Solid state compressor control system

A control system for connection with a fluid compressor driven by a motor, the compressor including loading valve means operative to adjust the capacity of the compressor by modulating fluid leakage from its discharge toward its inlet to maintain a desired suction pressure at the inlet, said system comprising:

• (a) means in said inlet for measuring fluid suction pressure and delivering a suction pressure signal;
• (b) means adjustable to select a compressor loading setpoint comprising a reference suction pressure signal;
• (c) a loading actuator and an unloading actuator, said actuators being connected to operate said loading valve means selectively and progressively to load and unload the compressor;
• (d) pressure signal comparator means for comparing said measured suction pressure signal with said reference suction pressure signal, said comparator means having separate outputs including a first output whose amplitude represents the degree of deviation from the setpoint when the measured suction pressure signal is greater than the reference suction pressure signal and including a second output whose amplitude represents the degree of deviation from the setpoint when the suction pressure signal is less than the reference suction pressure signal;
• and
• (e) means for converting the first and second outputs when present, respectively, to first and second control signals comprising trains of pulses whose duty cycles vary according to the amplitudes of said respective outputs, said means for converting said first and second outputs comprising separate comparators each having one input connected to one of said outputs, a triangular wave generator connected to the other input of each comparator, one comparator delivering an output control signal pulse which is present whenever one of said inputs exceeds the momentary level of the triangular wave, and the other comparator delivering an output control pulse whenever the other of said inputs exceeds the momentary level of the triangular wave, and means to establish a threshold level at the inputs to said comparators to provide a deadband within which said first and second outputs are too small to provide output control signals from the comparators;
• and said control signals being coupled respectively to said loading and unloading actuators and operative to control said actuators to load or unload the compressor in increments in response to the durations of said pulses.