GB1498829A - Digital coding of angles - Google Patents

Abstract

1498829 Measuring angular position SOC D'ETUDES, RECHERCHES ET CONSTRUCTIONS ELECTRONIQUES 26 March 1975 [28 March 1974] 12711/75 Headings G1A and G1N (A) A digital angle coder comprises a fixed reading head TF, a reading head TMO relatively movable about a first axis through the angle to be coded, a member 5 rotatable at constant speed about an axis coincident with the first and carrying a plurality of marks adapted to be read by the reading heads which in co-operation therewith produce two series of read-mark signals, and angle-detector means for accumulating the time intervals between the signals of one series and the signals of the other series during one or more complete revolutions of the member 5. The heads TF, TMO may be magnetic reading heads and the marks recordings on a magnetic track. To compensate for any misalignment of the two axes a second movable reading head TMA is provided and the mean of the readings of the two heads T MO , T MA is taken. In use, e.g. in a theodolite, the phase control circuit 15, Fig. 2 and origin setting 16 are activated and with the arm 6 in the first position the mean phase displacements read by circuits 23, 33 are stored in memories 24, 34. During rotation of the arm 6 to the second position the difference in numbers of pulses read by the fixed and moving heads are registered by the reversible counters 22 and 32, together with the sign of angular separation, and provide a coarse measurement of the angle. Pulses coincident in time are separated by anti-coincidence circuits 21, 31. A final mean phase displacement is then taken and subtractors 26, 36 supply the difference between this and the stored initial value to the circuit 18. Circuit 18 provides the mean of the fine measurements of angle from circuits 26, 36 and the mean of the coarse measurements from circuits 27, 37. In a second embodiment, utilizing optical methods, the fixed reading heads TFO, TF180 each comprise a photo-emission cell 420, e.g. an LED, a reflective grid 421 or 423, and a photo-receptive cell 422, e.g. a phototransistor. Coincidence between the marks on a grid with similar reflective marks 51 on a rotating disc 50 of low or zero reflectivity material causes a minimum in reflected light. For coarse measurements the auxiliary fixed head T 1 and the auxiliary moving head T2 read single marks 53, 54 respectively on the disc. The speed of the disc may be related to the frequency of the counter clock 1 or the clock may be enslaved to the disc speed which is maintained substantially constant.