OPERATOR AS ELEMENT IN CONTROL SYSTEM 



135 



Recently, technological developments 

 stimulated by the demands of World War II 

 have highlighted the inadequacy of these 

 approaches to the problem. The complexity 

 of much of the equipment has required 

 operator skills which can hardly be termed 

 a "minimum role." This same trend toward 

 complexity has resulted in the design of 

 equipment which demands manipulations 

 that even carefully selected and highly 

 trained personnel cannot perform satis- 

 factorily. These developments, along with 

 the expense in time and money involved in 

 elaborate programs of selection and training, 

 have emphasized the need for including 

 operator characteristics in the original design 

 layout. Solution of the general problems of 

 equipment design based on this approach 

 will depend upon the establishment of broad 

 research programs founded on a considera- 

 tion of (1) fundamental operator-control 

 relationships and (2) the relationships be- 

 tween operator-control combinations and the 

 over-all task. 



The Operator as an Element in 

 A Control System 



In designing a control system for a specific 

 application, the designer necessarily has a 

 clear, definite goal in mind; the system is to 

 perform some given task, and it must do so 

 with some minimum desired quality of 

 performance. Although the control system 

 in some instances may consist of only one 

 simple element, more often it consists of a 

 number of elements linked together. The 

 designer today has a choice of innumerable 

 devices from which to draw elements for his 

 system. His choice depends to a great ex- 

 tent on the nature of the task which the 

 system must perform. For example, if 

 rapid control action is required, certain 

 classes of elements and linkages are im- 

 mediately rejected as too slow. If the 

 maximum load is large, certain low-power 

 or fragile elements must be rejected or, if 

 they are included, other elements must be 

 incorporated into the system in a way which 



will prevent the low-power elements from 

 being subjected to extreme loads. In addi- 

 tion, the designer's choice will be affected 

 by the conditions under which the system 

 will be required to operate. Extremes of 

 temperature, probability of jolting, space 

 restrictions, etc., are examples of such con- 

 ditions. In general, the designer needs to 

 know the nature of the task and the expected 

 operating conditions, on the one hand, and 

 on the other he needs to know the operating 

 characteristics of the available elements, the 

 operating limits of these elements, and the 

 susceptibility of these elements to the ex- 

 pected conditions. 



The control systems in which we are 

 interested in this chapter are those systems 

 which include a human operator as one of 

 the elements. In line with the approach 

 here adopted, the operator is an element to 

 be considered in the original design of the 

 system. If the designer is to include the 

 operator in the design, he will require the 

 same sort of information about the operator 

 as he does about the other elements. He 

 will need to know his operating characteris- 

 tics and limits and his susceptibility to task 

 conditions. 



Certain features of the human operator as 

 a system element need to be pointed out. 

 First, the inclusion of the human operator 

 is not usually a matter of choice but rather 

 a matter of necessity. As an element he 

 must be accepted with all his limitations 

 and characteristics, and the choice of other 

 elements becomes largely a matter of com- 

 pensating for and supplementing these char- 

 acteristics and limitations. Second, differ- 

 ences between operators are greater than 

 is the case with other classes of elements. 

 Third, unlike other elements, frequent inter- 

 change of operators is the rule. Fourth, the 

 operator is seldom used simply as an element 

 in a single control system. Ordinarily he 

 functions intermittently as an element in 

 several systems and, in addition, may be 

 required to perform other functions than 

 those of control. Fifth, the link between 



