POSITIONAL ENCODING OF ROTATING MEMBERS

Abstract: An arrangement for generating signals representative of the motion of a rotating member, for example a printer platen (1) driven by an electric motor (8). The position of a shaft is noted by mounting a sectored magnetic disc (9) on it which has alternating north and south poles about its periphery. Two linear Hall effect sensors (11, 12) are mounted half an odd integral number of sectors apart adjacent the disc and connected to circuitry which converts their outputs to a useful signal. The circuitry preferably includes compensation to enable the outputs of the sensors to be matched and to stay matched despite temperature variation.

Description: Positional encoding of rotating members This invention relates to positional encoding of rotating members. There are many applications in engineering and technology in which it is desired to monitor the rotation of a shaft. Particularly in the case of apparatus which is electromechanical, numerous sensor systems have been developed enabling the movement of a shaft to be converted into an electrical signal and numerous systems developed to process that electrical signal into useful information. , A widely used system is optically to encode the movement of the shaft e.g. by mounting on the shaft a disc having alternate light transmitting and light blocking sectors. Such optical systems tend to require maintenance and a light source and are subject to error due to contamination by dirt, particularly in hostile environments. We have now found that particularly useful and robust systems may be developed using a magnetic encoding system. According to the present invention there is provided a method of encoding the movement of a shaft which comprises locating on the shaft a circular magnetised disc having about its periphery alternating north and south poles, locating adjacent the periphery of the disc a pair of linear Hall effect sensors, the angular spacing of the sensors correspond-, ing substantially to half an odd integral number of sectors, each sector subtending the angle between successive north and south poles about the periphery of the disc, and feeding the outputs of the Hall effect sensors to circuitry adapted to match the outputs and produce an output signal consisting of two signals in quadrature and representative of the motion of the shaft. Hall effect sensors have previously been proposed for use in encoding applications, for example as described in British Patent Specification 2098743. In the present invention, in contrast to the system there described, a multipole magnetic disc is used in conjunction with a pair of linear Hall effect sensors located at specifically defined positions on its periphery, giving a signal set of greater usefulness. The compensation circuitry attached to the linear Hall effect sensors preferably enables at least . differences in offset voltage for the two sensors, and differences in the output of the two sensors as the magnetic poles go past them,to be compensated. It is highly preferred that in addition to ihose two types of compensation, the circuitry furthermore provides means for compensating for the effects of the drift of offset voltage with variation in temperature and most preferably in addition circuitry to compensate for the variation in output amplitude in dependence on variations in ambient temperature. The mechanical arrangement employed may be any convenient mechanical arrangement for the purpose. The magnetic disc, which may have any suitable • number of poles distributed about its periphery, for example 10 to 20 poles in most cases, may be mounted directly on the shaft the rotation of which is to be measured or may be mounted on its own shaft which is coupled thereto e.g. using spur gearing. Magnetic discs having alternating poles about their periphery can be manufactured from ferrite. By way of example, one application of the present invention is illustrated in the accompanying drawings in which: Figure 1 is a general arrangement diagram showing a platen on a printer unit such as might be used as an output to a computer system or word processing system, and Figure 2 is a circuit diagram showing Hall effect sensors and the appropriate signal processing circuitry thereof. Referring to the drawings and referring particularly to Figure 1 this shows some of the components of a daisywheel printer. A cylindrical platen 1 against which the paper to be printed on is held is rotatably mounted on a shaft 2 in suitable bearings not shown. At the end of a shaft is a fixed spur gear 3 which is driven by the smaller diameter section 4 of a double spur gear 5, the larger toothed section of which 6 is in turn driven by a toothed output shaft 7 of an electric motor 8. The opposite end of output shaft 7 carries a magnetised disc 9 and the motor is energised by suitable windings enabling output shaft 7 to be driven in either sense and accordingly enabling the platen 1 to be driven to feed paper on it up or down as desired. Located adjacent disc 9 is one end of a printed circuit board 10 which has mounted adjacent its end two Hall effect sensors 11 , 12 which are located adjacent the periphery of disc 9. Disc 9 has 16 magnetic poles spaced evenly about its periphery, OMPI y j * WIPO ~ ,Λ,J 8 north and 8 south, and the sensors 11 , 12 are spaced apart by an angle corres onding to half the angle subtended by an integral number of poles. Thus when one of the sensors is radially aligned with one of the poles, the other sensor is radially aligned with the zero field position precisely intermediate two poles. Accordingly, as motor 8 is driven and shaft 7 and disc 9 rotated, the signals which can be derived from sensors 11, 12 are 90° out of phase with one another and both approximately sinusoidal. The circuitry for matching those signals and producing useful phase quadrature signals therefrom is shown in Figure 2. Referring to this Figure, it will be seen that the outputs of the linear Hall effect sensors 11 and 1 are fed each to the negative input of a summing amplifier, IC4b and IC4c respectively. In both cases, the same amplifier input is also fed with a dc voltage derived from a temperature dependent current source consisting of IC3 and its associated components including amplifier IC4a. The variation in voltage derived from the IC3 circuit compensates for the variation with temperature of the offset voltage of the two Hall effect sensors 11 and 12. Because of the variation between semiconductor components, the dc offset value associated with each of the sensors 11 and 12 will be different but these may be compensated for respectively by offset adjustment potentiometers VR1 and VR2 each of which constitutes part of a resistor chain ed with a constant voltage derived from temperature compensated zener diode D1. The voltages from VR1 and VR2 are applied to the positive inputs of summing amplifiers IC4c and IC4b respectively. The circuitry described thus far accordingly produces a pair of signals 0^ and 0 2 on the outputs of IC4e and IC4b which are in phase quadrature and in respect of which the initial sensor to sensor tolerance of the dc offset value has been removed and the temperature variation of that dc offset has also been compensated. The output of IC4b is also symmetrical. In order to match the signals more exactly to one another, the amplitude of one output signal can be matched to that of the other, for example by adjusting the output amplitude of one signal only rather than providing adjustments for both. In the circuit illustrated VR3 enables the output of a summing amplifier IC4d to be adjusted. Adjustment may be to produce an identical amplitude with the signal output from IC4c or it may be adjusted to provide a signal amplitude desirable for feeding to further processing circuitry. In the case of the specific example under discussion, the signal derived from IC4c is used by subsequent electronics to detect direction information i.e. which sense shaft 7 is turning and is not further processed. Diode D2 clamps the output signal from IC4c which is accordingly dc offset. The signal output from IC4d on the other hand is used in a linear servo control loop and accordingly needs to be maintained as constant as practicable for reliable operation of that control loop. With this in mind, in addition to final output amplitude adjustment being obtainable via potentiometer VR3, which operates linearly due to the symmetry of the output signal of IC4b, the input to summing amplifier IC4d is via a negative temperature co-efficient thermistor R8 which compensates for the variation of signal amplitude with variation in ambient temperature. The circuit shown in Figure 2 accordingly processes the raw signals received from sensors 11 and 12 into two signals in phase quadrature which appear on two output lines at the end of printed circuit board 10. Power is fed to printed circuit board 10 and the signals extracted via a 5 pin connector. The remaining details of the circuit are evident from Figure 2. OMPI ^

Claims: CLAIMS 1. A method of encoding the movement of a shaft which comprises locating on the shaft a circular magnetised disc having about its periphery alternating north and south poles, locating adjacent the periphery of the disc a pair of linear Hall effect sensors, the angular spacing of the sensors corresponding substantially to half an odd integral number of sectors, each sector subtending the angle between successive north and south poles about the periphery of the disc, and feeding the outputs of the Hall effect sensors to circuitry adapted to match the outputs and produce an output signal consisting of two signals in quadrature and representative of the motion of the shaft. 2. Apparatus including a rotatable shaft and means for determining the angular movement of the shaft including movement encoding means comprising a circular magnetised disc having about its periphery alternating north and south poles, a pair of linear Hall effect sensors located adjacent the periphery of the disc, the angular spacing of the sensors corresponding substantially to half an odd integral number of sectors, each sector subtending the angle between successive north and south poles about the periphery of the disc, and circuitry connected to the outputs of the Hall effect sensors and adapted to match the outputs from the sensors and produce an output signal consisting of two signals in quadrature and representative of the motion of the shaft. 3. Apparatus according to claim 2 and including compensation circuitry attached to the linear Hall effect sensors and capable of compensating differences in offset voltage for the two sensors. 4. Apparatus according to claim 2 or 3 and including compensation circuitry enabling differences in the outputs of the two sensors to be compensated. 5. Apparatus according to any one of claims 2 to 4 and including compensation circuitry to compensate for the effects of the drift of offset voltage with variation in temperature for at least one of the sensors. 6. Apparatus according to any one of claims 2 to 5 and including compensation circuitry to compensate for the variation in output amplitude in dependence on variations in ambient temperature of at least one of the sensors. 7. Apparatus according to any one of claims 2 to 6 wherein the magnetic disc has 10 to 20 poles around its periphery. 8. Apparatus according to claim 7 wherein the magnetic disc is manufactured from ferrite. 9. Apparatus according to any one of claims 2 to 8 wherein the magnetic disc is mounted on a shaft coupled to the shaft the rotation of which is to be measured. 10. A printer unit including a platen and including apparatus according to any one of claims 2 to 9 for sensing and determining platen movement and position. OMPI

Inventors: ASPIN GORDON JAMES, NEWMAN PERRAN VINCENT LEONARD, TEAPE JOHN WILLIAM, THOMPSON-BELL IAN, ASPIN, GORDON, JAMES, NEWMAN, PERRAN, VINCENT, LEONARD, TEAPE, JOHN, WILLIAM, THOMPSON-BELL, IAN
Applicants: PRUTEC LTD, PRUTEC LIMITED
Application year: 1984
Main classification: H03M1
IPC codes: H03M1