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An Introduction to Temperature Controls

Classic ThermostatTemperature control in manufacturing is a quintessential part of proper product formation. If the temperature slips above or below the ideal range needed for a particular stage in a manufacturing process, the results can be harmful—improperly adhered coatings, a weakened base material, or an overall compromised component—so it becomes increasingly important that the manufacturer not only determine the proper temperature for each stage, but also monitor the temperature inside the machine and receive appropriate feedback. 

Temperature controllers in manufacturing operations serve exactly this function: they ensure that a machine is running properly by gauging the temperature at different stages in the process and comparing the data to the programmed temperature specifications. As a result, manufacturers can quickly and easily discover temperature related machine malfunctions, and treat them as necessary. 

There are three general kinds of temperature controllers that are used to monitor temperature during manufacturing processes: on-off, proportional, and PID controls. 

On/Off Temperature Controls

An on/off temperature control is the least expensive of the control types, and also the most simple in terms of how it works. The control is either on or off—if the temperature drops below a certain point, the control signals to the machine to turn raise the temperature. Likewise, if a temperature goes above a certain point, the control is triggered to tell the machine to lower the temperature. A common example of on/off systems is a household thermostat. When the temperature drops below a certain point, the controller triggers the heater to raise the temperature back to the programmed value. With air-conditioning it works the other way: if the temperature rises past a certain point, the controller triggers the air-conditioner, dropping the temperature back to the programmed norm.

On/off controls are often used in processes where the temperature change is very slow, and precise control of temperature isn’t necessary.

Proportional Control

Unlike on/off controls, which only respond when a set limit is reached, proportional controls are designed to respond to temperature change before it slips out of the desired range. Essentially, proportional controls increase or decrease the power supply as the temperature reaches its upper or lower limit, or setpoint, which slows or speeds the heater and helps stabilize the temperature. 

The temperature range in which proportional controls either lessen or increase the power supply to slow or speed heating is known as the “proportional band.” If a temperature reaches the lower or upper setpoints, the control then functions as a full on/off control—the temperature is either turned fully on to increase the temperature, or fully off to drop the temperature. When the temperature is within the proportional band, and the power supply is decreased or increased, the heat is raised or lowered in relation to how far the temperature is from the setpoint. 

PID (proportional-integral-derivative) Control

This control combines proportional control with integral and derivative control (PID). Operating within a proportional band in the same way a proportional control does, a PID system has two added features that enhance overall temperature regulation. The proportional feature allows the control to react to the current circumstances and adjust accordingly. The integral value takes into consideration the sum of recent events (in other words, past proportional control rhythms) and the derivative value determines the appropriate reaction based on the rate at which past rhythms have been changing. Combined, the three use current data, past data, and the rate at which data is changing to set a case-specific algorithm to control temperature. By compensating for temperature error between the process variable and the setpoint, a steady temperature can be maintained.

Considerations

When deciding which kind of control is best for a specific process, there are several things to keep in mind. First of all, consider the kind of input sensor (thermocouple or RTD) and the temperature range the process calls for. Secondly, consider the form the ouput should be represented in: electromechanical relay, SSR, or analog output. Thirdly, decide what kind of control algorithm is necessary (on/off, proportional, PID). Lastly, consider the number and type of outputs needed for the application, such as heat, cool, alarm, and limit. Once these factors have been determined, it will be much easier to determine which type of temperature controller is suited to a specific application.

For more information, visit the University of Exeter's Feedback and Temperature Control page.

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