Note: Descriptions are shown in the official language in which they were submitted.
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SPATIAL MEASURING DEVICE
SPECIFICATION
Field of the Invention
This invention relates to measuring devices, and
more particularly to articulated arm coordinate
measuring machines for measuring three-dimensional
objects.
Background of the Invention
The ever-increasing complexity and
sophistication of commercial and industrial
components and assemblies, and the need for quality
assurance during certain interim stages in their
manufacture and assembly, along with the typical
market pressures for cost reduction have prompted
new methods of measuring complex -three-dimensional
parts in a fast, precise, accurate and reliable
manner .
This is particularly true in the area of
commercial or industrial quality assurance where
numerous complex mechanical components are
separately created with exacting tolerances for
later assembly. In order to efficiently ordinate
these systems, method and apparatuses for measuring
the components have been devised.
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An example coordinate measuring machine is
~l~scribed i~ ~~ft~~, tl.S. Patent Na. 3,9~:~:,~~ahd
U.~. Fatent i~o. 5..402, 58~x ~ In gaeg~~~a,l,
these devices comprise an a~tieuan
terminating in a probe. The arm has a plurality of
rigid transfer members connected end-to-end with a
series of joints. The position of the probe in
space at a given instant in time can therefore be
calculated by knowing the length of each member and
the specific position of each of the joints. The
operator simply manipulates the arm and takes
readings at various points on the artifact being
measured. Each of these readings is recorded and
processed by an electronic recorder/analyzer to
obtain the desired dimensions of the artifact.
The probe can be manipulated to reach any
point in space within substantially a sphere
centered on the base of the arm. This is called
the measurement sphere of the machine. Any article
being measured must therefore lie within the
measurement sphere.
A critical step in the measurement process is
determining the position of each of the joints at a
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given instant in time. Typically, the joints are
broken down into singular rotational degrees of
freedom, each of which is measured using a
dedicated rotational transducer. Each transducer
outputs an electrical signal which varies according
to the movement of the joint in that degree of
freedom. The signal is carried on wires through
the arm tv the recorder/analyzer.
In order to maximize precision, it is
important that the transducers be mechanically
coupled to the joint as directly as possible. This
usually requires that the transducers be
incorporated into the joints of the arm.
In recent times, the transducer of choice is
an optical encoder. A detailed description of the
operation of optical encoders may be found in the
optical encoder sales brochures available from
Heidenhain Company of Traunrent, Germany. In
general, each encoder measures the rotational
position of its axle by coupling is movement to a
pair of internal wheels having successive
transparent and opaque bands. Light is shined
through the wheels onto optical sensors which feed
a pair of electrical outputs. As the axle sweeps
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through an arc, the output of the analog encoder is
substantially two sinusoidal signals which are 90
degrees out of phase. Coarse positioning occurs
through monitoring the change in polarity of the
two signals. Fine positioning is determined by
measuring the actual value of the two signals at
the instant in question. Maximum accuracy is
obtained by measuring the output precisely before
it is corrupted by electronic noise.
A problem with prior designs involves a loss
in precision due to signal degradation as the
signal is transmitted down wires to the
recorder/analyzer. A machine which minimizes
signal degradation is therefore preferable.
Another problem with current designs is a
restriction on the movement of the joints due to
wiring. In prior designs, the arm and its joints
carry wiring for transmitting power and signals to
and from the probe and transducers. Although the
wiring is flexible, the rotational freedom of the
joints must be restricted to prevent over-coiling.
Usually, the structures employed to restrict
this motion are in the form of single or multiply
"stacked" end-stops. Each "stack" of end-stops
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ideally allows up to 360 degrees of motion before
stopping. However, it has been found that using
more than one or two stacks increases cost and/or
reduces precision to the point of being
prohibitive.
Encountering any mechanical end-stop results
in delays and inconvenience to the operator since
the arm must be "unwound". Additionally, there is
wear and tear on the wiring itself . An arm which
is not so restricted would therefore be preferable.
The accuracy and resolution of any coordinate
measuring machine is dependent on the rigidity of
the members and the accurate transmission of motion
to the joints and their transducers. In the past,
this required that the arm be made to expensive
exacting tolerances, and made of strong materials
such as steel with numerous complicated structural
supports. This often translated into an increase
in weight requiring complicated counterbalance
mechanisms to obviate operator fatigue. These
mechanisms tend to adversely affect precision.
Therefore, a light-weight, rigid arm is preferable.
Finally, as with any machine, a certain degree
of modularity is preferable so that repairs may be
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made quickly. Failed sub-assemblies may simply be
replaced with a new sub-assembly, instead of
attempting to repair the sub-assembly in situ.
Therefore, an arm having interchangeable sub-
assemblies is preferable.
Accordingly, there is a need for an
inexpensive, light-weight, high precision
coordinate measuring arm which offers unrestricted
probe motion from any given orientation, and offers
modular interchangeability to promote fast repair
and size configurability.
Summary of the Invention
The principal and secondary objects of this
invention are to provide an inexpensive, light-
weight, high precision, sleek profile coordinate
measuring arm which offers unrestricted probe
motion from any given orientation.
It is a further object of the invention to
provide a modularized arm having interchangeable
sub-assemblies and greater user configurability.
These and other valuable objects are achieved
by a multi-jointed arm having: 1) swiveling joints
which are unlimited by one or more end-stops; 2)
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signal conditioning means closely located to the
joint position transducers; and 3) stable transfer
membe..s which comprise a freely rotating shaft
mounted within an outer sheath, wherein both shaft
and sheath extend substantially the length of the
member.
Hrief Description of the Drawing
Figure 1 is a three-dimensional diagrammatic
perspective stylized illustration of a coordinate
measuring device showing the range of motion of the
joints according to the invention.
Figure 2 is a cross-sectional view of the
preferred arm according to the invention.
Figure 3 is a three-dimensional illustrative
view of the preferred structure of the transducers
used in the invention.
Figure 9 is close-up partial cross-sectional
view of a transfer member and its adjacent joints.
Figure 4a is a diagrammatic cross-sectional
end view of Figure 4 taken along line 4a-4a
Figure 5 is a three-dimensional illustrative
- view of a dual projection yoke for mechanically
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connecting adjacent joints.
Figure 6 is a three-dimensional illustrative
view of a pedestal for connecting a hinge joint to
a transfer member.
Figure 7 is a cross-sectional view of a hinge
joint taken along a plane defined by the joint's
rotational axis and the major axis of its adjacent
transfer member.
Figure 8 is a three-dimensional illustrative
view of a portion of a slip-ring sub-assembly.
Figure 9 is a cross-sectional view taken
perpendicular to the axis of rotation of the slip-
ring sub-assembly shown in Figure 8.
Figure 10 is a top plan view of a portion of a
slip-ring sub-assembly according to the invention.
Figure 11 is an end view of the slip-ring sub-
assembly of Figure 10 taken along line 11-11.
Figure 12 is a diagrammatic cross-sectional
side view taken parallel to the axis of rotation of
a portion of a slip-ring sub-assembly.
Figure 13 is a diagrammatic functional block
diagram of the electronic components of the
invention.
Figure 14 is an electronic block diagram of
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the major components of a member mounted printed
circuit board of the invention.
Description of the Preferred Embodiment of the Invention
Referring now to the drawing, there is shown
in Figure 1 a three-dimensional diagrammatic view
of the spatial coordinate measuring machine arm 1
according to the invention. The arm comprises a
stationary support base 2, three rigid transfer
members 3,9,5 and a probe 6 which are
interconnected with a plurality of joints
7a,7b,7c,7d,7e,7f. Each of the joints is dedicated
to a single degree of freedom and allows rotational
movement about a dedicated axis l0a-lOf which is
fixed relative to that joint.
The joints can be divided into two categories,
namely: 1) those joints 7a,7c,7e which allow the
swiveling motion associated with a specific member
(hereinafter, "swiveling joints"), and 2) those
joints 7b,7d,7f which allow a change in the
relative angle formed between two adjacent members
or between the probe and its adjacent member
(hereinafter, "hinge joints").
Each of two pairs of joints 7b,7c and 7d,7e,
I
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are connected via a yoke, 8 and 9 respectively,
and constitute, respectively, a second joint
assembly and a third joint assembly, the joint 7a
constituting a first joint assembly and the joint
7f constituting a fourth joint assembly. Each
hinge joint 7b,7d,7f is connected to its adjacent
transfer member 3,4,5 by means of a pedestal
11,12,13, respectively. The pedestal precisely
orients the rotational axis of its hinge joint
orthogonally to the major axis of its adjacent
transfer member.
The swiveling joints 7a,7c,7e are unlimited
in their range of motion. This capability is
accomplished without using any "stack" type
structures described earlier. This is in stark
contrast to prior designs. The preferred hinge
joints 7b,7d,7f however, are limited by
interference between adjacent members. Although
this interference is not required, as shown by
Raab (U. S. Patent No. 5,402,582), which utilizes
offset or skewed adjacent members; it has been
found that the preferred approach offers a
symmetrical design without sacrificing rigidity,
and a sleeker profile which more conveniently
allows access through narrower passageways. Non-
symmetrical designs must cope with greater
momentary forces generated by the orthogonally
arranged members.
Figure 2 shows cross-sectional view of the
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preferred coordinate measuring arm. Six joints
7a-7f are connected by three transfer members 3-5,
two yokes 8,9, and three pedestals 11-13.
The invention is able to achieve unlimited
motion in the swiveling joints by eliminating the
need for a continuous, flexible wire running
between the joints on opposite sides of a
swiveling joint. The preferred means for
conducting power and signals through these joint
replaces wires with a multi-conductor electrical
slip-ring sub-assembly. Therefore, there is a
slip-ring sub-assembly 14,15,16 operatively
associated with each swiveling joint 7a,7c,7e.
The slip-ring sub-assembly will be described in
detail later.
Each hinge or swiveling joint has its own
dedicated motion transducer in the form of an
optical encoder 17a-17f. Hinge joint encoders
17b,17d and 17f are shown hidden in Figure 2.
Since each encoder forms an integral part of each
joint by providing rotational bearings, it is
important that the encoder be structurally rugged.
As shown in Figure 3, a encoder 17c comprises
a generally cylindrical body 20 having an outer
housing comprising a cup-shaped casing 21 and a
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circular face plate 22. A central axle 23 is
rotatively mounted to the body on internal bearings
and extends through the face plate. An electrical
cable 24 carries power to the encoder and output
signals from the encoder through an aperture 25 in
the housing.
The preferred encoder is a ROD 450.0000 - 5000
model analog optical encoder available from
Heidenhain of Traunrent, Germany. These encoders
offer structural ruggedness by having a large
central axle and bearing. Ideally, this encoder is
capable of resolutions of about 5000 cycles per
revolution.
Digital encoders are available and could be
used in the invention; however, currently available
designs are not preferred. Some "digital" encoders
which use simple clipping circuits to generate a
square, rather than sine wave output, suffer from
inferior resolution. True digital encoders having
onboard digitizing circuitry currently suffer from
being too bulky. However, as circuit board sizes
decrease, adequate encoders may become available.
Current digital encoders are also more expensive
- and less versatile with respect to the type of
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systems they are capable of interfacing.
Because the encoder housing forms part of the
outer surface of the arm, slight modifications to
the encoders may be required. Power and output
wires should not be exposed and should therefore
run through portions of the housing having contact
with internal cavities of the arm. Means for
mounting the encoders to the arm may require
additional modifications, however, preferred
mounting occurs primarily through screw attachment
to the structurally rugged face plate.
The preferred structure of the transfer
members and their connection to adjacent joints
will now be described with reference to Figures 4,
4a and 5. Each transfer member 4 is in the form of
a dual concentric tubular structure having an inner
tubular shaft 30 rotatively mounted coaxially
within an outer tubular sheath 31 by means of a
first bearing 32 mounted proximately to a first end
of the member 4 adjacent to a hinge joint 7d, and a
second bearing comprising a slip-ring sub-assembly
15 in combination with the encoder 17a of an
adjacent swiveling joint 7c located at an opposite
- end of the member.
i
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The shaft 3o is mounted to the hinge joint 7d
by fastening means such as nut and screws 33-
connecting the shaft end flange 3~! to base plate 35
of pedestal i2. The sheath 31 is mounted to the
swiveling joint 7a by fastening means such as nut
and screws 36 connecting the sheath end flange 37
to the yoke 8.
By extending the first bearing 33 out to the
maximum practical distance away from the encoder of
the swiveling joint i7a, the amount of axis
misalignment between encoder and the swiveling
shaft are kept to a minimum, thereby increasing
precision.
The shaft 30 comprises a rigid cylindrical
tubular body 40 having opposite ends to which are
bonded first and second end-caps 42,42 The end
caps are glued to the body in such a way that the
resulting member is precisely and accurately
balanced. The preferred method of assuring this
balance involves allowing the glue to cure while
the assembled member is being revolved.
Portions of the interior surface of the shaft
body contacting the end-caps may be scored or
otherwise grooved to provide a more positive
i
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bonding surface for the glue. Likewise, relevant
portions of the end-caps may be scored in lieu of
or in addition to the body scoring. The sheath 3f
comprises a rigid tubular body 43 having similarly
attached first and second end-caps 45,44.
The end-caps of both the sheath and shaft are
structured specifically to interface their
respective joints. Therefore, the first end-cap ~2
of the shaft 3o provides an outer surface for
mounting the first bearing 3Z, and a flange 34 for
attachment to the pedestal 12. Likewise, the first
end-cap 1S of the sheath 31 has female coupling
flange ~17 for engaging the outer surface of the
first bearing 32.
The second end-cap 41 of the shaft 30 is
integral with the central axle 50 of the slip-ring
sub-assembly 15 dedicated to swiveling joint 7c.
An access hole 51 extends through a side wall
of both the sheath 31 and. the second end-cap 14 of
the sheath to allow for the loosening of the split
collar clamp 53 which firmly couples the central
axle 50 of the slip-ring to the swiveling joint
encoder axle 23. This allows for quick decoupling
during repair or reconfiguration.
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The end-caps are preferably made from an
easily machinable, inexpensive, light-weight, rigid
material such as aluminum.
The use of cylindrical tubes for both sheath
and shaft is preferred because they offer
construction simplicity, rigidity, light weight,
and space inside for the printed circuit board 46
feature of the invention. Also, as shown in Figure
4a, they allow a concentric mounting of a shaft
tubular body 40 having an outer diameter 48
approaching the inner diameter 49 of the sheath
tubular body 43, thereby increasing rigidity while
maintaining low weight and a sleek profile.
Therefore, the shaft tubular body outer diameter is
preferably at least 50%, and most preferably at
least 75% of the inner diameter of the sheath
tubular body.
In the preferred approach the closeness of
these two diameters is limited by the interposition
of the sheath's first end-cap 45 between the
diameters. Although the end-cap could be designed
to bond with the outer surface of the sheath body,
thereby allowing the two diameters to be closer, it
has been found that the preferred design is less
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expensive with respect to machining tolerances
while maintaining adequate rigidity.
The tubes are preferably made from a light
weight, rigid material such as epoxy bonded carbon
graphite which inexpensively offers a strength to
weight ratio in excess of that of steel. Another
advantage of carbon graphite is that it has a low
thermal expansion coefficient. Although
temperature transducers are commonly used in
coordinate measuring machines so as to compensate
for the thermal expansion of the arm and the
article being measured, errors in compensation are
reduced in arms having a lower overall thermal
expansion coefficient.
The inherent rigidity and light weight of the
coaxially mounted sheath/shaft member wherein the
shaft has a wide diameter, alleviates the need for
any additional structural bracing or duplex-style
or spaced bearing sets. Also, the shaft and sheath
may be made from lighter, inexpensive, and easily
machined material such as aluminum, or the
preferred carbon graphite. A lighter overall arm
provides less operator fatigue, reducing the need
for complex counterbalances or springs which may
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reduce precision. However, the invention can be
easily modified to incorporate counterbalance
mechanisms. Also, the amount play or backlash
inherent in a coupled system is significantly
reduced ~~y having direct coupling between the
various components.
The preferred structure of the yokes and
pedestals will now be described with reference to
Figures 5-7. Each of the dual projection yokes 8,9
comprises a fork structure with a base 60 and dual
support projections 61 and 62. The base 60 is
mounted to the face plate of the encoder 17e of a
swiveling joint using fastening screws 69. Each
support projection mounts to the pivoting axle of
an adjacent hinge joint by means of a releasable
stock-type split clamp 63,64, each having a hole
65,66 sized to allow engagement with the axle. The
tightness of the clamp is adjusted by end screws
67,68.
Each pedestal ii,i2,i3 comprises a circular
base plate 35 which mounts to the first end-cap 42
of a member shaft using fastening means such as
nuts and screws 33. The base plate 35 is integral
with two parallel, facing circular coupling plates
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?0,?i spaced apart to allow for the placement of an
encoder between them. Each coupling plate
comprises a stock-type split clamp TOa,?la for
mounting bearings ?5,76. The tightness of the
clamp is adjusted by end screws which engage the
lower portions of the plates through recesses
?0b,?ib.
One coupling plate 71 attaches directly to the
encoder's face plate 22a using screw fastening
means ?3. The bearing ?6 allows the encoder
axle 23a to freely rotate with respect to the
pedestal. The other coupling plate ?0 supports the
encoder 1?d from the back using an O-ring spacer ?~
contacting the encoder and a bearing ?5 mounted to
the coupling plate. The second bearing ?5 allows a
dummy axle ?? to freely rotate-with--respect-to-.the
pedestal. The dummy axle ?? and the encoder axle
23a form the pivoting axle for this hinge joint.
Protective end-guards ?e,?9 are attached to the
axles using screw fastening means.
The pedestal i2 also provides a bracket 80 for
mounting the printed circuit board ~6 of the hinge
joint's adjacent member. Wiring 83 soldered to the
circuit board ~16 passes to the encoder 1?d through
i
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an opening 81 in the base plate 35 and an O-ring
type bushing 82 traversing the space between the
encoder and pedestal.
Wiring 58 constituting a multi-conductor
cable between the printed circuit board (PCB) 46
and its adjacent slip-ring sub-assembly connects
to the PCB using an electrical connector 59
common in the art.
A similar yoke and pedestal connects joints
7b and 7c.
A yoke having encoder straddling dual
projections, although not necessary, is preferred
because it strengthens the joint making it less
susceptible to moment forces acting on the joint.
However, referring back to Figure 2, it has
been found that the joint 7f closest to the probe
6 does not require a dual projection yoke due to
the light loading of this joint. However, a
larger probe may require a dual projection yoke.
The half yoke 90 with a single support projection
91 also provides the operator greater comfort and
ease of use while grasping the probe.
A preferred slip-ring sub-assembly will now
be described in detail with reference to Figures
4 and 8-12. Electrical signals are transmitted
through the
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swiveling joints using a slip-ring sub-assembly 16.
Each slip-ring sub-assembly comprises a central
cylindrical shaft or axel 50 along which has been
mounted a plurality of cylindrical contact rings
102 each made of copper, bronze or other
inexpensive electrically conductive material. The
material need not be especially durable because of
the light loading and low rate of revolution to
which the sub-assembly will be subjected.
Adjacent contact rings are separated from each
other by spacers 103 of insulating material. Each
contact ring has a central, v-shaped groove 104
embedded circumferentially around its outer
surface. The groove is sized to provide tracking
and electrical contact for a portion of a contact
wire 105b stretched transversely across the contact
ring. The contact wire is made from an
electrically conductive, ductile, corrosion
resistant and somewhat strong material. The
preferred material is silver because of its
availability and positive wear characteristics when
contacting copper or bronze.
Each wire is in the form of a loop mounted on
two prongs 106,107 of a support rack which
AMENDED SHEET
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~~~~.a~~: ~~
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straddles the contact ring and axle combination.
Therefore, each wire has first and second portzs~ns
108,109 contacting the top and bottom of the
contact ring 102. The wire is biased toward the
ring by means of a spring 110 located on a section
of the loop contacting one of the prongs 106 of the
support rack. On an adjacent wire 105a, the spring
portion 111 of the loop is located on the opposite
prong i07 so that the spring forces acting on the
slip ring are balanced. The wires are kept in
place by the V-shaped grooves and U-shaped channels
112 set into the surface 113 of the prong facing
away from the axle 50.
The slip-ring axle 50 is rotatively mounted
concentrically within a pair of circular support
spacers 115,116 which are each connected to
opposite ends of the prongs 106,107 of the wire
loop support rack. The spacers are additionally
braced by bracing members (removed in Figure 10)
attached by screw means ii4. The outer cylindrical
surface of each of the support spacers is sized to
lodge the slip-ring sub-assembly concentrically
within the second end-cap of the sheath.
The slip-ring sub-assembly 16 is mechanically
ap~E,~~rj~a s!~E~
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~~;~;a~~, ~ ~ 19 NOV '97
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coupled to a swiveling joint encoder axle by means
of a slotted female coupling 117 and a removable
split collar clamp 53. This allows for quick
decoupling during repair or reconfigurati~r~. Note
that the fastening screws 118,119 of the split
collar clamp engage from opposite directions so as
to provide balanced axial symmetry, and to allow
for screwdriver access from one orientation by
simply revolving the axle 50 one-half turn (180
degrees).
Now will be described the preferred electrical
connection between slip-ring sub-assembly and
adjacent electrical components: The connection
between the adjacent swiveling joint 7a and the
contact wires 105a,105b is in the form of two
groups of connecting wires, each associated with a
mated pair of detachable electrical connectors
54a,5~b,55a,55b located on opposite sides of the
slip-ring sub-assembly.
For each contact wire loop 105b there is a
connecting wire 56b (one for each loop) soldered to
the loop at a point proximate to the supporting
rack prong 107 which does not contact the spring
110. Therefore, under this arrangement, the
AAAEt~DED SHEET
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connecting wires are divided into two groups. One
group (containing 56b) leads from the loops to the
upper connector 54b, and the other group
(containing 56a) leads to the lower connector 55b.
Electrical connection between the contact
rings 102 and a more distal PCB can be in the form
of connecting wires 125x-125d (one for each contact
ring) running through axial channels 126 in the
slip-ring sub-assembly axle 50. The channels are
provided by a crenellated tube 127 concentrically
mounted around the axle 50 beneath the contact
rings 102. Electrical contact is made to a ring
inserting the end 128a,128b of a connecting wire
into a radial well 129 set into the inner surface
of each contact ring. The connecting wires 125a-
125d congregate at the distal end 130 of the sub-
assembly where they bundle into a multi-conductor
cable 58 leading to a more distal PCB 46.
Other means such as metallization of the
supporting rack prongs and/or the sub-assembly axle
and other various components may be used to form
electrical contact between the slip-ring sub-
assembly and the wiring leading to the adjacent
swiveling joint or the adjacent PCB.
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Each preferred slip-ring sub-assembly has nine
conductive contact rings t02.
The preferred electronic means for_
communicating the output of the probe and encoders
to the recorder/analyzer will now be described with
reference to Figures 2 and 13-14.
Analog signals emanating from the encoders are
fed via wiring to signal :conditioning units
203,2o4,Z05 each mounted on a printed circuit board
("PCH") located in the adjacent transfer member
more proximate to the base: Each of the PCHs
residing in a transfer member are siz~d and
dimensioned to fit within the tubular confines of
the inner shaft. '
Therefore, signals from- the probe i (if tie=e
__ are .any and encoder 17f flow on wiring to unit 205. -_..
Signals from encoders i7e and 19d flow to PCB Z04.
And, signals from encoders 17a and i~b flow to PCH
203. Signals from encoder i9a flow to a PCH Z0Z
located on the base 2.
Each circuit board comprises microprocessor
means for reconditioning the analog signals from
the encoders into digital information to be sent to
the recording and analysis system.
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The PCBs therefore, are capable of receiving
and digitizing analog data from their dedicated
encoders or probe, and communicating with the
recorder/analyzer.
In general, coarse positioning of the encoders
is continuously monitored by a counter operating in
the microprocessor to count the polarity changes in
the encoder's output. Fine positioning occurs only
when the microprocessor receives a latching signal
301 from the recorder/analyzer. A latching signal
may be caused by any number of events such as the
operator pushing a button on the probe or events
generated by the recorder/analyzer software.
A latching signal 301 causes the micro
processor on each of the PCBs to initiate a fine
positioning reading of all the encoders in sync at
a specific instant in time. The precise voltage
from each of the signals of each encoder is
digitized and combined with the counter to derive a
2p digitized fine position data word which is stored
in the microprocessor until it can transmit its
data to the recorder/analyzer.
Referring now to Figure 14, of the lines
running through the slip-rings, two lines are
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PCTIUS 9 7 l 06 3 9 3
~ 9 NOV 97
.~.~r-J ~
devoted to ground 302, and another 305 as a shield.
Two lines 303 supply power at +10 volts DC to the
microprocessors, their attendant circuitry and the
encoders. Double lines are used increase
reliability and to divide power supply currents.
This allows for the use of lighter gauge and hence
lighter weight and more flexible wiring.
Another line 304 supplies +12 volts DC to
power the probe 6 if needed. One sync line 301 is
devoted to carrying the latching signal. The two
remaining lines are devoted to carrying multiplexed
data 310 to and from each of the PCBs. Two lines
are used to support the preferred RS-485 data
communication standard.
Analog data from the encoders comprises an
index line 321 which signals when the zero position
of the encoder is reached, and the two sinusoidal
lines A 322 and B323. Comparators 324 effectively
transform these signals into square waves monitored
by a programmable field programmable gate array
(FPGA) logic module 330 thereby tracking the coarse
positioning of the encoders. The FPGA module also
interprets any latching signal arriving on the sync
line 301 and directs a voltage hold circuit 325 to
A~IFNDED SrtEEt
r_.__ ~___. _ _ _.__ .._. ._.____. ~___
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initiate a hold of the current voltage outputs of
the encoders. The hold circuit maintains this
voltage long enough for the programmable
microprocessor 340 to digitally sample the held
voltage value. Programming for the microprocessor
is stored in a memory module 350. Command
communication carried on data 326 and address lines
327 between the microprocessor 340, the FPGA 330
and the memory module 350 is routed by a
programmable data and address line monitoring unit
360.
The specific identification of the components
in Figure 14 is provided only for example. Those
familiar with the art could substantially modify
the components shown and their interactions without
departing from the PCBs primary functions and
hence, the invention.
By digitizing the signal from the encoders as
physically close to the encoders as possible,
interference is kept to a minimum. In this way
resolution is improved over prior designs using
preamplifiers set close to analog encoders.
Another advantage of this approach is that it
allows the use of fewer electrical connectors
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(three per member), thereby increasing reliability.
Connectors are provided only at one end of the PCB
and at the slip-ring sub-assemblies. If data was
further multiplexed and/or modulated upon the power
lines, fewer connectors may be necessary.
Another advantage of the present invention is
that it allows for modular disassembly and
replacement of component parts. For example to
replace encoder 17c one would first loosen split
collar clamp 53 through holes 51,52. Next one
would remove screws 36 fastening the sheath 31 of
member 4 to the yoke 8 of joint 7c. The member 4
would then separate from the joint 7c, and
electrical connector pairs 54a,54b and 55a,55b
would disconnect. Encoder-to-yoke mounting screws
69 would then be exposed. Removing these screws
would allow for removal of the encoder save for
wiring leading to the circuit board in member 3.
Completely removing the encoder 17c would then
require cutting the wiring or accessing the circuit
board in member 3.
Therefore, when a single encoder or processor
goes bad, it can be replaced on site. This feature
also allows for members of varying sizes to be
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interchanged. The size of the measurement sphere
is therefore only limited by the physical abilities
of the operator.
Although the preferred embodiment utilizes a
multi- conductor slip-ring, it should be noted that
all signals to and from the joints of the arm may
be further multiplexed and modulated upon the power
supply line. Therefore, the slip-ring may be
reduced to as few as two conductors, one being the
power/signal line, and the other being a ground.
It should also be noted that power and signal
transmission means other than a slip-ring may be
used. For example, electromagnetic transmission
through the air may be accomplished using radio
waves, microwaves, light waves, or infrared waves.
Fiber optic cables having swivel connectors may
also be adapted for use in the arm.
