Note: Descriptions are shown in the official language in which they were submitted.
WO 94/14569 PCTIUS93112344
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CONTROLLER FOR CNC-OPERATED MACHINE TOOLS
The present invention relates to a controller and a
method for optimization of metal-working on CNC-operated
machine tools, especially on CNC-operated milling machines
and machining centers.
While CNC-operated machine tools have existed for
years, their efficiency and usefulness has been limited by
their incapability to take into account many factors in the
programming stage which influence production efficiency,
including: number of workpieces in a run, operating cost,
tool replacement time, tool cost, etc. In addition, the
rigidly deterministic nature of CNC-operated machine tool
programming is incapable of allowing for unforseeable
changes in real-time cutting conditions such as depth and
width of metal cutting, tool wear, non-uniformity of
workpiece blank, etc.
It is one of the objects of the present invention to
overcome the limitations and disadvantages of today's
CNC-operated machine tools and to provide an optimizing
controller for machine tools, in particular for CNC-operated
milling machines and machining centers, which calculates the
optimal cutting modes according to production efficiency
criteria, and automatically provides adaptive feed and
spindle speed control responding to real-time cutting
conditions, maintains a constant and presettable spindle
torque and/or tool life, ensures optimal machining
operation, prevents tool breakage and indicates tool
status.
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According to the invention, this is achieved by
providing a controller for optimization of metal-working on
CNC-operated machine tools, having a main drive powering the
tool spindle of said machine tools and feed drives powering
the feed mechanism of said machine tools, said feed drives
being controllable to produce a feed rate determined either
by a predetermined setting of the cutting torque produced by
said tool spindle, or by said controller overriding said
setting in a teaching mode of said controller, comprising a
first unit for monitoring the torque of the main drive of
said machine tool to establish the actual, instantaneous
cutting torque; a second unit for setting the rated cutting
torque in said teaching mode in dependence on said
main-drive torque as monitored; a third unit for calculating
the feed rate required to maintain said cutting torque at a
constant level and controlling the feed drive of said
machine tool; a fourth unit responsive to said monitored
main-drive torque and providing feed rate limiting signals
to said third unit for protecting the tool against breakage,
characterized in that said unit for calculating said feed
rate is addressed by a compensator unit responsive, on the
one hand, to signals from a comparator unit comparing said
torque as set with the actual, instantaneous torque as
indicated by said first unit and, on the other hand, to
signals from an identifier unit calculating the
instantaneous cross-sectional area of the cut in response to
signals from both said first, main-drive torque monitoring
unit and said feed-rate calculating unit, said compensator
unit facilitating a high-precision stabilization of said
torque.
The invention furthermore provides a method for
optimization or metal-working on CNC-operated machine tools
having a main drive powering the tool spindle of said
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machine tools and feed drives powering the feed mechanism of
said machine tools, said feed drives being controllable to
produce a feed rate determined by a predetermined setting of
the cutting torque produced by said tool spindle, or by said
controller overriding said setting in a teaching mode of
said controller, comprising the steps of monitoring the
torque of the main drive of said machine tool to establish
the actual, instantaneous cutting torque; setting the rated
to cutting torque in said teaching mode in dependence on said
main-drive torque as monitored; calculating, in a feed rate
calculating' unit, the feed rate required to maintain said
cutting torque at a constant level and controlling the feed
rate of said machine tool; providing feed rate limiting
signals to a feed~rate calculating unit for protecting the
tool against breakage; comparing, in a comparator unit, said
torque as set, with said actual, instantaneous torque;
calculating, in an identifier unit, the instantaneous
cross-sectional area of the cut in response to signals
produced by both said main-drive torque monitoring unit and
said feed rate~calculating unit; feeding the signals from
said two units to a compensator unit, and feeding the
signals from said compensator unit to said feed rate
calculating unit, thereby achieving high-precision
stabilization of~said cutting torque.
In another aspect, the present invention provides an
adaptive controller for a numerically controlled machine
including a machine tool, the controller comprising: a load
monitor operative to monitor at least one torque parameter of
the machine tool; and a control unit operative to receive said
at least one torque parameter and to provide high precision
control thereof.
In yet another aspect, the present invention provides a
controller for improving operation of a numerically controlled
operated machine having a feed drive rate and a cutting torque
which at least partly determines the feed drive rate, the
A
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controller comprising: a feed rate computation unit operative
to compute a feed rate which maintains the cutting torque at
a constant level; and a feed drive control receiving input from
the feed rate computation unit and operative to control the
feed drive rate of the machine.
In yet another aspect, the present invention provides an
apparatus for controlling the operation speed of a machine tool
on a workpiece based on a predetermined criterion, the
apparatus comprising: a desired operation speed computation
unit operative to receive at least one work situation
characteristic and to compute a desired operation speed based
on the predetermined criterion; and an operation speed
controller operative to adjust an operation speed of the
machine tool to conform to said desired operation speed.
In yet another aspect, the present invention provides a
control method for a numerically controlled machine operating
on a workpiece having thin wall sections, the method
comprising: analyzing harmonics of the workpiece to detect a
thin wall section thereof; and modifying at least one machine
operating characteristic so as to protect a detected thin wall
section and restoring said machine operating characteristic
when the thin wall section has ended.
In yet another aspect, the present invention provides a
machine tool status display apparatus comprising: a work
situation memory operative to store work situation information
including at least one machine tool operation characteristic
and at least one workpiece characteristic; a machine tool
status computation unit operative to compute a current machine
tool status based on the work situation information and a
previous machine tool status; and a machine tool status display
operative to display output from the machine tool status
computation unit.
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In a further aspect, the present invention provides a
system for adaptively controlling a feed rate F of a mi'~li.ng
cutter relative to a workpiece, the milling cutter
constituting part of a machine tool having a main drive, t:he
system comprising: (a) a torque monitor for monitoring an
actual main drive cutting torque M; (b) a torque comparat:or
for calculating DM where DM=Mo-M and Mo is a predetermined
reference main drive cutting torque established for t:he
milling cutter and the workpiece material; and (c) a feed
rate controller for determining the feed rate F as a
function of DM; wherein said feed rate controller includes
means to calculate an instantaneous cross-sectional area p
of a cut of the workpiece being worked on by the mill__ng
cutter and determines the feed rate F as a function of p to
substantially stabilized M such that DM--Ø
In a still further aspect, the present invention
provides a method for adaptively controlling a feed rate F
of a milling cutter relative to a workpiece, the milling
cutter constituting part of a machine tool having a main
drive, the method comprising the steps of: (a) monitoring an
actual main drive cutting torque M; (b) calculating 0M where
DM=Mo-M and where Mo is a predetermined reference main drive
cutting torque established for the milling cutter and she
workpiece material; and (c) determining the feed rate F as a
function of DM; wherein step (c) includes calculating an
instantaneous cross-sectional area p of a cut of the
workpiece being worked on by the milling cutter and
determining the feed rate F as a function of p to
substantially stabilized M such that 0M--->0.
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The invention will now be described in connection with
certain preferred embodiments with reference to the
following illustrative figures so that it may be more fully
understood.
With specific reference now to the figures in detail,
it is stressed that the particulars shown are by way of
example and for purposes of i_Llustrative discussion of the
preferred embodiments of the present invention only, and are
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presented in the cause of providing what is believed to be
the most useful and readily understood description of the
principles and conceptual aspects of the invention. In this
regard, no attempt is made to show structural details of the
invention in more detail than is necessary for a fundamental
understanding of the invention, the description taken with
the drawings making apparent to those skilled in the art how
the several forms of the invention may be embodied in
practice.
In the drawings:
Fig. 1 is a block diagram of a first embodiment of the
controller according to the invention;
Fig. 2 is a diagram illustrating the effect, on the
feed-rate values and the torque values, of the
compensator unit;
Fig. 3 is a block diagram of a second embodiment of the
controller according to the invention; and
Figs. 4 and S illustrate a third and a fourth embodiment,
respectively, of the controller according to the
invention.
The principal input parameters of the first and second
embodiments of the controller according to the present
invention are one or more of the main-drive parameters which
are proportional to the cutting torque M. The principal
output parameter is a signal determining the feed rate F as
a function of M, the task fulfilled by the invention being
to maintain this torque at a steady level determined in
dependence on the properties of the specific milling cutter
used. The required values can be found in appropriate
tables.
WO 94114569 2 U ~ PC'f/US93/12344
Another concept of the present invention is the
teaching mode in which, instead of the maximum rated cutting
torque Mo, a maximum torque Mo' is determined during the
machining of one or several of the first identical
workpieces. The teaching mode is particularly effective for
large runs of identical workpieces.
Another important parameter used by the controller
according to the invention is p[mm2], designating the
cross-sectional area of the cut (for short, area of cut),
which is the product of the cut width (b) and cut depth
(h).
Referring now to the drawings, there is seen in Fig. 1
a block diagram of a first embodiment of the controller
according to the invention, comprising a housing 2
attachable to a CNC-operated milling machine and
accommodating the various units of the controller, and a
panel 4 which is accessible to the operator.
On the panel 4 is located a switch 6 for selecting:
initiation of the Teaching Mode (TM) ("Initiate"); "Run" for
Mo settings determined in the teaching mode, and operation
with predetermined Mo settings ("without TM"). In the
latter, the value for Mo is set on the selector 8. Other
elements on panel 4 include a starting button 10 and a tool
status indicator 12 which lights up, or provides, e.g., an
acoustic warning, when the tool is worn beyond a certain
limit.
There is seen a monitoring unit 14 in which the
instantaneous main-drive cutting torque M (as applied by the
milling cutter) is monitored.
WO 94114569 PCT/US93/12344
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'The signal .4 from the monitoring unit 1=~ is fed to a
nwnber of other units of the controller:
a) the unit 16 for setting the rated cutting torque Mo
for application in the teaching mode;
b) a tool protection unit 18 which supplies feed rate
limiting signals to a feed rate calculator 20;
c) a unit 22 for identifying the instantaneous value
of ~, also addressed by the signal from the feed
rate calculator 20, and
d) a comparator unit 24 which compares the set torque
Mo with the actual, instantaneous torque M.
According to the position of the mode switch 6, a logic
element 26 provides the comparator unit 24 with the Mo value
as determined either by unit 16 or by the manual selector 8.
The controller also includes a self-diagnostic unit 28
interposed between the start button 10 on the panel 4 and
the feed rate calculator 20. When the button 10 is pressed,
the unit 28 performs a test of the entire system and, if the
latter is found operational, provides an enabling signal to
the feed rate calculator 20.
The heart of the controller is constituted by a
compensator unit 30 in cooperation with the
already-mentioned p-identifier unit 22.
The following is an explanation of the considerations
underlying the compensation principle.
The feed rate is determined by the difference !~M
between the set value Mo or Mo'and the actual value M.
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The metal-cutting process (as static process) can be
represented by the expression:
M - AFy pY
where:
o - the already-mentioned area of cut;
F - feed rate, and
A, y, Y = coef f icients depending on tool type and meta7.-
working conditions.
Seeing 4M as the error of cutting torque stabilization,
it can be c~effined as:
4~1~f=tLlo-M=Mo 1- K'K~AP
1+K,K~Ap
where:
K~ - CNC gain (static), and
K1 - current monitor gain.
However, in real-life machining, a «1/K1K~A, as a
result of which ,OM = Mo, or M = 0, making it impossible to
achieve cutting torque stabilization with medium and
small v -values .
In order to secure for M independence from changes
of c, it is necessary to provide a compensator unit with
variable gain Kk:
B
Kx
P
with H being a constant.
WO 94/14569 PCT/US93/12344
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To calculate Kx it is thus necessary to determine p at
every instant throughout the cutting process, which is done
by unit 22 according to the assumption that p is
proportional to the ratio D M/F a, where « is determined for
each material to be cut.
The effect of the compensator unit is shown in Fig. 2,
in which the solid curves 32 and 34 indicate the values of F
and M/Mo as functions of p ( specifically, of the cut height
h) with compensation, and the dashed curves 36 and 38
indicate the same values F and M/Mo without compensation.
The feed rate of the machine tool is obviously
controlled by the output F of the feed rate calculator 20.
Fig. 3 shows another embodiment of the controller
according to the invention. This embodiment differs from
the previous embodiment in that the controller is
inaccessible to the operator, being addressed only by the
CNC program. Added elements in this embodiment are a
program interface 40 linking the controller to the CNC
program and a memory unit 42 for the rated torque Mo of a
number of different tools N (as marked MN3 - MN25) to be
used in the machining process, with MNo and MN1 signifying
selection of the teaching mode and MN2 - without teaching
mode. The rest of the unit is identical with the units of
the previous embodiment and operate in the same manner.
The embodiment illustrated in the block diagram of Fig.
4 is intended for the optimization of machining operation on
the basis of either one or the other of two criteria:
1) maximum metal removal per unit time (mm3/min);
2) minimum cost of removal of unit volume of metal
($/min).
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It is possible to select a compromise between these
criteria.
The embodiment of Fig. 4 comprises all the units
described in connection with Figs. 1 and 3 (except for the
panel 4 and its elements), as well as some additional units
to be described further below.
While the first criterion is taken care of by the
"F-loop" comprised of units 20, 22, 24 and 30 (Figs. 1 and
3) and is conditional upon M - Mo, the second criterion
requires the introduction of an additional unit, 44, which
constitutes the operative part of an "S-loop", inasmuch as
it is meant to control the speed (S) of the tool spindle.
This unit consists of a calculator 44, which realizes the
expression:
A3
S =
F~' a"T as
p o
;here:
A3 - coefficient dependent on the specific tool
used;
coefficients depending on the material
machined;
a - area of cut, supplied by the identifier
unit 22,
F - feed rate, and
To - tool service life required for selected
optimization criteria.
The first criterion is conditional upon the relationship:
WO 94114569 PCTIUS93I12344
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1
T~ _ ( - 1) . .
m
The second criterion is conditional upon the
relationshix~:
1 D
To = ( - 1 ) ( T + )
m 5
where:
m - coefficient depending on the specific tool used and
material machined;
- auxiliary or idle time (min);
D - cost of tool ($);
B - cost of machining per min ($/min).
The calculator 44 has five inputs:
a) coefficients A3 for the tools N3-N25 ( from memory
46 addressed by input NIN3-MN25);
b) coefficients q 3 , ~ 4 ~ a 5 for four different groups
of materials (from memory 48 addressed by input
MN26-NIN28 ) ;
c) signal F (from calculator unit 20);
d 1 area of cut p ( from the identifier unit 22 ) , and
e) projected tool service life To (from unit for
calculation of To).
Input MNo initiates the teaching mode and input MN1
runs the teaching mode for all tool diameters.
The outputs of the controller of this embodiment are
the same as with the previous embodiment (tool status and
feed rate control signal F), with the addition of the speed
control signal S.
PCT/US93/12344
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The embodiment represented in Fig. 5 has all the
features described in the previous three embodiments, with
the addition of two further features, namely, a circuit
suppressing machine tool vibrations and chatter, and a
circuit facilitating the finish machining, at high
precision, of thin wall sections of workpieces.
The first of these features comprises a vibration
analyzer 50 addressed by any suitable transducer 51
responding to vibrations and chatter of the machine. The
output of the transducer 51 is analyzed by unit 50, which
produces a signal fed to the feed rate calculator 20 which,
in response, modifies the feed rate F to the degree required
to suppress the vibrations, returning it to the original
rate once this has been achieved.
The problem with thin sections is their elastic
deformability under the cutting pressure of the milling
cutter. Thus milling an aluminum wall of a thickness of,
e.g., 2.5 mm and a length of 200 mm, taking a cut of a depth
of 0.5 mm at a feed rate of 500 mm/min, a cutter speed of
1000 rpm and a tool diameter of 12 mm, will produce an error
of 0.04 mm, while milling a section of a thickness of 10 mm
at identical cut depth, feed rate, speed and tool will
produce an error of only 0.005 mm. This difference is, of
course, due to the "giving in", and subsequent spring-back,
of the thin section, necessitating a reduction of the feed
rate when the milling cutter arrives at such a thin
section.
This not only complicates the CNC-program, but it is
also difficult to determine at which point, after a heavy
section, the thin section effectively begins. Also, a worn
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cutter will increase the deforming force which, with a new
cutter, would be much smaller.
It is the task of the present embodiment to
automatically reduce the feed rate the moment wall
deformation is detected.
It was found that certain harmonics of the feed-drive
current are reduced during the milling of thin walls, due to
the change of frequency characteristics of the electrical-
mechanical loop of which the thin section is a part. Thus,
based on a dispersive analysis of feed-drive current
'ignals, it is possible to form special signals indicating
the effective beginning and ending of a thin section. These
signals are used to reduce the feed rate during the
machining of such thin sections, thus increasing the
accuracy of the machining operation.
The added circuit of the embodiment of Fig. 5 comprises
a suitable sensor 52 responsive to the feed-drive current,
feeding an analyzer 54 for analyzing the harmonics of the
feed-drive current, which analyzer addresses a signal
transducer 56 producing signals that, fed to the feed rate
calculator 20, modify the output signal of the latter,
reducing the feed rate whenever the sensor 52 and analyzer
54 indicate the effective beginning of a thin section, and
restoring the previous feed rate when the sensor 52 and
analyzer 54 indicate the ending of this section.
The embodiment of Fig. 3 is particularly suitable for
CNC- operated machining centers using a pre-programmed
sequence of different tools, and is more efficient than the
previous embodiment, particularly due to the provision, as
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shown in Fig. 3, of the memory unit 42 which eliminates the
need to reset the controller each time a tool is changed.
It will be evident to those skilled in the art that the
invention is not limited to the details of the foregoing
illustrated embodiments and that the present invention may
be embodied in other specific forms without departing from
the spirit or essential attributes thereof. The present
embodiments are therefore to be considered in all respects
as illustrative and not restrictive, the scope of the
invention being indicated by the appended claims rather than
by the foregoing description, and all changes which come
within the meaning and range of equivalency of the claims
are therefore intended to be embraced therein.
