Other terms related to the use of computers in control processes and devices will be found in Section C1 Programming concepts and Section E7 Communications technology.
The use of computers to make machines do what we want them to – called control – grew out of the extensive use of mechanical devices and electromagnetic switches (relays). In telephone exchanges, traffic lights and lifts control used to be achieved by the use of mechanically operated switching systems. Other methods were designed to control the operation of machines, made up of devices that reacted to conditions in the environment such as temperature, pressure, speed, position. Large manufacturing plants were built controlled by these methods.
The development of microprocessors and microcomputers made it possible to improve the reliability of existing systems and to make them more flexible. In a modern airliner the computer system is capable of controlling take-off and landing as well as automatically maintaining course, speed and altitude. The computers used in cars take over part of the control of the car engine from the driver. The operation of washing machines and video equipment is frequently managed by microprocessors. Control systems can also be used to improve efficiency (for example saving energy) or to be environmentally friendly (for example reducing pollution).
Control may be exercised remotely, in which case control signals are sent from the controller to the device. These signals may be transmitted in any of the ways that are used to pass data, for example through wires, or as radio or infrared signals.
Control systems may be either passive or reactive. In a passive system the controlled device, once it has been set going, will perform a predetermined set of activities regardless of the circumstances. A reactive system will vary its behaviour in response to feedback from different situations. By using programmable chips, connected to suitable sensors, systems respond to the information provided by these sensors. Combining control and observation, through the use of sensors, has made it possible to extend the scope of automatic process control, for example in the manufacture and packaging of chemical products. Fault tolerant systems are able to continue operation in the event of the failure of some of their components although the quality of the output may be degraded. This is in addition to the system characteristics of robustness and reliability.
Virtual reality systems give the user the illusion that they are transported into an environment created by the computer. Computer-controlled graphics are used to generate scenes of various levels of realism that the user can interact with and control.
Automation
is the use of machines or systems to perform tasks as an alternative to using people.
Actuator
is any device that can be operated by signals from a computer or control system causing physical movement, for example, devices for opening windows in a computer-controlled ventilation system.
Servo mechanism
also known as: servo
is a mechanical mechanism for remote control of machines. A simple form is the motor that operates the control surface of a radio-controlled model aircraft, where the person flying the model plays an active part continuously adjusting the position of the control levers. Servos can be controlled electronically through computer circuits that may incorporate feedback to achieve automatic control; in these situations human participation may be very limited.
Data capture
including: sampling, data logging
is the sampling (collection at specified intervals) of output from external sensors (see also polling). This data may be used to control a process. Data logging is the capture and storage of data for later use; thus data captured in a control process may be logged for later analysis of the process.
Fault tolerance
is the ability of a system to recognise and to cope with unusual events. Examples include a traffic light system where failure to display instructions to the pedestrian by one of the two displays will not stop the use of the crossing system. Alternatively in a packet switching network system if a package is lost it will be resent.
Feedback
including: closed loop, open loop
is the use of data from sensors as input to the controlling program. In this way the result of previous actions becomes input that contributes to selecting the next action. If the response to the feedback is automatic (there is no human operator involvement), the process is called closed loop; if an operator is involved or feedback is not used, it is called open loop. In most situations where feedback is used to control position, for example stacking boxes on shelves, the correct position is achieved by an iterative process, in which a move is followed by a position check, each move bringing it closer to the required position, until the correct position is reached.
is a method of controlling an aeroplane in flight. The flaps, rudder and other control surfaces of the aeroplane are operated by motors. These motors are controlled by (electrical) signals that are created as a result of actions by the pilot. This kind of flight control system involves the use of computers to analyse the pilot’s intentions and thus work out the right amount of movement of the control surfaces; the computers can override the pilot in situations that would endanger the aeroplane.
Numeric control
also known as: computer numeric control (CNC)
generally refers to the automatic control of machines such as lathes and milling machines. Some numerically controlled machines simply obey a preset program of instructions, while more advanced machines can react to feedback from sensors (see below).
Paper tape
including: paper tape punch, paper tape reader
is sometimes still used as the means of program and data input to machine tools. A pattern of holes punched in the tape is used to represent the data. In order to prepare the tape a paper tape punch is attached to the computer on which the program and data are prepared while a paper tape reader is used as an input device to the machine tool.
Process control
including: integrated manufacturing
is the automatic monitoring and control of an industrial activity by a computer that is programmed to respond to the feedback signals from sensors (see below). The operation controlled may be as small as a single machine packing boxes or as extensive as the control of an automated bakery, where the mixing, cooking and packaging are controlled within a single integrated manufacturing process.
Robot
including: robot arm
is a computer-controlled mechanical device that is sufficiently flexible to be able to do a variety of tasks. Robots are frequently used to do jobs where consistent performance is required (such as paint spraying motor cars) or where there is some danger to humans performing the task (such as the handling of toxic materials). A robot arm is a relatively simple fixed robot capable of picking things up, positioning them etc.
Sensor
including: analogue sensor, digital sensor, passive device, active device
is a transducer that responds to some physical property such as pressure, rate of flow, humidity, the proximity of ferrous metal. The electrical output from the sensor may be either analogue, an analogue sensor, or digital, a digital sensor. Some sensors, called passive devices, require no external electrical source. Those that require an external voltage are called active devices.
Stepper motor
also known as: stepping motor
is an electric motor that moves in small rotational steps. Suitably controlled and geared, a stepper motor can provide very small discrete movements, for example the movement of the paper rollers and the print head in a printer. The control circuits may involve the use of feedback.
Telemetry
is the use of communications (usually radio) to monitor sensors to achieve control of machines and instruments at a distance. For example, the control of satellites and space probes, or the monitoring and control of the performance of Formula 1 racing cars, where the technicians can monitor the engine control system on the car from the pits while the race is in progress.
VIRTUAL REALITY
Virtual reality
including: immersive virtual reality, non-immersive virtual reality
is a computer-generated environment that provides the user with the illusion of being present in that situation. Virtual reality is produced by providing feedback to our various senses: vision, hearing, movement and sometimes smell. As the user moves or acts, the image seen will change along with appropriate sound and movement. It usually requires high-powered computers.
Immersive virtual reality systems provide feedback to as many senses as possible by using specialised equipment. They attempt to provide the user with a very realistic situation and are used for training in critical and stressful situations. Examples include the training of aircraft crew (a cockpit simulator using a cabin mounted on hydraulic jacks to provide movement) and training maintenance engineers to work in nuclear reactors (using specialised peripherals such as data gloves, and headsets).
Non-immersive virtual reality systems use limited feedback to provide the user with the perception of a particular situation without attempting to convince the user that it is real. This can be done relatively cheaply using common equipment. The environment is displayed on a standard monitor using 3D graphics and can be controlled using a simple pointer device such as a special scalpel that a sculptor may use to carve a sculpture. Other examples include vehicle driver training and the ergonomic evaluation of a shop layout.
is an input device worn on the hand by a user of a virtual reality system. Typically, it enables the position of the fingers and the orientation of the hand to be sensed by the system. In most systems, when the user ‘touches’ a simulated object, there is no physical sensation returned to the hand. Greater realism is possible in systems that do provide this kind of feedback. See also haptic feedback, below.
Haptic feedback
provides the user with a sense of feel and touch. When the user touches a virtual object the input device stops moving freely and provides resistance to the user. An example is the sculptor who uses a special scalpel that moves freely until it starts cutting the virtual object.
Headset
is an input and output device worn on the head by a user of a virtual reality system. It gives the wearer the impression of being within the computer-generated scene, with sound provided through headphones and vision through small video screens in goggles worn over the eyes. The headset senses changes of position of the head and inputs this to the computer so that the simulation can be changed appropriately. Since the user is isolated from external sounds and vision there is a very strong sense of being within the simulated environment.
Virtual reality cave
is a small room where the walls consist of back-projected displays, giving the user a more immersive experience without the need for a headset. For example, a driving simulator may consist of a car body at the centre of a virtual reality cave.