IDEC FC5A-F2M2 User Manual page 25

PID M M F
ODULE AIN UNCTIONS
In reality, the ideal temperature control shown in Figure 1 on the previous page is almost impossible to
achieve due to a number of complicated factors such as thermal capacity, static characteristics, dynamic
characteristics and disturbances.
Figure 2 is regarded as an optimal temperature control result. Depending on the usage and objective, for
some temperature control applications, suppression of overshoot is required even if the temperature
rises very slowly as shown in Figure 3. For some temperature control applications, it is necessary to
stabilize the temperature as quickly as possible by raising the temperature rapidly even if overshoot is
generated as shown in Figure 4. In general, however, Figure 2 is regarded as an optimal temperature
control. The PID module is designed to raise the process variable (PV) to the set point (SP) as quickly as
possible in order to stabilize the process variable (PV) at the set point (SP) so as to perform the optimal
temperature control. If the temperature fluctuates due to sudden disturbances, the PID module responds
to the fluctuation with speedy response in the shortest possible time and performs quick control to
stabilize the temperature.
Set point (SP)
Set point (SP)
Figure 4. The temperature rises rapidly; however, the control stabilizes after overshoot and undershoot.
Characteristics of the Control Target
To perform optimal temperature control, it is necessary to have a good knowledge of the thermal
characteristics of the PID module, sensors, actuators as well as control targets. For example, the PID
module controls a constant temperature oven and its temperature can rise up to 100°C. Even if the set
point (SP) of the PID modules is configured as 200°C, the temperature of the constant temperature oven
rise only up to 100 °C due to its static characteristic.
The characteristic of the control target is determined by the combination of the following 4 factors.
1. Thermal capacity:
This represents how the target is easily heated, and has a relation with the volume size of the control
target.
2. Static characteristic:
This represents the capability of heating, and is determined by the size of the heater capacity.
3. Dynamic characteristic:
This represents the rising characteristic (transitional response) during initial heating. This is a
complicated process involving heater capacity, furnace capacity size and sensor location.
4. Disturbance:
Any change in control temperature causes disturbance. For example, the change of ambient
temperature or supply voltage can cause disturbance.
4-2
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Temperature
Figure 3. Stable but slow temperature rise control
Temperature
Characteristics of Control Target
Thermal Capacity
Disturbance
FC5A MicroSmart PID Module User's Manual FC9Y-B1283
Time
Time
Static
Characteristic
Dynamic
Characteristic