136 



DESIGN OF CONTROLS 



the operator and the rest of the system often 

 is not fixed or controlled as are other links 

 in the system. This arises mostly from the 

 practice of including many unit tasks in the 

 over-all task of a single operator. These are 

 all features which the designer must consider 

 in the case of the human element but which 

 rarely have their counterpart in the case of 

 the other elements. They should be viewed 

 as unique features, however, rather than as 

 insurmountable difficulties. 



The relationship between these features 

 and the problems posed by operating con- 

 ditions also deserve comment. To some 

 extent, the designer of a control system may 

 counteract the disadvantages inherent in 

 operating conditions by selecting equivalent 

 elements which are not adversely affected 

 by those conditions. In general, this prin- 

 ciple can not be used in the case of the human 

 operator, for, as was stated above, he is 

 usually included as a matter of necessity 

 rather than choice. The designer may also 

 build protective devices into his system to 

 eliminate or diminish the effects of adverse 

 conditions. This principle is applicable to 

 the human element as well as to the others. 

 However, the last three "unique features" 

 hsted above are essentially operating condi- 

 tions, and the manner in which they must 

 be taken into account is not covered by 

 either of the above principles. 



The effects of these conditions which are 

 inherent in the use of a human element may 

 be modified by fitting the manipulative re- 

 quirements to the expected conditions. For 

 example, in many military tasks the operator 

 is under stress and his characteristics may 

 tend to change as a result. The degree of 

 this stress varies. If a gunner is tracking an 

 approaching plane, the nearness of the plane 

 may increase the stress and initiate certain 

 reflex or habitual responses. Adjustment 

 of the control device should be arranged to 

 coincide with these stress reactions rather 

 than to oppose them. Again, the fact that 

 the operator can not be "bolted do^\^l" dic- 

 tates against the requirement of protruding 



devices, if sudden displacement of the opera- 

 tor is a probabihty. Also, such principles 

 as standardization of controls, shape coding, 

 and design of movement sequences in the 

 over-all task may influence the choice be- 

 tween a rotating knob and a lever device. 

 In short, although the designer's primary 

 concern is the accomplishment of the proper 

 arrangement of elements such that control 

 data which are fed into the system at one 

 end will be transformed into the proper out- 

 put at the other end, he must also be con- 

 cerned with the expected operating condi- 

 tions when selecting the elements for his 

 system. 



In the development of equipment requir- 

 ing a human operator, the problems con- 

 nected with the design of a specific control 

 system which forms a part of the equipment 

 may have their source either in the nature 

 of the control task or in the expected operat- 

 ing conditions. These two factors are inter- 

 related and the importance of the problems 

 relative to each varies from case to case. 

 The next section will include a discussion of 

 these interrelationships, a classification of 

 problems according to source, and a classifi- 

 cation of control tasks. 



The Relationships of Design Problems 



TO Design Criteria and to Types of 



Control Adjustments 



Design Criteria and the Classification of 

 Design Problems 



The great variety of tasks in which the 

 operator manipulates "controls" and the 

 great variety of control devices combine to 

 produce innumerable design problems. For 

 purposes of discussion and systematic re- 

 search it is necessary that these problems be 

 classified. Before examining a suggested 

 classification let us consider an hypothetical 

 case of design evaluation. 



The placement of the control in relation 

 to the operator is included in this discussion 

 as one of the variables in control design. 

 As such a variable its evaluation may serve 



