Aircraft altitude control system

Claim 1

In a system for sampling and holding analog signals corresponding to the altitude of an aircraft and providing a control signal for maintaining the aircraft at the altitude corresponding to the level of the altitude signal when sampled, altitude transducer means for providing an analog altitude signal corresponding to the altitude of the aircraft, a source of clock pulses, digital counting means, digital-to-analog converter means connected to the counting means for providing an analog output signal having a level corresponding to the count registered by the counting means, gating means connected between the source of pulses and the counting means for passing pulses from the source to the counting means in the presence of an enabling signal and blocking the passage of pulses in the absence of the enabling signal, bistable means connected to the gating means and having first and second output states, said bistable means being adapted for delivering an enabling signal to the gating means when in its first output state, means for resetting the counting means to an initial level and setting the bistable means to its first output state in response to an externally applied command signal, comparator means responsive to the altitude and output signals for delivering a signal to the bistable means for switching the same to its second output state to remove the enabling signal from the gating means when the output signal reaches the level of the altitude signal, thereby blocking the delivery of further pulses to the counting means so that the count registered by the counting means and the output signal remain at levels corresponding to the altitude of the aircraft at the time the command signal is applied, and means responsive to the altitude and output signals for providing an altitude correction signal corresponding to the difference between the altitude signal and the output signal when the bistable means is in its second output state.