Analog Output, Pid Control Mode - Walchem W600 Series Instruction Manual

Set point
Proportional Band
Minimum Output
Maximum Output
Hand Output
Off Mode Output
Error Output
Input
Direction
5.3.18

Analog Output, PID Control Mode

The PID algorithm controls an analog (4-20 mA) output using standard Proportional-Integral-Derivative
control logic. The algorithm provides feedback control based on an error value continuously calculated as the
difference between a measured process variable and a desired set point. Tuning settings specify the response for
proportional (the size of the error), integral (the time that the error has been present), and derivative (the rate
of change for the error) parameters. With proper tuning, the PID control algorithm can hold the process value
close the set point while minimizing overshoot and undershoot.
Normalized Error
The error value versus set point that is calculated by the controller is normalized and represented as percent of
full scale. As a result, tuning parameters entered by the user are not dependent upon the scale of the process
variable and the PID response with similar settings will be more consistent even when using different types of
sensor
inputs.
The scale used to normalize the error is dependent upon the type of sensor selected. By default, the full nominal
range of the sensor is used. This range is editable by the user if tighter control is desired.
PID Equation Formats
The controller supports two different forms of the PID equation as specified by the Gain Form setting. The two
forms require different units for entry of the PID tuning parameters.
Standard
The standard form is more commonly used in industry because its time-based settings for the integral and de-
rivative coefficients are more meaningful. This form is selected by default.
Parameter
K
p
T
i
T
d
Output (%) = K
Parameter
e(t)
Enter the sensor process value at which the output % will be the programmed minimum %.
Enter the sensor process value away from the set point at which the output % will be
the programmed maximum %.
Enter the lowest output %. If the output should be off at the set point, this will be 0%.
Enter the highest output %.
Enter the output % desired when the output is in Hand mode.
Enter the output mA value desired when the output is in Off mode, or being Interlocked, or
during a calibration of the sensor being used as an input. The acceptable range is 0 to 21 mA.
Enter the output mA desired when the sensor is not giving the controller a valid signal.
The acceptable range is 0 to 21 mA.
Select the sensor input to use for proportional control.
Select the control direction.
Description
Gain
Integral Time
Derivative Gain
1
e(t) +
T
p
i
Description
Current Error
Units
unitless
seconds or seconds/repeat
seconds
f
e(t)dt + T
d
73
de(t)
dt
Units
% of full scale