A temperature controller is an instrument that controls temperatures, often without extensive operator involvement. In a temperature controller system, the controller accepts a temperature sensor as an input, such as an RTD or thermocouple, and compares the actual temperature with the required control temperature or setpoint. The output is then provided to a control element.
For example, a temperature sensor sends an input to the controller, whose output is linked to a control element, such as a fan or heater. The selection of a suitable controller involves analyzing the whole temperature control system, as well as the controller.
What Are the Different Types of Controllers, and How Do They Work?
There are three types of controllers: PID, proportional, and on-off. The type of controller to be used to control the process depends upon the system being controlled.
The simplest form of temperature control device is an on-off controller. There is no middle state in the output from the device - it is either on or off. When the temperature crosses the setpoint, the on/off controller switches on the output. The output is on for heating control when the temperature is below the setpoint, and off above the setpoint.
As the temperature crosses the setpoint to change the output state, the process temperature cycles continually, going from below the setpoint to above, and back below. A "hysteresis" or an on-off differential is added to the controller when cycling is rapid and can damage the valves and contactors.
The on/off differential requires the temperature to exceed the setpoint by a specific amount before the output is turned off or on. The differential also prevents the output from “chattering” or making quick continual switches when the cycling below and above the setpoint takes place very quickly.
The on/off control is often used when precise controlling is not a requirement, in systems that cannot handle energy being frequently turned on or off, in systems where the mass is so large that temperature changes very slowly, or for temperature alarms.
A limit controller is a special type of an alarm on/off controller, where a latching relay, which is reset manually, shuts down a process if a specific temperature is reached.
The cycling effect associated with on/off control can be eliminated using proportional controls. In a proportional controller, the average power provided to the heater is reduced when the temperature approaches the setpoint. This slows down the heater so that it won't overshoot the setpoint, but will approach the setpoint and maintain a stable temperature.
This proportioning action can be achieved by turning the output off and on for short durations. The “time proportioning” varies the ratio of the “on” time to “off” so that the temperature is controlled. The proportioning action takes place around the setpoint temperature within a "proportional band."
Outside of this band, the controller works like an on/off unit with the output fully off (above the band) or fully on (below the band). Within the band, the output is turned off in the ratio of the measurement differences from the setpoint.
At the setpoint (the proportional bands setpoint) the output on:off ratio is 1:1, indicating that the off-time and on-time are equal. When the temperature is further from the setpoint, the off- and on-times vary in proportion to the temperature difference. The output is on for a longer time when the temperature is below the setpoint, and is off for a longer period when the temperature is too high.
The third type of controller provides proportional control with integral and derivative control, or PID . The PID controller combines proportional control with two additional adjustments, which enable the unit to automatically compensate according to system changes.
The adjustments, integral and derivative, are expressed in time-based units, and are referred to according to their reciprocals: RESET and RATE. Using trial and error, the proportional, integral, and derivative terms are separately “tuned” or adjusted to a specific system.
The PID controller provides the most stable and accurate control, and is ideal for systems with a relatively small mass and for those systems that rapidly react to energy changes in the process.
The PID controller can be used for systems that have frequent load changes and where the controller has to automatically compensate due to frequent changes in setpoint, the mass to be controlled, or the availability of the energy. Known as autotune controllers, OMEGA presents several types of controllers that can automatically tune themselves.
The instrument panel is cut to accommodate the temperature controller as they are generally mounted inside the panel. Most controllers are designed to standard DIN sizes, because temperature controllers need to be interchangeable. The common DIN sizes are illustrated below:
Panel Cutouts: Standard industrial sizes
Choose a Controller for Your Application
These are the most basic type of controllers, which feature on-off control action designed to deliver the functionality of general purpose PID controllers, but at a price suitable for on/off applications.
Autotune PID Controllers
PID controllers offer extremely tight control, but the PID algorithm has to be adjusted. Autotune controllers are able to provide that function.
Generally, each control loop contains a single input and at least a single output. OMEGA provides a wide range of multiloop controllers that can handle more than a single control loop. The CN1507 supplied by OMEGA can handle up to 7 control loops.
Safety Limit Controllers
This controller is an off-off controller with a latching output. When the output changes state a manual reset should be done to change it back. Generally, safety limit controllers are used as redundant controllers so that a process can be shut down when undesirable limits are reached.
This adjustable temperature switch is suitable for those applications where an economical solution to temperature control is required. Temperature switches are less complex and can be set up more easily compared to more advanced electronic controls.
How to Choose a Process Controller?
The controller represents a part of the entire control system, and the entire system should be inspected when choosing the right controller. When choosing a controller, the following items should be taken into account:
- Type of output needed (SSR, electromechanical relay, SSR, analog output)
- Type of input sensor (RTD, thermocouple, RTD) and temperature range
- Control algorithm required (on/off, PID, proportional)
- Type and number of outputs (cool, heat, limit, alarm)
This information has been sourced, reviewed and adapted from materials provided by OMEGA Engineering Ltd.
For more information on this source, please visit OMEGA Engineering Ltd.