Incremental optical encoder

Abstract: Incremental optical encoder comprising a luminous source 1, an encoder disk 4 having windows 5 and a sensor 3 with at least two photosensitive elements. The light intensity from the source may be varied by use of a U-shaped wire 8 having a branch 12 positioned between the luminous source at the photosensitive elements.

Description: The present invention relates to an incremental optical encoder. The principles of optical incremental encoders are well known. In summary, a thin disc is provided with a multiplicity of slits around its peripheral edge. The number of slits and their spacing determines the degree of control which can be exercised on the motor upon which the encoder is mounted. On one axial side of the disc is mounted a light emitting diode which is made to send a beam of light perpendicular to the face of the disc. When this light beam is opposite a slit in the disc the light passes through to land on a light sensitive sensor which changes logical state from zero to one. As the disc rotates so the slit moves on and the light beam is cut off by the disc material between the slits. The light sensor returns to logic level zero. It is common to provide two light emitting diodes and two light sensitive elements each side by side on opposite sides of the disc. By precise placing of the two sets of electronic devices they can be made to give up/down signals 90 degrees out of phase with one another. Because of this it is possible to use the two signal channels to determine the direction of rotation of the motor. This is essential for full servo control. Since both the up edges and the down edges of each signal can be used then there are four signals for each disc slit. This further enhances servo control. One problem faced by designers of these encoders is concerned with the fact that the light emitting diodes propagate a dispersant beam. Some degree of masking can reduce this. However the fact that the disc can move axially between the emitters and receivers can cause a dispersed beam to reach a sensor. The consequence of this is that the time when the sensor is at logic level zero and the time at logic level one are not equal. This is often called the signal duty cycle. If these duty cycles are allowed to vary substantially then the states of both sensors cannot be compared to sense direction any more since one of the signals, having a short up time may go down before the other sensor which may have a long up time hence spoiling the sequence. An incremental optical encoder has been proposed in US Patent No.3400275 which uses a single light source which is split into two light beams by a cover or housing in which the light source is fitted and a parabolic reflector. The cover has a window through which light can pass. By rotating the light source and cover with respect to the reflector, the relative intensities of the two light beams or the collimation of the light is varied. The reflector directs the light beams to respective photo cells which are suitably spaced from each other to avoid interference by the other light beam. The encoder also has a rotatable slotted encoder disc and a slotted plate which acts as a mask for the encoder disc. This is used to also reduce interference between the two light beams. This is relatively complex encoder. Moreover, because the light source is movable and is required to be moved during calibration, calibration must take place before the encoder is completely assembled making it virtually impossible for the encoder to be produced by an automated assembly process. According to the invention, there is provided an incremental optical encoder for a rotating shaft, comprising an encoder disc disposed on said shaft having at least one window, a radiation source disposed on one side of said disc, a sensor disposed on the other side of said disc for receiving radiation from the source and means for regulating the intensity of radiation received by said sensor, wherein the radiation source is fixed and is arranged to produce a single beam of radiation directed at the sensor and wherein the regulating means comprises a movable opaque element which is fine enough to intercept only a fraction of the radiation destined for the sensor to allow a fine modulation of the intensity of the radiation on the sensor. In a preferred embodiment of the invention, the regulating means comprises a wire which is introduced into the light path to compensate the duty cycle and which can be positioned independently of the light source, which remains fixed. The detailed description of an embodiment of the invention with accompanying reference drawings, follows : Figure 1, is an elevated view of the embodiment of the invention. Figure 2, is a view from above Fig. 1. Figure 3, is an elevated enlarged view from the side of both Fig. 1 and Fig. 2. An embodiment of the incremental optical encoder of the invention will now be described, with reference to the above figures and comprises:- A luminous source 1 having an electroluminescent diode equipped with a lens 7, a holder 2 for the luminous source on which the source is fixed, a sensor 3, consisting of four sensitive zones formed by photosensitive elements 9, at angular positions of O°, 90°, 180°, and 270°, an encoder disk 4 having windows 5, the disk being fixed on a shaft 6 and means 8 for regulating the distribution of the luminous intensity on the sensitive zones (photo-sensitive elements) 9. The regulation means comprises a wire 8, generally of a U-form of which the base 10 forms an axis of rotation, allowing a branch 12 to swivel by manual action of an opposed branch 11 in the directions indicated by the arrows in Fig. 2. The base 10 can be seated in the guiding slot 13, in order to facilitate operation. The wire 8 is of opaque material and has dimensions such that the branch 12, which is placed between the luminous source and the photosensitive elements 9 will project a shadow on one or more of the elements thereby allowing a fine modulation of the intensity of the light on these elements. In Fig. 3, this effect can be seen more precisely. It is to be noted that the luminous radiation from the luminous source 14 is a pinpoint and even though it is designed to propagate itself parallel to the axis 14 of the lens, can very well in practice diverge. The branch 12 of wire 8 will intercept a part of the luminous radiation and consequently project a shadow on one or more of the elements 9. Preferably, the regulation means 8 comprises a steel spring, fine enough to intercept only a fraction of the light destined for the sensors, which allows a fine modulation of the intensity of the light on the elements 9. Of course other materials or form could be just as suitable for the regulation means 8. Tests have been carried out with the help of commercially available elements comprising an LED and a sensor of four photosensitive elements. For normal use according to the instructions of the manufacturer, a duty cycle of the elements may be less than 50% (a duty cycle is the relation between the time during which the signal from the sensor of the channel is on level 1 and the period of the signal). This occurrence is an intrinsic inconvenience and provokes an error of the phase between the two channels, concerning the direction of rotation. If the light on one of the photosensitive elements is reduced, this provokes a change of its level as compared to the level of the complementary element. Thus, using the regulation means as described above, the duty cycle of one complementary element relative to the other may be changed by reducing the light in the direct line of the LED on the two photosensitive elements in the centre of the sensor. This way the duty cycles can be matched. The wire 8 is therefore positioned in such a way that it can intercept the radiation in the direct line of the LED. Nevertheless, the wire 8 can be pivoted, according to each system's own characteristics, so that the intensity of the light on the photosensitive elements can be regulated in an optimal way, in order to obtain the desired duty cycles of the signals. Regulation may be carried out and controlled with the help of an oscilloscope, with the wire 8 being fixed in position with the help of a drop of glue, for example.

Claims: An incremental optical encoder for a rotating shaft, comprising an encoder disc (4) disposed on said shaft having at least one window (5), a source of radiation (1) disposed on one side of said disc (4), a sensor (3) disposed on the other side of said disc for receiving radiation from the source (1), the sensor comprising at least two sensitive zones, and means (8) for regulating the intensity of radiation received by said sensor characterised in that the radiation source (1) is fixed and is arranged to produce a single beam of radiation directed at the sensor and in that the regulating means comprises a movable opaque element (8) which is fine enough to intercept only a fraction of the radiation destined for the sensor (3) to allow a fine modulation of the intensity of the radiation on the sensor (3). An incremental optical encoder as claimed in claim 1, characterised in that said element (8) is of elongate form. An incremental optical encoder as claimed in claim 2, characterised in that said element (8) is U shaped having a connecting limb (10) defining an axis of rotation substantially parallel to the axis of said shaft, a first limb (12) for intercepting said radiation and a second limb (11) for enabling the position of the first limb (12) to be varied. An incremental optical encoder as claimed in claim 3, characterised in that said first limb (12) is disposed between said radiation source (1) and said disc (4). An incremental optical encoder as claimed in any one of claims 1 to 4, characterised in that said regulating means (8) is comprised of metal wire. An incremental optical encoder as claimed in any one of the preceding claims, characterised in that said radiation source (1) comprises an LED. An incremental optical encoder as claimed in any one of the preceding claims, characterised in that said sensor (3) comprises four photosensitive elements (9) each arranged for receiving said radiation such that adjacent photosensitive elements are 90° out of phase.

Inventors: HEYRAUD MARC, HEYRAUD, MARC
Applicants: JOHNSON ELECTRIC SA, JOHNSON ELECTRIC S.A.
Application year: 1989
Main classification: H03M1
IPC codes: H03M1, G01D5