Device control refers to one piece of equipment controlling the operation of another piece of equipment by sending signals to it. The controlling equipment is usually a computer a circuit board containing a microprocessor A control system consists of everything involved in the control of a device - the processor, the programs, the interfaces and the devices themselves.

Examples of control systems a washing machine - the user selects a sequence which a microprocessor in the washing machine carries out in order. It sends signals to switch the heater, pumps and motors on and off at the right time intervals. a fully automatic camera - a processor receives light readings from light sensors. When the shutter release button is partly pressed, the processor sends control signals to adjust the aperture and shutter speed. Then when the shutter release button is completely depressed the film will be correctly exposed.

The signals sent by a computer to a device are called control signals.

An output port is a connecting point on a computer through which it can send control signals.

A relay is a switch which can be switched on and off by an electrical signal. It can be used in the device being controlled to switch lights and motors on and off.

An actuator is a device which can produce a movement when given an electrical signal. It receives the control signals and converts them into movement.

An interface is some hardware, and possibly also some software, that is used to connect two devices or systems to enable them to communicate.

Control systems often need interfaces between the computer and the controlled device. This happens if the type of signal sent or received by the computer is not the same type as that of the device. In particular the interface may need to include analogue-to-digital or digital-to-analogue converters.

Sensors and feedback

A simple control system can work by a processor sending control signals to a device which then responds accordingly. However, in most control systems the computer also receives signals back. These help the processor to decide what to do next. Such signals come from:

• sensors or instruments. eg an infra-red sensor which sends a signal if a beam across a gateway is broken.
• switches or buttons used by a user or an operator. eg a button pushed by a pedestrian waiting to cross at traffic lights.

A sensor is a monitoring device which measures some physical quantity and sends signals back to the processor.

Examples of sensors :

• light sensor - used on street lighting systems
• heat sensor - used in domestic heating systems
• movement sensor - used in burglar alarm systems
• proximity sensor - used in industrial robots
• sound sensor - used to monitor heart beats
• pressure sensors - used in traffic light systems

An input port is a connecting point on a computer through which it can receive signals from sensors and switches.

Feedback generally means using output from a system to influence the input. Data from sensors is received by the processor and this data helps it to decide what control signals to send. The processor uses feedback to keep the control system stable.

Components of a control system.

A control system normally requires a processor which has:

• storage for program and data
• a control program which can send control signals and deal with feedback from sensors
• input ports to receive data from sensors
• output ports to send control signals
• interfaces to convert signals between the processors and the rest of the system.

Application of sensors and feedback.

Automatic control of a greenhouse.

The situation - A large nursery has a number of greenhouses which need constant attention. To save workers having to continually check them, an automatic computer-controlled system is introduced. Each greenhouse needs to be kept at constant temperature and constant humidity. These can be controlled by: opening or closing overhead ventilators - opening the ventilators decreases the temperature and humidity, while closing the ventilators increases them; switching a fan on or off - on decreases temperature; switching a heater on or off - on increases temperature; putting misters on or off - on increases humidity.

Data Capture - The greenhouse has suitably placed sensors to measure -

• temperature;
• humidity.

The temperature sensor is digital but the humidity sensor is analogue.

Role of the computer - The computer receives readings from the sensors. Readings from the humidity sensor are passed through an analogue-to-digital converter. It is programmed to check humidity and temperature at 10-second intervals and take appropriate actions eg.

• if temperature is too low then switch heater on; shut the ventilators slowly, checking humidity and temperature; switch the fan off if necessary;
• if humidity is too low, then put the misters on until humidity recovers.

Process control

Process control means automatic control of an industrial process. A computer is used to control an operation by monitoring readings from sensors and sending control signals when necessary. Characteristics of process control - It is a real-time operation - input from sensors is processed and control signals are sent back almost immediately. It is an example of the use of feedback - the sensor input is used to adjust the process if it is out of balance. The computer usually controls the supply of materials and the timing of each part of the process. Some more sophisticated systems allow for 'learning' to take place. The computer 'remembers' how the best results were obtained and attempts to reproduce those results.

A Process Control system is an example of a real-time system.

Requirements for process control In a computerised process control application the following items are generally required - a controller - usually a dedicated computer or a microprocessor-based circuit; sensors to provide information on the process under control. If a sensor is analogue an analogue-to-digital converter is needed; actuators to carry out control actions in response to signals from the processor. If the actuator is analogue, a digital-to-analogue converter is required; display devices (eg monitor screens, LED or LCD displays) so that a human operator can check the system;

Application involving process control.

Manufacture of Glass

Molten glass has to be cooled at just the right rate - otherwise poor-quality glass is produced.

Input :

• Data from heat sensors
• Signals from the control operator

Process :

• Analyse data to calculate rate of cooling

Output :

• Control signals to heaters or cooling fans to regulate the temperature of the glass.
• Readings to display monitors