Abstract: A security system comprises electronic means for generating a group of pulses of predetermined frequency, which group has a unique identity specified by the particular durations of its constituent pulses; means for transmitting that group of pulses; means for receiving that group of pulses; and means responsive to that received group of pulses only when it has said unique identity. Preferably the pulses are bursts of high frequency waves, and the means responsive to received pulses includes means for establishing tolerance intervals for the ends of corresponding pulses.

Abstract: In a follow-up servo system for the electrical control of a friction clutch of, say, a motor vehicle from the throttle control by way of an electro-mechanical actuator, with automatic take-up of the wear of the mating friction surfaces, whenever the clutch plate (i.e. the axially-movable part) is in its fully-engaged (FE) position with respect to the cooperating axially-fixed part a signal is repetitively derived to represent the corresponding just-touching (JT) position of the plate under the then conditions of wear. To this signal is added a signal from the throttle control within a range corresponding to the JT-FE range of the plate, thereby forming a demand signal corrected for wear. From a comparison of it with a measurement signal representing the actual position of the plate an error signal is derived for application to the actuator in the appropriate sense.