Linear capacitance displacement transducer

Claims

What is claimed is:

1. A coaxial variable capacitor for measuring linear displacement between a first and second member of an assembly comprising:

(a) a first cylindrical capacitor plate composed of electrically conductive material;

(b) a second cylindrical capacitor plate composed of electrically conductive material which slidably engages the first cylindrical capacitor plate further comprised of:

(1) a hole extending into but not through the second member;

(2) an extended rigid hollow transducer tube substantially the length of the hole comprised of:

(i) a longer section having a first diameter;

(ii) a second section having a second diameter larger than said first diameter of said first section;

(iii) a rounded shoulder joining the smaller diameter and larger diameter sections of the tube;

(3) a mounting flange; and

(4) means for attaching the mounting flange to the transducer tube

(c) a first dielectric material interposed between said first and second capacitor plates and covering the entire surface of one of said plates;

(d) means for attaching said first cylindrical plate to the first member of an assembly; and

(e) means for rigidly securing or attaching said second cylindrical capacitor plate to the second member of an assembly

wherein relative movement of the first and second members of the assembly causes corresponding relative movement of said first and second capacitor plates which results in a variable capacitance between the plates which is linearly related to the relative linear displacement of the first and second members of the assembly.

2. The coaxial variable capacitor of claim 1 wherein the dielectric material is fixedly attached to the outer surface of the first cylindrical capacitor plate.

3. The coaxial variable capacitor of claim 2 wherein the first dielectric material is a Teflon.

4. The coaxial variable capacitor of claim 3 wherein the dielectric material is TFE5.

5. The coaxial variable capacitor of claim 1 wherein the dielectric material is fixedly attached to the inner surface of the second cylindrical capacitor plate.

6. The coaxial variable capacitor of claim 5 wherein the first dielectric material is a Teflon.

7. The coaxial variable capacitor of claim 4 wherein the dielectric material is TFE5.

8. The coaxial variable capacitor of claim 7 wherein the solid rod first cylindrical capacitor plate is composed of stainless steel.

9. The coaxial variable capacitor of claim 1 wherein the first cylindrical capacitor plate is a solid rod.

10. The coaxial variable capacitor of claim 1 wherein the transducer tube is composed of stainless steel.

11. The coaxial variable capacitor of claim 1 wherein the means for attaching said first cylindrical capacitor plate to the first member of an assembly comprises a floating mounting assembly which permits radial and angular displacement of the first cylindrical capacitor plate.

12. The coaxial variable capacitor of claim 11 wherein the floating mounting assembly further comprises:

(a) a first cylindrical capacitor plate mount composed of electrically insulating material;

(b) a means for rigidly fastening the cylindrical capacitor plate to said mount;

(c) a retainer flange having a hole sufficiently large that part but not all of said mount may pass freely through it;

(d) a retainer cover having a hole larger than the diameter of the first cylindrical capacitor plate and through which the first cylindrical capacitor plate passes when the retainer cover is fastened to the retainer flange but which hole is smaller than the mount, and which retainer cover, when attached to the retainer flange, defines, in conjunction with the retainer flange, a movement space of height greater than the thickness of the mount and width cross sectional area larger than the greatest width cross sectional area of said mount;

(e) means for fastening the retainer cover to the retainer flange to form a housing;

(f) means for resiliently forcing said mount into slidable contact with the housing formed by the retainer flange and retainer cover; and

(g) means for fastening the fastened retainer flange and retainer cover to the first member of an assembly;

wherein the mount is restrained between the retainer flange and retainer cover so that it may be slidably displaced about said movement space and at the same time be free to tilt against the action of the resilient forcing means.

13. A coaxial variable capacitor for measuring linear displacement between a first and second member of an assembly comprising:

(a) a first cylindrical capacitor plate composed of electrically conductive material;

(b) a second cylindrical capacitor plate composed of electrically conductive material which slidably engages the first cylindrical capacitor plate;

(c) a first dielectric material interposed between said first and second capacitor plates and covering the entire surface of one of said plates;

(d) means for attaching said first cylindrical plate to the first member of an assembly further comprising a floating mounting assembly which permits radial and angular displacement of the first cylindrical capacitor plate; and

(e) means for attaching said second cylindrical plate to the second member of an assembly

wherein relative movement of the first and second members of the assembly causes corresponding relative movement of said first and second capacitor plates which results in a variable capacitance between the plates which is linearly related to the relative linear displacement of the first and second members of the assembly.

14. The coaxial variable capacitor of claim 13 wherein the floating mounting assembly further comprises:

(a) a first cylindrical capacitor plate mount composed of electrically insulating material;

(b) a means for rigidly fastening the cylindrical capacitor plate to said mount;

(c) a retainer flange having a hole sufficiently large that part but not all of said mount may pass freely through it;

(d) a retainer cover having a hole larger than the diameter of the first cylindrical capacitor plate and through which the first cylindrical capacitor plate passes when the retainer cover is fastened to the retainer flange but which hole is smaller than the mount, and which retainer cover, when attached to the retainer flange, defines in conjunction with the retainer flange a movement space of height greater than the thickness of the mount and width cross sectional area larger than the greatest width cross sectional area of said mount;

(e) means for fastening the retainer cover to the retainer flange to form a housing;

(f) means for resiliently forcing said mount into slidable contact with the housing formed by the retainer flange and retainer cover; and

(g) means for fastening the fastened retainer flange and retainer cover to the first member of an assembly

wherein the mount is restrained between the retainer flange and retainer cover so that it may be slidably displaced about said movement space and at the same time be free to tilt against the action of the resilient forcing means.

15. The coaxial variable capacitor of claim 13 used in a linear displacement transducer as the variable capacitor with a capacitor controlled oscillator further comprising:

(a) a fixed resistor-variable capacitor input wherein the voltage across the variable capacitor serves as the input voltage to the oscillator;

(b) an isolation amplifier;

(c) a first voltage comparator which generates an output to a flip flop circuit when a lower predetermined voltage is reached by the input;

(d) a second voltage comparator which generates an output to said flip flop circuit when a higher predetermined voltage is reached by the input;

(e) said flip flop circuit responsive to the outputs of the comparators whereby the flip flop alternately changes state upon the input to the oscillator reaching a lower and then higher predetermined voltage generating a square wave

wherein the square wave output voltage from the flip flop circuit drives the fixed resistor of the input resistor-capacitor circuit so that the difference in time for the voltage on the input variable capacitor to reach the predetermined voltage values set at the comparators determines the period of the square wave output such that the period of the square wave output of the oscillator is a linear measure of the capacitance of the variable capacitor and is locked in phase with the charging and discharging of the input resistor-capacitor circuit.

16. A coaxial variable capacitor for measuring linear displacement between a first and second member of an assembly comprising:

(a) a first cylindrical capacitor plate composed of electrically conductive material;

(b) a second cylindrical capacitor plate composed of electrically conductive material which slidably engages the first cylindrical capacitor plate;

(c) a first dielectric material interposed between said first and second capacitor plates and covering the entire surface of one of said plates;

(d) means for attaching said first cylindrical plate to the first member of an assembly;

(e) means for attaching said second cylindrical plate to the second member of an assembly

(f) a third cylindrical capacitor plate composed of electrically conductive material which slidably engages the second cylindrical capacitor plate;

(g) means for attaching said third cylindrical capacitor plate to the first member of an assembly wherein said third cylindrical capacitor plate encircles the first cylindrical capacitor plate for its entire length;

(h) a second dielectric material interposed between the first and third cylindrical capacitor plates; and

(i) means for electrical conduction between the second and third cylindrical capacitor plates

wherein relative movement of the first and second members of the assembly causes corresponding relative movement of said first and second capacitor plates which results in a variable capacitance between the plates which is linearly related to the relative linear displacement of the first and second members of the assembly.

17. The coaxial variable capacitor of claim 16 wherein the first cylindrical capacitor plate is a solid rod.

18. The coaxial variable capacitor of claim 17 wherein the solid rod first cylindrical capacitor plate is composed of stainless steel.

19. The coaxial variable capacitor of claim 16 wherein the second cylindrical capacitor plate comprises the second part of the assembly having a hole such that the interior surface of the assembly formed by said hole serves as the capacitor surface.

20. The coaxial variable cylinder of claim 19 wherein the second cylindrical capacitor plate is formed from the piston rod and its attached piston head of a hydraulic cylinder.

21. The coaxial variable capacitor of claim 20 wherein the hole in the second cylinder capacitor plate is lined with a transducer tube further comprising:

(a) an extended hollow tube the length of the second capacitor plate comprised of:

(i) a longer section having a first diameter;

(ii) a second section having a second diameter larger than said first diameter of said first section;

(iii) a rounded shoulder joining the smaller diameter and larger diameter sections of the tube;

(b) a mounting flange;

(c) means for attaching the mounting flange to the transducer tube; and

(d) means for rigidly securing or attaching the mounting flange to the second cylindrical capacitor plates.

22. The coaxial variable capacitor of claim 21 wherein the transducer tube is composed of stainless steel.

23. The coaxial variable capacitor of claim 16 wherein the first dielectric material is fixedly attached to the outer surface of the first cylindrical capacitor plate.

24. The coaxial variable capacitor of claim 23 wherein the first dielectric material is a Teflon.

25. The coaxial variable capacitor of claim 24 wherein the dielectric material is TFE5.

26. The coaxial variable capacitor of claim 16 wherein the first dielectric material is fixedly attached to the inner surface of the second cylindrical capacitor plate.

27. The coaxial variable capacitor of claim 26 wherein the first dielectric material is a Teflon.

28. The coaxial variable capacitor of claim 27 wherein the Teflon dielectric material is Teflon TFE5.

29. The coaxial variable capacitor of claim 16 wherein the means for attaching said first cylindrical capacitor plate to the first member of an assembly comprises a floating mounting assembly which permits radial and angular displacement of the first cylindrical capacitor plate.

30. The coaxial variable capacitor of claim 29 wherein the first cylindrical capacitor plate of the floating mounting assembly is attached to the cylinder head of a hydraulic cylinder.

31. The coaxial variable capacitor of claim 29 wherein the floating mounting assembly further comprises:

(a) a first cylindrical capacitor plate mount composed of electrically insulating material;

(b) a means for rigidly fastening the cylindrical capacitor plate to said mount;

(c) a retainer flange having a hole sufficiently large that part but not all of said mount may pass freely through it;

(d) a retainer cover having a hole larger than the diameter of the first cylindrical capacitor plate and through which the first cylindrical capacitor plate passes when the retainer cover is fastened to the retainer flange but which hole is smaller than the mount, and which retainer cover, when attached to the retainer flange, defines in conjunction with the retainer flange a movement space of height greater than the thickness of the mount and supporting cross sectional area larger than the greatest cross sectional area of said mount;

(e) means for fastening the retainer cover to the retainer flange to form a housing;

(f) means for resiliently forcing said mount into slidable contact with the housing formed by the retainer flange and retainer cover; and

(g) means for fastening the fastened retainer flange and retainer cover to the first member of an assembly

wherein the mount is restrained between the retainer flange and retainer cover so that it may be slidably displaced about said movement space and at the same time to be free to tilt against the action of the resilient forcing means.

32. The coaxial variable capacitor of claim 31 wherein the insulator of the floating mounting assembly is composed of nylon.

33. The coaxial variable capacitor of claim 31 wherein the insulator of the floating mounting assembly is composed of delrin.

34. The coaxial variable capacitor of claim 16 wherein the means for attaching the second cylindrical plate to the second member of an assembly comprises integrally forming the second cylindrical capacitor plate from the second member of an assembly by drilling a hole in the second part of the assembly.

35. The coaxial variable capacitor of claim 34 wherein the second cylindrical capacitor plate is attached to the second member of an assembly by being integrally formed from the piston rod and its associated piston head of a hydraulic cylinder.

36. The coaxial variable capacitor of claim 35 wherein the piston head has a hole completely through it and the piston rod has a hole in its longer dimension but not through its entire length.

37. The coaxial variable capacitor of claim 16 wherein the third cylindrical capacitor plate is the housing surrounding said first and second plates.

38. The coaxial variable capacitor of claim 37 wherein the housing forming the third cylindrical capacitor plate is the piston cylinder wall of a hydraulic cylinder.

39. The coaxial variable capacitor of claim 16 wherein the means for attaching the third cylindrical capacitor plate is to form the third plate out of the housing surrounding the first and second plates.

40. The coaxial variable capacitor of claim 39 wherein the housing forming the third cylindrical capacitor plate is the piston cylinder wall of a hydraulic cylinder.

41. The coaxial variable capacitor of claim 16 wherein the second dielectric material is hydraulic fluid.

42. The coaxial variable capacitor of claim 16 wherein the means for electrical conduction between the second and third cylindrical capacitor plates is simultaneous metal to metal contact between a metal hydraulic piston rod and its metal support bearing and between the metal support bearing and the metal bearing head of a hydraulic cylinder.

43. The coaxial variable capacitor of claim 1 used in a linear displacement transducer as the variable capacitor with a capacitor controlled oscillator further comprising:

(a) a fixed resistor-variable capacitor input wherein the voltage across the variable capacitor serves as the input voltage to the oscillator;

(b) an isolation amplifier;

(c) a first voltage comparator which generates an output to a flip flop circuit when a lower predetermined voltage is reached by the input;

(d) a second voltage comparator which generates an output to said flip flop circuit when a higher predetermined voltage is reached by the input; and

(e) said flip flop circuit responsive to the outputs of the comparators whereby the flip flop alternately changes state upon the input to the oscillator reaching a lower and then higher predetermined voltage generating a square wave

wherein the square wave output voltage from the flip flop circuit drives the fixed resistor of the input resistor-capacitor circuit so that the difference in time for the voltage on the input variable capacitor to reach the predetermined voltage values set at the comparators determines the period of the square wave output such that the period of the square wave output of the oscillator is a linear measure of the capacitance of the variable capacitor and is locked in phase with the charging and discharging of the input resistor-capacitor circuit.

44. The coaxial variable capacitor of claim 16 used in a linear displacement transducer as the variable capacitor with a capacitor controlled oscillator further comprising:

(a) a fixed resistor-variable capacitor input wherein the voltage across the variable capacitor serves as the input voltage to the oscillator;

(b) an isolation amplifier;

(c) a first voltage comparator which generates an output to a flip flop circuit when a lower predetermined voltage is reached by the input;

(d) a second voltage comparator which generates an output to said flip flop circuit when a higher predetermined voltage is reached by the input;

(e) said flip flop circuit responsive to the outputs of the comparators whereby the flip flop alternately changes state upon the input to the oscillator reaching a lower and then higher predetermined voltage generating a square wave

wherein the square wave output voltage from the flip flop circuit drives the fixed resistor of the input resistor-capacitor circuit so that the difference in time for the voltage on the input variable capacitor to reach the predetermined voltage values set at the comparators determines the period of the square wave output such that the period of the square wave output of the oscillator is a linear measure of the capacitance of the variable capacitor and is locked in phase with the charging and discharging of the input resistor-capacitor circuit.

45. A linear displacement transducer formed from a coaxial variable capacitor and a capacitor controlled oscillator comprising:

(a) a first cylindrical capacitor plate composed of electrically conductive material;

(b) a second cylindrical capacitor plate composed of electrically conductive material which slidably engages the first cylindrical capacitor plate;

(c) a first dielectric material interposed between said first and second capacitor plates and covering the entire surface of one of said plates;

(d) means for attaching said first cylindrical plate to the first member of an assembly; and

(e) means for attaching said second cylindrical plate to the second member of an assembly

wherein relative movement of the first and second members of the assembly causes corresponding relative movement of said first and second capacitor plates which results in a variable capacitance between the plates which is linearly related to the relative linear displacement of the first and second members of the assembly;

a fixed resistor-variable capacitor input wherein the voltage across the variable capacitor serves as the input voltage to the oscillator;

(g) an isolation amplifier wherein said oscillator is isolated from said resistor-variable capacitor input so that the voltage of the resistor-variable capacitor input is not altered by any current drain by said oscillator;

(h) a first voltage comparator which generates an output to a flip flop circuit when a lower predetermined voltage is reached by the input;

(i) a second voltage comparator which generates an output to said flip flop circuit when a higher predetermined voltage is reached by the input; and

(j) said flip flop circuit responsive to the outputs of the comparators whereby the flip flop alternately changes state upon the input to the oscillator reaching a lower and then higher predetermined voltage generating a square wave

wherein the square wave output voltage from the flip flop circuit drives the fixed resistor of the input resistor-capacitor circuit so that the difference in time for the voltage on the input variable capacitor to reach the predetermined voltage values set at the comparators determines the period of the square wave output such that the period of the square wave output of the oscillator is a linear measure of the capacitance of the variable capacitor and is locked in phase with the charging and discharging of the input resistor-capacitor circuit.

46. A capacitor controlled oscillator comprising:

(a) a fixed resistor-variable capacitor input wherein the voltage across the variable capacitor serves as the input voltage to the oscillator;

(b) an isolation amplifier wherein said oscillator is isolated from said resistor-variable capacitor input so that the voltage of the resistor-variable capacitor input is not altered by any current drain by said oscillator;

(c) a first voltage comparator which generates an output to a flip flop circuit when a lower predetermined voltage is reached by the input;

(d) a second voltage comparator which generates an output to said flip flop circuit when a higher predetermined voltage is reached by the input; and

(e) said flip flop circuit responsive to the outputs of the comparators whereby the flip flop alternately changes state upon the input to the oscillator reaching a lower and then higher predetermined voltage generating a square wave

wherein the square wave output voltage from the flip flop circuit drives the fixed resistor of the input resistor-capacitor circuit so that the difference in time for the voltage on the input variable capacitor to reach the predetermined voltage values set at the comparators determines the period of the square wave output such that the period of the square wave output of the oscillator is a linear measure of the capacitance of the variable capacitor and is locked in phase with the charging and discharging of the input resistor-capacitor circuit.

47. The capacitor controlled oscillator of claim 46 wherein the isolation amplifier is an operational amplifier with feedback that draws insufficient current from the input capacitor to distort the voltage across the input capacitor, and provides sufficient current at the same voltage as across the input capacitor to drive the comparators.

48. The capacitor controlled oscillator of claim 46 wherein the first voltage comparator generates an output to the flip flop when 1/3 of the driving voltage is reached.

49. The capacitor controlled oscillator of claim 46 wherein the second comparator generates an output to the flip flop when 2/3 of the driving voltage is reached.

50. The capacitor controlled capacitor of claim 46 wherein the first and second voltage comparators have response times which are at least 100 times faster than the period of oscillation of the capacitor controlled oscillator.

51. The method of measuring capacitance comprising the following steps:

(a) combining a fixed resistor and a variable capacitor into a resistor-capacitor circuit with one side of the capacitor being grounded;

(b) applying the voltage across the capacitor of the resistor-capacitor circuit to the input of an oscillator;

(c) isolating the input voltage across the capacitor so that the voltage is not altered by any current drain;

(d) supplying sufficient current at a voltage identical to the voltage across the input capacitor to at least two comparators to drive the comparators;

(e) comparing the voltage across the input capacitor to two predetermined voltage values;

(f) generating an output signal to a flip flop each time the voltage across the input capacitor reaches either of the two predetermined levels;

(g) changing the state of the flip flop each time it receives a signal indicating the voltages across the input capacitor reaches either of the two predetermined levels; and

(h) applying the output of the flip flop to drive the input resistor-capacitor circuit thereby forming an oscillator

wherein the difference in time for the voltage on the input variable capacitor to reach the predetermined voltage values set at the comparators determines the period of the square wave output so that the period of the square wave output of the oscillator is a linear measure of the capacitance of the variable capacitor and is locked in phase with the charging and discharging of the input resistor-capacitor circuit.

BACKGROUND OF THE INVENTION

Field of Invention:

The present invention relates generally to the field of the precision measurement of displacement of mechanical assemblies, and more particularly to a displacement transducer utilizing precision electronic measurement of the capacitance of a variable capacitor attached to a movable assembly. The invention is adaptable for use in the measurement of the linear displacement of a hydraulic cylinder piston rod.

It is frequently necessary in equipment of all kinds to know the exact location of one movable part in relation to another part. To provide this information, various position indicating transducers have been devised which usually attach to the exterior of or otherwise sense the position of the movable part.

Initially, many position sensing devices used potentiometers to generate an electrical signal proportional to displacement. The current through or voltage across the potentiometer is used as a measure of linear displacement. Potentiometers come in many forms, both linear and circular, and means are well known in the prior art to link the potentiometers to reflect linear displacement. However, due to their resistive nature, potentiometers are susceptible to inaccuracies caused by temperature variations and wear. Also it has been found difficult to build potentiometers of high resolution to measure large linear displacements.

As machinery is more and more automated with electronic controllers, there is a growing necessity to obtain reliable positional information which can be provided to the controllers. Preferably the information will be presented in digital form. An example of a time modulated position transducer using inductive sensing is Technar Incorporated's 11000 series sensors. However, these sensors are limited to about an inch of linear displacement.

When the movable part whose position needs to be known is a piston rod of a hydraulic cylinder, it is often impractical or undesirable to have an external position sensing transducer. Internally mounted devices have been built and are now commercially available which will provide either a direct indication of the position of a hydraulic cylinder piston or provide such information to a closed loop hydraulic cylinder control system. One currently available example of this type of device is the magnetostrictive transducer of Temposonics, Inc. based on U.S. Pat. No. 3,898,555 by Tellerman. Using this device, a pulse-carrying conductor tube is mounted down the center of the piston rod. A magnet located on the piston head interacts with the magnetic field generated by an electrical pulse sent down the tube to produce a local torsional strain in the magnetostrictive tube which travels the length of the tube at ultrasonic speeds. A measurement of the time between electrical pulse generation and detection of the ultrasonic pulse gives an absolute indication of the position of the piston head. The linear displacement of very long hydraulic cylinders may be measured by this type of device However, the repetition rate at which measurements can be determined is dependent on the displacement being measured. The above mentioned device requires a complex electro-mechanical assembly resulting in significant costs for the device which may well be in excess of the cost of the hydraulic cylinder itself.

The SH25 hydraulic cylinder displacement transducer made by Hvilsted a/s gives an absolute position indication using an inductive sensing element built into the piston rod. However, it is limited to a 250 cm stroke length. The present invention may be mounted inside a hydraulic cylinder or externally to an actuation arm and accurately determines the absolute position of a hydraulic piston using relatively simple mechanical and electronic components and can, therefore, be manufactured at significantly reduced costs. In addition, the output virtually instantaneously reflects the displacement no matter how long or short a displacement is being measured.

Capacitance measurements have been used in the prior art to measure changes in linear dimensions. Patent Nos. 3,729,985 by Sikorra and 4,197,753 by Harting and Egger disclose strain gauges in which linear displacement is detected and measured by a differential capacitance detector. In these patents, a plate common to two capacitors is mounted either on a fixed or movable member while the two non-common plates are mounted on the other member. In Sikorra, the capacitors form half of a bridge circuit. Movement is measured by measuring the unbalanced output of the bridge circuit as the capacitor values change. Harting and Egger drive the differential capacitors with opposite ends of a center tapped secondary winding of a carrier transformer. The varying output signal from the movable plate is amplified and further processed by means known in the art.

Similarly patent No. 4,054,049 by Egger discloses a capacitance based linear displacement sensor for use in a thermal extensometer which utilizes a differential capacitance measured by a bridge circuit. It should be noted that the above cited patents utilize capacitance measurement to detect very small changes in linear dimension induced by thermal or strain distortions.

The sensitivity of capacitance changes has also been utilized in patent No. 3,365,946 by Hall. In this patent, a moveable central dividing plate between two unequal gas filled enclosures forms a common plate of two capacitors. The movement or displacement of the central plate changes the capacitance of both capacitors in an opposite direction, thus providing a differential capacitance measurement of linear displacement. Again, a bridge circuit is used to detect the changes in capacitance. In all the above cited patents, differential capacitance measurements were employed to measure very slight changes in linear dimension Previous to the present invention, the measurement of the capacitance of a single variable coaxial capacitor to measure precisely relatively large linear displacements has not been known.

SUMMARY OF THE INVENTION

The present invention utilizes a single variable coaxial capacitor in conjunction with a coordinated specially designed electronic circuit to measure linear displacement. The variable coaxial capacitor is formed by having one cylindrical plate fastened to a stationary member and another cylindrical plate fastened to a moveable member of an assembly. The outer surface of the inner plate or the inner surface of the outer plate may be coated with a dielectric material of high permittivity. The inner cylinder slidably engages within the outer cylinder so that, as the movable cylinder is displaced, the overlap area of the two cylinders changes as a function of linear displacement. The change in overlap area linearly changes the capacitance of the capacitor. A measure of the capacitance is, therefore, a measure of the displacement of the two cylinders with respect to each other. Precision measurement of the capacitance is achieved by the present invention with a capacitor controlled oscillator whose period in locked onto the capacitive changes The period of the square wave output from the oscillator is a linear measure of the capacitance, and, therefore, of the linear displacement. Any change in capacitance (displacment) is immediately reflected as a change in the period of the square wave. The period is converted into both analog and digital signals indicating the linear position of the transducer for use by typical system controllers

Specifically, for use with hydraulic cylinders, the moving plate of the capacitor is integrally formed from the hydraulic cylinder piston head, piston rod, and cylinder walls themselves, while the fixed plate is mounted inside an end of the hydraulic cylinder and extends into the cylinder. The fixed plate is a cylindrical probe in the form of an elongated metal rod. A hole drilled down the center of the hydraulic cylinder piston head and rod of sufficient diameter so that the probe may be slidably engaged within it, allows the interior walls of said hole and of the cylinder to serve as the movable second plate of the capacitor. A cylindrical teflon jacket surrounding the probe serves as a first dielectric medium and the hydraulic fluid serves as a second dielectric medium. As the piston is displaced linearally, a proportionally greater or lesser area of the probe is enclosed within the piston head and rod and the probe in exposed to a proportionately greater or lesser area of the cylinder walls, thus causing a change in the capacitance between the two cylindrical plates To accommodate any positional inaccuracies in the movement of the piston head and piston rod, the probe is mounted within the hydraulic cylinder head in a floating assembly.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a coaxial variable capacitor showing the inner cylindrical plate, the dielectric material on the inner cylindrical plate, the outer cylindrical plate, and the electrostatic shielding cylinder.

FIG. 2 is a cross-sectional view of a variable capacitor mounted in a typical hydraulic cylinder showing the inner plate or probe mounted on the hydraulic cylinder head and penetrating the movable piston head and rod.

FIG. 3 is a cross-sectional view of the probe in its floating mount.

FIG. 4 is a cross-sectional view of the transducer tube which lines the hole in the piston head and rod.

FIG. 5 is an end-on view of the probe mounting assembly, showing the cutout through which hydraulic fluid flows past the assembly.

FIGS. 6A-6C shows a resistor-capacitor circuit and the typical charging and discharging curves seen for different values of capacitance when the resistance is fixed.

FIG. 7 is a schematic diagram of the capacitor controlled oscillator used to measure the capacitance of the variable coaxial capacitor.

FIGS. 8A-8D shows the signals present in the capacitor controlled oscillator of FIG. 7 at various times.

FIG. 9 shows a block diagram of the signal processing electronics which may be used with the present invention to provide information to electronic machinery controllers.

DETAILED DESCRIPTION AND PREFERRED EMBODIMENT OF THE INVENTION

FIG. 1 shows a cross-sectional view of a coaxial variable capacitor made up of a solid central cylinder 39 which has affixed to its surface a dielectric material 40. Surrounding cylinder 39 is hollow cylinder 41 which is shown with a wall thickness 42. Either cylinder 39 or cylinder 41 may be fixed and the other movable. Surro