Follow-up servo systems for clutches

A follow-up servo system for controlling by means of an actuator system, and in response to signals from a demand member, the location of an axially displaceable part of a mechanical friction clutch with respect to the co-operating axially fixed part of the clutch, includes measurement transducer means for deriving a signal to represent the actual instantaneous axial location of the displaceable part in relation to a reference point, means for supplying a signal to represent the axial location of the displaceable part in relation to the reference point when the displaceable part is in its instantaneous just-touching (JT) position with the fixed part, by combining in the appropriate manner a signal from the measurement transducer means when the displaceable part is in its fully-engaged (FE) position with the fixed part, and a constant signal provided by means within the system and representing the distance between the JT and FE positions of the displaceable part, demand transducer means for deriving from the demand member an electrical signal representing the instantaneous setting of the demand member, this signal also representing the desired instantaneous axial separation betwen the displaceable and fixed clutch parts as demanded by the demand member, summation means responsive to and combining both the signal representing the JT position of the displaceable clutch part and the signal representing the desired instantaneous axial separation between the displacement and fixed clutch parts, and so deriving the required instantaneous axial location of the displaceable part in relation to the reference point, comparator means for comparing the signals representing the required and the actual instantaneous axial location of the displaceable part in relation to the reference point and, in response to the detection of any difference therebetween other than a predetermined value, to provide an error signal indicative of the difference, and connections for applying the error signal to the actuator system to bring about a movement of the displaceable clutch part in the direction for nulling the signal error.