Digital servovalve and method of operation

In a digital feedback control system for controlling a load whose output is controllable by flow of fluid therethrough, the combination comprising:

• a. servovalve means including a valve operating member movable therewithin;
• b. fluid circuit means adapted for connection between a source of pressurized fluid and said servovalve means and between said servovalve means and the load, for providing fluid communication between the source of pressurized fluid and said servovalve means and between said servovalve means and the load, the rate and direction of fluid flow through said servovalve means and thereby through the load being dependent on the position of said operating member in said servovalve means said valve operating member being movable to a null position in which it prevents fluid flow in said servovalve;
• c. stepper motor means operatively connected to said valve operating member to change the position of said valve operating member by steps in response to pulses applied to said stepper motor means;
• d. digital feedback means adapted to be connected to the fluid-controlled load to sense changes in the output of the load and generate digital feedback signals indicative of changes in the load output;
• and
• e. digital controller means, adapted to be connected to a source of digital command signals indicative of a desired load output, and connected to said digital feedback means for reception of the digital feedback signals, for determining the actual load output from the feedback signals, comparing the actual load output with the desired load output, and generating and applying to said stepper motor means signals to position said valve operating member, in response to the difference between the actual load output and the desired load output, for flow of fluid through the load to reduce the difference between the actual load output and the desired load output, said digital controller means including position-storing means for storing a digital number, monitoring the step signals generated and applied to said stepper motor means by said digital controller means, and incrementing and decrementing the digital number in accordance with the signals applied to said stepper motor means by said digital controller means, the position of said valve operating member being represented by the digital number, a predetermined null valve of the digital number representing the null position of said servovalve, said digital controller means further including nulling means operable to monitor the digital feedback signals received by said digital controller means, to generate and apply to said stepper motor means signals for positioning said valve operating member to counter any changes in output indicated by said digital feedback signals, and to reset the position-representing digital number to the predetermined null value when no further changes in output are indicated by the feedback signals, said digital controller means generating and applying to said stepper motor means signals to cause said position-storing means to increment or decrement said stored digital number to equal the predetermined null value when the desired load output equals the actual load output, fluid flow in said fluid circuit means thereby being prevented when the actual and desired load outputs are equal.