Capacitance Transducer

This invention relates to a capacitive load transducer for measuring the amount of tension in a linkage.

Resistance-type strain gauges have been used to detect the draft loads in a tractor hitch, such as disclosed in U.S. Pat. No. 3,814,188. Such strain gauge sensors have suffered from a lack of sensitivity, temperature stability and durability. A dual capacitor draft sensor, with analog signal conditioning circuitry, is disclosed in U.S. Pat. No. 3,995,696. This draft sensor proved costly due to the required number of capacitor plates and the structure of its shaft-like strain member.

It is an object of this invention to provide an improved draft sensor with a simpler and less expensive strain member structure.

It is another object of this invention to provide a draft sensor of the capacitive type with only a single variable capacitor.

A further object of this invention is to provide a capacitive draft sensor with signal conditioning circuitry which may be easily interfaced with a digital control system.

These and other objects are achieved, according to the present invention, in that it provides an electronic capacitive draft sensor with a single variable capacitor. A pair of deformable members include spaced-apart central portions. The transverse spacing between the central portions varies according to the longitudinal tension applied to the ends of the deformable members. Separate disc-shaped supports are pivotally attached to each central portion via ball and socket connections. Each support holds a metal capacitor plate in spaced-apart relationship to each other. Rods cooperate with bores in the supports to maintain the capacitor plates in parallel alignment. A signal-conditioning circuit utilizes a commercially available voltage-to-frequency converter to obtain an output signal with a frequency which is proportional to the tension applied to the draft sensor.

FIG. 1 is a partial cross-sectional view of applicants' draft sensor;

FIG. 2 is a cross-sectional view of applicants' draft sensor along lines 2--2 of FIG. 1;

FIG. 3 is an electrical schematic of the signal conditioning circuit of this invention.

Referring to FIGS. 1 and 2, a strain or tension-measuring device 10 includes a pair of deformable members 12 and 13. Linkages 4 and 7 may be attached to opposite ends 17 and 18 of the device 10 so that the longitudinal tension in the linkages may be measured by the device 10. A bore 3 extends through end 17 so that a linkage 4 may be bolted thereto. A space is maintained in the end 18 via a spacer 5. A threaded bore 6, for receiving the other linkage 7, extends into the spaced end 18. The split ends of end 18 are clamped together by a bolt 8 to tightly grip the linkage 7 in the threaded bore 6. Deformable members 12 and 13 include laterally spaced-apart central portions 14 and 15, each with a centrally positioned threaded bore 16 extending laterally therethrough. Each bore 16 rotatably receives a threaded adjusting screw 19. The reduced diameter inner end 20 of each adjusting screw 19 is slip-fit into bores 22 which extend through spherical ball members 24.

A pair of identical rubber boots or seals 26 may be secured together by bolts 27 to enclose the volume circumscribed by the spaced-apart central portions 14 and 15 and to protect the enclosed volume from contamination from the environment. A groove 25 in the boots provides an exit path for an electrical cable (not shown) from circuit board 60. As best seen in FIG. 2, the boots 26 include a complimentary pair of recesses which cooperate to define an alignment notch 28.

A capacitive transducer 29 is carried in the volume enclosed by the central portions 14 and 15 and the rubber boots 26. The transducer 29 includes a pair of identical generally disc-shaped electrically insulating supports 30 which may be formed of an insulating material such as plastic. Each support 30 includes a raised rim 32 with an alignment tab 34 extending radially from the outer peripheral surface thereof. Each support 30 also includes three evenly distributed larger raised cylindrical necks 36 (two of which are shown in FIG. 1) extending from the outwardly facing side thereof. Each support 30 also includes three smaller necks 37 (two of which are shown in FIG. 1) extending from the inwardly facing side thereof in axial alignment with necks 36. Alignment bores extend laterally through the supports 30 and the necks 36 and 37. Each bore includes a smaller diameter portion 38 interconnecting larger diameter portions 35. Each inwardly located larger diameter bore portion 35 receives a compression spring 39 which is biased to urge the supports 30 apart. Each support 30 also includes a central raised cylindrical portion 41 on the side outwardly facing thereof. Each central raised portion includes a spherical socket 40 formed therein for pivotally receiving a corresponding one of the ball members 24 so that the supports 30 can pivot relative to their corresponding central portions 14 and 15. The adjusting screws 18 can be turned manually to adjust the spacing between the supports 30.

A pair of generally disc-shaped electrically conducting plates 50 are mounted by any suitable manner for movement with the supports 30. Each plate 50 includes three clearance bores 52, one of which is shown in FIG. 1, for lining up with the alignment bores 38 in the supports 30 and for registering with the corresponding small necks 37 of the support 30 to prevent rotation of the plates 50. Three alignment pins 56 extend through bore portions 35, 38 and 52 and slidably engage the walls of the smaller diameter portions 38 of the alignment bores in the supports 30. The alignment pins 56 cooperate with the walls of the bore portions 38 to maintain a parallel relationship between the plates 50. The plates 50 form the plates of a parallel plate variable capacitor Ct, the capacitance of which varies as a function of the separation between plates 50, and thus, as a function of the separation between central portions 14 and 15 of deformable members 12 and 13, and thus, as a function of the longitudinal tension applied to ends 17 and 18 of the sensor 10.

A circuit board 60 is supported by the rubber boots 26 in a position adjacent the transducer 29 and between the central portions 14 and 15 of the deformable members 12 and 13. The circuit 70 is assembled onto the circuit board 60, with the exception of the variable capacitor Ct.

A resistor R1 is coupled between ground and the base of grounded collector transistor Q1. A resistor R2 couples the base of Q1 to a regulated D.C. supply of +5 volts. A resistor R3 couples the emitter of Q1 with +5 volts. The emitter of Q1 is also coupled to the base of transistor Q2. A resistor R4 couples the emitter of Q2 with ground. The collector of Q2 is coupled to ground via capacitor C1 and is coupled to pins 1 and 6 of voltage-to-frequency (V/F) converter 76, such as National Semiconductor's LM231A. For a detailed description of the operation of V/F converter 76, the reader is referred to the manufacturer's specification literature for the LM231A.

Pin 2 of converter 76 is grounded via resistor R5. Pin 3 is connected to +5 volts via resistor R6. Pin 4 is grounded. Pin 5 is coupled to +5 volts via resistor R7 and is coupled to ground via variable capacitor Ct of transducer 29. Pin 7 is coupled to ground via capacitor C2. Pin 7 is also coupled to ground via series connected resistors R8 and R9 and is coupled to +5 volts via series connected resistors R8 and R10. Pin 8 is coupled to +5 volts.

The circuit 70 is similar to a circuit suggested in the specification literature for the LM231A V1F converter. As described in that literature, the converter 76 generates a square wave output signal at pin 3 with a frequency F defined by the equation:

The lateral separation between the central portions 14 and 15 of the deformable members 12 and 13 is proportional to the amount of longitudinal tension applied to ends 17 and 18 tending to pull the ends apart. The plates 50 of variable capacitor Ct move with their respective central portions 14 and 15 and thus, the separation d, between the plates 50, may be described by the following equation:

where T is the tension and k.sub.1 is a constant.

It is well known that the capacitance Ct of a closely spaced parallel plate capacitor is defined by the equation:

where .epsilon. is the permittivity of the dialectric between the plates 50 and s is the area of the plates.

Combining and rearranging equations (1), (2), and (3), we derive:

Thus, since the bracketed term [k.sub.1 /(R7.times..epsilon..times.s)] is a constant, the sensor 10 and the circuit 70 cooperate to generate an output signal at pin 3 of V/F converter 76 with a frequency F which is directly proportional to the tension T.

Pin 6 of V/F converter 76 provides for a threshhold input to a comparator amplifier (not shown) which is internal to the converter 76. If the level of this threshhold input is uncompensated for temperature variations, then the output frequency F will be undesirably dependent upon the temperature of the converter 76.

As described hereinafter, the applicants have provided a temperature compensation feature for improving the operation of the basic circuit suggested in the LM231A specification literature. This improvement comprises the addition of transistors Q1 and Q2 as previously described. Transistors Q1 and Q2 are temperature sensitive. Their respective gains vary with changes in temperature to vary the D.C. level of a threshhold voltage applied to pin 6 to compensate for temperature variations internal to the converter 76.