A study was run using both a stable, third-order task and an adaptive first-order unstable task singly and in combination to test the effects of 2 min hypoxia (22000 ft) on human operator. The results indicate that the RMS error in the stable task does not change as a function of hypoxic stress whereas the error in an unstable task changes significantly. Models involving human operator parameter changes and noise injection are discussed

Holden, F. M.

Iay, C. N.

Replogle, C. R.

English

NASA Technical Reports Server

34. Application of Manual Control Theory to the 
Study of Biological Stress 
CLYDE R. REPLOGLE, FRANK M. HOLDEN, AND CARROLL N. DAY 
Wright-Putterson Air Force Base 
Current techniques used to evaluate the effects of stress on the human operator may be gen- 
erally divided into two classes: man-machine systems effectiveness models and factor-analytic 
techniques using test batteries. Each of these methods have inherent difficulties which may be 
solved by manual control techniques. The use of mission effectiveness metrics provide the 
essential estimate of system performance but due to the broad range of human operator adapta- 
tion, the effects of stress are not observed until catastrophic failure. On the other hand, task 
batteries can be constructed which are sensitive to changes induced by stress but from which 
little extrapolation to system effectiveness can be made. It is desirable, therefore, to have a task 
sensitive to stress induced changes in the human operator while measuring quantities which can 
be used in an analysis of mission effectiveness and reliability. 
A study was run using both a stable, third-order task and an adaptive first-order unstable 
task singly and in combination to  test the effects of 2 min hypoxia (22 000 ft). The results indi- 
cate that the RMS error in the stable task does not change as a function of hypoxic stress 
whereas the error in an unstable task changes significantly. Models involving human operator 
parameter changes and noise injection are discussed. 
INTRODUCTION 
This report discusses an experiment the pur- 
pose of which is to deminstrate that measure- 
ment of human operator parameters is a sensitive 
technique for evaluation of human operator per- 
formance under stress. The task consisted of an 
adaptive unstable compensatory tracking task 
and a stable third-order plant compensatory 
tracking task. The stresses used consisted of 
three simulated altitudes utiIizing oxygen- 
nitrogen mixtures equivalent to sea level, 12 000 
f t ,  and 22 000 f t .  
Research into the effect of stress on the human 
operator may, in general, be broken into three 
classes : systems effectiveness evaluation for mis- 
sion oriented tasks, behavior testing for changes 
in the operator, and physiological examination. 
Substantial difficulties have risen in the ability 
to predict a decrement in operational capability 
from all three approaches. The general pattern 
found in heat, hypoxic, acceleration, vibration, 
vestibular, toxic, and workload stresses is physio- 
logical and operator compensation to some limit 
and then rapid decreases in systems effectiveness. 
The man in a control situation will utilize his 
maximum capability to reduce or minimize closed 
loop error until the environment, task, mission 
goal or psychological factors, overwhelm him or 
terminate the control task. For this reason, mea- 
surement of system effectiveness alone, while 
providing necessary mission capability informa- 
tion, is not predictive of the decrement. Behav- 
ioral and physiological measures are more 
sensitive to changes in the human operator but 
only with great difficulty (if at all) can be related 
back to changes in operational errors. The task 
of identification of compensation and limits of 
human operator control parameters then falls 
to the control engineer. Not all of the techniques 
used to study manual control may be adapted 
to this area. The duration of many stresses may 
be as short as 30 sec to 1 min. Further the degree 
of compensation or adaptation may be large in 
this time frame. Techniques which need a 30 sec 
285 
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286 SEVENTH CONFERENCE ON MANUAL CONTROL 
analysis period or which must be done off-line 
have a t  best limited uses. 
To evaluate the performance of a man in a 
control situation, the investigator is obligated 
to evaluate how the man is achieving effective 
control. If the stresses upon the man are affecting 
his ability to control the weapons system and the 
man can switch to alternative modes of control 
while maintaining closed loop error at a mini- 
mum, then evaluation of the man as an input- 
output processor will show the changes in his 
operating behavior while analysis of closed loop 
error will be uninformative. If changes in the 
man’s mode of control are correlated with the 
stresses upon the man then they ought to be 
correlated with the psychological and physio- 
logical changes induced by the stresses. 
TASK DESCRIPTION 
The unstable tracking task is a first-order sys- 
tem with a single pole to the right of the imag- 
inary axis. The first-order unstable tracking task 
as described by Jex et al. (ref. 1) was modified 
in the following manner (fig. 1). The absolute 
value of the displayed error was filtered with a 
UNSTABLETASK 
FIGURE 1.-Modified unstable tracking task. 
first-order linear filter and then divided into a 
constant K. This provides a real time function 
called b. The function b is always positive and is 
used as the pole in the unstable element. By 
appropriate choices of the values for the constant 
K and the time constant in the first-order filter, 
one can prevent skilled trackers from losing 
control of the task while at the same time forcing 
him to maintain a low mean squared error. 
The above modification of the unstable track- 
ing task provides the following advantages. The 
operator while controlling such a system can 
optimize one of two variables. He can either 
optimize the displayed error which he is in- 
structed to do or he can optimize the difficulty 
of the controlled element by operating a t  a point 
which provides a manageable unstable element. 
In either case in order for the operator to effec- 
tively control the system, he must either adjust 
his internal gain or he must maintain an effective 
time delay which is smaller than the value of the 
unstable pole at all times. Jex et al. have 
observed that the unstable tracking task forces 
a mode of controlling by the operator which is 
consistent with pure gain controlling. This is 
true as long as the operator maintains an effective 
time delay shorter than the value of the unstable 
pole. The distribution of the moving pole b 
or the distribution of the displayed error there- 
fore provides a measure of either the operator’s 
gain or his effective time delay. Jex has shown 
that in his formulation of an unstable element 
the critical value of the unstable pole is propor- 
tional to the effective time delay of the operator 
and Leviston has demonstrated an increased 
sensitivity in the unstable task to injected noise. 
The stable tracking task was chosen to be rep- 
resentative of the pitch axis dynamics of a high 
performance aircraft. The dynamics of this stable 
tracking task are typical of a third-order system 
with a pole a t  the origin. The forcing function 
used in the stable tracking task consisted of the 
sum of five sine waves whose frequencies were 
chosen to simulate Gaussian white noise buffet- 
ing (fig. 2). 
If the operator’s performance a t  22 000 f t  is 
different than his performance a t  sea level, then 
the modification of the unstable tracking task 
as described above should indicate either a 
change in the mean value of the signal b or a 
APPLICATION O F  MANUAL CONTROL THEORY 287 
r ---- 1 
Sin (w3t) 
Sin Cw,t) =-+ 
Sin (w5t) 
STABLETASK 
~ 
FIQURE 2.-Stable tracking task representative of the 
pitch axis control system of a high performance 
aircraft. 
HPA Pitch 
Dynamics 
change in his rms error. Either of these changes 
or both in combination indicate that there is an 
internal change in the structure of the human 
operator. The operator has no control over the 
dynamics represented in the stable tracking task. 
It is, therefore, anticipated that at altitude no 
effect on rms error will be observed. If the oper- 
ator is forced by the affects of altitude to change 
his internal structure as an operator, he can 
do so while operating the stable task and main- 
taining identical or similar mean squared error 
distribution. 
EXPERIMENTAL DESIGN 
The experiment is designed to evaluate the 
performance of an operator in a compensatory 
tracking task at  sea level followed by the same 
task at  a simulated altitude of either 12000 or 
22 000 ft .  Each run of the experiment consisted 
of two periods of tracking. Each period was pre- 
ceded by one minute of prebreathing at the 
indicated altitude followed by one minute of 
tracking. Randomization of the order of presen- 
tation of the simulated altitudes and tasks to  the 
subjects was done in order to minimize the effects 
of learning and anticipation of experimental 
factors. 
Six subjects were evaluated. Each subject per- 
formed the same task on two different occasions. 
Three Compensatory tracking tasks were em- 
ployed. The first was a single axis stable tracking 
task. The dynamics of the stable tracking tasks 
were representative of a high performance air- 
craft pitch axis control system. The second task 
was an unstable single axis tracking task modi- 
fied as described below. The third task consisted 
of a dual axis tracking task combining the stable 
tracking task on the vertical axis and the unsta- 
ble tracking task on the horizontal axis. The data 
were sampled at  a rate of 50 samples per sec. 
The data were reduced by calculating the mean 
error and the variance of the error for the dis- 
played error signals, and the mean unstable pole 
and the variance of the unstable pole in the 
unstable tasks. I n  addition, empirical probability 
entity functions were generated for the displayed 
error and the unstable pole-signals. The mean 
squared error scores and the variance of the mean 
squared error scores were subjected to analysis of 
variance utilizing a random variable design. 
RESULTS 
The results are straightforward and require 
little interpretation. Referring to table 1, it is 
TABLE 1.-Results of Analysis of Variance Comparing the Diflerences in the 
Various Scores at Ground Level and 22 000 f t  Altitude Across Subjects 
Task 
Main effect Interactions 
Score d f= l  df=5 F ratio 
Stable Rms error 1.69 11.99 .8457 
Unstable Rms error 0.01 64.97 .00092 * 
Combined Mean 6( t )  6.98 5.54 7.56t 
* p  > 0.99 
t p  > 0.95 
288 SEVENTH CONFERENCE ON MANUAL CONTROL 
apparent that the RMS error in the stable axis 
was not affected by the 22000 ft altitude. The 
RMS error in the unstable tracking task is sig- 
nificantly affected by altitude, and the mean 
operating point b in the combined task unstable 
axis is significantly affected by the altitude of 
22 000 ft. 
Further analysis of the data will include iden- 
tification of the linear portion of the human 
operator as a function of altitude. The analysis of 
variance will be repeated on the parameters of 
the operators. It is anticipated that examination 
FIGURE 3.-Representative empirical probability density 
function of closed loop error in the stable task at sea 
FIQURE 4.-Representative empirical probability density 
function of closed loop error in the stable task at 
22 000 feet. 
of the internal parameters of the operator while 
performing the stable tracking task will demon- 
strate an effect due to the simulated altitude. 
Support for this statement arises as a result of 
examining the data for the unstable tracking 
task at  22 000 ft. It is apparent that the majority 
of the operators in the unstable axis attempted 
to maintain the average operating points b at an 
identical point irrespective of altitude while 
allowing mean squared error to change. 
Examination of the empirical probability den- 
sity curves (figs. 3 through 8) verifies that the 
FIGURE 5.-Representative probability density functions 
for b( t )  and closed loop error e(t)  in the unstable task 
4 3 I I D  1 1 1  , .I11 
FIGURE 6.-Representative probability density functions 
for b(t)  and closed loop error e@) in the unstable task 
at 22 000 feet. 
APPLICATION OF MANUAL CONTROL THEORY 289 
P, 
I \  
I \  
I 1  
I I  
I 1  
I I  
I I  
I I 
I 
I I 
I 
I I 
I 
I I 
FIGURE 7.-Representative empirical probability density 
functions of vertical axis closed loop error e( t )  and 
horizontal axis b(t)  function at ground level. 
error curve for the unstable task has a broader 
variance at  altitudes. The b curves however shift 
their mean to a lower value at  higher altitudes. 
The variance due to interaction was too large to 
demonstrate significance in the change of the 
mean b at altitude. If the mean b scores are 
normalized however a student’s T test shows 
significance at the 0.01 level. 
Figures 3 and 4 are the empirical probability 
density functions for the displayed error in the 
stable tracking task. There is relatively little 
difference between the curves a t  the control 
altitude, sea level, and at the 22 000 f t  altitude. 
Figures 5 and 6 are the probability density 
functions for both b and e in the unstable track- 
ing task. Not only does the shape of the error 
curve e change but also the mean of the b curve 
is lower at 22 000 ft and has a broader variance 
at 22 000 ft. Figures 7 and 8 are the probability 
density functions for the combined task a t  con- 
trol and a t  sea level and at  22 000 ft. Again, there 
is relatively little change in the error curves for 
the vertical axis whereas the mean value for the 
b curve shifts to a lower point at 22 000 ft .  
The purpose of these experiments is not to 
evaluate a unique way of identifying a perfor- 
mance measure. Rather, the purpose of these ex- 
periments is to demonstrate that control theory 
principles can be used to identify changes in 
human operator performance as a result of 
FIGURE 8.-Representative empirical probability density 
functions of vertical axis closed loop error e( t )  and 
horizontal axis b( t )  function at 22 000 feet. 
environmental factors. The data clearly demon- 
strates that standard measurements of error are 
insensitive to changes in human operator per- 
formance at  22000 ft. That changes in the 
human operator occur at  22 000 ft is evidenced 
by the changes in the unstable tracking task 
parameters. One of the exciting aspects of the 
experiments was that the same changes and 
effects were suggested at  12000 ft. The effects 
at 12 000 ft, however, were severely masked by 
the large amount of subject variance and 
interaction variance. 
SUMMARY 
Although the performance of a man-machine 
control system is best represented as a function 
of the closed loop system error, the compensa- 
tory actions of the human operator will obscure 
the effects of stress on the man until the point is 
reached where his compensation is not effective. 
In  order to predict this point instead of merely 
observing it, the parameters of the human oper- 
ator, the effect of stress on these parameters, and 
the range of control of these parameters must be 
described. 
REFERENCE 
1. JEX, H. R.; MCDONNELL, J. D.; AND PHATAK, A. V.: 
A Critical Tracking Task for Man-Machine Research 
Related to the Operator’s Effective Delay Time. Part 
I: Theory and Experiments with a First-Order Diver- 
gent Controlled Element. NASA CR-616, Nov. 1966. 


Application of manual control theory to the study of biological stress

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