VARIABLE REACTANCE POSITION DETECTOR

Abstract: A position sensor (10) includes a capacitive position transducer (14) in series with an inductor. A generator (30) applies a current to the series combination at the resonant frequency thereof and the voltage across the capacitor provides an indication of the sensed position. The series resonance provides a filtering function essentially removing harmonics of the resonance frequency. It also boosts the voltage across the capacitor, thereby increasing the sensitivity of the sensor.

Description: VARIABLE REACTANCE POSITION DETECTOR BACKGROUND OF THE INVENTION Field of the Invention This invention relates to a novel position sensor in which a reactive position transducer is incorporated in a resonant circuit tuned to the drive frequency of the sensor. More particularly, it relates to a sensor driven by a square wave, in which the resonant circuit is tuned to the fundamental frequency of the square wave and thus provides a high degree of attenuation at harmonics of that frequency. A typical application of sensors to which the invention is directed is a galva- nometer unit comprising a servo-controlled, limited-rotation motor. An angular position sensor connected to a shaft of the motor provides a feedback signal for a servo loop that controls the instantaneous angular position of the shaft. The transducer is a variable reactance device, such as a compasitive transducer arranged in a differential capacitor configuration. The relative values of a pair of capacitances are linearly related to the angular position of the shaft, whereas the total capacitance is essentially independent of the shaft position. The galvanometer units are used in devices such as optical scanners, laser cutting tools and the like and it has become increasingly desirable to reduce the sizes of these devices. In turn this has imposed a requirement that the sizes of the galvanometer units be decreased, with a resulting requirement that the position sensors be miniaturized. However, this reduces the capacitances in the sensor and the sensitivity of a capacitance transducer is roughly proportional to these capacitances. The decrease in capacitance can be offset to some extent by increasing the frequency. However, it is desirable to obtain a sensitivity that is greater than what can be obtained solely by a fre- quency increase. The invention is therefore directed to a reduction in the size of the sensor without a corresponding reduction in its sensitivity SUMMARY OF THE INVENTION A sensor incorporating the invention includes a square-wave generator and a series-resonant circuit comprising a capacitive position transducer and an inductor. The sensor is a differential capacitor and its total capacitance is essentially independent of changes in the sensed position. The capacitance and inductance resonate at the fundamental frequency of the generator and therefore serve as a filter that removes the harmonics of that frequency. Furthermore, in a series resonant circuit the voltage across each of the reactive elements is much greater than the voltage across the circuit. Accordingly, the voltage across the capacitor is much greater than the drive voltage. This eliminates the requirement for a transformer that would otherwise be used to obtain an acceptable sensitivity when the size of the transducer is reduced. Equivalently, the sensitivity of the sensor increases with the current through the transducer. The low impedance provided by series resonance results in a much larger current for a given drive voltage and the transducer can therefore be driven with the voltages common to integrated circuits. The circuitry can therefore be found on a semiconductor chip, which maintaining the desired sensitivity. BRIEF DESCRIPTION OF THE DRAWINGS The invention description below refers to the accompanying drawings, of which: The drawing is a diagram of a position sensor incorporating the invention. DETAILED DESCRIPTION OF AN ILLUSTRATIVE EMBODIMENT As shown in the drawing, a position sensor generally indicated at 10 is employed to sense the angular position of a shaft 12 of a galvanometer motor (not shown). The sensor includes a capacitator position transducer 14, a drive circuit 16 that powers the transducer, and a sensing circuit responsive to the output of the transducer 18. The transducer 14, shown in exploded view, includes stators 20 and 22 that enclose a rotor 24 affixed to the shaft 12 for rotation therewith. The illustrated rotor is constructed of dialetric material and is configured with four radially extending lobes 24a. The stator 20 includes a drive electrode (not shown) covering the surface facing the rotor 24 and stator 22. The stator 22 includes pairs of electrode segments 28a, 28b facing the rotor 24 and stator 20. The segments 28a are connected together as are the segments 28b to provide a pair of inputs for the sensing circuit 18. The drive circuit 16 comprises a generator 30 whose output is amplified by an amplifier 31. The output of amplifier 31 is applied between ground and the series combination of an inductor 32 and the capacitance of the transducer 14. Preferably, in the neutral position of the shaft 12, each of the rotor lobes 24 overlies equal portions of a pair of electrode segments 28a, and 28b, so that the capacitances between the stator 20 and the segments are roughly equal. Accordingly, equal capacitative currents from the generator 30 pass through the transducer 14 to the respective input terminals of the sensing circuit 18. On the other hand, if the rotor 12 is displaced from its neutral position, the area of one of the underlying electrode segment pair opposite each of the lobes 24 increases while the underlying area of the other segment in the pair decreases. The relative capacitive currents through the electrodes in each pair change accordingly and this is reflected in the output of the sensing circuit 18. More specifically, the sensing circuit 18 includes a pair of identical current amplifiers 34. Each of the amplifiers comprises an operational amplifier 36 whose non- inverting input terminal 36a is grounded. A feedback resister 38 is connected between an output terminal 40 and the inverting input terminal 36b of the amplifier 36. A diode 42 is connected as shown, between an output the terminal of the transducer 12 and ground. A second diode 43 is connected in series as shown. During positive half-cycles of the output of the generator 30, current flows through the transducer 14 to each of the input terminals 36b of the amplifiers 38, these input terminals being maintained at ground potential by virtue of feedback to the resist- ers 38. The voltages at the output terminals 40 thus represent the currents through the respective transducer electrode segments 28a, 28b and thus the capacitanes between the stator 20 and these electrodes segments. During negative half-cycles of the generator output currents flow through the diodes 42 to ground. The difference between the voltages at the terminals 40 that represents the angular position of the shaft 12. The total transducer capacitance between the stator 20 and ground is essentially independent of the position of the shaft 12. This capacitance resonates with the reactance provided by the inductor 32 at the frequency of the output of the generator 30. The resonant combination thus has a very small impedance at that frequency and a sub- stantial current from the generator therefore passes through the transducer. The sensitivity of the position sensor, which depends on the current through the transducer, is thus materially increased by the resonance. Preferably, the generator 30 is a square wave (clock) generator. The resonant circuit thus provides an additional function, namely that of filtering the harmonics of the fundamental frequency of the generator output. The amplitude-frequency characteristic of the current through the transducer 14 exhibits a sharp cutoff at frequencies above the resonant frequency and thus essentially eliminates these harmonics from the transducer current. The frequency of the generator 30 is preferably as high as is practical, e.g., 24 MHz. This contributes to the ability to maintain sensitivity of the position sensor while reducing the size of the transducer and it also permits the use of a small "air core" inductor 32 which can be included on a semi-conductor chip containing the drive circuit 16 and the output circuit 18. The position sensor can thus be made small and at the same time inexpensive while maintaining the sensitivity required for highly accurate position measurements. While a conventional amplifier 31 will provide the advantages of the inventions, we prefer to use the illustrated amplifier, which provides an additional boost to the sensitivity of the sensor 10 and aids in filtering out the harmonics of the drive frequency. Specifically, the amplifier comprises a field effect transistor (FET) 50. The output of the generator 30 is applied between the FET gate 50a and the grounded source 50b. Power is applied to the FET drain 50c through an inductor 52. The inductor 52 resonates with the capacitances of the FET 50 to boost the voltage at the drain 50c, which is connected to the inductor 32. A diode 54 is connected as shown to provide a return path around the FET 50 for reverse current flow in the reso- nant circuit comprising the inductor 52 and FET 50. The voltage at the drain 50c thus has the half-sinusoid form shown at 56. As an example of circuit values that provide the advantages of the invention, the transducer 14 may have a capacitance of approximately 10 picofarads between the stator 20 and ground. Assuming a clock frequency of 16 MHz, the inductor 32 may then have a value of 9 microhenries. With a FET 50 capacitance of 15 picofarads, the inductor 52 may have a value of 15 microhenries. These inductance valves are approximate, since stray capacitances and inductances may have a significant effect on circuit operation and thus will ordinarily be compensated by adjustment of the values and configurations of the inductors. What is claimed is:

Claims: CLAIMS i L A position transducer for monitoring the position of an object sensor, 2 said sensor comprising: 3 A. a transducer comprising first and second electrically reactive sensing 4 elements whose relative impedances vary inversely as a function of 5 the position of the object, the sum of the impedance being essentially 6 invariant; 7 B. a reactive element that resonates with said impedance sum at a reso- 8 nant frequency; 9 C. a drive circuit; ιo 1. providing an electrical output current at said resonant frequency, n and; 12 2. connected to pass the output current through the series combina- 13 tion of the reactive elements; and ι 4 D. an output circuit responsive to the relative currents passing through is the sensing elements. ι 6 2. The sensor defined in claim 1 in which said drive circuit includes a 17 square wave generator, the fundamental frequency of the generator be- 18 ing said resonant frequency, the series resonant circuit serving to attenu- 19 ate the harmonics of the resonant frequency from the current passing 20 through said transducer. 2i 3. The sensor defined in claim 1 in which: 22 A. said transducer is a capacitance transducer, the sensing elements 23 comprising first and second capacitances; and 24 B. said reactive element is a first inductor. 25 4. The sensor defined in claim 2 in which: 26 5. The transducer of claim 3 in which said transducer comprises 27 A. first and second spaced apart stators, wherein 28 1. said first stator comprises a common conductive surface serving 2 as a first electrode and 0 2. said second stator comprises alternating, spaced-apart second and i third electrodes; 2 B. a rotor for mechanical connection to said object, said rotor compris- 3 ing dialetric segments, each of which is disposed between said first electrode and one each of said second and third electrodes. 5 6. The sensor defined in claim 5 in which said output circuit is responsive 6 to the combined currents through said second electrodes and the com- 7 bined currents through said third electrodes. 8 The sensor defined in claim 4 in which: 9 A. The drive circuit includes an amplifier connected to amplify the output of 0 said generator to provide said electrical output current; and i B. the amplifier includes: 2 1. a field effect transistor having a gate connected to the square-wave gen- 3 erator and a source and draw connected to said first inductor and to a 4 common reference point, and 5 2. a second inductor connected between a power supply and the junction of 6 said first inductor and said transistor, said second resonating insulating 7 with the capacitance of said transistor to boost the voltage at said junc- 8 tion.

Inventors: NUSSBAUM MICHAEL B, NUSSBAUM, MICHAEL, B.
Applicants: GSI LUMONICS INC, GSI LUMONICS, INC.
Application year: 2000
Main classification: G01D5
IPC codes: G01D5, G01R29