The control device is the means by which information on a decision made by a human operator is transferred to the machine. The decision may, for example, be taken on the basis of previously-read information devices, on the basis of information from other sources, or from some form of cognitive process.
Functionally, controls may be divided into the following categories:
1. Switching ‘on’ or ‘off’, ‘start’ or ‘stop’.
2. Increase and reduction (quantitative changes).
3. Spatial control (e. g., continuous control upwards, downwards, to the left, or right).
4. Symbol/character production (e. g., alphanumeric keyboards).
5. Special tasks (e. g., producing sound or speech).
6. Multifunction (e. g., controls for communicating with computers).
Examples of control types: (1) include the starting or stopping of motors, or switching lamps on or off. Control types (2) may consist of an accelerator pedal to increase and reduce the flow of fuel to the engine. Traditionally the best-known example of spatial control (3) is the steering of a car. Examples of character production (4) include typewriting and telegraphy. Different forms of controls (5) are used for the production of sound. Of special interest here are the machines that are beginning to appear for the production and replication of human speech (see Section 3.6).
Of particular importance in control room design are the types of controls (6) used in conjunction with computers. Controls operated by hand are of particular use where great accuracy is required in the control movement. Hands are considerably better at carrying out precision movements than feet. Where a very high degree of accuracy of movement is required, it is best for only the fingers to be used.
Other alternatives are also possible for the design of special controls. For example, if one has a large crank, or two cranks coupled in parallel that have to be controlled by both hands, very fine control movements can be made.
Because the power available from the leg is considerably greater than that from the arms, foot controls are suitable for manoeuvring over long periods or continuously. Foot controls are also valuable where very large pressures are needed, as the body weight can be added to the force of the strong leg musculature. It may also be necessary to use foot control devices where the hands are occupied in other tasks. However, it should be noted that it may be necessary to use hand controls where there is insufficient space to accommodate foot controls.
When designing traditional types of controls, it is possible to design them in such a way that they naturally represent the changes one wishes to bring about in the process. For example, a lever that is pushed forwards may determine the forward direction of movement of a digger bucket. Or the flow in a pipe can be stopped by turning a knob that lies on a line drawn on the panel. In this way the design of the control increases the understanding of the current state of the process.
For communication with computers, the keyboard is often chosen for carrying out all the different control functions. Technically, it is often easy to connect a keyboard to a computer system. Other control devices also exist for communicating with computers, such as light pens. However, a particular failing of this type of multipurpose control device is that the control movements in themselves have no natural analogy with the changes that they aim to bring about in the process.