Temperature controllers are used for controlling and maintaining the temperature of any equipment, room, or closed space. Temperature controllers are of two principal types; Open loop and closed loop. Open-loop controllers generate the control output independent of the input signal. In contrast, closed-loop controllers generate the control output based on the delta between the sensor input signal and the controller setpoint.

Temperature controller working principal:

The temperature controller has a sensor like RTD or Thermocouple that senses the temperature. The temperature controller then compares setpoint temperature and sensed temperature. Suppose there is a deviation in setpoint temperature and sensed temperature. In that case, the controller sends the single to bring up or bring down the temperature in cases of heating or cooling, respectively.

Types of controllers:

There are various types of controllers available in the market.

General-purpose temperature controller:

A general-purpose temperature controller in various DIN sizes has multiple outputs and programmable output functions. These controllers can also perform PID control for excellent general control situations. They are traditionally placed in the front panel. Also, there is a display for easy operator accessibility. 

Modern digital temperature controllers can automatically calculate PID parameters, resulting in optimum thermal system performance using built-in auto-tuning algorithms. As a result, these are widely used for controlling most typical processes in the industry.

Limit:

The limit controller cannot control the temperature on its own. However, it provides safety limit control against high or low temperatures, and these devices are used in process control applications. Users can define upper or lower limits by entering setpoint values. If the pre-set temperature limit is exceeded, the output switches OFF to ensure that the system is safe. These are commonly used in ovens, furnaces, combustion plants, steam systems, and food heating equipment.

Profile:

Profiling controllers, also known as ramp-soak controllers, have several programmed setpoints and the time to sit at each setpoint. Programming the rate of change of temperature is called a ramp. Soak is the time to hold the temperature at the given setpoint. One ramp or one soak makes one segment. A profile controller can have several segments to allow complex temperature profiles. These profiles, also known as recipes, are stored in the profile controller for later use. Profile controllers execute ramp-and-soak profiles, such as temperature changes and hold and soak/cycle duration, without operator supervision. Profile controllers are used in heat treating, annealing, environmental chambers, and complex process furnaces.

Multi-loop:

Multi-loop controllers control processes that need more than one control loop within a single system. For example, several heating systems are designed to have multiple heating zones within a single piece of equipment. These zones need to be controlled using independent sensors and control outputs but depend on each other. In such scenarios, a multi-loop temperature controller is used to control these zones independently while maintaining coherence between them. In addition, multi-loop controllers are used for controlling large furnaces, multi-step heating processes, post-weld heat treatment.

Control Methods:

There are various control methods that a temperature controller can use.

ON/OFF Control Action:

On-Off control is the most straightforward feedback control. The control signal is either ON or OFF. If the process value is lower than the set point, the output is turned ON, and power is supplied to the heater. If the process value is higher than the setpoint, the output is turned OFF, and the system will shut off power to the heater.

Advantages:

• Easy to use
• Cost-effective

Disadvantages:

• Oscillatory control
• Precise control is challenging to achieve

PID Control:

P action (or proportional control action) controls the output variable proportional to the deviation between the process value and setpoint.

I action (or necessary action) increases or decreases the controlled variable according to the size and duration of the deviation.

D action (or derivative action) provides a controlled variable in response to sudden changes in the process value because of factors such as an external disturbance. Control will quickly return to the original status. 

PID control is a combination of proportional, integral, and derivative control actiFor example, the. The temperature is controlled smoothly here by proportional control action without hunting, automatic offset change is made by integral control action, and quick response to an external disturbance is made possible by derivative control action.

Advantages:

• Smooth control
• Very Precise control can be achieved

Disadvantages:

• Expensive
• Appropriate power control devices are required.

Applications:

Temperature controllers are used in various industries to manage manufacturing processes or operations. Some examples are:

• The temperature controllers are used in ovens and heat-treating applications within furnaces, ceramic kilns, boilers, and heat exchangers.

• The temperature controllers are used In the packaging industry as machinery equipped with seal bars, glue applicators, hot melt functions, shrink wrap tunnels, or label applicators. They must operate at designated temperatures and process time lengths.

• The temperature control is used in portable chillers, hoppers and dryers, and moulding and extruding equipment in the plastic industry.

• The temperature controllers are used in the healthcare industry for laboratory and test equipment, autoclaves, incubators, refrigeration equipment, and crystallization of growing chambers and test chambers.

• The temperature controllers are used in brewing, blending, sterilization, and cooking and baking ovens in the food processing industry.