Temperature controllers are used in situations where temperatures need to be kept within a specified range or at a specific setpoint regardless of the environmental conditions.
This can involve either heating, cooling or both.
The accurate control of temperature is greatly important across a wide variety of applications. 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 others. Temperature controllers are also useful in commercial and
even residential settings. In fact, one of the simplest types of temperature controllers is the thermostat used to keep your house warm in the winter and cool in the summer.
Thermostats work in the same manner as all other temperature controllers. A temperature sensor, often some type of thermocouple or RTD (resistance temperature detector), senses the
actual temperature and provides input to the controller/thermostat. When the temperature is found to deviate from the setpoint, the controller/thermostat generates an output signal
to activate other temperature regulating devices such as heating elements (or furnace in the case of a thermostat) or refrigeration components (or air conditioning unit in the case
of a thermostat) to bring the temperature back to the setpoint.
By carefully monitoring the process temperature and taking corrective action when it deviates from the setpoint, temperature controllers greatly assist in reducing variability,
increasing efficiency, and ensuring safety.
Temperature control system components
Temperature controllers are arranged into control systems (also known as control loops) that consist of the controller, any associated sensors, a power supply, the final control element,
as well as any necessary load handling devices.
As we know, temperature controllers seek to maintain the temperature at a preset point.
Sensors provide the input signal to the controller. Thermocouples and RTDs are the most common temperature sensors. Controllers use the output from these sensors and compare it to
the setpoint.
The final control element refers to the device that acts upon orders from the controller. It can be a heater that is activated when the sensor finds a temperature lower than the
setpoint or refrigeration equipment activated when the temperature is higher than the setpoint.
Temperature controllers, certain sensors, and final control elements require power to operate. A power supply is an integral element to control loops
Control loops regularly feature additional instruments. Transmitters or signal conditioners are often used to isolate, filter, amplify, or convert a sensor input signal when conditions
dictate it. Control loops also frequently include data acquisition devices for archiving information related to the process.
Load handling devices are often needed when the final control element requires more power to operate than can be supplied by the controller.
Types of Control Action
Depending upon the unit, temperature controllers are capable of providing multiple types of control which are suited to the requirements of different applications.
On-Off Control
On-off control, also called hysteresis control, is the simplest type of control. As expected, on-off controllers switch abruptly between two states with no middle state. They are for
use with equipment that accepts binary input, for example a furnace that is either completely on or completely off.
On-off controllers only switch output when the set point has been crossed. In the case of heating control, the controller switches on when below the set point and off when above
the set point. To prevent rapid cycling of the system which can cause damage, hysteresis or on-off differential, is added to the controller operations. The differential prevents
cycling by exceeding the setpoint by a small amount before the controller switches on or off.
On-off controllers are often used in applications that don’t require precise control, in systems which cannot handle having the energy turned on and off frequently, where the mass
of the system is so great that temperatures change extremely slowly, or for temperature alarms.
PID Control
PID control uses three control terms; proportional (P), integral (I), and derivative (D) to help the controller’s algorithms provide a more accurate response to deviations from
the set point.
When a controller receives input that a process variable has varied from the set point, instructions are sent to the final control element for correction. For example, a controller
receives a signal from a thermocouple that a process temperature is too low prompting the controller to turn on a heater to bring it back up to temperature.
Simple on-off control often leads the final control element to overshoot the set point, especially when the original deviation was small. Repeatedly overshooting the set point
causes the output to oscillate around the setpoint in either a constant, growing, or decaying sinusoid. The system is unstable if the amplitude of the oscillations continuously
increase with time.
PID controllers use the algorithm derived from their three control terms to maintain system stability by limiting overshoot and resulting oscillation. The proportional variable
controls the rate of correction so that it is proportional to the error. The integral and derivative variables are time-based and help the controller automatically compensate
to changes in the system. The derivative variable considers the rate at which the error is increasing or decreasing while the integral variable uses knowledge of accumulated
errors to the length of time the process is not at the set point. This information is used to correct the proportional value.
PID controllers are generally considered the most useful type of controller. They are widely used in industrial settings. Though each of the variables must be tuned to a particular
system, PID controllers provide very accurate and stable control.
In order to make PID controllers even more responsive to real-world situations, many manufacturers have incorporated fuzzy logic (or fuzzy control)
into the instruments. Fuzzy logic is a mathematical system that attempts to emulate human reasoning. Rather than the binary logic of standard controllers, fuzzy logic introduces
continuous variables which provide an effective means of capturing the approximate, inexact nature of the real world.
This ability enables controllers with fuzzy logic to make quick, subtle changes that significantly improves response to fast-changing variables independent of the programming done
by the operator. For example, as heaters, valves and other final control elements age, they show signs of wear and no longer respond in the same way as they did when new. Fuzzy
logic recognizes this and automatically compensates.
Profile Control:
Profile Control refers to controlling a changing temperature against time. Users input the desired time and temperature profile with the help of extensive instruction set like jump,
loop, loop with count apart from ramp and soak control.
Profile control is especially useful for cycling applications which require multiple temperature profiles as well as specific on and off periods.
Limit Control:
Limit control involves an independent switch which will shut down the system if the temperature crosses a preset threshold. Limit controllers are for use in processes where for
safety or quality issues, the temperature must be kept within specified tolerance levels.
Limit controllers are designed to work in conjunction with another controller and require a manual rest to acknowledge an error has occurred.
Things to consider when selecting a temperature controller:
- What type of input is provided by the sensors?
- What type of control is needed?
- What power supply is available to power the controller?
- What amperage and voltage is required for the load?
- What type and number of outputs are needed to control load or load handling devices?
- What size controller is required?
- Are there any mounting requirements for the controller?
- What are the environmental conditions the controller will be exposed to?
- Are any auxiliary functions required such as communications, remote setpoint, retransmission, etc.?
- What temperature range is required?
If you have any questions regarding process controllers, please don't hesitate to speak with one of our engineers by e-mailing us at sales@instrumart.com or calling 1-800-884-4967.
