Capacitive displacement measuring instrument

• US 4,586,260 A
• Filed: 05/29/1984
• Issued: 05/06/1986
• Est. Priority Date: 05/29/1984
• Status: Expired due to Fees

• Current Assignee: LS Starrett Company
• Original Assignee: LS Starrett Company
• Inventors: Baxter, Larry K., Buehler, Robert J.
• Primary Examiner(s): NOT, DEFINED
• Application Number:
  US06/614,818
• Time in Patent Office: 707 Days
• Field of Search: 33/125 C, 33/143 L, 33/147 N, 33/172 E
• US Class Current: 33/706
• CPC Class Codes: G01B 3/205 provided with a counter for digital indication of the measured dimension

Abstract

A scale and slider in a linear displacement measuring instrument have spaced opposed parallel surfaces bearing aligned patterns of thin flat conductive elements. The scale pattern consists of equally spaced parallel bars transverse to displacement with enlarged aligned central portions which form collectively a single set of coupling electrodes. A pair of aligned sensor electrodes are fixed on the surface of the slider opposite the coupling electrodes of the scale and matching arrays of transmitter elements are arranged on opposite sides of the sensor electrodes and grouped axially in space quadrature. Alternate interdigitated sets of the transmitter electrodes on the slider are excited with oppositely phased square wave signals. The signals picked up by the sensor electrodes on the slider are fed to separate coarse plate counting and fine plate interpolation circuits. In the fine count circuit, the displacement-dependent amplitude of the signals from first one and then the other of the sensor electrodes is amplitude demodulated and digitally converted by a dual slope integrator. The reference signal for the return slope is furnished by placing cophase signals on the transmitter electrodes. Battery life is prolonged by staged power down modes which remove bias voltages from selected analog components while retaining limit values.

Claims

1. A capacitive displacement transducer, comprising

a scale member bearing a transducer surface,

a slider member bearing a transducer surface,

means for mounting said members for relative movement over a displacement interval throughout which said transducer surfaces are in predetermined spaced opposition,

a pattern of spaced elongated conductive elements fixed on said transducer surface on said scale member extending over said displacement interval arranged transversely to the direction of relative movement between said members,

a pattern of conductive elements fixed on said transducer surface of said slider member including two elongated coupling electrodes in spaced alignment parallel to the direction of movement opposite said elements on said scale member, and at least one array composed of two interdigitated sets of transmitter electrodes arranged parallel to said coupling electrodes, said array being divided lengthwise into two parts in space quadrature with each other at least coextensive respectively with said coupling electrodes in the direction of the length of said coupling electrodes,

means for furnishing electrical connections to said slider electrodes including means for electrically connecting said transmitter electrodes, respectively, in an external circuit and means for electrically connecting said two coupling electrodes respectively in an external circuit.

2. The transducer of claim 1, wherein said scale elements are bars having integral intermediate portions of increased surface area, said intermediate portions being aligned with respect to each other to form a single set of coupling electrodes.

3. The transducer of claim 2, wherein said slider member coupling electrodes being arranged opposite said scale member coupling electrodes.

4. The transducer of claim 3, wherein said slider pattern includes another array of two interdigitated sets of transmitter electrodes symmetrical with said one array.

5. The transducer of claim 4, wherein said arrays are arranged in parallel on opposite sides of said slider member coupling electrodes.

6. The transducer of claim 5 further comprising means for electrically interconnecting each of the corresponding sets of transmitter electrodes in two respective groups, and

means for connecting said two groups of transmitter electrodes, respectively, in an external circuit.

7. The transducer of claim 1, wherein, at the junction between the two parts of said array, the spacing between adjacent pairs of transmitter electrodes is approximately 25% greater or less than the otherwise uniform spacing between the remainder of said transmitter electrodes in both said sets.

8. The transducer of claim 1, wherein said two coupling electrodes are of approximately equal length.

9. The transducer of claim 1, wherein said scale elements are straight parallel bars.

10. The transducer of claim 1, wherein said scale elements are identical in shape and orientation.

11. The transducer of claim 1, wherein said scale elements are equally spaced.

12. The transducer of claim 2, wherein said scale elements are parallel bars having an elongated rectangular geometry with said intermediate portion also being of rectangular shape with increased width.

13. The transducer of claim 12, wherein said rectangular intermediate portion of each bar is centered on the axis of the outer rectangular end portions of each bar.

14. The transducer of claim 13, wherein said intermediate rectangular portion is approximately centered between the ends of the bar.

15. A capacitive displacement transducer system, comprising

a scale member bearing a transducer surface,

a slider member bearing a transducer surface,

means for mounting said members for relative movement over a displacement interval throughout which said transducer surfaces are aligned in predetermined spaced opposition,

a pattern of spaced conductive elements fixed on said transducer surface of said scale member extending over said displacement interval,

a pattern of spaced conductive elements fixed on said transducer surface of said slider member in alignment with said pattern of conductive elements on said scale element, said pattern of conductive elements on said slider member being divided lengthwise into X and Y groups arranged in space quadrature with each other,

means for applying multiphase high frequency excitation signal to said spaced elements on said slider member,

means for producing an X signal indicative of the signal capacitively coupled from the X group of conductive elements to the subjacent conductive elements of said scale member,

means for generating a Y signal indicative of the signal capacitively coupled from the Y group of conductive elements to the subjacent conductive elements on said scale member,

coarse counting means including

first demodulator means for sampling the relative polarity of said X and Y signals for producing outputs indicative thereof, and

logic means responsive to the outputs of said demodulator means for keeping a count proportional to the number of conductive elements on said scale member traversed by said slider member indicative of gross displacement, and

fine counting means for converting the analog amplitude of said X signal to a digital value.

16. The system of claim 15, wherein said fine counting means further includes means for applying a cophase signal to at least some of said slider elements during a cophase interval and means for comparing the amplitude of said X signal during normal excitation to the amplitude of said Y signal during said cophase interval for producing a normalized amplitude signal.

17. The system of claim 15 wherein said fine counting means includes

second demodulator means having an input for producing an analog output indicative of peak-to-peak signal levels at the input,

X/Y control means for alternately connecting the input of said second demodulator means to receive said X and Y signals respectively in accordance with the timing signal,

dual slope integrator means for integrating the output of said second demodulator means,

switch means for resetting said integrator means output to a reference level on command,

means for generating a cophase signal at said excitation frequency and for applying said cophase signal to a selected one of said X and Y groups of elements on said slider member when enabled,

control means for issuing timing signals for establishing alternating X signal and Y signal measuring cycles, each comprising a fixed integration period followed by a variable reverse integration period including means for causing said integrator means to integrate said peak-to-peak signal level for said fixed integration period and then enabling said cophase signal generator,

means responsive to the return of said integrator means output to said reference level during the generation of said cophase signal for issuing a signal indicative of the end point of said reverse integration period,

means for measuring the duration of said reverse integration period and for producing a digital output indicative thereof,

means for storing counts respectively indicative of the most recent digital outputs of said measuring means corresponding to the X signal and the Y signal, and

logic means for producing an output indicative of fine displacement of said slider member relative to said scale member accordingly to said stored counts.

18. The instrument of claim 17, wherein said fine counting means further includes

means responsive to the direction of change in the level of the output of said integrator means during said fixed integration period for producing a polarity signal indicative thereof,

means responsive to said polarity signal for applying said cophase signal to the corresponding X or Y group of slider elements during the following reverse integration period,

whereby said reverse integration returns said integrator means output to said reference level regardless of the direction of change in the level attained during said fixed integration period.

19. The instrument of claim 18, further comprising

an X buffer amplifying means connected between said X signal producing means and said X/Y control means,

a Y buffer amplifying means connected between said Y signal generating means and said X/Y control means,

means for alternately supplying bias voltage to said X and Y buffer amplifying means in accordance with said alternating X and Y measurement cycles.

20. The instrument of claim 19, wherein said first demodulator means in said coarse counting means includes first and second X and Y comparators each having a separate bias supply,

said polarity means including a third comparator having a separate bias supply,

said integrator means including an operational amplifier having a separate bias supply,

bias circuit means for producing the corresponding bias voltages for said X and Y buffer amplifiers, first, second and third comparators and integrator means operational amplifier, and

idle mode logic means for interrupting each of said bias supplies except for said first and second comparators on command,

whereby the displacement measuring instrument adopts an idle mode to conserve energy while remaining responsive to relative movement of said slider member via said coarse counting means.

21. The instrument of claim 20, further including

timer means for initiating said idle mode upon the expiration of a predetermined period without relative movement of said slider member.

22. The instrument of claim 21, wherein said timer means further includes means for interrupting all of said bias supply voltages after a predetermined period in said idle mode.