Transducer for measuring workpieces

A caliper-type transducer for measuring a workpiece includes a pair of non-crossing pivot arms each having a contact end and a transducer end. The arms are interconnected for frictionless pivotal movement by a fixed pivot block and spring members which act to limit the displacement of the arms to complemental displacement in the plane of measurement. One arm carries two reactive elements mounted in spaced opposed relationship such that the transducer end of the other arm may be disposed therebetween to vary the reactances according to movement of the caliper ends which cause corresponding movement in their respective transducer ends. The change in reactances of the reactive elements are differentially sensed as the arms are displaced from an initial position when measuring the workpiece.

INTRODUCTION 
This invention relates to an electromechanical measuring apparatus for 
gauging inside or outside dimensions of a workpiece, and more particularly 
to devices yielding an output linearly related to displacement. 
BACKGROUND 
There are many prior art devices which measure part dimensions as a 
function of the gap or distance between spaced elements which contact the 
part. The sensing elements of these devices include various electrical, 
magnetic and thermally actuated transducers. 
Simple coil and core transducers are sometimes used in gauging fixtures; 
however, the output signal of these gauges is of an exponential form and 
must be modified electronically to be useable. 
Linear variable displacement transducers (LVDT) are magnetically operative 
devices which generate a linear output signal relative to displacement. 
U.S. Pat. No. 2,196,806 to Hoadley discloses an early embodiment of an 
LVDT. However, due to their mode of operation, these devices are typically 
of such configuration that they do not readily lend themselves to gauging 
applications in which measurements must be accurately taken from 
workpieces having particular configurations, e.g., a cylindrical bore, 
which are not easily accessible by the LVDT's. 
Thermally actuated transducers are able to provide a linear output but the 
range of measurement is limited. 
Several different structural combinations exist for communicating the 
displacement of spaced elements when contacting a part to a transducing 
means. Some of these combinations include interlocking gears, scissor type 
pivot pins and various complicated combinations. However, they are 
generally subject to wear and environmental influences that affect 
accuracy and reliability. Finally, the prior art devices are typically 
designed for gauging either internal or external dimensions exclusively. 
BRIEF SUMMARY OF THE INVENTION 
In accordance with the present invention, a simple and rugged caliper type 
measuring device is provided which is capable of producing a highly linear 
electrical signal representing measured dimensions and which is readily 
adaptable to take measurements from workpieces having configurations not 
readily accessible to known LVDT's. In general, this is accomplished in a 
device having two rigid arms pivotably interconnected in non-crossing 
relationship mediate the ends thereof such that relative movement of the 
contact ends produces a proportional relative movement between the 
opposite ends. Said opposite ends are provided with signal generating 
means, such as variable reactive circuit elements, which produce an 
electrical signal quantity related to the measured dimensions. 
In the preferred form, one of the arms is provided with a reverse-cursed or 
J-shaped end bearing opposed inductor coils. The corresponding end of the 
other arm includes a ferromagnetic pole piece which is disposed between 
such coils to vary the inductive reactances according to movement of the 
arms in the measurement plane. The outputs of the coils are differentially 
sensed by means of a simple bridge circuit and amplifier, wherein the 
resulting signal quantity is made both linear and of high signal-to-noise 
ratio. Moreover, since the coils and the pole piece move in unison with 
their respective arm portions which contact the workpiece, the initial 
setup and alignment of the device is not as critical in comparison to 
other gauges. 
In accordance with another aspect of a preferred embodiment of the 
invention, the arms are reversible to allow the taking of both internal 
and external measurements and have portions which may be varied depending 
on the workpiece to be measured. To accomplish this, each arm is 
constructed in two portions, a transducer portion and a caliper portion. 
The caliper portion may have a wide variety of configurations to 
accomodate different workpieces and is reversibly alternately attachable 
to the transducer portion to change the orientation of its contact points 
to take internal and external measurements. The caliper ends may be 
attached to the device so as to preset the arms with a bias inward or 
outward to enable the device to take external or internal measurements in 
a wide variety of applications.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now to FIGS. 1, 2 and 4, the preferred embodiment of the 
invention is shown to comprise a first arm 10, a second arm 12, and a 
pivot block 14 which interconnects the two arms in non-crossing 
relationship. Arm 10 is connected to pivot block 14 by transverse reed 
spring 16 and parallel reed spring 18. Arm 12 is connected to the pivot 
block 14 by transverse reed spring 16 and parallel reed spring 20. In 
FIGS. 1, 2 and 4 the thickness of the reed springs is enlarged for 
clarity. This combination of reed springs 16, 18, and 20 flexibly connect 
the arms 10 and 12 to the pivot block 14 thereby limiting the relative 
movement of the arms to complemental displacement toward and away from 
each other within the measurement plane. 
Arm 10 comprises a caliper end 22 and a transducer end 23, the pivot block 
14 being mediate the two ends. Similarly, arm 12 comprises caliper end 24 
and transducer end 25, the pivot block 14 being mediate these two ends. 
The transducer end 23 of arm 10 is constructed in a J-shape to allow the 
mounting of inductors 26 and 28 in an opposed spaced relationship. 
Inductors 26 and 28 are preferably bobbinless coils mounted in ferrite pot 
cores. The transducer end 25 of arm 12 is S-shaped to allow the 
disposition of first and second ferromagnetic pieces 30 and 32 comprising 
a pole piece attached to one end thereof, between the inductors 26 and 28. 
In this arrangement the movement of the arms 10 and 12 produces an 
increase in the impedance of one inductor and a decrease in the impedance 
of the opposed inductor. As shown in FIG. 3 inductors 26 and 28 are 
electrically connected in opposite legs of bridge circuit 34. The change 
in voltage at bridge terminals 36 and 38 is sensed in the 
amplifier-comparator circuit 40 to yield an input for the display device 
42, shown in FIG. 1. 
The amplifier-comparator circuit 40 in the preferred embodiment may be a 
Type 741 operational amplifier or an equivalent. Transducing by inductive 
reactive elements, although advantageous, may be replaced with other 
elements whose electrical characteristics vary depending upon the relative 
displacement of an activator component. For example, the transducer end of 
the first arm could carry two complementally variable capacitors with the 
transducer end of the second arm carrying corresponding activator plate 
elements. 
As shown in FIG. 4 transducer end 23 comprises a rectangular segment 46 for 
carrying inductor 26 and an L-shaped segment 48 for carrying inductor 28 
in a spaced opposed relationship. Wires 50 and 51 leading from inductors 
26 and 28 are sheathed in insulative tubing 52 and 53 or channeled in a 
wire feed through to prevent short circuiting through the arm. 
The caliper end 22 is T-shaped in design as shown in FIG. 4 and the cross 
member 58 is mechanically attached, as shown in FIG. 2 by screws to the 
outer end of the rectangular segment 46. The rod portion 62 of the caliper 
end 22 extends from cross member 58. A standard carbide ball type contact 
point 64 is attached to a tapped hole in the side of caliper end 22 to 
engage a workpiece (not shown). The caliper end 22 is thus demountably 
attached to the transducer end, and may be attached with the contact point 
disposed for internal or external measurements. 
First and second ferromagnetic pieces 30 and 32 are cemented to transducer 
end 25 on opposite surfaces between and facing the inductors 26 and 28. 
Thin plates of a high, magnetic permeability material known as .mu. metal 
are preferred. Alternatively, a ferrite slug may be imbedded in second arm 
transducer end 25 instead of the two ferromagnetic pieces shown in the 
preferred embodiment. 
As shown in FIG. 4, caliper end 24 is T-shaped in design and cross-member 
76 is mechanically attached, as shown in FIG. 2, to transducer end 25. The 
rod portion 80 of caliper end 24 extends from cross-member 76, with a 
standard form carbide ball type contact point 82 attached to a tapped hole 
in the side of the caliper end 24 that is to engage a workpiece (not 
shown). It should be noted that the two-part arm construction detailed 
herein is not the only format for implementing the invention. It is 
preferred, however, as it lends itself to a dual function gauge adaptable 
to making either internal or external measurements. The configuration of 
rod portions 62 and 80 of caliper ends 22 and 24, respectively, can be 
varied to accomodate different workpieces. Since the caliper ends 22 and 
24 are detachable, these ends can be interchanged and yet still use the 
major portion of the device which operates in the same manner regardless 
of the configuration of the caliper ends. The mechanical symmetry inherent 
in the use of two arms of similar dimensions minimizes the effect of any 
elongation of the arms due to thermal expansion. Moreover, the utilization 
of two substantially identical inductors on the same mechanical base, 
complementally varied by a common activating element on transducer end 25 
and connected in a bridge circuit to provide differential sensing serves 
to provide compensation for temperature changes, variations in line 
voltage and component aging. 
As viewed in FIG. 1 pivot block 14 is substantially T-shaped with the 
cross-member of the T having a face 84 perpendicular to the longitudinal 
axes of the first and second arms 10 and 12. The rod portion 86 of the T 
parallel to the longitudinal axes of the arms. In the preferred embodiment 
the height of the pivot block 14 extends beyond the height of the first 
and second arms in one direction perpendicular to the measurement plane to 
permit the arms to move freely. The portion of the pivot block 14 
extending beyond said arms has a mounting means comprising two tapped 
holes provided therein to secure the device to a base 77 shown in FIG. 4 
and in dotted lines in FIG. 1. The device may be mounted in a wide range 
of orientations without affecting the measurement accuracy. 
As shown in FIGS. 2 and 4 the transverse reed spring 16 is disposed 
perpendicular to said first and second arms and is compliant only to 
parallel movement of said arms, as shown in the dotted lines. The middle 
of transverse reed spring 16 is fastened to the end of the rod portion 87 
of the pivot block 14. The first and second transverse reed spring ends 88 
and 90 extend to the first and second arms 10 and 12. First transverse 
reed spring end 88 is clamped between caliper end 22 and transducer end 
23. The two screws passing through cross-member 58 provide the clamping 
force but are spaced from spring end 88. Second transverse reed spring end 
90 is similarly clamped between caliper end 24 and transducer end 25. As 
shown in FIG. 4 transverse reed spring 16 is rectangular in shape having 
each of its four corners notched to permit passage of fasteners from the 
respective caliper end 22 and 24 to transducer 23 and transducer end 25, 
respectively. The notches allow the arms to be attached to the transverse 
reed spring in a range of alignments. 
As shown in FIG. 2 first and second parallel reed springs 18 and 20 are 
disposed parallel to the longitudinal axes of said first and second arms 
so as to be compliant only to divergent and convergent movement of the 
arms, as shown in the dotted lines. The first parallel reed spring 18 is 
fastened to the inner surface of the transducer end 23 of the first arm 10 
and is connected to the pivot block cross-member end surface 92. The 
second parallel reed spring 20 is similarly fastened to the inner surface 
of transducer end 25 of the second arm 12 and is connected to the pivot 
block cross-member end surface 94. The springs 18 and 20 terminate 
slightly inboard of spring 16 to avoid binding when the arms are moved. 
The resiliency of the reed springs determines the gauging tension of the 
device. The tension may be varied by changing the strength of the reed 
spring or changing the distance spanned by the parallel reed springs 18 
and 20 between the pivot block and the respective arms or by varying the 
thickness of reed springs 18 and 20. 
The frictionless reed spring pivot mechanism provides substantial 
advantages over the prior art in that it is insensitive to foreign 
material, requires no lubrication, is not subject to frictional wear, has 
desirable durability and can be inexpensively manufactured. 
The device of the present invention is characterized in that it can be used 
for a wide variety of different gauging applications. As one example the 
caliper type transducer is set at an initial position corresponding to the 
dimensions of a master or part of ideal size. Due to the unique 
construction of this device, the initial position is set by clamping the 
caliper end portions 22 and 24 and the outer end portions of the 
transducer ends 23 and 25 to the transverse reed spring 16, which is 
biased in a particular direction, e.g. the reed springs are tensioned to 
close down for OD measurements and open up for ID measurements. Once 
clamped in this orientation the arms are placed under load at the desired 
dimension via use of the master part, and the associated electronics are 
zeroed. In machining operations for which this device finds particular 
utility it is usually only desirable to follow the progress of a machining 
operation as it approaches the desired dimension. After the desired 
dimension is obtained, there is no need to measure reduction beyond the 
desired dimension. For this reason the device of the preferred embodiment 
may be zeroed with the pole piece of the transducer end of the second arm 
12 in close proximity to one inductor, and set in the initial position 
with the pole piece in close proximity to the other inductor. In this way 
the full range of measurement of the device may be realized. For example, 
in an operation to reduce an outside dimension from 35 mm to 30 mm the 
arms would be clamped in position at 35 mm and the gauge would be zeroed 
at the desired dimension of 30 mm using a master part. To maximize this 
range of measurement the pole piece of second arm transducer 25 can be 
placed in close proximity to inductor 26 when clamped in the initial 
position and in close proximity to inductor 28 when zeroed at the desired 
dimension. If the device is zeroed with the pole piece of transducer end 
25 substantially centered between inductors 26 and 28, bidirectional 
deviations may be measured. Since the pole piece and inductors 26, 28 are 
freely movable and mutually react with complemental displacement of their 
respective arms, the workpiece may be misaligned with the device to some 
degree without deleteriously affecting the accuracy of the measurement. 
Still other applications of this device will become readily apparent to 
one skilled in the art. 
Regardless of the application, as the contact points 64 and 82 engage the 
workpiece they are displaced from an initial position if the measured part 
varies in dimension from the master part, which displacement is 
transferred the length of the rigid arms 10 and 12 through the pivotal 
motion of the arms around pivot block 14. The movement of contact points 
64 and 82 is reflected in the complemental displacement of the transducer 
ends 23 and 25 causing a change in the impedance of each inductor, due to 
the change in spacing between the pole piece and the inductors. The 
inductors are electrically connected in bridge circuit 34 wherein the 
resulting change in impedance causes a change in voltage at bridge 
terminals 36 and 38. The output of the inductors are differentially sensed 
in amplifier-comparator circuit 40 to yield an output that is displayed on 
the display device 42. The display device may be of the electronic column 
gauge type disclosed in U.S. Pat. No. 4,038,756, or any other suitable 
type. This type of gauge has nulling circuit suitable for setting the 
caliper type transducer. As noted above, the output of the device is 
extremely linear and is not affected by changes in line voltage, 
temperature fluctuations, and component aging due to the differential 
sensing technique. 
FIG. 5 shows a preferred electrical circuit which may be used in place of 
the circuitry of FIG. 3. This circuit utilizes a transformer T1 having a 
primary winding 100 and a secondary winding 102. The secondary winding is 
center tapped as represented by 104 such that each half of the secondary 
winding serves as one leg of the inductive bridge network. Coils 106 and 
108 of the transducer complete the bridge circuit. This circuit permits 
the use of a relatively inexpensive single-ended amplifier 110 to 
differentially sense the output of the transducer as activator element 25 
is moved relative to the coils 106 and 108 since both the center tap 104 
and the noninverting input of the amplifier 110 are grounded. Since the 
probe excitation voltage source, .sup.e in, is isolated via transformer T1 
from the sensing circuitry, noise is minimized and greater flexibility in 
designing the grounding systems for the circuitry is obtained. 
It is to be understood that the invention has been described with reference 
to a specific illustrative embodiment, that various modifications are 
possible, and that the foregoing description is not to be construed in a 
limiting sense.