Temperature Controllers

Many applications need the temperature of a material to be controlled to give required results. This control can be carried out manually, by taking a temperature reading and turning heaters on or off , or more effectively, automatically with temperature controllers. Temperature controllers have evolved greatly over the past 20 years from a basic mercury or bimetallic switch through to today’s micro-processor controllers with l.c.d. / l.e.d. displays with P.I.D. and fuzzy logic.

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Control Functions

Did you know?

Most industrial temperature controllers are made around a DIN (Deutsches institute fur Normung) sizing system :-

DIN = 192mm x 192mm
1/4 DIN = 96mm x 96mm
1/32 DIN = 24mm x 48mm

One of the most basic temperature control functions is an open loop, 2 state control system. In this system a controller would be set on a time cycle to turn a heater on & off for a set period of time.

Eventually this system will get to a point where the cooling effect will match the heating period. The system will hold an approximate fluctuating temperature, it is pot luck as to what temperature this will cycle around.

To remove this unpredictability temperature controllers use a closed loop system, which incorporates a temperature sensor, (either a thermocouple, Platinum resistance sensor or thermistor) to give an electrical representation of the temperature to provide feedback into the controller and give it a point of reference to control to.

On/Off Control

An on/off controller only changes the state of the control output when the temperature sensor (Process Variable) crosses the required temperature (Set-point). The control output only has two states - fully on or fully off.

In a heating application (reverse acting control), when the process variable (PV) is below the setpoint (SP) then the control state or heater will be on, and when the PV is higher than the SP then the control state will be off. As the temperature has to cross the setpoint before the heaters turn on and off, the temperature will continually cycle. The difference between maximum and minimum temperatures and the period of the cycling depends mainly on the process characteristics and response time.

PID Control

Did you know?

Whilst most controllers use P.I.D. references as standard other similar terms can be used
Proportional Band is also known as Gain
Integral is also known as Reset
Derivative is also known as Rate

PID control incorporates three seperate parameters; Proportional, Integral and Derivative values. A PID controller controls by calculating the difference between the process variable and the setpoint and then adjusting the output accordingly without necessarily crossing the setpoint.

The Proportional value determines the area around which the heat input is approximately in balance with the process heat demand. It is rare that the heat input required to maintain the setpoint is 50% of the maximum available, therefore, this band will give a stable temperature either above or below the setpoint.

The Integral introduces an offset to this proportional band to bring this into line with the required setpoint.

The Derivative monitors how quickly the system responds to a variation in the system and controls how quickly the integral adjusts the offset of this proportional band.

Outputs

Controllers can be fitted with a variety of outputs, depending on application speeds and power ratings. Control relays can be used for slower processes, and would normally operate a second power relay or contactor for heater switching. Solid state drives are more popular as they are more able to handle high speed switching and have an increased reliability due to the absence of moving parts.

These will normally operate an external SSR (Solid State Relay) for power handling. The other common option is a 4-20mA control output. Whilst both the relay and SSR are variations of on/off signals, all be it with PID control, the 4-20mA enables the controller to provide a proportional output which is ideal for operating control valves and variable systems.

Calibration of temperature controllers

Temperature controllers can either be calibrated, as a system with temperature probe, between -50 & +255°C, in our UKAS laboratory, which is regularly audited by UKAS or we can offer an on site, signal injection calibration using equipment which is certified to UKAS or NPL (National Physical Laboratory) standards.

Temperature Controller Manufacturers

Charnwood Instruments have expertise in the supply, commissioning and service of most manufacturers temperature controllers and work in particular partnership with Invensys Eurotherm, West Controls and Cal Controls to provide the correct solution for your control requirements.