GB2137755A

GB2137755A - Position Encoder System having Quadrature Outputs

Info

Publication number: GB2137755A
Application number: GB08405569A
Authority: GB
Inventors: Raymond Brooke
Original assignee: Computer Memories Inc
Current assignee: Computer Memories Inc
Priority date: 1983-04-04
Filing date: 1984-03-02
Publication date: 1984-10-10
Also published as: GB8405569D0; JPS59197821A

Abstract

First and second sinusoidal outputs of an encoder system 20, nominally in phase quadrature, are subjected to sum and difference processing 24, 26 in order to obtain a pair of sinusoidal outputs which are precisely 90 DEG out of phase. The phase relationship enables both output signals to the utilized to provide position information, thus doubling the resolution of the basic encoder. Amplifier 28 is used to equalise the amplitudes of the outputs. <IMAGE>

Description

SPECIFICATION Position Encoder System Having Quadrature Outputs Background of the Invention 1. Field of the Invention This invention relates to encoders which provide a position indicating signal which varies in amplitude with the position of an object, e.g., the rotational position of a motor shaft. More particularly, tne present invention relates to encoders of the type which provide a pair of sinusoidal output signals as a position indication.

2. Description of the Prior Art Position encoders which provide a pair of sinusoidal output signals are well known in the art.

Typically, such devices generate electric signals in response to angular or linear displacement of two relatively movable members. The signals vary sinusoidally as the members move with respect to each other, with one-half period of the sinusoidal output signal corresponding to a predetermined increment of motion. When two output signals are provided, they nominally correspond to sine and cosine functions, i.e. they are 900 out of phase with respect to one another. The null (i.e., zero-crossing) positions of one of the signals is used as a position indication, while the other signal is used to determine direction or velocity of movement In prior art encoders, the sine and cosine outputs in fact are not exactly 900 out of phase.

Generally, such encoders have a phase error of between 30 and 150. As a result, errors occur in the generation of velocity and the direction signals. More importantly, the phase error prevents the use of both of the sinusoidal output signals together to provide a position indication, since if combined the signals would not provide equally spaced nulls.

Summary of the Invention The present invention is directed to an encoder system which provides two separate sinusoidal outputs which are precisely 900 out of phase. The system may be used in conjunction with many different types of encoders (e.g. optical or capacitive), with the only requirement being that the encoder provide two sinusoidal outputs of equal peak-to-peak amplitude. The output signals of the encoder are subjected to sum and difference processing. The resultant output signals are precisely 900 out of phase. An additional amplifier may be provided to amplify one of the output signals in order to equalize the peak-to-peak amplitudes of the signals resulting from the processing.

By providing two sinusoidal outputs which are precisely 900 out of phase, both of the outputs can be used to provide position information. As a result, the resolution of the encoder is doubled over encoders in which only one of the sinusoidal outputs is used for position information. Furthermore, distortion in velocity and direction information caused by phase errors in prior art devices is eliminated.

Brief Description of the Drawings The invention will now be described with reference to the accompanying drawings wherein: Fig. 1 is a diagram showing the phase relationship between two sinusoidal outputs of a typical prior art position encoder which is used with the invention; Fig. 2 is a block diagram of the encoder system of the present invention; Fig. 3 is a vector diagram illustrating the sum and difference processing of the signals of Fig. 1; and Fig. 4 is a diagram showing the sinusoidal output signals provided by the present invention.

Description of the Preferred Embodiment The following description is of the best presently contemplated mode of carrying out the invention. This description is made for the purpose of illustrating the general principles of the invention and is not to be taken in a limiting sense. The scope of the invention is best determined by appended claims.

Referring to Fig. 1, a prior art position encoder such as a rotary or linear encoder having a pair of relatively movable plate elements provides "sine" and "cosine" outputs 10 and 12, respectively.

Encoders of this type are well known in the art and may be optical, capacitive or other type of encoder.

In one embodiment of the invention, a Sensor Technology Model STRE 1601 optical encoder is employed. The output signals are sinusoidal signals which vary as the plates are moved with respect to each other. For example, in a rotary encoder, one period of the sine output 10 or cosine may represent 1.80 of rotation. The sine output 10 has a zero-crossing or "null" positions indicated by arrows 14.

These nulls are equally spaced and may be employed to provide position indications. Thus, in the example given, each null corresponds to 1.8 of relative rotation of the encoder plates.

The period of the cosine wave 1 2 is equal to that of the sine wave 10. Although the cosine waveform 12 is nominally 900 out of phase with respect to the sine waveform 10, in actuality there is a phase error which is typically from 30 to 1 50. This error is indicated at -1 6 in Fig. 1, which shows a peak of the cosine waveform 12 offset from a zero-crossing of the sine waveform 1 0. Generally, the cosine waveform is used to obtain velocity and direction information, whereas the sine waveform alone is used to provide position information. The error 16 results in distortion or "ripple" in the velocity and direction signals. Furthermore, the error prevents both the sine and cosine outputs from being used to provide position information.If the cosine signal were precisely 900 out of phase with respect to the sine signal the nulls of both signals could be used for position information since ail of the nulls would then be equally spaced. The present invention is directed to a system for providing such outputs.

Fig. 2 is a block diagram of the present invention. An encoder 20 of the type which provides outputs such as those shown in Fig. 1 is used to monitor the position of a device such as a motor 22.

The encoder provides two sinusoidal outputs having equal peak-to-peak amplitudes. These outputs are indicated as A sinwt and A sin(wt+). These signals are summed by a summing amplifier 24 to provide a first system output signal and subtracted from one another by means of a differential amplifier 26 in order to provide a second system output signal. The output signals have different peak-to-peak amplitudes and an additional amplifier 28 may be provided to equalize the amplitudes. In the present embodiment of the invention, the amplifiers 24, 26 and 28 are all integrated circuit operational amplifiers.

The sum and difference processing of the output signals of the encoder 20 results in signals which are precisely 900 out of phase with respect to each other despite the fact that the encoder outputs of the encoder 20 are not 900 out of phase. The addition (sum processing) of the two sinusoidal output signals of the encoder is as follows: A sin t+A sin(est+4s)=2A cos#/2 # sin( # +#t) (1) 2 =k, sin( +a)t) (2) 2 where k,=2A cos Similarly, the subtraction (difference processing) of the signals is as follows:: wt-wt- 2cl)t+0 A sin cot- A sin(t+si)=2A sin ( ) cos ( ) (3) 2 2 =-2A sin (-) cos (-+cot) (4) 2 2 -k2 cos (-+cot) (5) 2 where k2=2A sin 0/2 From equations (2) and (5), it can be seen that the signals resulting from the sum and difference processing of the encoder outputs will be signals which are precisely 900 out of phase. The sum signal will-have null values at a)t=- ,- +' - +2#, etc.

2 2 2 The difference processing will result in signals having a null value at 7t 37r a)t=- + , + 2 2 2 2 etc. Thus, the two output signals will have nu!l values which are--precisely -900 out of phase.

The sum and the difference processing of the encoder signals is indicated in vector form in Fig. 3.

The signals of Fig. 1 can be considered to be rotating vectors 10 and 12 separated by an angle 0. By adding the vectors 10 and 12, a vector 30 will result. This vector bisects the angle between the vectors 10 and 12. By subtracting the vector 12 from the vector 10 (i.e., adding the vector 32, which is the negative of the vector 1 2, to the vector 10) a vector 34 results which bisects the angle between the vectors 10 and 32. The vectors 30 and 34 are thus precisely 900 out of phase. As can be seen in Fig. 3, the amplitudes of these vectors are not equal and the amplifier 28 may be employed to equalize the final output signals of the system.

Fig. 4 shows the output signals of the summing amplifier 24 and differential amplifier 26. These signals are precisely 900 out of phase and therefore the null points of the two signals are all equally spaced. Thus, both signals can be utilized to provide position information. The ability to use both of the signals results in a resolution which is double that when only a single output signal can be employed. In addition, the precise phase relationship eliminates ripple in velocity and direction signals obtained from the cosine output of the encoder.

In sum mary, the present invention provides two sinusoidal outputs which are precisely 900 out of phase from an encoder which provides two outputs having a phase error. The only requirements of the encoder is that the peak-to-peak amplitudes of the sinusoidal outputs be equal and that there be zero DC offset voltage. The resolution of an encoder can thus be doubled in a simple and inexpensive fashion and distortion in velocity and direction determinations eliminated.

Claims

1. A system for use with a position encoder which provides first and second sinusoidal encoder signals having equal peak-to-peak amplitudes and equal periods, said system comprising: summing means for adding the first and second encoder signals to provide a first sinusoidal output signal; and difference means for taking the difference between the first and second encoder signals to provide a second sinusoidal output signal, wherein the first and second output signals are precisely 900 out of phase irrespective of the phase relationship of the encoder signals.

2. A system as claimed in Claim 1 further including amplifier means, coupled to the output of one of the summing means and difference means, for equalizing the peak-to-peak amplitudes of the first and second output signals.

3. A system as claimed in Clairn 1 or 2 wherein the summary means and difference means each comprises an operational amplifier.

4. A system as claimed in Claim 2 or Claim 3 when appendant to Claim 2 wherein the summing means, difference means and amplifier means each comprises an operational amplifier.

5. An encoder system, comprising: a position encoder which provides first and second sinusoidal encoder signals having equal peakto-peak amplitudes and equal periods; summing means for adding the first and second encoder signals to thereby provide a first sinusoidal output signal; difference means for subtracting the second encoder signal from the first encoder signal to thereby provide a second sinusoidal output signal which is precisely 900 out of phase with respect to the first output signal.

6. An encoder system, comprising: a position encoder which provides a first varying encoder signal Asin t and a second varying encoder signal Asin(wt+4); a summing amplifier for adding the encoder signals to obtain a first output signal k1 sin(/2+cot); a difference amplifier for subtracting the second encoder signal from the first encoder signal to obtain a second output signal k2 cos(4/2+wt).

7. An encoder system as claimed in claim 6 including an amplifier coupled to the output of one of the summing and difference amplifiers to equalize the peak-to-peak amplitudes of the output signals.

8. A-system for use with a position encoder, substantially as hereinbefore described with reference to the accompanying drawings.