An on/off valve and actuator, for example, can be operated directly from a thermostat. Another example is the operation of high limit safety controls, which have a 'snap' action to close valves or to switch off fuel supplies.
However, when the control requirements become more sophisticated, a controller is needed to match these requirements.
The controller receives a signal, decides what action is needed and then sends a signal to the actuator to make it move.
In the age of the microchip, integrated circuits and computers, the functions performed by the controller can be very complex indeed.
However, since an analogy between the human brain and controllers/computers has been made in previous Tutorials, the renowned IBM motto can be paraphrased:
To summarise, the controller will not solve all problems. It must be properly selected and commissioned, subjects which will be dealt with later.
Although most controllers are now electronic digital/microprocessor based, a range of pneumatic controllers is commercially available. These might be used in hazardous areas where the risk of explosion precludes the use of electrics/electronics. It is possible to make electrical equipment 'intrinsically safe' or explosion-proof or flameproof, however, there is usually a substantial cost implication.
As previously mentioned, the functions carried out by the controller can be very complex and it is beyond the scope of this publication to list them in detail, or to explain how they operate.
The major variations that require consideration are as follows:
Single loop controller
Operates one valve/actuator from a single sensor.
Multi-loop controller
May operate more than one valve/actuator from more than one sensor.
Single input/output
Can accept only one signal from the sensor and send only one to the actuator.
Multi-input/output (multi-channel)
Can accept several signals and send out several signals.
Real time
May include a time clock to switch at pre-determined, pre-set times.
Elapsed time
May switch at some predetermined, pre-set length of time before or after other items of plant have been switched on or off.
Ramp and dwell
Using temperature as an example, the capability to raise the temperature of a controlled medium over a specified time period and then to hold it at a pre-set value. Such controllers frequently incorporate a series of ramps and dwells.
Figure bellow, shows a typical electronic, single loop controller. This has P + I + D action , suitable for 110 or 230 volt supply, and shows a pneumatic single loop controller with P action.
Different models can be selected to control either temperature or pressure.
Electronic single loop controller
Pneumatic single loop temperature controller
A single loop controller, which has the ability to perform ramp and dwell functions, may have a typical sequence pattern like the one shown in Figure bellow. This shows a series of ramps (temperature change) and dwell (maintaining temperature) functions, carried out over a period of time.
Typical multi-sequence ramp and dwell pattern
One term frequently found in control literature is 'Programmable Logic Controller (PLC)'. In a batch process, the controller must trigger a sequence of actions, for example, turning valves or pumps on or off. In some cases the whole sequence is on a timed basis, but often the various steps may be triggered by a specific condition being reached and maintained for a certain time period; for example a certain temperature being reached or a vessel filled. These sequences can be controlled by a PLC, a microcomputer-based device that utilises standard interfaces for sensors and actuators to control the process.
Another type of complex controller is the plant room controller, which might be used to control the boiler, pump, heating control valve, HWS valve, as well as providing a number of other features.
Another type of complex controller is the plant room controller, which might be used to control the boiler, pump, heating control valve, HWS valve, as well as providing a number of other features.
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