil
assembly 337 are disposed in close proximity to one another to provide a
powerful direct drive to the cutting head 23 for rapid, and smooth translation
along the Y-axis.
CA 02319293 2000-09-14
The bridge 24 also includes an optical track 363 mounted to an upper
surface thereof. The Y-axis feedback element 137 generally comprises a
position
sensor indicated generally at 362. The cutting head includes a C channel
attached
to a rear surface thereof to dispose a read head 364 proximate the optical
strip
5 363 for detecting position and velocity data for the cutting head 23. The
CNC
system 120 utilizes the feedback from position sensor 362 to accurately
control
the translation and position of cutting head 23 at speeds suitable for cutting
both
thick and thin material.
Cutting head 23 is mounted to the front face of supporting truck 355 via
10 linear guides or ways 400 (FIG. 12) which allow the cutting head 23 to be
selectively positioned in the vertical or Z axis direction. Linear bearings
401 are
bolted to the front face of supporting truck 355 and the ways 402 are bolted
to the
back face of cutting head 23.
Turning attention to FIG. 13, the base 24a of the bridge 24 is shown in top
15 elevation. The bridge bottom base structure 24a includes two opposing ends
396,
397 configured for attachment to the trucks 21 l, 212 as shown in FIGS. 3 and
12.
The ends 395, 396 each have on opposing sides a plurality of counter sunk
apertures 395 that cooperate with the apertures 295 and the trucks 211, 212
for
bolting the bridge base 24a fo the trucks. The bridge bottom base structure
24a
20 includes at each end 396, 397 an aperture 390, 391 respectively for
receiving the
dowel pins (not shown) which are sized to tightly fit within the apertures
290,
291 in the respective trucks. These apertures and dowel pins are used to
initially
position the bridge 24 on the trucks and connect the truck 211, 212 to the
bridge
24 such as to allow bridge 24 to be squared with the X axis. The dowel pins
are
25 an interference fit with the needle bearings providing a preloaded pivotal
connection. The combination of a preloaded pinned connection at base end 396
CA 02319293 2000-09-14
41
of bridge 24 and a slideable preloaded pinned connection at base end 397 makes
it easier to square bridge during machine assembly.
Aperture 390 and base end 396 includes a needle bearing 398 (shown in
FIG. 13A) which allows for the bridge 24 to rotate relative to the truck on
which
it rests. Thus, the trucks 211, 212 may be positioned to square the X axis
without
twisting or deflecting the bridge 24 and bridge bottom base structure 24a.
Similarly, the hole 391 at the end 397 of base 24a is provided with a
needle bearing 399. The aperture 391 actually is formed in a small plate 394
which is slidably disposed in a slot 392. When a truck is moved during
squaring ,
the length between the apertures 290, 291 in the trucks increases or
decreases.
Thus the aperture 391 formed in plate 394 is allowed to slide in the Y
direction to
increase or decrease the distance between holes 390, 391. With this structure,
the
bridge 24 is connected to the trucks 211, 212 via dowel pins connecting
apertures
290, 291 and apertures 390, 391. Bridge 24 is set in place on the trucks 211
and
212, then adjusted and squared. Then the bridge base 24a is rigidly affixed to
the
trucks via bolts extending through apertures 395 and 295 in the bridge bottom
base structure and trucks respectively, without stressing the bridge and
without
extra structure allowing angular twisting of the bridge as compared to Graf et
al.
U. S. Patent No. 5,854,460.
Fig. 14 is a plan view of the machine 20 having a load/unload station 410.
A workpiece is loaded on a pallet 31 removably positioned at the load station
410. The pallet is slidably disposed so that it can be drawn directly into the
machine 20 for processing the workpiece. After processing, the pallet 31 is
returned to the load station 410 for unloading. The pallet is readily
accessible for
manual loading and unloading and is easily adaptable for automated material
handling systems. This adaptability is due in part to direct loading and
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42
unloading, i.e. the workpieces need not be worked around machine structure for
placement within the machine for processing.
It will now be appreciated that what has been provided is a laser-equipped
machine tool configured to cut heavy plate at production speeds. A high power
laser, preferably 3 to 4 kilowatts or more, and most preferably at least 6
kilowatts, provides the cutting power. A beam delivery system couples the beam
from the laser to the focal optic in the cutting head. The beam delivery
system
has an adjustment mechanism to compensate for distortion in the output coupler
caused by the high power laser beam. Preferably the mechanism is automatically
adjusted by the CNC so that the beam size is precisely maintained in the
presence
of transient distortions in the optics. The cutting head which delivers the
cutting
beam and assist gas stream to the workpiece is adjustable by the CNC along x,
y
and z axes, so as to control machine motion to make the programmed cuts. The
cutting head also has the ability to adjust the position of the focal spot
with
respect to the plate, a feature which is significant in the cutting of heavy
plate.
These features which combine to provide the ability to cut heavy plate at
production speeds can also afford significant utility when the machine is used
for
thinner sheet materials. The cutting head is preferably provided with the
ability
to exchange optics such that a relatively long focal length lens can be used
with
thicker materials, but shorter focal length lens can be substituted for
cutting
thinner materials. When cutting thinner material, the cutting speed
significantly
increases. The x and y axes include linear motors to precisely traverse the
bridge
and cutting head along the axes at speeds suitable for heavy plate, and at
higher
speeds suitable for thinner plate. The flying optic configuration and a
comparatively light weight bridge structure allow high rates of acceleration
and
deceleration, high cutting speeds and very rapid traverse speeds. The result
is a
CA 02319293 2000-09-14
43
machine configured to cut heavy plate, but which can, if desired, also operate
on
lighter sheet metal at commercial production rates.
The foregoing description of various preferred embodiments of the
invention has been presented for purposes of illustration and description. It
is not
intended to be exhaustive or to limit the invention to the precise forms
disclosed.
Obvious modifications or variations are possible in light of the above
teachings.
The embodiments discussed were chosen and described to provide the best
illustration of the principles of the invention and its practical application
to
thereby enable one of ordinary skill in the art to utilize the invention in
various
embodiments and with various modifications as are suited to the particular use
contemplated. All such modifications and variations are within the scope of
the
invention as determined by the appended claims when interpreted in accordance
with the breadth to which they are fairly, legally, and equitably entitled.
