IDEC MicroSmart Pentra - Manual (Page 26)

PID M ODULE M AIN F UNCTIONS
FC5A MicroSmart PID Module User’s Manual FC9Y-B1283 4-3
Fixed Value Control
The PID module provides 2 control modes, one is the fixed value control and the other is the program control.
The fixed value control is a standard temperature control which performs to eliminate the deviation between
the single set point (SP) and process variable (PV). The program control allows you to define the set point
(SP) that changes as the time progresses so that the process variable (PV) can be controlled to match the set
point (SP) changing as the time progresses. For detail about the program control, see 4-9.
Control actions that can be used for fixed value control and program control are described below.
ON/OFF Control Action
In the ON/OFF control action, when the process variable (PV) is lower than the set point (SP), the control
output is turned on, and when the process variable (PV) exceeds the set point (SP), the control output is
turned off. Overshoot, undershoot, and hunting are generated. ON/OFF control is suitable for processes
which do not require accuracy.
If the proportional band or proportional gain of the PID module parameter is set to 0, the control action
becomes ON/OFF control.
Overshoot, Undershoot
As the temperature of the control target
rises as shown in the figure on the right,
the process variable (PV) sometimes
exceeds the set point (SP) greatly.
This is called overshoot. If the process
variable (PV) drops below the set point
(SP), this is called undershoot.
Hunting
The control result oscillates as shown
in the figure on the right. This is the
the hunting.
P Control Action (Proportional Action)
P control action outputs the manipulated variable (MV) in proportion to the deviation between the process
variable (PV) and the set point (SP) within the proportional band. The control output is ON until the
process variable (PV) reaches the point A that is determined by the proportional band. If the process
variable (PV) exceeds the point A (enters the proportional band), the control output starts turn on/off
according to the control period and the manipulated variable (MV). If the process variable (PV) exceeds
the set point (SP), the control output is completely turned off. While the process variable (PV) rises from
the point A to the set point (SP), the control output ON time decreases and the control output OFF time
increases. Compared to ON/OFF control action, there is no overshoot in P control action, and hunting
becomes less frequent; however, the offset is generated. The P control action is suitable for processes
such as gas pressure control or level control, in which there is no dead time.
If the integral time and derivative time of the PID module parameter are set to 0, the control action
becomes the P control action.
Set Point (SP)
Temperature
Time
Control Output
ON
OFF
Undershoot
Overshoot Hunting
Set Point (SP)
Temperature
Time
Proportional Band Offset
Point A