GB2116313A

GB2116313A - Sine wave generator for position encoder

Info

Publication number: GB2116313A
Application number: GB08205558A
Authority: GB
Inventors: Douglas James Rogers
Original assignee: Ferranti PLC
Current assignee: Ferranti International PLC
Priority date: 1982-02-25
Filing date: 1982-02-25
Publication date: 1983-09-21
Also published as: DE3305921A1; GB2116313B; JPS58157203A; FR2522164A1

Abstract

Optical means for generating a sinusoidal waveform having a low harmonic content includes a first member 17 bearing at least one pattern 11 of optical transmissivity which varies sinusoidally along an axis (14). A second member 18 carries at least one aperture 13 of width along the axis (14) lying between one quarter and three quarters of the wavelength of the sinusoidal pattern. A light-sensitive device is provided which is responsive to light passing through the aperture and the pattern from a light source, and means are provided for producing relative motion between the two members in a direction parallel to the axis (14), the signal being indicative of the relative position (displacement) of the members. One member may be circular for rotational motion. A further sinusoidal pattern 90 DEG displaced from those shown enable direction of movement to be determined. The pattern 11 may be provided by a shaped opaque strip, a variable density film or coating, or a member of parallel lines of ranging mark/space ratio. <IMAGE>

Description

SPECIFICATION Optical means for generating a sinusoidal waveform having a low harmonic content Optical means are frequently used for generating electrical signal waveforms which are subsequently processed by electronic circuitry.

Position encoders are one example of instruments using such means, and usually comprise two relatively movable members each carrying a pattern of some sort. A light source and a lightsensitive device co-operate with the two members to produce an electrical signal which may be used to determine the direction and extent of relative movement of the two members.

For most purposes a square-wave electrical signal is suitable, particularly if it is being used to operate digital circuitry. However, there are situations in which complex analog electronic circuitry is used to operate on the signals. In such circumstances it is preferable for the opticailyderived electrical signal to be sinusoidal, with as low a harmonic content as possible.

According to the invention there is provided optical means for generating a sinusoidal waveform having a low harmonic content, which includes a first member bearing a pattern of optical transmissivity which varies sinusoidally along an axis, a second member bearing an aperture having a width along said axis of between one-quarter and three-quarters of the wavelength of said sinusoidal pattern, a lightsensitive device responsive to light passing through said aperture and said pattern from a light source, and means for producing relative motion between said first and second members in a direction parallel to said axis.

Also according to the invention there is provided a position encoder which includes means as specified in the preceding paragraph in which the first member is carried by one of two bodies movable relative to one another in a direction parallel to said axis, and in which the second member is carried by the other of the two bodies.

The invention will now be described with reference to the accompanying drawings, in which: Figure 1 shows the form of the pattern on the first member; Figure 2 illustrates the optical arrangement; Figure 3 shows a practical form of the invention; Figure 4 shows detail of an alternative form of the embodiment of Figure 3; Figure 5 shows part of a shaft encoder incororating the invention.

Referring now to Figure 1 ,the first member 10 of opaque material I or carrying an opaque coating, has formed on it a pattern in the form of a variable-width slit 11 with sine-wave edges. A second member 12 has formed in it an aperture 13 having a width equal to about one-half of a wave-length of the sinusoidal pattern. If the two members are moved relative to one another in a direction parallel to the length of the slit 11, as indicated by the arrow 14, then the amount of light passing through both the pattern and the slit will vary in a sinusoidal manner.

Figure 2 shows the other components necessary to generate a sinusoidal electrical signal. The two members 10 and 12 are arranged adjacent and parallel to one another with a light source 1 5 on one side and a light-sensitive device 1 6 on the other side. Such arrangements are well known, and optical elements such as lenses have been omitted.

In order to generate the required waveform it is necessary to produce the relative motion between the two members. This is most easily done by making one of them in the form of a disc, which may then be rotated whilst the other member is stationary. Since the aperture 13 is easiler to form it is convenient to produce a disc bearing a number of apertures, whilst the stationary member carries the sinusoidal pattern. Figure 3 shows such an arrangement, with a stationary member, or index, illustrated at 1 7 and the rotatable disc or scale at 1 8.

In practice, in order to avoid errors due to unevenness of the light source or of the response of the light-sensitive device over its sensitive area, it is convenient to have a number of apertures in the scale overlapping a number of sinusoidal patterns on the index. The centres of the apertures must be spaced apart by the wavelength of the pattern so that the variations of light passing through each aperture are in phase with one another. In the case of a rotating scale the apertures must be arranged radiaily, and not exactly parallel to one another, and the sinusoidal patterns must be similarly arranged with wavelengths matched to the spacing of the apertures. Figure 4 shows part of such a scale and index. As shown in Figure 4, the index 1 7 carries three sinusoidal patterns 11 arranged parallel to one another.The scale 18, of which only part is shown has a number of apertures 13, four in the case illustrated, each of which extends over one half-cycle of each of the sinusoidal patterns. A single light-sensitive device responds to the light passing through all four apertures. This is a known technique for removing such errors. It is also possible to apply known techniques for removing d.c. offset voltages from the sinusoidal output. One technique involves using the same light-sensitive device to respond also to light passing through a second set of sinusoidally-varying strips. These co-operate with a set of slots arranged so that the variations in light received by the light-sensitive device from the two sets of strips are in anti-phase.

The width of the apertures 1 3 need not be exactly one-half of the wavelength of the sinusoidal pattern. Widths between one-quarter and three-quarters of a wavelength are suitable, but the centres of the apertures must be one wavelength apart. The particular advantage of the half-wavelength aperture is the complete cancellation of the even harmonics from the output of the detector and in the reduction of the odd harmonics by a factory equal to their harmonic number. The presence of harmonics is due to imperfections in the shape of the sinusoidal pattern. Other slit widths reduce the harmonic content by a lesser extent.

As has already been suggested, the sinewavegenerating means described above may be used as a position encoding device. In the case of the embodiment of Figures 3 and 4, these may be used as shaft angle encoders, using appropriate electronic circuitry. However, in order to determine direction of rotation of the disc, it is necessary to add a further pattern, or set of patterns. These are also sinusoidal envelopes, but displaced by one-quarter of a wavelength so as to provide, in conjunction with a separate set of apertures and a separate light-sensitive device, a cosine wave output The combination of the sine and cosine waveforms enables the direction of rotation of the disc to be determined. Figure 5 shows the relevant parts of the scale and index for a shaft encoder producing sine and cosine outputs.A similar arrangement may be used for a linear position encoder, in which case the scale is a straight elongated member with parallel apertures, rather than a disc bearing radial apertures.

The required sinusoidally-varying optical transmissivity required may be provided by other means than the transparent strip with sinusoidal edges. For example, it is possible to produce the same effect using a number of parallel lines of varying mark/space ratio. Alternatively a film or coating having sinusoidally-varying density may be used.

It will be appreciated that the requirement for producing a low harmonic sine wave depends upon relative movement between the two members referred to, and it does not matter which of them is moved and which is stationary. The sine (or cosine) waveform produced is very low in harmonic content and may be processed electronically with little or no distortion.

Claims

1. Optical means for generating a sinusoidal waveform having a low harmonic content, which includes a first member bearing a pattern of optical transmissively which varies sinusoidally along an axis, a second member bearing an aperture having a width along said axis of between one-quarter and threequarters of the wavelength of said sinusoidal pattern, a lightsensitive device responsive to light passing through said aperture and said pattern from a light source, and means for producing relative motion between said first and second members in a direction parallel to said axis.

2. Optical means as claimed in Claim 1 in which the pattern of sinusoidally-varying optical transmissivity comprises a variable width transparent strip having edges formed by two sinusoidal waveforms of the same wavelength and in antiphase.

3. Optical means as claimed in either of Claims 1 or 2 in which one of said first and second members is in the form of a rotatable disc the other member being stationary relative to the disc.

4. Optical means as claimed in Claim 3, in which the stationary member is the first member carrying said pattern, the rotatable disc carrying a plurality of apertures the centres of which are spaced apart from one another by the wavelength of said pattern.

5. Optical apparatus as claimed in any one of the preceding claims in which each aperture has a width along said axis of one-half of the wavelength of the sinusoidal pattern.

6. Optical apparatus as claimed in any one of Claims 1 to 5 in which the first member carries a number of sinusoidal patterns in phase with one another, part of each pattern being aligned simultaneously with said aperture.

7. Optical means substantially as herein described with reference to Figures 1 to 4 of the accompanying drawings.

8. A position encoder which includes optical means as claimed in any one of the preceding claims in which the first member is carried by one of two bodies movable relative to one another in a direction parallel to said axis and in which the second member is carried by the other of said two bodies.

9. A position encoder as claimed in Claim 8 in which the first member also carries a second pattern of optical transmissivity which varies sinusoidally along the said axis, the first and second patterns being 900 out of phase with one another.

10. A position encoder substantially as herein described with reference to Figure 5 of the accompanying drawings.