Adjustable gain controller for valve position control loop and method for reducing jitter

A method of reducing mechanical and hydraulic jitter in an electrohydraulic valve actuator which is part of a valve position control loop in combination with a steam turbine, said valve actuator controlling the flow of steam through said valve, the valve's regulation being set relatively low and said control loop responding quickly to steam flow condition error signals, said error signals being amplified by the reciprocal of the valve's regulation value thereby establishing a relatively high gain for said control loop, the method of reducing the jitter caused by a digitized portion of said control loop comprising the steps of:

• (a) initially selecting a value of A between -1.0 and +1.0;
• (b) initially selecting a value of n from the group consisting of 0,1 and 2;
• (c) calculating a gain factor G from the relationship expressed by
• G=A+(1-A)(E/REG).sup.n
• wherein G is limited to positive values, E is the normalized value of said error signal and the value of REG is said valve's regulation value;
• (d) integrating the amplified error signals over a preselected time period;
• (e) multiplying said amplified error signals by said gain factor to produce resultant signals;
• (f) obtaining the derivative of said resultant signals with a preselected gain multiplier;
• (g) multiplying said resultant signals by a preselected constant;
• (h) summing the integrated, amplified error signal, the derivative of said resultant signal and the proportional output resultant signal together;
• (i) applying the summed signal to said electrohydraulic valve actuator;
• (j) monitoring the mechanical and hydraulic jitter of said electrohydraulic valve actuator;
• and
• (k) repeating all the above steps with different values of A and different values of n until the jitter is reduced to an acceptable level and said valve responds to an error signal by establishing the desired steam flow condition within an acceptable period of time.

In combination with a computer-based control system for a steam turbine, wherein a steam valve is controlled directly by a valve positioning subloop which responds to analog electrical signals input thereto, and said computer-based control system generates digital error signals which are the difference between the actual steam flow condition through said valve and the desired steam flow condition through said valve, an adjustable gain controller as part of the digitized portion of a valve position control loop tied to said positioning subloop and said control system, said controller comprising:

• an integral controller receiving said error signals and integrating said error signals over a preselected time period;
• an electronic operator which includes means for initially selecting a value of A between -1.0 and +1.0 and means for initially selecting a value of n from the group consisting of 0, 1 and 2, said electronic operator generating a gain factor G from the relatioship expressed by
• G=A+(1-A)(E/REG).sup.n
• wherein G is limited to positive values, E is the normalized value of said error signal and the value of REG is the valve's regulation value, and said electronic operator multiplying said amplified error signals by said gain factor;
• a proportional controller in parallel with a derivative controller, both receiving the electronic operator's output signals;
• means for summing the outputs of said integral controller, said derivative controller and said proportional controller together;
• means for converting said summed output into an analog electrical signal and inputing said analog signal into said positioning subloop;
• and
• means for adjusting both said value of A and said value of n to reduce the mechanical and hydraulic jitter in said positioning subloop which is caused by said digitized portion of said control loop and yet maintaing the relatively quick response of said control loop to said error signals.