IDEC MicroSmart Pentra - Manual (Page 27)

PID M ODULE M AIN F UNCTIONS
4-4 FC5A MicroSmart PID Module User’s Manual FC9Y-B1283
•
If the proportional band is narrowed (Proportional gain is made larger)
Because the control output starts turning on/off at around the set point (SP), the time until the process
variable (PV) reaches the set point (SP) is shortened, and the offset is small; however, hunting is
frequent. If the proportional band is greatly narrowed, the control action becomes similar to the ON/OFF
control action.
•
If the proportional band is broadened (Proportional gain is made smaller)
Because the control output starts turning on/off at the significantly low temperature from the set point
(SP), overshoot or hunting is reduced; however, it takes time for the process variable (PV) to reach to
the set point (SP), and the offset between the process variable (PV) and the set point (SP) becomes
broadened.
The offset caused by the P control action can be corrected by configuring the reset value. If the reset
value is configured, the proportional band range can be shifted as shown in the figure below. The reset
value can be automatically calculated by the auto-reset function.
PI Control Action (Proportional + Integral Action)
I (Integral) action automatically corrects the offset caused by P control action, and temperature control is
performed at the set point (SP). However, it takes time for the process variable (PV) to be stable if the
process variable (PV) is changed rapidly due to disturbance. PI control action is suitable for the
processes in which the temperature slowly changes.
If the derivative time of the PID module parameter is set to 0, the control action becomes the PI control
action.
• If the integral time is shortened too much, the integral action becomes strong. The offset can be
corrected in a shorter time; however, hunting with a long cycle may be caused.
• If the integral time is extended too much, the integral action becomes weak and it takes time to correct
the offset.
Output Manipulated
Variable
100
50
0
Proportional Band
Set Point (SP)
A B C
Proportional band area that
can be configured with reset
function.
Offset can be corrected within
±proportional band from the
set point (SP).
Output Manipulated Variable
A: 100%
B: 50%
C: 0%
Set Point (SP)
Temperature
Time
Proportional Band
Disturbance