ROLE OF OPERATOR IN MANUALLY OPERATED CONTROL SYSTEMS 



143 



''transmission elements" and reserve the 

 term "transmission system" for a combina- 

 tion of these elements. Generally, in such 

 combinations the elements are so linked that 

 the output of one element constitutes the 

 input to the next element in the series. 

 Thus, in the present case the control data 

 constitute the input to the operator and 

 the resulting movement constitutes the out- 

 put of the operator as well as the input to the 

 control device. An additional type of link- 

 age may occur in which the output of an 

 element at some point in the series is fed 

 back into the system at some previous point 

 in the series. Systems containing such 

 linkages are appropriately called feedback 

 systems. In the discussion which follows 

 we will have occasion to consider a special 

 class of systems of this type, namely, servo 

 systems. 



"A servo system is a feedback system in 

 which the actual output is compared with 

 the input, which is the desired output, and 

 the driving element is activated by the 

 difference of these quantities" (16). There 

 are numerous examples of man-machine 

 combinations in which the operator con- 

 trols the output of the machine by compar- 

 ing the actual output with the desired output 

 and then making a compensatory adjustment 

 of the controls to correct the difference. 

 The control of a plane's attitude or a ship's 

 heading or a reticle's position in respect to 

 the target are all familiar examples of closed- 

 cycle control systems, or servo systems, in 

 which the human operator serves as a bio- 

 mechanical link. Although it is almost im- 

 possible to find a humanly operated control 

 system in which the operator receives no 

 information concerning the result of his 

 manipulations, the manner and degree to 

 which this feedback affects the operations 

 may vary from the critical role played in the 

 examples above to the minor role exempli- 

 fied by the radio operator's casual check of 

 his instrument to make sure that turning 

 the power switch actually turned on the 

 power. 



The output of a transmission element 

 depends on three factors: (1) the characteris- 

 tics of the input, (2) the transmission char- 

 acteristics of the element, and (3) the 

 characteristics of the load against which 

 the element operates. Thus, the output of 

 the operator is determined by the nature of 

 the control data, by his own transmission 

 characteristics, and by the characteristics 

 of the load inherent in the design of the 

 device which he manipulates. System vari- 

 ables involved in each of these factors will 

 be taken up separately, but the interrelation- 

 ships which exist between them must neces- 

 sarily be considered in any research work 

 or practical application. 



The concept of "the operator" also needs 

 a brief comment. At the level of analysis 

 employed thus far in this chapter the opera- 

 tor, as an individual, has been considered a 

 transmission element. However, for design 

 purposes it is important to realize that he 

 is actually a complex network of numerous 

 transmission systems. He possesses a num- 

 ber of different types of receptors or sense 

 organs each of which is peculiarly sensitive to 

 a certain class of inputs. Likewise, his 

 fingers, toes, hands, feet, arms, legs, and 

 head are all outlets for utilizable movement 

 patterns. His possibilities are further mul- 

 tiplied by the fact that the correlation of a 

 given input with a particular output (move- 

 ment pattern) is not "built into" the opera- 

 tor except perhaps at the reflex level. It is 

 only by means of instructions, training, and 

 system design that certain of his movement 

 patterns may be standardized for a given 

 class of inputs in a particular situation. 

 Recognition of the multiplicity of trans- 

 mission systems embodied in the operator 

 necessarily leads to the reaUzation that in- 

 put design and "operator" design deserve 

 the same detailed consideration that has 

 recently been focused on the design of 

 manipulative devices, for the role of the 

 operator is determined by the interrelation- 

 ships between design characteristics of each 

 of the three elements. This fact is impor- 



