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 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. They may be in the form of

• short electrical pulses sent down a cable
• a simple voltage in a cable
• radio, microwave, laser or infra-red pulses.

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.

A digital-to-analogue converter is an interface to convert a digital signal to an analogue one. It converts a set of binary signals to one single varying voltage.

An analogue-to-digital converter is an interface to convert an analogue signal (eg from a sensor) to a digital one (for the computer).

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.

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

Examples of sensors

• A thermocouple for measuring temperature. This is a device which produces a voltage proportional to its temperature. It is an analogue sensor because it produces a continuous range of voltages.
• A pressure pad at a traffic light. This produces a signal if a car goes over it. It is a digital sensor because it is either on or off.
• An infra-red sensor on a camera. An infra-red beam is produced by the camera and reflected back from an object. The reflection is detected by a sensor on the camera and the processor in the camera can work out whether or not the object is in focus.
• An analogue light sensor. This responds to the amount of light falling on it. The voltage produced is greater as more light falls on it.

Feedback generally means using output from a system to influence the input. Data from sensors is received by the processor and these data help 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.

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; a printer to provide hard copy when required.

Application involving process control.

Control of gas pipelines and pumping stations.

The situation - As consumers use gas, the pressure has to be maintained at a fixed level throughout the grid. This is done by pumping gas to those areas where the pressure is dropping.

Data capture - each pumping station has a microprocessor and pressure sensors. Signals are sent from these by microwaves.

The role of the computer - The whole grid is controlled by a computer at the board's headquarters. It sends a microwave signal to a pumping station asking for pressure readings. The microprocessor there sends results back. These are analysed and the main computer sends signals to the microprocessor to adjust the pumps.

Data logging.

Data logging is the automatic recording of data as it is produced.

Characteristics of a data logging system

• A process is monitored by instruments or sensors.
• Often the sensors are connected to an interface board which in turn is connected to the computer.
• The computer controlling the system samples the readings at regular time intervals
• The readings are recorded, usually by storing them on backing store.
• The data is analysed. This may happen continuously or after the data has been collected.
• Results may be displayed continuously:
•  
  • as a set of numbers on a printer, OR
  • as a constantly changing screen display, OR
  • as a graph.

APPLICATION: involving data logging

Monitoring conditions inside a petrochemical plant

Data capture - Sensors monitor the temperature in the boiler, the flow at two critical points and the output produced by the plant.

The role of the computer - The computer takes a sample of each of the four readings at one minute intervals. These values are stored in a disc file. Graphs of each of the readings are displayed all the time on the computer's screen. The four graphs are adjusted each minute to take account of the new reading. Each hour, the values stored are used to produce statistics of the performance of the plant.