Electrical control systems
An electrical control system is a physical interconnection of devices that influences the behaviour of other devices or systems. A simple electronic system is made up of an input, a process, and an output. Both input and output variables to the system are signals. Examples of such systems include circulation pumps, compressors, manufacturing systems, refrigeration plant and motor control panels.
Input devices such as sensors gather and respond to information and control a physical process by using electrical energy in the form of an output action. Electronic systems can be classed as ‘causal’ in nature. The input signal is the ‘cause’ of the change in the process or system operation, while the output signal is the ‘effect’, the consequence of the cause. An example is a microphone (input device) causing sound waves to be converted into electrical signals and being amplified by a speaker (output device) producing sound waves.
Electronic systems are commonly represented as a series of interconnected blocks and signals. Each block is shown with its own set of inputs and outputs. This is known as block-diagram representation.
Electrical systems operate either on continuous-time (CT) signals or discrete-time (DT) signals.
A CT system is where the input signals are continuous over time. These tend to be analogue systems producing a linear operation with input and output signals referenced over a set time period, such as between 13:00 and 14:00.
A DT system is where the input signals are a sequence or series of signal values defined in specific time intervals, such as 13:00 and 14:00 separately.
Control systems are one of two different types, either an open loop system or a closed loop system.
 Open loop control system
An open loop control system is one in which the output does not feedback to the input to correct variations. Instead, the output is varied by varying the input. This means that external conditions will not impact on the system output. An example is a timer-controlled central heating boiler which is switched on between certain preset times regardless of the thermal comfort level of the building.
The benefits of open loop systems are that they are simple, easy to construct and generally remain stable. However, they can be inaccurate and unreliable due to the output not being corrected automatically.
 Closed loop control system
A closed loop control system is one in which the output has an effect upon the input to maintain a desired output value. It achieves this by providing a feedback loop. For example, a boiler may have a temperature thermostat which monitors the thermal comfort level of a building and sends a feedback signal to ensure the controller maintains the set temperature.
Closed loop systems have the advantage of being accurate, and can be made more or less sensitive depending on the required stability of the system. However, they are more complex in terms of designing a stable system.
 Types of control
There are a number of different types of control:
 Manual control
This system uses no automatic controls, the link is provided by the human operator.
 Semi-automatic control
A sequence of operations is carried out automatically after being started by a human operator. An example is starting an electric motor.
 Automatic control
The human operator is replaced by a controller which monitors the system in comparison with a desired value, using feedback loops to take corrective action if necessary.
 Local control
A level, hand wheel or other attachment fixed on the unit ‘locally’ is used as a means of alteration and control.
 Remote control
The regulating unit is connected to an actuating device mounted some distance away by means of power transmission through electrical linkages. For example, a remote control to turn on an air-conditioning unit.
 On/off control
The regulating unit can occupy only one of the two available positions of ‘on’ or ‘off’. An example is the on-off switch for lights.
 Step-by-step control
More than two positions can be occupied by the regulating unit but the action occurs in steps rather than being continuous.
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