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Introduction of temperature controller
Jan 20, 2020

How to control process temperature accurately and reliably?

In order to control the process temperature accurately without a large number of operators' intervention, the temperature control system relies on the controller to receive the signals from temperature sensors such as thermocouples or RTDS as input. It compares the actual temperature with the required control temperature or set value, and then provides the output to control the heating element.

Controller is a part of the whole control system. When choosing the right controller, the whole system should be analyzed. The following should be considered when selecting the controller:

1. Type of input sensor (thermocouple, RTD) and temperature range

2. Type of output required (electromechanical relay, SSR, analog output)

3 required control algorithm (on / off, proportion, PID)

4 number and type of output (heating, cooling, alarm, limit)

What are the types of controllers and how they work?

There are three basic types of controllers: on-off, proportional, and PID. Depending on the system to be controlled, the operator can use one or another to control the process.

On / off

On off controller is the simplest temperature control device. The output of the device is not on or off, and there is no intermediate state. The on-off 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 is changed by the temperature passing the set value, the process temperature will continue to cycle and fluctuate back and forth at the set value. When this cycle occurs quickly, in order to prevent the contactor and valve from being damaged, a on-off difference or "lag" can be added to the controller operation. This difference requires that the temperature exceeds the set value by a certain amount before the output is cut off or turned on again. The on-off difference prevents the output from "oscillating" (i.e. fast engagement and frequent switching when the temperature is cycled very quickly up and down the set value).

On-off control is usually used in the situation where precise control is not needed, in the system where energy can not be switched on and off frequently, in the situation where the temperature changes very slowly due to the high quality of the system, or as a temperature alarm.

A special type of on-off control used as an alarm is a limit controller. This controller uses a lock-in relay that must be manually reset to close the process when a specific temperature is reached.


The proportional control is designed to eliminate the cycles associated with on-off control. The proportional controller reduces the average power supplied to the heater as the temperature approaches the set point. This has the effect of slowing down the heater heating so that it does not exceed the set value, but only approaches the set value and maintains a stable temperature. This proportional effect can be achieved by switching on and off the output at short intervals. This "time ratio" is used to change the ratio of the "on" time to the "off" time to control the temperature. The proportional action occurs in a "proportional band" near the set temperature. Beyond this proportional band, the controller functions in the same way as the on-off controller, and the output is full on (below the proportional band) or full off (above the proportional band). However, in the proportional band, the on and off of the output are proportional to the difference between the measured value and the set value. At the set value (the midpoint of the proportional band), the output on-off ratio is 1:1, that is, the on time and the off time are equal. If the temperature is away from the set value, the on time and off time will change in proportion to the temperature difference. If the temperature is lower than the set value, the output on time is longer; if the temperature is too high, the output off time is longer.

The scale band is usually expressed as a percentage or degree of full scale. It can also be called gain, which is the reciprocal of the proportional band. Note that in time proportional control, the heater operates at maximum power, but cycles on and off, so the average time is variable. In most devices, the cycle time and / or proportional band can be adjusted so that the controller can better match a specific process.

In addition to electromechanical and solid-state relay output, the proportional controller can also provide proportional analog output, such as 4-20 mA or 0-5 VDC. In these outputs, the actual output level is changed, not the on-off time, just like the relay output controller.

One of the advantages of proportional control is its simple operation. It requires only a small adjustment (manual reset) by the operator at the first start-up or when the process conditions change significantly to bring the temperature to the set value.


The third controller provides proportional and integral and differential control, or PID. This controller combines two additional adjustments in the proportional control to help the device automatically compensate for changes in the system. These two adjustments (integral and differential) are expressed in time-based units and are also referred to as reciprocal "reset" and "rate" respectively.

Proportion, integral and differential terms must be adjusted or "adjusted" specifically for a specific system using "trial and error method". It provides the most accurate and stable control among the three types of controllers, and is most suitable for systems with relatively small mass and rapid response to changes in the energy added to the process. This kind of controller is recommended to be used in the system with frequent load changes and in the system which needs automatic compensation of the controller due to frequent changes of setting value, available energy or quality to be controlled.

What are the effects of rate and reset, and how do they work?

Rate and reset are the methods used by the controller to compensate for temperature excursion and drift. When using the proportional controller, it is very rare to maintain the heat input of 50% of the set temperature; the temperature will rise or fall from the set value until a stable temperature is obtained. The difference between the stable temperature and the set value is called the offset. The offset can be compensated manually or automatically. Using manual reset, the user will move the proportional band to stabilize the process at the set temperature. Automatic reset (also known as integral) will calculate the time integral of the deviation signal, which is added with the deviation signal to move the proportional band. This automatically increases or decreases the output power to return the process temperature to the set value.

The rate or differential function provides the controller with the ability to move the proportional band to compensate for rapidly changing temperatures. The amount of movement is proportional to the rate of temperature change.

PID or three mode controller combines proportional, integral (reset) and differential (rate) functions, which usually need to be used when controlling complex processes. The controller can also have two proportional outputs, one for heating and the other for cooling. This type of controller is required for processes that may require heating to start but then generate excess heat at some time during operation.

What are the different output types of controllers?

The output of the controller can be one of several forms. The most common forms are time scale and simulation scale. The time proportional output power to the load is a percentage of the fixed cycle time. For example, for a 10 second cycle time, if the controller output is set to 60%, the relay will be energized (closed, powered) for 6 seconds, then de energized (open, non powered) for 4 seconds. There are three different forms of time proportional output: electromechanical relay, three terminal bidirectional thyristor switch or AC solid-state relay or DC voltage pulse (driving external solid-state relay). Electromechanical relay is usually the most economical one. It is generally used in systems with cycle time greater than 10 seconds and relatively small load.

The purpose of selecting AC solid state relays or DC voltage pulses is to improve reliability because they do not contain any moving parts. It is recommended for processes requiring short cycle times, which require an additional relay located outside the controller to handle the typical loads required by the heating elements. These external solid-state relays are usually used with the AC control signal of the AC solid-state relay output controller, or with the DC control signal of the DC voltage pulse output controller.

Analog proportional output is usually analog voltage (0-5 VDC) or current (4-20 mA). The output level of this output is also set by the controller; if the output is set to 60%, the output level will be 60% of 5V, that is 3V. For 4 to 20 mA output (16 Ma range), 60% is equal to (0.6 x 16) + 4, i.e. 13.6 ma. This kind of controller is often used with proportional valve or power controller.

What factors should be considered when selecting controllers for a specific application?

When choosing the controller, the main factors include the required control accuracy and the difficulty of process control. In order to facilitate the setting and reduce the initial cost as much as possible, the simplest controller that can produce the desired results should be selected.

For simple processes with proportional (but not too small) heaters and no fast circulation, on-off controllers can be used. For systems subject to circulation, or for systems where the heater does not match (too large or too small), a proportional controller is required.