Abstract: A pneumatic control system for a reversible stepping actuator has an analog pneumatic control signal and an analog signal indicative of actual position of the actuator. Imbalance in these signals opens one or the other of a pair of comparator valves having inlets connected to a source of pneumatic pressure. Opening of either of the comparator valves enables a pulse timer which converts the pneumatic pressure from either of the comparator valves to pulses of pneumatic pressure applied to one or the other of the operators of the reversible stepping actuator. An oscillating pneumatic timer may be used to produce the pressure pulses, or an electric timer can be used to "chop" one of the pneumatic pressures into periodic pulses.

Abstract: A pneumatic pressure regulating system controls a choke having a hydraulic operator actuator for selectively opening or closing the choke to control pressure in a well casing. The system applies periodic pulses of pneumatic pressure for opening an output valve in one direction when the actual casing pressure is greater than a selected control pressure for opening the choke, and applies periodic pulses of pneumatic pressure for opening the output valve in another direction when casing pressure is less than the set pressure for closing the choke. The interval between sucessive pulses is adjusted to be long enough for the pressure in the casing to equilibrate before the next pulse occurs.

Abstract: A pneumatic system is used for controlling the position of a hydraulically-actuated drilling choke to limit the maximum pressure in the casing of an oil well. Pressure in the casing is used directly to switch a converter valve that applies air pressure to the pressure limiting system. The air pressure opens the choke to relieve casing pressure. The system traps a volume of air at a pressure corresponding to the choke position and uses this pressure as a memory signal for resetting the choke to its preset position after casing pressure has decreased below its allowable maximum. The system also uses pneumatic pressure to assure that the memory is not lost before the choke is reset.