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Operation of temperature controller
Jan 27, 2020

There are three basic types of controllers: switch controller, proportional controller and PID controller. Depending on the system to be controlled, the operator will be able to control the process using one type or another of controller.

Switch controller

The switch controller is the simplest temperature control device. The output of the device has only two states of "on" and "off", without any intermediate state. The switch controller switches the output only when the temperature exceeds the set value. For heating control, the output is "on" when the temperature is lower than the set value, and "off" when the temperature is higher than the set value.

Since the output state will be changed only when the temperature exceeds the set value, the process temperature will continue to cycle from below the set value to above the set value, and then return to below the set value. If this cycle occurs quickly, a switch differential or "hysteresis" is added to the controller during operation to prevent damage to the contactors and valves. This difference needs to be turned off or on again after the temperature exceeds the set value by a certain number of degrees. If the cycle is very fast up and down the set value, the switch differential prevents the output from switching "repeatedly" or quickly.

Switch control is usually used without precise control, in systems that cannot handle frequent energy opening and closing, and when the system is very large and the temperature changes are extremely slow, or for temperature alarm.

A special type of switch control used for alarms is the limit controller. The controller uses a latching relay that must be manually reset and is used to end a process when a specific temperature is reached.

Proportional controller

Proportional control is designed to eliminate cycles associated with switch control. The proportional controller reduces the average power supply to the heater as the temperature approaches the set point. This slows down heater heating so that the temperature does not exceed the set point, but will approach the set point and maintain a stable temperature. This proportional control operation can be achieved by opening and closing the output in a short time interval. This time proportional control controls the temperature by changing the ratio of the on time to the off time. The proportional control operation occurs in the "proportional band range" near the set point temperature. Beyond this proportional band, the controller is used as a switch controller whose output state is full open (below the proportional band) or full close (above the proportional band). However, in this proportional band range, the output status is determined to be on or off according to the ratio of the measurement difference from the set value. At the set value (the midpoint of the scale band), the output on / off ratio is 1:1; that is to say, the on time and the off time are equal. If the temperature is far from the set value, the opening time and closing time will be different depending on the proportion of temperature difference. If the temperature is lower than the set value, the opening time is longer; if the temperature is too high, the closing time is longer.

The scale band is usually expressed as a percentage or degree of full scale. Also known as gain, gain is the reciprocal of the proportional band. Please note that during time proportional control, the heater applies full power, but cycles between on and off,

So the average time is different. In most proportional controllers, the cycle time and / or the proportional band are adjustable so that the controller can better match the specific process.

In addition to electromechanical and solid state relay outputs, the proportional controller can also be used for proportional analog outputs, such as 4 to 20 mA or 0 to 5 VDC. With these outputs, the actual output levels are different, not just the on and off times, as with relay output controllers.

One of the advantages of proportional control is that it is easy to operate. It may require a small amount of adjustment (manual reset) by the operator to set the set point temperature at initial start-up, or to adjust when process conditions change significantly.

Proportional controllers are also needed for systems prone to large-scale temperature cycling. It is necessary to determine whether simple proportional control or PID proportional control is needed according to the required process and accuracy.

The process with long lag time and large maximum rise rate (such as heat exchanger) requires a large scale of proportional band to eliminate the oscillation. A large scale band may cause a large offset with the load. To eliminate these offsets, use automatic reset (integration). Differential (rate) operation can be used in the process of long time delay to accelerate the recovery speed after process interference.

PID controller

The third controller (PID controller) can provide integral and differential control for the proportional controller. The controller combines the proportional control with the other two adjustments to help the device automatically compensate for changes in the system. These adjustments (integral and differential) are expressed in time-based units; they can also be expressed by their reciprocal (reset and rate, respectively).

The proportional, integral and differential conditions must be adjusted or "set" individually for a particular system using the trial error method. Among the three types of controllers, PID controller can provide the most accurate and stable control, and is most suitable for systems with relatively small mass. These systems can respond to the changes of energy added to the process quickly. In the system where the load changes frequently and the controller is expected to compensate automatically due to the frequent changes of setting value, energy provided or quality to be controlled, PID controller is recommended.

There are other features to consider when selecting the controller. These features include: automatic setting or self setting, in which case the instrument will automatically calculate the proportional band, ratio value and reset value suitable for precise control; serial communication, in which case the controller can "talk" with the host computer for data storage, analysis and setting; alarm, alarm can be locked (manual reset) or non locked (automatic reset) Alarm can be set to trigger when the process temperature is high or low, or when the deviation from the set value is detected; timer / event indicator can be used to mark the elapsed time or the end / start of the event. In addition, relays or thyristor triggered output controllers can be used with external switches such as SSR solid state relays or magnetic contactors to switch large loads up to 75 A.