Capacitance-type measuring device for absolute measurement of positions

• US 4,879,508 A
• Filed: 05/31/1988
• Issued: 11/07/1989
• Est. Priority Date: 04/04/1986
• Status: Expired due to Term

• Current Assignee: Mitutoyo Corporation
• Original Assignee: Mitutoyo Corporation
• Inventors: Andermo, Nils I.
• Primary Examiner(s): Eisenzopf, Reinhard J.
• Assistant Examiner(s): Harvey, Jack B.
• Application Number:
  US07/200,368
• Time in Patent Office: 525 Days
• Field of Search: 324/61 R, 324/61 P, 340/870.37, 33/125 C, 341/15
• US Class Current: 324/690
• CPC Class Codes: G01B 7/003 for measuring position, not involving coordinate determination coordinate measuring G01B7/004
• CPC Class Codes: G01D 5/2415 adapted for encoders
• CPC Class Codes: H03M 1/645 for position encoding, e.g. using resolvers or synchros H03M1/485 takes precedence

Abstract

Capacitive-type measuring apparatus for making absolute measurements comprises first and second support members which are relatively displaceable with respect to a measurement axis; an array of first transmitter electrodes disposed on the first support member; an array of first receiver electrodes disposed on the second support member such that differing portions of the first receiver electrode array are capacitively coupled with the first transmitter electrode array in dependence on the relative positions of the support members; an array of second transmitter electrodes disposed on the second support member so as to be spatially offset from and electrically connected to corresponding first receiver electrodes; and a second receiver electrode arrangement disposed on the first support member for sensing the voltage distribution of the electric field created by the second transmitter electrode array in response to excitation of the first transmitter electrode array with N excitation signals. The degree of offset of a second transmitter electrode relative to its corresponding first receiver electrode is a predetermined function of the distance of the second transmitter electrode from a reference position. The first receiver electrodes define a fine wavelength, the first transmitter electrodes are arranged to occupy unique relative fine wavelength segment positions, and the second receiver electrode arrangement is adapted to produce outputs in response to predetermined combinations of excitation signals applied to the first transmitter electrodes which permit different transfer functions to be sensed, thereby permitting different resolution measurements to be made with the same electrode arrays which can be combined to obtain high accuracy absolute position measurements over an extended measuring range.

Claims

1. Capacitive-type measuring apparatus comprising:

first and second support members, said support members being relatively displaceable with respect to each other, and at least one of said support members being displaceable relative to a measurement axis;

an array of first transmitter electrodes disposed on said first support member in alignment with said measurement axis;

an array of first receiver electrodes disposed on said second support member in alignment with said measurement axis and such that differing portions of said first receiver electrode array are capacitively coupled with said first transmitter electrode array in dependence on the relative positions of said supporting members; and

an array of second transmitter electrodes disposed on said second support member in relative alignment with said first receiver electrode array, each of said second transmitter electrodes being electrically connected to a corresponding one of said first receiver electrodes so as to be offset from the corresponding first receiver electrode by an amount which is a predetermined function of the position of the second transmitter electrode relative to a reference position on said measurement axis.

2. The measuring apparatus of claim 1 further comprising:

second receiver electrode means disposed on said first support element in relative alignment with said first transmitter electrode array for sensing said electrode offset.

3. The measuring apparatus of claim 2 wherein said first transmitter electrodes are spaced from each other at predetermined intervals relative to the measurement axis, and at least one group of first transmitter electrodes is defined by at least two adjacent first transmitter electrodes;

said second receiver electrode means comprises at least one elongated second receiver electrode having a shape which varies relative to the measurement axis in correspondence with the measurement axis distance spanned by each first transmitter electrode group.

4. The measuring apparatus of claim 3 wherein said first transmitter electrodes are spaced at uniform predetermined intervals with a pitch P_t1, and said at least one first transmitter electrode group defines a transmitting wavelength W_t1 ;

and said at least one second receiver electrode has a shape which varies periodically over a receiving wavelength W_r2 having a predetermined relationship to said transmitting wavelength W_t1.

5. The measuring apparatus of claim 4 wherein said at least one second receiver electrode spans an integral number of transmitting wavelengths W_t1.

6. The measuring apparatus of claim 4 wherein said first receiver electrodes are uniformly spaced from each by a pitch P_r1 defining a scale wavelength W_f.

7. The measuring apparatus of claim 6 wherein said second transmitter electrodes are spaced uniformly from each other by a pitch P_t2 different from pitch P_r1 such that the variation in said offset of said second transmitter electrodes relative to the corresponding first receiver electrodes is a linear function of the position of each second transmitter electrode relative to said reference position;

and wherein said receiving wavelength W_r2 =W_t1 (P_t2 /P_r1).

8. The measuring apparatus of claim 4 wherein said second receiver electrode shape is substantially sinusoidal.

9. The measuring apparatus of claim 4 wherein said second receiver electrode shape is substantially triangular.

10. The measuring apparatus of claim 4 wherein said second receiver electrode shape is substantially rectangular.

11. The measuring apparatus of claim 3 wherein said first transmitter electrodes are spaced at uniform predetermined intervals with a pitch P_t1, and said at least one first transmitter electrode group defines a transmitting wavelength W_t1 ;

and said second receiver electrode means comprises an elongate array of discrete second receiver electrodes spaced from each other such that the pitch between adjacent pairs of second receiver electrodes defines a receiving wavelength W_r2 having a predetermined relationship to said transmitting wavelength W_t1.

12. The measuring apparatus of claim 1 wherein said first receiver electrodes are spaced from each other relative to the measurement axis by a pitch P_r1 defining a scale wavelength W_f, and at least one group of first transmitter electrodes is defined by N adjacent electrodes, where N is an integer greater than two in value;

said at least one first transmitter electrode group defining a transmitting wavelength W_t, and the first transmitter electrodes in each group are positioned within the group so as to respectively occupy predetermined group positions which span a distance greater than one wavelength W_f, and such that each group position corresponds to the relative position of a different one of a group of relative fine wavelength segment positions obtained by dividing the transmitting wavelength W_t into intervals corresponding to the fine wavelength, and dividing each interval into N equal segments.

13. The measuring apparatus of claim 12 further comprising excitation signal means for selectively applying N periodically varying excitation signals to the respective electrodes in each first transmitter electrode group in a first order of connection according to the sequence of positions of the first transmitter electrodes relative to each other in each first transmitter electrode group, and in a second order of connection according to the sequence of relative fine wavelength segment positions in which the respective first transmitter electrode group positions are arranged.

14. The measuring apparatus of claim 13 wherein the N signals differ from each other incrementally and are connected in numerical order to the respective first transmitter electrodes in each first transmitter electrode group in said first order of connection, and wherein said second order of connection is different from said first order of connection.

15. The capacitive-type measuring apparatus of claim 12 wherein said second receiver electrode means produces complementary first and second outputs in response to excitation signals applied to said first transmitter electrode array;

and the measuring apparatus further comprises signal processing means for selectively combining said first and second outputs to produce a measurement signal equivalent to the signal which would be produced f said second receiver electrode had an elongate nonvarying planar configuration.

16. Capacitive-type measuring apparatus comprising:

first and second support members, said support members being relatively displaceable with respect to each other, and at least one of said support members being displaceable relative to a measurement axis;

an array of first transmitter electrodes disposed on said first support member in alignment with said measurement axis;

an array of first receiver electrodes disposed on said second support member in alignment with said measurement axis and such that differing portions of said first receiver electrode array are capacitively coupled with said first transmitter electrode array in dependence on the relative positions of said supporting members;

an array of second transmitter electrodes disposed on said second support member in relative alignment with said first receiver electrode array, at least one group of first receiver electrodes being respectively electrically connected to a corresponding at least one group of second transmitter electrodes, each of said second transmitter electrodes in said at least one second transmitter electrode group being electrically connected to a corresponding one of said first receiver electrodes so as to be offset from the corresponding first receiver electrode by an amount which is a predetermined function of the position of the second transmitter electrode relative to the center of said at least one second transmitter electrode group.

17. The measuring apparatus of claim 16 further comprising:

second receiver electrode means disposed on said first support element in relative alignment with said first transmitter electrode array for sensing said electrode offset.

18. Capacitive-type measuring apparatus comprising:

first and second support members, said support members being relatively displaceable with respect to each other, and at least one of said support members being displaceable relative to a measurement axis;

an array of first transmitter electrodes disposed on said first support member in alignment with said measurement axis;

at least one array of first receiver electrodes disposed on said second support member in alignment with said measurement axis and such that differing portions of said at least one first receiver electrode array are capacitively coupled with said first transmitter electrode array in dependence on the relative positions of said supporting members;

an array of second transmitter electrodes disposed on said second support member in relative alignment with said at least one first array of first receiver electrode, each of said second transmitter electrodes being electrically connected to a corresponding one of said first receiver electrodes so as to be offset from the corresponding first receiver electrode by an amount which is a predetermined function of the position of the second transmitter electrode relative to a reference position on said measurement axis;

the spatial range occupied by the second transmitter electrodes, over which the degree of electrode offset between the second transmitter electrodes and the first receiver electrodes changes by a predetermined amount, defining a first measurement range;

an array of third transmitter electrodes disposed on said second support member in relative alignment with said at least one array of first receiver electrodes,

at least one group of first receiver electrodes being respectively electrically connected to a corresponding at least one group of third transmitter electrodes, each of said third transmitter electrodes in said at least one third transmitter electrode group being electrically connected to a corresponding one of said first receiver electrodes so as to be offset from the corresponding first receiver electrode by an amount which is a predetermined function of the position of the third transmitter electrode relative to the center of said at least one third transmitter electrode group, and such that the degree of electrode offset changes over said third transmitter electrode group by said predetermined amount, and the spatial range occupied by said at least one third transmitter electrode group defining a second measurement range smaller than said first measurement range.

19. The measuring apparatus of claim 18 further comprising:

second receiver electrode means disposed on said first support element in relative alignment with said first transmitter electrode array for sensing said electrode offset between said second transmitter and said first receiver electrods; and

third receiver electrode means disposed on said first support element in relative alignment with said first transmitter electrode array for sensing said electrode offset between said third transmitter electrodes in said at least one third transmitter electrode group and the corresponding first receiver electrodes.

20. The measuring apparatus of claim 19 wherein said first transmitter electrodes are spaced at uniform predetermined intervals relative to the measurement axis with a pitch P_t1, at least one group of first transmitter electrodes is defined by at least two adjacent first transmitter electrodes, said at least one first transmitter electrode group defining a transmitting wavelength W_t1 ;

said first receiver electrodes are uniformily spaced from each other by a pitch P_r1 defining a scale wavelength W_f ;

said second measuring range is an integral multiple of said scale wavelength W_f ; and

said predetermined amount of electrode offset change is substantially equal to said transmitting wavelength W_t1.

21. The measuring apparatus of claim 20 wherein said third transmitter electrodes in said at least one third transmitter electrode group are spaced from each other by a uniform pitch P_t3 different from pitch P_r.

22. The measuring apparatus of claim 21 wherein said pitch P_t3 is less than said pitch P_r1.

23. The measuring apparatus of claim 21 wherein said pitch P_t3 is greater than said pitch P_r1.

24. The measuring apparatus of claim 18 wherein said first receiver electrodes are spaced from each other relative to the measurement axis by a pitch P_r1 defining a scale wavelength W_f, and at least one group of first transmitter electrodes is defined by N adjacent electrodes, where N is an integer greater than two in value;

said at least one first transmitter electrode group defining a transmitting wavelength W_t, and the first transmitter electrodes in each group are positioned within the group so as to respectively occupy predetermined group positions which span a distance greater than one wavelength W_f, and such that each group position corresponds to the relative position of a different one of a group of relative fine wavelength segment positions obtained by dividing the transmitting wavelength W_t into intervals corresponding to the fine wavelength, and dividing each interval into N equal segments.

25. Capacitive-type measuring apparatus comprising:

first and second support members, said support members being relatively displaceable with respect to each other, and at least one of said support members being displaceable relative to a measurement axis;

electrode array means disposed on said first and second support members in relative alignment with said measurement axis for providing a plurality of discrete signal transmission paths each having a capacitive transfer function which varies in dependence on the relative positions of said first and second support members with respect to each other, said electrode array means comprising a transmitter electrode array having at least one group of N transmitter electrodes spaced from each other relative to the measurement axis, wherein N has a value which is an integer multiple of four, said electrode array means producing an output signal in response to excitation signals applied to said transmitter electrode array;

means for generating N groups of N excitation signals for application to the respective N electrodes of said at least one group of transmitter electrodes in a predetermined spatial order, each of said groups of excitation signals comprising two sets of excitation signals which are phase inverted with respect to each other and in spatial phase quadrature when applied to said transmitter electrodes, and the relative spatial phase positions occupied by the respective sets of excitation signals in each group of excitation signals successively incrementally changing from one group to the next;

means for selectively applying a first pair of first and second ones of said groups of excitation signals to said at least one group of transmitter electrodes to produce successive first and second electrode array means output signals;

means for demodulating said first and second output signals to produce first and second demodulation signals, and for performing a dual ramp integration of said first and second demodulation signals wherein said first demodulated signal is integrated for a predetermined time interval by integrator means, and said second demodulated signal is integrated by said integrator means until the integrator output returns to a reference level, said first and second ones of said groups of excitation signals being selected such that said integration of said second demodulation signal causes integration of the integrator output in the reverse direction from said integration of said first demodulation signal output;

means for measuring the integration time of said second demodulated signal integration and for producing an output when the integration time exceeds a predetermined limit value;

means responsive to said output of said integration time measuring means for resetting said demodulation and dual ramp integration means, for repetitively selectively applying a different further pair of first and second ones of said groups of excitation signals to said at least one group of transmitter electrodes, and for causing said demodulation and dual ramp integration means to integrate the resultant first and second demodulation signals until there is no output from said integration time measuring means.

26. The measuring apparatus of claim 25 wherein said signal transmission path transfer functions are substantially sinusoidal;

and said predetermined limit value for said integration time of said second demodulated signals corresponds to the range over which the ratio of said first and second electrode array means output signals is substantially linear.

27. Capacitive-type measuring apparatus comprising:

first and second support members, said support members being relatively displaceable with respect to each other, and at least one of said support members being displaceable relative to a measurement axis;

electrode array means disposed on said first and second support members in relative alignment with said measurement axis for providing a plurality of discrete signal transmission paths each having a capacitive transfer function with first and second components, said first component varying according to a predetermined function of the displacement between said support members over a first predetermined wavelength, and said second component varying according to a predetermined function of the displacement between said support members over a second predetermined wavelength shorter than said first predetermined wavelength.

28. The measuring apparatus of claim 27 wherein said first transfer function components vary in dependence on the positions of the associated signal transmission paths relative to a reference position on one of said support members.

29. The measuring apparatus of claim 28 further comprising filtering means for selectively sensing variations caused by one of said first and second transfer function components in output signals produced by said electrode array means in response to excitation signals applied thereto.

30. The measuring apparatus of claim 29 wherein said filtering means comprises detector electrode means included in said electrode array means, said detector electrode means producing first and second outputs which individually vary in accordance with said first transfer function component and which when combined produce a signal which varies in accordance with said second transfer function component.

31. The measuring apparatus of claim 30 wherein said detector electrode means comprises first and second complementary elongate electrode elements which each has a shape that varies according to a predetermined function relative to the measurement axis.

32. The measuring apparatus of claim 30 wherein said detector electrode means comprises a plurality of electrode pairs spaced relative to the measurment axis such that the electrodes in each pair occupy complementary phase positions.