IDEC MICROSmart FC6A Series Ladder Programming Manual page 413

PID Control
PID control performs adjustment operations to cancel the deviation between a single set point and the process variable that is
present in normal temperature control. The types of PID control that can be used with the FC6A Series MICROSmart are as follows.
■Proportional control
Proportional control outputs the manipulated variable in the proportional band that is proportional to the deviation between the set
point and the process variable. The output is turned on until the process variable reaches the A point (the proportional band), and
when it exceeds this (it enters the proportional band), the control output starts to be turned on and off at the control cycle, and
then when the set point is exceeded, the control output is turned completely off. Along with the rise in temperature from the A
point to the set point, the control output on time becomes shorter and the off time becomes longer. An offset always occurs with
Proportional control. The P action is suitable for processes with no wasted time such as gas pressure control and level control.
When set to Proportional control, integral time and derivative time are automatically ignored.
Temperature Proportional band
Set point
A point
• When the proportional band is made smaller, the control output turns on or off from near the set point, so the time until the process variable
rises in temperature to the set point decreases and the offset also becomes smaller, but hunting increases.
• When the proportional band is made larger, the control output turns on or off from a temperature considerably lower than the target
temperature, so auto tuning and hunting eliminate, but it takes time until the process variable rises in temperature to the set point and the
offset between the set point and the process variable becomes larger.
■PI control
The I action automatically corrects the offset generated by Proportional control. However, for rapid changes in temperature due to
disturbances, it will take time until the temperature stabilizes. The PI action is suitable for temperature control with a slow rate of change.
When set to PI control, derivative time is automatically 0.
Proportional band
Temperature
Set point
• When the integral time is decreased, I (integral) control becomes more powerful and the offset can be corrected in a short amount of time,
but this may cause long cycles of hunting.
• When the integral time is increased, I (integral) control becomes weaker and it takes time to correct the offset.
■PD control
Compared to Proportional control, PD control increases the response to rapid temperature changes due to disturbances, stabilizes
control in a short amount of time, and attempts to improve the transient response characteristics. PD control is suitable to
temperature control with a fast rate of change.
When set to PD control, integral time is automatically 0.
Temperature Proportional band
Set point
• When the derivative time is decreased, the D (derivative) control becomes weaker and the response to rapid temperature changes becomes
slower. The action to control rapid temperature increases also becomes weaker, so the rise in temperature time to the set point will become
faster, but overshooting the set point will occur more easily.
• When the derivative time is increased, the D (derivative) control becomes more powerful and the response to rapid temperature changes
becomes faster. The action to control rapid temperature increases also becomes more powerful, so the rise in temperature time to the set
point will become slower, but overshooting the set point will occur less easily.
Offset
Time
Disturbance
Time
Disturbance
Time
FC6A S MICROS L
ERIES MART
P M
ADDER ROGRAMMING ANUAL
19: PID C
ONTROL
FC9Y-B1726
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NSTRUCTION
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