Low Temperature Topics | Temperature Control

Time: 2020.03.10Source: View: 1668

A complete temperature control system usually includes a temperature controller, a heater, a temperature sensor, cables, etc. Among them, the basic function of the temperature controller is to compare the actual temperature with the set value and output a signal to control the heater to maintain the set value temperature.
When selecting a controller, the following two factors should be considered:
1. The operating temperature range of the instrument
2. The type of input sensor (thermocouple, PT100, diode thermometer, thin film sensor, etc.)

PID algorithm

Today's closed-loop automatic control technology is based on the concept of feedback to reduce uncertainty. Feedback includes measurement, comparison, and execution. The PID algorithm is the most commonly used. After making correct measurements and comparisons, the system can be better corrected through PID adjustment.

PID's three letters mean the following:

1) P: Proportion.

Typically, the temperature range is expressed in % or degrees of full scale, called proportion or proportional band. Within this range, the controller acts proportionally, that is, the controller's output is proportional to the input error signal. The wider the proportional band, the larger the area around the set value where proportional action occurs. Sometimes it is also expressed in terms of gain, which is the inverse of the proportional band.

2) I: Integration.

Integral, also known as reset, adjusts the proportional bandwidth according to the set point to compensate for the offset (fixed deviation) from the set point, that is, it adjusts the controlled temperature to the set point after the system stabilizes.

3) D: Differentiation.

Differential, also known as rate, senses the rate at which the system temperature rises or falls and automatically adjusts the proportional band to minimize undershoot or overshoot.

How to adjust PID value

After setting the required temperature value according to the target, set the PID parameters. Usually, set the P value first, I input 0 (first find the appropriate P and then adjust I); D input 0 (generally not adjusted).

Observe the heating curve, the following situations may occur:

(a)P value is too large

If the P value is too large, the temperature will keep oscillating and cannot stabilize. Since the P value obtained near the set value may not be accurate, you should first turn off the heater, lower the temperature away from the set value, reduce the P value, and then reheat and observe the temperature rise curve.

（b）P value is too small

The P value is too small and the set temperature cannot be reached. Use the method in (a) to increase the P value and then observe the heating curve.

(c)P value is appropriate

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Through PID adjustment, the temperature accuracy can reach the mK level.
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Low Temperature Topics | Temperature Control