Temperature Controllers - an overview

From Bronze Age smelting to modern cryogenics, temperature control always has and always will play a huge role in fabrication and process control. These days it is hard to imagine a single commercial or consumer product that has not deployed accurate temperature control as some facet of its manufacturing process; with many products relying on temperature control to fractions of a degree for their manufacture.

The use of temperature controllers is widespread. In domestic applications we see them in simple on-off guises in room and radiator thermostats and refrigerators. In industry their use is prevalent in multiple vertical sectors across a great many applications. In the materials industry they are used for plastic moulding, metal smelting, ceramic firing and composite curing. In packaging they are deployed for sealing, forming and wrapping, while the food and beverage industries use them for cooking, chilling and processing. At the more accurate end of the scale the life sciences industry uses them for testing, fermenting, chilling, incubating and analysis while general manufacturing uses controllers for a huge range of purposes, including solder baths, heat treatment part forming and shrink-wrapping.

With such a widespread applications base, it is not surprising that users are spoilt for choice when it comes to suppliers, technologies and sensors types. Many of the world’s leading automation vendors, including ABB, Omron, Panasonic, Schneider Electric, West Instruments and RS, offer a variety of temperature controller types, most of which come in enough variants to allow users to almost tailor them to specific applications.

There are many controller types available, with the primary differentiator being their method of operation. At the top of the specification pyramid is open or closed loop. Open loop is the simpler approach, delivering a control signal with no reliance on an input signal to act as a basis for on-off decisions. Closed loop, on the other hand, uses an incoming signal to regulate its output according to a predetermined value set by the user or a higher-level controller.

Closed loop controllers can then be split into three sub varieties: on/off, proportional and PID. A simple room thermostat is a good example of an on/off controller: in industry its equivalent might be a curing oven that processes products that simply need to be kept warm within a wide temperature range. Proportional controllers are more accurate. Instead of using a simple on/off signal they can deliver signal levels proportional to the level above and below the predetermined temperature pre-set; the idea being that eventually they will reach a steady state with little or no fluctuation (hysteresis). PID (proportional-integral-derivative) controllers are the pick of the bunch when accuracy and tuning is concerned. PID controllers use the difference between the temperature level and the desired set point and then cycle the power until this difference is minimised. Like proportional controllers, they apply power proportional to the offset from the set point, but in addition they use an integral function to correct the offset error, while the derivative function suppresses overshoot.

When it comes to delivering signals to temperature controllers, the choice is a little simpler. Users have a primary choice between thermocouples, thermistors or resistance temperature devices (RTD). Thermocouples use a temperature differential across two dissimilar materials to generate a current (the thermoelectric effect) proportional to the temperature. Thermistors and RTD devices on the other hand are fabricated from materials whose resistance varies in proportion to temperature. Thermocouples come in a huge range of types and varieties, as do RTD devices, with RS brand devices making up more than 50% of the range, so once again the user is able to fine tune the selection to best match their application.

RS offers over 190 different on/off controllers from 13 different suppliers, including Baumer, Carlo Gavazzi, Honeywell, Jumo, Omron and its own brand. These vary incredibly across a price range that covers tens to hundreds of pounds, reflecting their relative accuracy, functionality and capability. From the simplest NTC (negative temperature coefficient) controller all the way up to fully featured on/off controllers that accept multiple input types (current, thermocouple, thermometer & voltage) and offer a wide sensing range, users can find units that not only match their application, but also their price point.

 

The PID offering from RS is more extensive, with over 660 units from 19 suppliers, including ABB, Eurotherm, Fuji, Omron, Panasonic, Schneider Electric, Siemens, West Instruments and the company’s own RS brand. Once again the price range defines the choice, with simple controllers being available from £75, with more advanced units on offer from over £1,000. The prove variation really does reflect on the capability of the units, with the higher cost units offering significantly wider and higher operating windows, often coupled with greater accuracy, broader input capabilities and enhanced signal processing.

 

Whatever the budget or application, the chances are that users will be able to source, specify and purchase a temperature controller that closely matches their application’s needs. Simplistic control can be handled by simpler technology, but the more complex an application, especially in terms of accuracy, repeatability and historic data capture, the more expensive the unit will be. This is a fact of life, but luckily, with such a well-served and mature market, engineers do not need to over specify or over design. There is an incredible choice out there, with RS alone offering over 850 units in the on/off and PID variants.