Factors to be Considered while Designing the Controls (with diagram) (2024)

The four factors to be considered in the design of controls are as follows: 1. Control Display Ratio (C/D ratio) 2. Directional Relation in Control and Display 3. Control Resistance 4. Operational coding of Controls.

1. Control Display Ratio (C/D ratio):

The C.D ratio is defined as the ratio between the movement of control device and the moving element of the display showing or representing the control movement. It should be obvious that this relationship shall be of considerable importance to the operator operating the control. This term is meaning full only in case of continuous controls.

An optimum C/D ratio will reduce the operation time. In the rapid or slowing movement any increase in C/D ratio would increase the time but in case of fine adjustment movement increased C/D ratio will reduce the time. The C/D ratio is illustrated in Fig 9.12.

2. Directional Relation in Control and Display:

There should be correct relationship between the direction of movement of control and that of the moving element of this display, if the control moves clockwise the pointer should also move clock wise. The reverse movement may confuse the operator.

An appropriate control and display movement relationship reduces reaction time, facilitates in taking quick decisions, speeds up movements, eliminates reversal errors and helps in reducing learning time, it is essential when a job is complex and control motion sequence Is irregular.

3. Control Resistance:

The force offered by the control to the intended movement is known as control resistance; it is offered by the control and keeps a relation with the resistance offered by the device being activated by the control. There is variety of resistances offered by controls. Some are negligible in terms of physical forces where as other are significant.

The main types are as follows:

(i) Inertia Force

(ii) Static and dynamic friction

(iii) Elastic or spring force.

(iv) Viscous damping force.

These resistances have the following effects on the operational performance of the control:

(a) It affects the smoothness of control operation.

(b) Accuracy and speed of control movement is affected.

(c) A very small control resistance may create/ give rise to accidental activation by virtue of unintentional load situations such as gravity and shock etc.

All types of resistance force have their benefits and limitations which should be considered by the designer. Spring or elastic resistance helps in return movement of the control because it is always directed towards zero position. It cannot be actuated easily by accident other than application of accidental force. This provides the operator the feel of control movement.

Inertia force opposes sudden changes in velocity there by reducing risk of accidental activation. The operator is provided with a true feel of movement but makes it difficult to do precise adjustments. Viscous damping force, reduces risk of accidental activation opposes quick movement of control and thus helps the operator in performing smooth control movements.

Frictional resistance plays different roles in dynamic and static: conditions. It decreases as soon as control becomes dynamic but it tends to keep control in position during static conditions.

4. Operational coding of Controls:

Coding means the technique of conveying information quickly; may be through colours numbers or letters etc. controls are required to be coded in order to identify them so as to reduce the overall operational time. Effective methods of contra coding are to regulate/control their size, shape, method of operation, position colour and labels.

The following rules in this regard should be followed:

(i) Shape coding:

It should have some relationship with the intended purpose of control. It acts as a visual display as well as a communication input. Many shapes have been standardized for control coding in many industries and government bodies.

(ii) Size coding:

It is not as effective as shape coding but may serve

the purpose in many cases specifically industries.

(iii) Position coding:

It is helpful in habit forming. Example is why electric bulbs switches are normally fitted near doors or at other convenient places at shoulder height.

(iv) Method of operation coding:

This rule is concerned with that control should be designed in such a manner that it would not be operated in a wrong direction.

(v) Colour coding:

It is a visual coding method and is very effective which can be combined with other coding techniques such as shape, size and level etc.

(vi) Labeling:

It is also an effective coding method for controls. A good level should be precise, complete, short and standard in case of notational and should be placed on or very close to control. But all this can be done if space for putting up the level and lighting as a visual aid is available.