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PID Explained - For more detailed information request the free guide!
On / Off Controllers
Starting at the simplest end, today’s on-off controllers are generally sized between 1/4 and 1/32 DIN and typically provide functionality over and above control in that many of even the lowest cost units can be used as alarm devices or high/low trip instruments as well as controllers.  

Features could include, for example, a latching alarm relay (which is ideally de-energised in the alarm condition) with a front-panel reset push button, setpoint stops (to allow the range of temperature settings to be limited), a deadband adjustment (say between 0.1% and 2% of span to provide for some hysteresis), and some form of display to indicate at least the temperature deviation from setpoint, or the setpoint itself.

Generally, front panel LED (or equivalent) status indication will show the output relay condition and there may be other indicators covering other options, or features, like power on and limits.  

The method of set-up and display has moved on from being analogue via a control knob or moving dial arrangement to being entirely digital using any of the LED, gas plasma discharge or advanced LCD technologies.

Looking at specifications, typically these units can accept wide operating input ranges typically 24V AC or DC as well as 85 - 264V AC; they can handle multiple input types (internal linearisers of one form or another will allow for adjustment either automatically, or through a series of range cards, to a subset of the thermocouple types and some of the platinum resistance thermometer variants as well); and they can offer a calibration accuracy of around 0.5% of span at zero and span.  With modern electronics, linearity of the setpoint would be of the order of 3% of span.

Automatic cold junction compensation and sensor break protection circuits are standard equipment.  

Outputs are typically electromechanical or solid state relays covering, for example, 2 to 5A, 85/264V AC, or 5A, 30V DC into a resistive load.

Proportional plus Derivative Controllers
These units offered similar functionality to that of the on-off controllers.  Additions might have included a manual reset facility, an adjustable high limit, low limit or band alarm and a second slave setpoint.  

Some also offered an option for a semi-independent cooling channel.  A few units went one stage further and featured what was sometimes referred to as an ‘inferred integral’ action, which was claimed to offer performance comparable to full PID controllers in some situations, but without the extra cost, or tuning difficulties.

Output options often extended beyond the standard on-off and time proportioning relays to include, for example, logic signals for solid state contactors, or triac outputs to drive loads either directly, or through a suitable contactor.

Proportional plus Integral plus Derivative    Controllers
Most of today’s modern controllers fall into this category, and it is here that the greatest variety and range of sophisticated options is to be found - and not necessarily with high price tags.  

By far the major slice of investment has gone into these units, and the advances are profound.  Virtually every aspect of the controllers has been looked at afresh in the light of new technology - both from the computing possibilities angle (in terms of improvements in the measurement itself using advanced digital techniques, and the complexity of the algorithms which the on-board microcomputer can effect for tighter and more flexible monitoring and control), and the new packaging made possible by the newer electronic assembly techniques.  

Typically, it is possible to take any single controller and, through the selection of the appropriate hardware and by following a  simple software configuration, design a controller that is ideally matched to virtually any temperature control problem.

The most modern units will generally feature low level, high input impedance, drift-correcting analogue to digital converter input circuitry.  These are typically capable of delivering high resolution (12 to 200,000 counts), high accuracy (0.25%) and high repeatability (0.25%) measurements to the microprocessor-based heart of the systems.  

Currently, Proportional plus Derivative application can be met by a modern PID controller by turning off the Integral term. Further, they are normally designed to provide between two and forty conversions per second, to handle fast moving rates of change of temperature.  

Common mode rejection is usually around ± one digit for a 240V AC signal applied to the input at the supply frequency, and the series mode rejection will usually be around 40 to 60dB, depending upon the type of controller and the range.

0°C reference stability and cold junction reference checks are carried out automatically several times per second.  

As a result, errors of less than 0.5 times the LSD (least significant digit) are commonly quoted, with thermocouple linearisation (performed digitally for the various sensor types) in the range of 0.2%, cold junction compensation rejection in the region of 20:1 and RTD linearisation (again digital) better than ±0.1 to 0.2°C, plus automatic three wire lead compensation for RTDs.  

Mains-borne noise rejection will normally be up to 2.5kV spikes with 50 nanoseconds rise times.  Typical figures for the effects of power supply variations are much less than 10 microvolts, or one digit for a 10% change.

Control actions are normally variable across the whole range of on-off, P, PI, PD, PID, PID heat/cool, reverse and direct acting.  In terms of the ranges of the three terms, these are commonly: proportional band 0.1 to 999%; integral time constant from off to 9,999 seconds  and derivative time constant in the range of off to 1 to 3,999 seconds  (although many offer derivative time constants only up to say 50 or 100 seconds on their standard controllers, the range being basically designed to suit the kinds of applications targeted by the manufacturer concerned).  

Instruments with an overshoot inhibiting facility, which usually presets the integral term for fastest approach directly to stable control (sometimes termed cutback or approach control), will normally have an adjustment from one digit to the full display range.  

Most also feature lock-out on the integral term, preventing integral action outside the proportional band, hence reducing the risk of saturation, while offering enhanced stability on the run up to setpoint.

Setpoint rate limiting is also often available to allow the setpoint to be ramped at a pre-configured rate from the current process temperature on power-up, or after setpoint changes, thus again preventing excessive overshoot and thermal shock.  Typical values range up to 0 to 99.99°C/min.  Cycle times can range from 0.2 to 100 seconds at 50% power for solid state relays and 10 to 100 seconds for electromechanical relays.

Outputs are again the logic and relay (on-off or time proportioning choices), but additionally, there is likely to be at least an optional triac heating output and a relay or logic cooling output or 4 to 20mA analogue output, plus one or more separate alarm channels available for setting over the full range (with their own relays for driving into resistive loads).  Isolation will also be provided between the main outputs, although the logic output may be connected to the sensor input card.

Further, there may well be the choice of two cooling output algorithms - linear and quadratic.  The former would handle fan assisted air cooling, while the latter would be aimed at water or oil cooling.  

Typically, there will be a variable gain provided from heating to cooling of 0.1 to 10 times the proportional band, as well as a variable cross-over point.  

Some controllers are also offered with the option of outputs suitable for driving motorised valves direct, typically using velocity-mode control to obviate the need for a feedback potentiometer.  Valve position can then be displayed and limits of travel set up directly on the controller.
Temperature Control Data
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