CA1305766C

CA1305766C - Encoder disc

Info

Publication number: CA1305766C
Application number: CA000613488A
Authority: CA
Inventors: Timothy M. Malcolm; Garland H. Latta, Jr.
Original assignee: Aerospace Controls Corp
Current assignee: Aerospace Controls Corp
Priority date: 1989-09-27
Filing date: 1989-09-27
Publication date: 1992-07-28
Anticipated expiration: 2009-09-27

Abstract

ABSTRACT OF THE DISCLOSURE

An encoder disc for a photoelectric shaft angle encoder includes a disc body which defines first and second regions. The first region is defined be-tween two circles of differing radii and offset centers positioned such that the two circles are substantially tangent at one point and the smaller circle is con-tained within the larger circle. The second region is situated adjacent to the first region, and the first and second regions have differing light transmission characteristics. The first region forms a measuring track which varies in width substantially sinusoidally around the disc body. Especially good results are ob-tained when the two circles are offset from the central axis of the disc body by substantially equal amounts.

Description

ENCODER DISC

BACK~ROUND OF THE INVENTION
This invention relates to an improved encoder disc for an encoder of the type having a scanning unit for scanning the disc to measure an angular position characteristic of the disc.
Encoders such as shaft angle encoders have been used for some time to provide an electronic signal indicative of the angular position of the shaft to which the encoder is mounted. Such encoders include discs having either absolute or incremental tracks, or a comhination of the two. Absolute tracks provide~a parameter that varies in accordance with the absolute position of the disc, while incremental tracks provide repetitive signals that can be counted to determine movement away from a reference position.
European Patent Application EP O 276 402 dis-closes an encoder disc which, as shown in Fi~ure 2, includes both incremental and absolute tracks. Note in particular ~he outermost track which varies in width in a linear manner between a minimum width at O degreas and a maximum width at 180 degrees. This width vari-ation is indicated in Figure 4, where the signal Ul is shown as triangular in shape.
Though the triangular waveform produced by the encoder disc of the above-identified EP O 276 402 3~3~5~
-- 2 ~

is suitable for some applica-tions, it is often prefera-ble to provide a measuring track which varies in width sinusoidally rather than linearly. Such sinusoidal waveforms eliminate the cusps of triangular waveforms and associated scanning difficulties. Additionally, processing systems for sinusoidal signals are commonly avai~able.
It is an object of the present invention to provide an improved encoder disc for an encoder of the type described above which provides such a sinusoidally varying measuring track in a particularly simple and cost effective manner.

SUMMARY OF THE INVENTION
. . . _ According to this invention, an encoder disc for an encoder of the type described initially above comprises a disc body having first and second reyions on the disc body. The first region is defined between two circles of differing radii and offset centers posi-tioned such that the smaller circle is contained within the larger circle. The second region is situated adja~
cent the first region, and the first and second regions have differing characteristics of a scanned parameter such as light transmission. The first region forms a measuring track which varies in width substantially sinusoidally around the disc body.
In the preferred embodiment described below, the measuring track is transparent and the surrounding region of the disc is opaque. Preferably, the disc body defines a central axis of rotation, and the cen-ters of the two circles are each offset by a substan-tially e~ual amount from the central a~is such that the two centers and the central axis are colinear with the central axis positioned between the two centers. This arrangement has been found to provide a measuring track ~3~

which approximates a sinusoidal variation in track width with surprising accuracy.
The invention itself, tog~ther with further objects and attendant advantages, will best be under-stood by reference to the followiny detailed descrip-tion, taken in conjunction with the accompanying draw-ings.

BRIEF DESCRIPTION OF T~E_DRAWINGS
Figure la is a plan vi~w of an encoder di c which incorporates a presently preferred embodiment of this invention.
Figure lb is a schematic representation of an encoder which incorporates the encoder disc of Fig-ure 1.
Figure 2 is a schematic representation of the disc of Figure 1, in which proportions have been exag-gerated for clarity of illustration.
Figure 3 is a geometrical construct used be-low to analyze the schematic representation of Figure 2.
Figure 4 is a graph showing errors associated with the encoder disc of Eig. l.

DETAILED DESCRIPTION OE THE PRESENTLY
PREFERRED EMBODIMENTS _ _ _ Turning now to the drawings, Figure la shows a plan view of an encoder disc which incorporates a presently preferred embodiment of this invention. This disc includes a disc body 10 which defines a central axis of rotation 12 and a periphery 14. Typically, the periphery 14 is at a fixed radius from the central axis 12. The disc body 10 defines four first regions 16a-16d, and a second region 18 which differs in char-acteristics of a scanned parameter such as light ~3~7~i~

transmission. Typlcally, one of the first and second regions 16a-16d, 18 is opaque, and the other is trans-parent. In this embodiment it is the firs-t regions 16a-16d that are transparent. The second region 18 is immediately adjacent to the first regions 16a-16d and in this embodiment surrounds them. The following dis-cussion applies equally to all of the first regions 16a-16d, and the reference number 16 wi]1 be used ge-nerically to refer to any of the first regions 16a-16d.
As best shown in Figure 2, the first region 16 is defined as the region between an inner circle 20 having a radius R1 and a center 22, and an outer circle 24 having a radius R2 and a center 26. The central axis 12 and the centers 22, 26 are colinear along an offset axis 32, with the central axis 12 posi-tioned between the two centers 22, 26.
Figure lb schematically shows the manner in which the encoder disc body 10 can be used in an encod er. As shown in Figure lb the encoder includes a scan-ning unit which is fixedly mounted with respect to the axis of rotation 12 of the di SG body 10. The scanning unit includes a set of lamps L which generate light that passes through the disc body 10 to respective light sensors S. The amplitude of a signal generated by one of the sensors S is proportional to the amount of light passing through the respective first region 16. This parameter varies as a function of the width of the first region 16, which width is measured with respect to the central axis 12 and is indicated by the reference symbol W in Figure 2. As the disc body 10 makes one complete revolution the signals generated by the sensors S vary from a minimum value at a selected angular position to a maximum value at the selected angular position plus 180 degrees and back to the mini-mum value. Surprisingly, it has baan discovered that ~L3g~5~

the width W of the first region 16 varies in a sinusoidal manner to an excellent approximation. Thus, the signal generated by the sensor S varies sinusoid-ally (to a close approximation) between the minimum and maximum values as the disc body 10 makes one complete revolution. As used herein, a sinusoida:L variation includes a sine wave with a DC offset.
The encoder disc of Figure 1 can be manufac-tured by a variety of methods, including the conven-tional photolithographic methods currently used to man-ufacture encoder discs. For example, one surface of the encoder disc body 10 can be plated with an opaque metal layer, and then photoresist techniques can be used to remove the opaque metal layer in the first re-gion 16 bounded by the inner and outer circles 20, 24.
One approach to abrication i5 to coat the opague metal layer with a photoresist, then to expose the photore~ist outside the outer circle 24 and inside the inner circle, and then to use conventional techniques to remove the metal layer between the two circles 20, 24. Another possible approach is to expose such a lay-er o~ photoresist between the circles 20, 24 in a raster scan so as to e~pose the antire first region 16.
The sketch of Figure 2 will be used to clari-fy the manner in which the width W of the first region 16 varies in an approximately sinusoidal manner. As shown in Figure 2, the distance between the two centers 22, 26 is indicated by the reference symbol a, while the distance between the central axis 12 and the center 22 is indicated by the symbol b. As shown, the width W
is mQasured along a radius procaeding from the central axis 12.
Using the notation defined in the enlarged geometrical construct of Figure 3, -the following geo-metrical identities are apparent:

~3~

h' = (a-b)sin~; (EQ 1) C' = (a-b)cos~; (EQ 2) C~2 - R Z~h~2 (EQ 3) C" can ~hen be expressed as follows:
C" = (R22-h'2~2 = (R22-[(a-b)sin~]2)~. (EQ 4) C, which equals C' plus C", can be expressed as follows:

C = C'+ C" = (a-b)cos~ + (R22-[(a-b)sin~]2)~. (EQ 5) Similarly, the following three geometrical identities obtain:
h" = bsin~; (EQ 6) s" = bcos~; (EQ 7) (s' ~ s")2 = Rl2 - hn2 (EQ 8) These identities can be used to calculate s' as follows:

s' = (Rl2-h"2)~ - s"; (EQ- 9) = (Rl2-~bsin~)2)~ - bcos~. (EQ- 10) The width W as shown in the geometrical con-struct of Figure 3 is equal to C s'. Using the identi-ties set out above, W can be expressed as follows:

W = c - s; ' = (a-~cos~+(R22-[(a-b)sin~]2)~ - (EQ. 11) (Rl2-(bsin~)2)~+bcos~;

= acosl~l+(R22-1(a-b)sinl~]2) 2 _ (EQ. 12) (Rl2-(bsin~)2)~.

~3bs~

For the specific case where b--0 (i.e. where the inner circle 20 is centered on the central axis 12) EQ 12 simplifies as follows:

W = acos~(R22-(asin~)2)~ - R~ (for b=0). (EQ. 13) Gi.ven that the desired formula for W is W=a+acos~, EQ 13 indicates an error equal to the fol-lowing:

Error = (R22-(asin~)2)~ - R1-a. (EQ 14) Similarly, when the two circles 20, 24 are symmetrically positioned with respect to the central axis 12, i.e. where a=2b and R2=Rl~2b, EQ 12 simpliies as follows:

W = acos~(R22-(b3in~)2)~ - ~Rl2-(bsin~)2)~. (EQ 15) In this case, W again is desired to equal a+acos0, and the error between the desired and actual values of W is indicated as follows:

Error = (R22-(bsin~)2)~ - (Rl2-(bsin~)2)~-a. (EQ 16) Analysis has shown that the error is mini-mized when b is selected to approximately equal ~a.
The minimum 2rror is found at a point where b is slightly less than ~a, where the offsets of the two circles (b and a-b) differ from one another by about 0.25%.
Figure 4 is a graph showing the magnitude of the worst case error for the situation where the two circles are -tangent at one point and 2b=a. In Figure 4 the X axis indicates the accuracy of the approximation (worst case percentage error~ and the Y axis indicates the r~tio of the maximum track width (a~ divided by the average of the radii of the two circles that define the track (~(Rl~R2)). Note that for a 4 bit encoder ~which requires an error less than one part in 16) a/~(RI+R2) must be less than 0.65. Similarly, for 8, 12 and 14 bit encoders (which require an error of less than one part in 256,4096 and 16,389, respectively) a/~(Rl+R23 must be less than 0.25, 0.06 and 0.03, respectively.
The values of Rl, R2 ~ a and b can be chosen to fit the application. Simply by way of example, the following table defines the dimensions of the embodi-ment of Eigure la in millimeters.
First 2R~ Center Region 2RI X Y
. . . _ . _ .
16a 42.05 0 -0.120 41.5~ 0 ~0.120 16b 40.05 0 +0.120 39-55 0 -0.120 16c 36.16 -0.120 0 35.50 ~0.120 o 16d 33.53 ~0.120 0 32.88 -0.120 0 From ths foregoing, it should be apparent that an encoder disc has been described which provides the desired sinusoidally varying measuring track in a particularly simple manner. Of course, the measuring tracks described above will often be combined with oth-er tracks, either absolute or incremental, on -the en~
coder disc. For example, four sets of the measuring tracks described above can be provided on an encoder disc at 0 degrees, 90 degrees, 180 degrees and 270 de-grees in order to produce a highly accurate substitute for a magnetic resolver or inductosyn. In many ~- 9 -applications it will be desirable to make the two cir-cles 20, 24 almost tangent at one point to minimize the unchanging portion of the width W. However, tangency is not required in all applications.
Of course, a wide range of materials and fab-rication techniques can be used to implement this in-vention. If desired, the first region :L6 can be opaque and the second region 18 transparent. Furthermore, this invention is not limited to use with optical en-coders, but can also be used with capacitive and induc-tive encoders. It is therefore intended that the fore-going detailed description be regarded as illustrative rather than limiting, and that it be understood that it is the following claims, including all equivalents, which are intended to define the scope of this inven-tion.

Claims

1. An encoder disc for an encoder of the type comprising a scanning unit for scanning the disc to measure an angular position characteristic of the disc, said encoder disc comprising:
a disc body;
first and second regions on the disc body, said first region defined between two circles of dif-fering radii and offset centers positioned such that the smaller circle is contained within the larger cir-cle, said second region situated adjacent said first region;
said first and second regions having differ-ing characteristics of a scanned parameter;
said first region forming a measuring track which varies in width substantially sinusoidally around the disc body.

2. The inventions of Claim 1 wherein one of the two regions is substantially opaque and the other is substantially transparent.

3. The invention of Claim 2 wherein the first region is substantially transparent.

4. The invention of Claim 1 wherein the two cir-cles are substantially tangent at one point.

5. The invention of Claim 1 or 4 wherein the two circles have centers offset by an amount a, and wherein the ratio of a divided by the average of the radii of the two circles is less than 0.65.

6. The invention of Claim 1 or 4 wherein the two circles have centers offset by an amount a, and wherein the ratio of a divided by the average of the radii of the two circles is less than 0.25.

7. The invention of Claim 1 or 4 wherein the two circles have centers offset by an amount a, and wherein the ratio of a divided by the average of the radii of the two circles is less than 0.06.

8. The invention of Claim 1 or 4 wherein the two circles have centers offset by an amount a, and wherein the ratio of a divided by the average of the radii of the two circles is less than 0.03.

9. An encoder disc for an encoder of the type comprising a scanning unit for scanning the disc to measure an angular position characteristic of the disc, said encoder disc comprising:
a disc body having a central axis of rota-tion;
first and second regions on the disc body which differ from one another in light transmission characteristics of a scanned parameter, said first re-gion defined between two circles of differing radii and offset centers positioned such that the smaller circle is contained within the larger circle, said second re-gion situated adjacent said first region, the central axis and the two centers being colinear along an offset axis with the central axis positioned between the two centers, the center of one of the circles being offset from the central axis by a first amount, the center of the other of the circles being offset from the central axis by an amount substantially equal to the first amount;

said first region forming a measuring track which varies in width substantially sinusoidally around the disc body.

10. The invention of Claim 9 wherein one of the two regions is substantially opaque and the other is substantially transparent.

11. The invention of Claim 10 wherein the first region is substantially transparent.

12. The invention of Claim 9 wherein the two cir-cles are offset from the central axis by amounts which differ from one another by about 0.25%.

13. The invention of Claim 9 wherein the disc body defines a periphery at a constant radius from the central axis.

14. The invention of Claim 9 wherein the two cir-cles are substantially tangent at one point.

15. The invention of Claim 9 or 14 wherein the ratio of two times the first amount divided by the av-erage of the radii of the two circles is less than 0.65.

16. The invention of Claim 9 or 14 wherein the ratio of two times the first amount divided by the av-erage of the radii of the two circles is less than 0.25.

17. The invention of Claim 9 or 14 wherein the ratio of two times the first amount divided by the av-erage of the radii of the two circles is less than 0.06.

18. The invention of Claim 9 or 14 wherein the ratio of two times the first amount divided by the av-erage of the radii of the two circles is less than 0.03.