WPI ATC2000, Instruction Manual

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4 World Precision Instruments
Fig. 4—The monitored temperature is compared with the set point (error), and then the
P, I and D algorithms are applied to the error value. The PID values are added together
to determine the command sent to the heater. The output changes the monitored
temperature, and the cycle repeats.
The default set point is 37°C, but you can configure it as needed. The controller
calculates the error, which is the difference between the present temperature and the
desired temperature.
Error = Monitored temperature – set point
Three individual components are derived by applying different functions to the error.
• The error is multiplied by the proportional gain factor (P) that is set in the
configuration menu. See “Setting PID Parameters” on page 13.
• The error is accumulated to constitute the integral, which is multiplied by the
integral gain factor (I) that is set in the configuration menu. See “Setting PID
Parameters” on page 13.
• The error is subtracted from the previous value of the error to determine the
difference. It is then multiplied by the differential term (D) that is set in the
configuration menu. See “Setting PID Parameters” on page 13.
These three components are added together to form the stimulus input applied to the
system. The system produces a temperature value which is fed back to be compared
with the set point once more. Then, the cycle repeats until the error is zero, meaning
that the output is at the desired set point.
Proportional Gain Effect
The proportional gain is fast acting, and it does not change with time when the error
is constant. The proportional gain alone brings the system close to the desired
value, because the proportional control limits the amount of fluctuation of the
temperature around the set point value as the controller stabilizes. However, if the
proportional control is used alone after the temperature has stabilized, then the
actual temperature approaches the set point, but never quite reaches it. Too much
proportional gain causes the system to oscillate, going above and below the set point.