RELATIONSHIPS OF DESIGN PROBLEMS TO DESIGN CRITERIA 



139 



shift levers, knobs for selection of different 

 radio frequency bands, and knobs for chang- 

 ing the operating range of radar equipment. 



Before continuing to the other categories 

 it might be well to remind the reader that 

 all classifications are subject to the difficul- 

 ties posed by the "borderline case" and that 

 this classification is no exception . However, 

 these difficulties do not impair the usefulness 

 of classifications for purposes of general dis- 

 cussion. 



The adjustments classified as Type B are 

 finite setting adjustments in which the pre- 

 cision of the setting is dependent upon the 

 operator. Continuous gradation of position 

 is provided and thus precision depends on 

 the operator's manipulations rather than on 

 mechanical positioning aids. Normally, the 

 frequency of settings is such that secondary 

 corrections may be utilized to compensate for 

 errors in the initial positioning attempt. 

 The data for these secondary corrections 

 arise from the difference between the desired 

 setting and the actual setting initially 

 accomplished. However, the data for the 

 primary settings arise from other sources and 

 are not a function of the preceding adjust- 

 ment. Typical examples of this type are 

 the tuning adjustments in the operation of 

 a radio, the focusing of optical instruments 

 on fixed objects or for fixed ranges, and the 

 operation of some models of torpedo data 

 computers. The toggle switch example at 

 the beginning of this discussion illustrates 

 the use of remote control and power amplifi- 

 cation to accomplish this type of adjustment. 



Adjustments of Type C are continuous 

 correction adjustments such as tracking or 

 steering operations. These adjustments are 

 error-reducing responses. The data are 

 presented to the operator as a misalignment 

 between two points and the adjustment con- 

 sists of reducing the misahgnment to zero. 

 Changes in the extent of the misalignment 

 are only partially under the control of the 

 operator. This is one of the principal char- 

 acteristics which distinguish Type C from 

 Type B adjustments. Type B adjustments 



are movements of a variable point to a fixed 

 position and the movement of the point is 

 completely under the control of the system 

 through which the adjustment is made. In 

 Type C adjustments, however, one or the 

 other of these conditions does not obtain. 

 The position to which the variable point 

 must be moved may be continuously chang- 

 ing, as in pursuit tracking, or the variable 

 point, although linked to the operator, may 

 be subject to forces external to the control 

 system, as in compensatory tracking. The 

 differences between tracking systems will be 

 analyzed further in the later sections. 



The relationships between types of ad- 

 justments and classes of design problems as 

 indicated in Table II require little comment. 

 Problems of Class I are encountered in all 

 types of adjustments, just as they are en- 

 countered in all human tasks. Problems of 

 Class II are usually encountered in all types 

 of adjustments because of the current prac- 

 tice of including several operations in the 

 task of any one operator. The exception is 

 Type C in which the need for continuous ad- 

 justment usually restricts the task to a single 

 operation or requires that additional opera- 

 tions be accomphshed without interruption 

 of the operation involving Type C adjust- 

 ments. The problems of Class III, how- 

 ever, bear a somewhat less general relation- 

 ship to the several types of adjustments. 



The extent to which Class III problems are 

 involved in a specific control adjustment is 

 proportional to the sensitivity of the control 

 system to operator movement characteris- 

 tics. Most control systems requiring ad- 

 justments of Types A-1 and A-2 are rela- 

 tively insensitive to the movement char- 

 acteristics of the operator. The rate at 

 which successive adjustments are required 

 is low and the deviations of variables such 

 as the extent, force, speed and direction of 

 the movement are either controlled or 

 filtered out. The reverse is true at the other 

 end of the classification. Type C adjust- 

 ments usually are accomplished through 

 control systems which are extremely sensi- 



