User manual

IEC61131 User and Reference Manual
April 22, 2008
326
The above parameters are fully described below.
Setpoint
The setpoint is a floating-point value representing the desired value of the process value.
The error value is the difference between the process value and the setpoint.
error = process value – setpoint (+/- deadband).
Process Value
The process value is a value that represents the actual state of the process being
controlled.
Gain
The proportional (P) part of the PID algorithm is the gain. A positive value of gain configures
a forward-acting PID controller and a negative value of gain configures a reverse acting
controller.
Reset Time
The integral (I) part of the PID algorithm is the reset time. This value, in seconds, controls
the reset gain (or magnitude of integral action) in a PI or PID controller. This is typically
referred to as Seconds Per Repeat. From the equation above it is seen that the integral
action of the PI or PID controller is a function of the reset time and the execution period
(cycle time). A smaller reset time provides more integral action and a larger reset time
provides less integral action. Valid range is any value greater than 0. A value of 0 disables
the reset action.
Rate Gain
The derivative (D) part of the PID algorithm is the rate time. This value, in seconds, controls
the rate gain (or magnitude of derivative action) in a PD or PID controller. From the equation
above it is seen that the derivative action of the PD or PID controller is a function of the rate
gain and the execution period (cycle time). A larger rate gain provides more derivative action
and a smaller rate gain provides less derivative action. Valid range is any value greater than
0. A value of 0 disables the rate action.
Deadband
The deadband parameter is used by the PID algorithm to determine if the process requires
the control outputs to be changed. If the absolute value of the error is less than the
deadband, then the function block skips execution of the control algorithm. This prevents
changes to the output when the process value is near the setpoint and can reduce wear on
the control elements. Valid range is any value greater than 0.The setpoint is a floating-point
value representing the desired value of the process value.
Full
The full setting is used in limiting the maximum output value of the PIDA function. If the PID
algorithm calculates an output quantity that is greater than the value stored in full, the
output quantity is set equal to the value stored in full. The full setting should always be
greater than the zero setting.