Capacitive position sensor including a scale with integral topographical features which effect the mutual capacitance between cursor electrodes

• US 5,394,096 A
• Filed: 11/16/1993
• Issued: 02/28/1995
• Est. Priority Date: 10/15/1991
• Status: Expired due to Fees

• Current Assignee: Hans U. Meyer
• Original Assignee: Hans U. Meyer
• Inventors: Meyer, Hans U.
• Primary Examiner(s): Wieder, Kenneth A.
• Assistant Examiner(s): TOBIN, C
• Application Number:
  US08/152,150
• Time in Patent Office: 469 Days
• Field of Search: 318/662, 324/660, 324/662, 324/665, 324/669, 324/686, 324/688, 324/690, 340/870.37
• US Class Current: 324/686
• CPC Class Codes: G01D 5/2415 adapted for encoders

Abstract

A capacitive position sensor has a scale (10), having hollow or raised topographical features, and a cursor (20) having an electrode array comprising at least two transmitting electrodes (21a, 21b) and at least one receiving electrode (21c) separated by shielding electrodes (21g). The coupling capacitances (Cac, Cbc) between transmitting and receiving cursor (20) electrodes are modified as a function of the position (x) of the scale topographic feature (11) relative to the cursor, the evaluation of the variation of the signals picked up by the receiving electrode permitting a precise measurement of the relative displacement between scale and cursor.

Claims

1. In a capacitive position sensor comprising a cursor (20) and a scale (10) separated from each other by a predetermined distance (H) and displaceable relative to each other along at least one measuring path (x), the cursor comprising at least two transmitting electrodes (21a, 21b) and at least one receiving electrode (21c), a first means (629) for generating electrical signals to be applied to the transmitting electrodes, second means (623, 624) for evaluating the signal picked up by the receiving electrodes so as to determine the relative position of the cursor (20) relative to the scale (10);

the improvement wherein the scale (10) comprises at least one raised or hollow topographical feature (11), each of said topographical features being an integral part of said scale, said second means processing the signals coupled from the transmitting electrodes (21a, 21b) on the cursor (20) to the receiving electrode (21c) on the cursor (20), the topographical feature (11) on the scale (10) being conformed so as to influence said signals coupled from the transmitting electrodes on the cursor to the receiving electrode on the cursor by modifying mutual capacitances (Cac, Cbc) between said receiving electrode on said cursor (20) and said transmitting electrodes on said cursor (20) when said cursor (20) and said scale (10) are displaced relative to each other along the measuring path (x), said mutual capacitances (Cac, Cbc) being indicative of a position of said cursor (20) relative to said scale (10).

2. Capacitive sensor according to claim 1, wherein the scale (10) comprises several said topographical features (11) arrayed along said at least one measurement path (x) so as to form a periodic texture, the cursor comprising at least one electrode array extending along said measuring path (x), and switching means for connecting the electrodes to at least two electrical signals (Va, Vb) in a periodic distribution.

3. Capacitive sensor according to claim 2, wherein the electrical signals (Va, Vb) applied to at least two transmitting electrodes (21a, 21b) are a.c. signals (Va, Vb) of the same amplitude and frequency but of opposite polarity, the transmitting electrodes being substantially coplanar with the receiving electrode (21c).

4. Capacitive sensor according to claim 3, wherein the scale (510) comprises an array of topographical features (511) forming a periodic texture having a periodicity (Tx) along at least one measuring path (x), the cursor (520) comprising at least one row of electrodes (521) extending along said path, a switching means (525) for connecting each electrode of said row to a respective one of:

the a.c. signals (Va, Vb), the inputs of a differential amplifier (522), and a shielding potential (Vg), according to a given signal distribution whose period corresponds to the period (Tx) of said periodic texture or to a multiple thereof, said switching means (525) allowing a displacement of said distribution along said electrode row (521).

5. Capacitive sensor according to claim 4, wherein the switching means (525) allow a signal distribution with at least one signal sequence comprising at least one electrode (521a) connected to a first a.c. signal (Va), at least one electrode (521g) connected to said shielding potential (Vg), at least one electrode (521c) connected to a first input (525c) of said differential amplifier (522), at least one electrode (521g) connected to the shielding potential (Vg), at least one electrode (521b) connected to a second a.c. signal, at least one electrode (521g) connected to the shielding potential (vg), at least one electrode (521d) connected to a second input of said differential amplifier (522), and at least one electrode (521g) connected to the shielding potential (Vg), the period of said signal sequence corresponding to two scale periods.

6. Capacitive sensor according to claim 2, wherein the cursor (420) comprises at least one pair of identical and coplanar receiving electrodes (421c, 421d) connected to both inputs of a differential amplifier (422), the transmitting electrodes (421a, 421b) lying nearer to one (421c) of the receiving electrodes than to the other (421d) of the receiving electrodes.

7. Capacitive sensor according to claim 6, wherein for each pair of transmitting electrodes (421a, 421b) lying nearer to said one (421c) of the receiving electrodes, the cursor (420) comprises another group of identical transmitting electrodes (421a'"'"', 421b'"'"') lying nearer to said other (421d) receiving electrode, the corresponding transmitting electrodes (421a, 421b'"'"';

421b, 421a'"'"') on both sides of the receiving electrodes (421c, 421d) being tied to a.c. signals of the same amplitude and frequency, but of opposite polarity, said scale (410) topographical feature (411) being shaped so as to modify by its displacement the coupling capacitances between the transmitting and the receiving electrodes and to obtain a differential signal between both inputs of the differential amplifier (422).

8. Capacitive sensor according to claim 3, wherein the cursor (620) comprises two rows of identical transmitting electrodes (621) aligned along a measuring path (x) and receiving electrodes (621c, 621d) extending along each of said rows, the switching means (629) providing the transmitting electrodes (621) of each row with a.c. signal distributions which are of the same amplitude and frequency, but of opposite polarity, said switching means (629) being adapted to switch the signals on said transmitting electrodes in sequence, thus causing a displacement of said distributions along said rows, each receiving electrode (621c, 621d) being connected to one input (622c, 622d) of a differential amplifier (622).

9. Capacitive sensor according to claim 8, wherein the differential amplifier (622) is connected to a comparator (623) for converting the amplified signal into a digital signal, which digital signal is demodulated in a demodulator (624) whose output signal controls a counting direction of an up-down counter (628) whose output signal generates, through a decoder (629) and a modulator (630), said a.c. signal distributions along both rows of transmitting electrodes (621).

10. Capacitive sensor according to claim 1, wherein the scale (310) comprises at least one topographical feature (311) of annular shape, the cursor (320) having at least one set of concentric electrodes comprising at least one annular receiving electrode (321c) lying between transmitting electrodes (321a, 321b) each connected to one of two a.c. signals (Va, Vb).

11. Capacitive sensor according to claim 1, wherein the scale (10) comprises topographical features (11) periodically arrayed with a given pitch along a first measuring path, the cursor (20) comprising sets of electrodes periodically arrayed along the same path, but with a slightly different pitch so as to produce a vernier effect.

12. Capacitive sensor according to claim 11, wherein the topographical features (11) of the scale (10) are periodically arrayed with a given pitch along a second measuring path, the cursor (20) sets of electrodes are also periodically arrayed along that second path with a slightly different pitch so as to produce a vernier effect along both of said first and second measuring paths.

13. Sensor according to claim 2, wherein the scale (910a) comprises topographical features (911a) periodically arrayed along two perpendicular measuring paths.

14. Sensor according to claim 13, wherein the cursor comprises a two-dimensional array of electrodes connected in rows parallel to the first path and in columns parallel to the second path, said rows of electrodes being connected to a first switching means, said columns of electrodes being connected to a second switching means.

15. Sensor according to claim 13, wherein the cursor (938a, 939b) comprises two electrode arrangements, which arrangements allow the determination of relative positions between the cursor and the scale along both perpendicular measuring paths.

16. Sensor according to claim 13, wherein the scale consists of a dielectric sheet (911b) having a smooth upper surface closest to the cursor, and a two dimensional array of cavities on the bottom surface of the scale.

17. Capacitive sensor according to claim 2, wherein the scale is one of:

a cylindrical plunger (810b) with a rack in it (811b), a pinion (810c), a rotating disk (810d) having topographical features and a micrometer screw (811c).

18. Capacitive sensor according to claim 2, wherein the scale (810) comprises topographical features (811) covered by a material (812) presenting a smooth surface to the cursor, said material forming a coating over said features (811).

19. Capacitive sensor according to claim 2, wherein the cursor (720a) is an integrated circuit comprising transmitting and receiving electrodes.

20. Capacitive sensor according to claim 2, wherein an electrode plane (721c) of the cursor (720c) is perpendicular to the scale (710c) face provided with topographical features.

21. Capacitive sensor according to claim 2, wherein the switching means (525, 629) is adapted to switch the signal on said transmitting electrodes in sequence, in such a manner as to move the signal distribution on the electrode array so as to track the displacement of the scale (510, 610) relative to the cursor (520, 620).

22. Capacitive sensor according to claim 2, wherein the switching means (525, 629) continuously displace the signal distribution on the electrode array so as to result in a constant speed scan, the relative position of said cursor (520, 620) and said scale (510, 610) being obtained by at least one of a zero crossing and phase detection method.

23. Capacitive sensor according to claim 1, wherein the scale (10) comprises a continuous conductive piece extending over the entire length of the scale (10).

24. Capacitive sensor according to claim 1, wherein the scale (10) is nonconductive.