Patent GB-1451371-A (GB)
Title: Follow-up servo systems
IPC: F16D48/06
Filed: 1973-09-29
Abstract: 1451371 Clutch control FERRANTI Ltd 27 Sept 1974 [29 Sept 1973] 45646/73 Heading F2L [Also in Division G3] In the control of a mechanical friction clutch by means of a follow-up servo system, a stored value JT representing the axial position of the movable clutch part corresponding to the just touching disposition of the mating surfaces is combined with a demand signal representing a desired position between just touching, and fully engaged, FE positions, the resultant being compared with the actual position to derive an error signal controlling an actuator which moves the movable part to null the error signal. The value JT is periodically updated during clutch usage, to correspond to the changing just touching position, due to wear. The clutch is used in a vehicle, in which case the demand signal is derived from the vehicle throttle setting, or in a machine tool. Digital servo arrangement, Fig. 1.-Store 17 contains a preset quantity, appropriate to the clutch being controlled, representing the displacement JT/FE between the just touching and fully engaged positions. Electronic switch 13 being closed whenever the clutch is fully engaged, electronic switch 14 operates periodieally to provide to counter 16 via A/D converter 15 a pulse train representing the fully engaged position FE, which is combined with the value JT/FE to provide the current just touching position JT for storage in unit 21. The stored value is transferred, whenever the clutch is in the fully disengaged position FD, to counter 23, where it is additively combined with the A/D converted throttle position signal TP; the conversion being such that a null digital output is provided until 15% of throttle opening is is reached: to provide the demand signal TP + JT fed to comparator 31 also receiving the actual position signal from A/D converter 33, the resultant error being fed to actuator system 35. Between positions JT and FE, the movable part is vibrated about a mean position corresponding to servo actual position, and switches 26 and 32 are closed at the beginning of each vibration cycle for a small proportion of the cycle. Analogue servo arrangement, Figs. 4, 6 (not shown).-The digitally stored value JT is converted to analogue form in D/A converter (61) and passed to amplifier (62) having a further input coupled to constant current source (64) resistor (63) being set to provide an analogue equivalent of the value JT/FE, the amplifier output, fed to throttle potentiometer (65) representing the value FE. The analogue signal JT is also connected via amplifier (66) to the potentiometer, ensuring that the signal on line (67), corresponding to the throttle position, varies between levels corresponding to JT, and FE, the signal on line (67) is fed to comparator amplifier (71) receiving also the actual position signal from wiper (12). A further potentiometer (81, Fig. 6) with a wiper (82) preset to 15% throttle opening may be provided, amplifier (83) connected thereto, and to the wiper (67) exerting over-riding control ensuring that the clutch remains fully disengaged until 15% throttle setting is exceeded. Actuator systems (1) digital, Figs. 2, 3 (not shown).-Between just touching, and fully engaged positions, the clutch is moved towards, resp. way from full engagement during the mark resp. space epochs of a variable mark/space ratio pulse waveform, of constant frequency determined by the capacity of counter (44) counted by clock pulses. Each zero crossing of the counter sets bi-stable (43) to initiate a mark phase, counter (41) being initially set, for each cycle to contain a value equal to half the capacity of counter (44), augmented or diminished as appropriate by the error signal value, and counted down to zero to initiate the space phase of the cycle. The pulse waveform passes to actuator (47) comprising a piston/cylinder moved in opposed senses by the mark resp. space phases of the waveform the mean position thus moving in accordance with the error signal to reduce it to zero. (ii) Analogue, Figs. 4, 6 (not shown). The analogue error signal form amplifier (71) is passed to amplifier (74), whose other input receives a sawtooth waveform from generator (73). The output of the amplifier (71) is a pulsed waveform of variable mark/space ratio controlling the clutch as in the digital case, change between mark and space epochs occurring at instants when the sawtooth waveform and error values are equal, see Fig. 5 (not shown). A single A/D converter, with timesharing, may be used instead of converters 15, 33 and 27.