138 



DESIGN OF CONTROLS 



were distinguished on the basis of different 

 sets of criteria for evaluating the effects 

 of design variables on the operator's per- 

 formance. That principle has been used as 

 the basis for a general classification of design 

 problems as presented in Table I. 



The corollaries of this classification include 

 generalized statements of the implications 

 noted in the hand wheel placement example. 



1. A single design variable may be in- 

 volved in problems of more than one class. 



2. The optimal values prescribed for a 

 given design variable by different sets of 

 criteria may not always coincide. 



3. In any particular application the im- 

 portance of a given class of problems varies 

 according to the relation of the control 

 operation to the expected operating condi- 

 tions and according to the type of adjust- 

 ment which is required. 



4. Problems of Classes I and II arise from 

 a consideration of effects of operating con- 

 ditions. Problems of Class III arise from a 

 consideration of operator movement char- 

 acteristics and the requirements of the 

 control task. 



Types of Control Adjustments and their 



Relation to the Classes of Design 



Problems 



The nature of the control task constitutes 

 the starting point for the design of a control 

 system. In the paragraphs above it was 

 noted also that the nature of the adjustment 

 (or control task) partially determines the 

 relative importance of the different classes 

 of design problems which may be met in any 

 specific instance. Table II presents a classi- 

 fication of control adjustments, along with 

 an additional column indicating the classes 

 of design problems which usually arise in 

 connection with each type of adjustment. 



Certain devices are typically used to ac- 

 complish certain kinds of adjustment. In 

 the following discussion this fact will be 

 utilized to illustrate the type of adjustment 

 being considered. However, it is important 

 to note that it is types of control adjustment 



that we have classified, and not types of 

 devices manipulated by the operator. For 

 example, a toggle switch may be the control 

 by which power for a drill press is turned on 

 or off. The same toggle switch may be used 

 to turn on or off the power to a motor which 

 positions a gun turret. In the former case 

 the switch is used for an on-off adjustment 

 (Type A-1). In the latter case the switch 

 is used for a finite setting adjustment (Type 

 B). 



Adjustments of Type A-1 normally en- 

 gage or disengage a source of power and 

 thus control its availability for further opera- 

 tions. The ignition key and starter button 

 of an automobile are examples of devices 

 requiring this type of adjustment. Various 



clutch arrangements in machine shops and 

 automotive vehicles are further examples, 

 as are light switches and the power switches 

 on innumerable electrical appliances. 



Type A-2 adjustments involve the selec- 

 tion of different amounts of power or differ- 

 ent operating equipment. As with Type 

 A-1, the adjustment consists of effecting a 

 desired setting of the control device, but in 

 the present case a greater choice of settings 

 is available. Wrong settings may more 

 easily occur and sequences of settings may 

 be required. In both types the precision 

 of the adjustment is accomplished by me- 

 chanical features of the equipment and thus 

 successive adjustments are not normally a 

 function of the precision of preceding ad- 

 justments. The levers controlling the speed 

 of submarine engines are examples of devices 

 used for power selection. Examples of 

 equipment selection devices include gear- 



