A temperature controller is a device used to hold a desired temperature at a specified value. The simplest example of a temperature controller is a common thermostat found in homes. For instance, a hot water heater uses a thermostat to control the temperature of the water and maintain it at a certain commanded temperature. Temperature controllers are also used in ovens. When a temperature is set for an oven, a controller monitors the actual temperature inside of the oven. If it falls below the set temperature, it sends a signal to activate the heater to raise the temperature back to the setpoint. Thermostats are also used in refrigerators. So if the temperature gets too high, a controller initiates an action to bring the temperature down.

Types of Controllers

Temperature controllers come in many different styles with a vast array of features and capabilities. There are also plenty of ways to categorize controllers according to their functional capabilities. In general, temperature controllers are either single loop or multi-loop. Single loop controllers have one input and one or more outputs to control a thermal system. On the other hand, multi-loop controllers have multiple inputs and outputs, and are capable of controlling several loops in a process. More control loops permit controlling more process system functions.

Common Controller Applications

Temperature controllers in industry work much the same way they do in common household applications. A basic temperature controller provides control of industrial or laboratory heating and cooling processes. In a typical application, sensors measure the actual temperature. This sensed temperature is constantly compared to a user setpoint. When the actual temperature deviates from the setpoint, the controller generates an output signal to activate other temperature regulating devices such as heating elements or refrigeration components to bring the temperature back to the setpoint.

Common Uses in Industry

Temperature controllers are used in a wide variety of industries to manage manufacturing processes or operations. Some common uses for temperature controllers in industry include plastic extrusion and injection molding machines, thermo-forming machines, packaging machines, food processing, food storage, and blood banks.

Digital Temperature Controller Circuit

A Digital temperature controller circuit is a precise temperature controller in medical, industrial and home applications. This system is better than analogue/thermostat system, which has poor accuracy. For example, it can use for temperature control of an incubator where maintaining a precise temperature is very important.

Features

Controllers can also have a number of additional optional features. One of these is communication capability. A communication link lets the controller communicate with a PLC or a computer. This allows data exchange between the controller and the host. An example of typical data exchange would be the host computer or PLC reading the process value.

A second option is a remote setpoint. This feature allows a remote device, such as a PLC or computer, to change the controller setpoint. However, unlike the communication capability mentioned above, the remote setpoint input uses a linear analog input signal that is proportional to the setpoint value. This gives an operator added flexibility by being able to change the setpoint from a remote location. A typical signal might be 4–20mA or 0–10VDC.

Another common feature supplied with controllers is the ability to configure them using special software on a PC connected via a communications link. This allows quick and easy configuration of the controller and also the option to save configurations for future use.

Another common feature is a digital input. The digital input can work together with a remote setpoint to select the local or remote setpoint for the controller. It can also be used to select between setpoint 1 and setpoint 2 as programmed in the controller. Digital inputs can also remotely reset a limit device if it has gone into the limit condition.

Other optional features include a transmitter power supply used to power a 4–20mA sensor. This power supply is used to supply 24VDC power at a maximum of 40mA.

In some applications, a dual-color display can also be a desirable feature, making it easy to identify different controller states. Some products also have displays that can change from red to green or vice versa depending on preprogrammed conditions, such as indicating an alarm condition. In this case, no alarm might be shown by a green display, but if an alarm is present the display would turn red.

 How it Works

 All controllers, from the basic to the most complex, work pretty much the same way. Controllers control, or hold, some variable or parameter at a set value. There are two variables required by the controller; actual input signal and desired setpoint value. The input signal is also known as the process value. The input to the controller is sampled many times per second, depending on the controller.

This input, or process, value is then compared with the setpoint value. If the actual value doesn't match the setpoint, the controller generates an output signal change based on the difference between the setpoint and the process value and whether or not the process value is approaching the setpoint or deviating farther from the setpoint. This output signal then initiates some type of response to correct the actual value so that it matches the setpoint. Usually, the control algorithm updates the output power value which is then applied to the output.

The control action taken depends on the type of controller. For instance, if the controller is an ON/OFF control, the controller decides if the output needs to be turned on, turned off, or left in its present state.

ON/OFF control is one of the simplest types of control to implement. It works by setting up a hysteresis band. For instance, a temperature controller may be set to control the temperature inside of a room. If the setpoint is 68° and the actual temperature falls to 67°, an error signal would show a –1° difference. The controller would then send a signal to increase the applied heat to raise the temperature back to the setpoint of 68°. Once the temperature reaches 68°, the heater shuts off. For a temperature between 68° and 67°, the controller takes no action and the heater remains off. However, once the temperature reaches 67°, the heater will again kick in.

Unlike ON/OFF control, PID control determines the exact output value required to maintain the desired temperature. The output power can range from 0 to 100%. When an analog output type is used, the output drive is proportional to the output power value. However, if the output is a binary output type such as a relay, SSR driver, or triac, then the output must be time proportioned to obtain an analog representation.

A time proportioned system uses a cycle time to proportion the output value. If the cycle time is set to 8 seconds, a system calling for 50% power will have the output on for 4 seconds and off for 4 seconds. As long as the power value doesn't change, the time values wouldn't change. Over time, the power is averaged to the 50% commanded value, half on and half off. If the output power needed to be 25%, then for the same 8 second cycle time, the output would be on for 2 seconds and off for 6 seconds.

                                                   Output time proportioning example

All things being equal, a shorter cycle time is desirable because the controller can more quickly react and change the state of the output for given changes on the process. Due to the mechanics of a relay, a shorter cycle time can shorten the life of a relay, and is not recommend to be less than 8 seconds. For solid state switching devices like an SSR driver or triac, faster switching times are better. Longer switching times, no matter what output type, allow for more oscillation in the process value. The general rule is that, ONLY if the process will allow it, when a relay output is used, a longer cycle time is desired.

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This guide has been written in reference to the blog published by instrumart.com

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