ROLE OF OPERATOR IN MANUALLY OPERATED CONTROL SYSTEMS 



145 



of the input in a tracking adjustment, there 

 are finite limits outside of which the velocity 

 is not perceptible. The continuity of the 

 velocity data as effective input will depend 

 therefore on whether or not the operating 

 range of pointer velocities remains within 

 these limits. (See also "Operating range of 

 input magnitude.") 



Homogeneity of data. Many tracking dis- 

 plays vary according to the number of input 

 characteristics which may function as effec- 

 tive data. In pursuit tracking^ the target 

 and reticle are both visible and thus the 

 * 'error" or relative position is visible. The 

 target exhibits position, velocity or rate, and 

 acceleration data. So does the reticle. 

 The "error" exhibits size, rate of change of 

 size, and acceleration of size changes. The 

 reticle may be instantaneously "on target" 

 in position but off in rate, so that a positional 

 error immediately develops. The reticle 

 may be "on" in rate but off in position, so 

 that a constant positional error exists as long 

 as rate is constant. Other possibilities are 

 obvious. In compensatory tracking only 

 the error and its derivatives are visible. 

 Displacement of a marker from a null posi- 

 tion indicates error. This technique also 

 allows separation of target movement and 

 its derivatives. The "error" may be in posi- 

 tion, rate, or acceleration. If it is rate in- 

 dication, then the operator knows nothing 

 about position except when it is constant but 

 may receive acceleration data from the rate 

 of error development. If it is acceleration 

 indication, then he knows nothing about 

 position and nothing about rate except when 

 it is constant. The rate of error develop- 

 ment now reflects higher derivatives of target 

 movement which are probably negligible for 

 purposes of control. 



The homogeneity of the input may be re- 



^ In pursuit tracking, sometimes called direct 

 tracking, the operator attempts to superimpose 

 an indicator on a moving target which is visible to 

 him. In compensatory tracking the operator 

 seeks to keep an indicator from drifting away from 

 a stationary zero position. 



garded in three ways: (1) in terms of the 

 amount of information displayed, (2) in 

 terms of the degree of complexity in the op- 

 erator's transmission task, and (3) in terms 

 of the possibility of intermittent selection of 

 effective input from the total input pattern. 



Operating range of input magnitudes. The 

 human operator is characterized by definite 

 limitations concerning the perception of 

 changes in a variable as well as the percep- 

 tion of the variable itseK. We mentioned 

 above that pointer velocities could exceed 

 perceptual limits. We are here concerned 

 with minimal values or the "thresholds" be- 

 low which the perception of differences is im- 

 possible. In a compensatory tracking task 

 the error indicator may move from the null 

 position by so slight an amount that the po- 

 sitional change is not effective as an input. 

 Or it may move at such a slow rate that the 

 latter is not perceptible. Or it may move at 

 a variable rate but with the variations (ac- 

 celerations) not perceptible. The deriva- 

 tives of target position to which these display 

 variables correspond may be utilizable as ef- 

 fective inputs in direct tracking, but the dis- 

 play characteristics may have filtered them 

 out by means of the operator's perceptual 

 thresholds. 



The continuity of the input as well as the 

 homogeneity of the input may be affected 

 by these perceptual thresholds. The aver- 

 age rate of a translatory displacement in- 

 creases with the amplitude of the displace- 

 ment. If the operating range of the 

 amplitudes is great enough, the amplification 

 or reduction characteristics of the transmis- 

 sion system between the operator's move- 

 ment and the pointer which he moves may 

 be adjusted so that (1) the rates of all pointer 

 movements exceed perceptual limits, or (2) 

 rate is perceptible over a portion of the am- 

 plitude range, or (3) the rates of all pointer 

 movements are perceptible. 



Variables in experimental input patterns. 

 Certain variables in input patterns have been 

 observed in experimental work which are 

 worth mentioning, although their applica- 



