Techmation deadband reset scheduling algorithm

02/01/1999


N ot intended to replace good valve selection and maintenance procedures, the Techmation Inc. (Scottsdale, Ariz.) deadband reset scheduling algorithm provides a good compromise between the competing requirements of steady-state stability and speed of response.

Deadband Reset Scheduling (DRS) is the name given to a modified PID control algorithm developed by Techmation. It has been successfully implemented on thousands of control loops on a wide variety of digital control systems, according to Techmation. The DRS algorithm simply modifies the PID controller integral setting between a fast setting and a slow setting depending on the size of the control error (SP-PV).




Where:

if |e| & errorgap then
Kr = 10 to 20
else Kr = 1

Where:

Kp = Controller Gain
Ti = Controller Integral Time
Kr = Reset Ratio Factor (user adjustable typically between 10 and 20)
e = Error (SP - PV)
errorgap = Width of Error Deadband around setpoint

Equation 1 - PI Time domain deadband reset scheduling equation

The DRS implementation increases the controller integral time setting (Ti) when the actual error (e) is smaller than the errorgap, thus, detuning the controller integral setting when the error is small. With the controller detuned near setpoint, the rate at which the controller integral action drives the controller output is too slow for a stick-slip cycle to be maintained. When the controller is forced to respond to an upset or setpoint change, the optimum tuning is used to insure fast and stable response.

Once implemented, set the errorgap equal to the range of oscillation in the PV signal. Since the absolute value of the error is compared to the errorgap this represents a 2X safety factor. Selection of Kr is made by observing the cycling at steady state and increasing Kr until the cycle disappears. Typically, a Kr setting between 10 to 20 is sufficient to stop the cycling.

Contact Techmation at:
Techmation Inc.
8070 E. Morgan Trail
Suite 150
Scottsdale, AZ 85258-1228
Phone: 602/968-9946
Fax: 602/443-0402
e-mail: techmatn@dancris.com





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