The indifference thresholds for the perception of tilt in the roll axis were experimentally determined in a moving base simulator under three tracking task difficulties. The threshold level determined in this experiment is approximately 5 to 7 degrees (.lg)

Junker, A. M.

Roark, M.

English

NASA Technical Reports Server

N79" 15620 .. !
4
THE EFFECTS OF CLOSED LOOP TRACKING ON A SUBJECTIVE
TILT THRESHOLD IN THE ROLL AXIS
By Marvin Roark_ and Andre_ Junker_* !
•Systems Research Laboratories, Inc.
Dayton, Ohio 454/+(I }
}
_*Aerospaee Medical Research heboratory
Wright-Patterson Air Force Base_ Ohio. 45433
SUMMARY
The indifference thresholds for the perception of tilt in the roll axis
were experimentally determined in a moving base simulator under three track-
ing task difficulties. The threshold level determined in this experiment is
approxlmately 5 to 7 degrees (.ig).
INTRODUCTION
In ground based simulatcs, unlike aircraft, false tilt cues may occur
when they are rolled. The amount of tilt which can be detected by the pilot
in the simulator, defined as the indifference threshold, appears to be a func-
tion of the task being performed. To eliminate the false tilt cues that occur
when the indifference threshold is exceeded, washout schemes are used to limit
the motions of the simulator. This is often accomplished by limiting the
amount of roll of the simulator while still allowing the acceleration and
other helpful cues to he felt by the pilot. In the past the washouts used
were based on what felt good to the pilot in the simulator. Very little data
is available relative to what this threshold is, see reference i, and what
factors, if any, alter the level of indifference. Data in the past has dealt
with determining the absolute threshold. One set of data, see reference 2,
does show that the "absolute" threshold increases when workload is increased.
This prompted an investigation to determine the effect of workload on the in-
difference threshold level for use in washouts and in motion related param- i_
eters of pilot modeling.
!An experiment was performed to determine the indifference threshold andthe interactive effect of the tracking task on the threshold value. Thisexporiment and associated results are presented in this paper. _
METHOD J
The baslc idea behind the experiment was to have a subject track a closed
loop disturbance hulling task and then superimpose a randotn appearing ramp
(tilt input) to the motion loop of the simulator. Figure i is a block diagram
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https://ntrs.nasa.gov/search.jsp?R=19790007449 2020-03-20T18:18:40+00:00Z
of the motion and visual systems used in the experiment showing the location
of the disturbance input and the tilt input to the motion plant in the system.
The only input was the disturbance input so the display was the negative of
the simulated plant position. When the subject could detect the tilt input,
he was to indicate the direction of the tilt via a hand-held indicator con-
taining a left and a right thumb actuated pushbutton. The subject was in-
structed to hold the pushbutton down until he no longer felt that he was
tilted. The time histories of the chair position, the tilt input and the
hand-held indicator slgnalswere recorded and later analyzed.
i The experiment was run on the Roll Axis Trackin B Simulator (RATS) at the
i Aerospace Medical Research Laboratory at Wright-Patterson Air Force Base.
The RATS is capable of simulating the roll dynamics of a high performance
aircraft, in this case, an F-16. The plant dynamics are described by equation
! i with limits of 180°/sec and 400@/sec 2. The cab contains a CRT mounted at
the axis of rotation which is through the head of the subject. The subject
viewed the display shown in figure 2 which was 26 inches away. The subject
used a force stick mounted on the right side of the cab to control the track-
ing task. To prevent the subject from experiencing any external cues, a
shroud inclosed the cab, white noise was injected in the helmet and the room
lights were extinguished. In addition, a harness was used to keep the subject
in his seat while the cab was in motion.
Three sum of sine inputs, simulating white noise passed through a low-
pass filter with a double pole at 2 radians, were used with RMS values of
.933, 1.40, and .467 pounds which resulted in 14, 21, and 7 degrees/second
root mean squared (RMS) variance in the visual error. A group of ten subjects
were exposed to the inputs in the order mentioned above. They ran 4 runs a
day for 3 days, with each run lasting 165 seconds. The subject_ had been
trained from a previous experiment, therefore, minimum training was required.
Data from the last two days was used for subsequent analysis. An example of
a data run can be seen in figure 3. The top trace is a time history of the
negative visual error (simulated plant position) seen by the subject. The
negative of the visual error is shown so that it can easily be compared to the
actual motion plant seen in trace 3. This was the tracking task he was trying
to null out by keeping the wings on the display level with the dashed refer-
ence llne. The second trace shows the offset signal that was added to the cab
position. The resulting cab position during the tracking task is shown in
trace 3. A run consisted of from zero to four of the offset signals to maxi-
mize randomness between runs. The offset signal itself occurred at a rate of
l°/sec with limits of ±20 degrees. The l°/sec value was chosen because it is
below the roll velocity threshold of 2@/sec, see reference I. The last graph
is the output of the hand-held indicator the subject used to indicate when the
tilt was felt.
In addition to the tracking data taken, a set of baseline data for each
subject was recorded. This data provides a baseline indifference threshold,
comparable to the absolute threshold data taken in previous experiments, The
subject was asked to sit quietly in the cab while the cab was tilted, using
the same tilt input described earlier. The subject was to indicate the direc-
tion of tilt as he did before. The tracking task was not present and the
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i.______ -..-. 1979007417-427
! !
, display remained fixed on the screen.
RESULTS !!
_ The results are sho_m in figures 4 through 7 and summarized in figure 8. ;
For each group, the offset angle, at the point when the indicator button was
pressed was recorded and placed in groups of half degree increments. The
number of points recorded in each half degree group was then plotted in his-
togram form with the mean and standard deviation shown for each group. The
mean values for the indifference threshold increases with the difficulty of
the task. Fewer data points were needed for the baseline data due to the
smaller amount of variance across subjects.
The summary shows a baseline level of 3.48 ° (.06g) which jumps to 6.46°
(.llSg) with the least difficult task and 7.56 ° (.132g) with the most
difficult. These results are discussed in the following section.
DISCUSSION
From the baseline data, the indifference threshold level is 3.48° (.06g).
This value can be compared to data taken from other experiments where the
absolute threshold was determined but, the results from this experiment are
3 to 30 times higher, see reference i. This is due to the conditions under
which the experiment was run. These conditions added extra loading to the
subjects tilt detection, similar to the kind of loading he would receive when
running in a simulator. Under these "real world" conditions, the simulator
environment, a useful measurement of the indifference threshold is made and
can be directly applied to washout designs and used in pilot modeling.
Figures 5 through 7 show the results when tracking tasks of varying
difficulties are added to the baseline conditions. The results of all four
conditions, summarized in figure 8, contained the type of trend as expected,
see reference 2. The summary shows a sharp Jump from the baseline to the
least difficult of the three tracking tasks and then increasing threshold
levels with the difficulty of the task. The various difficulties were obtain-
ed by increasing the tasks RMS value which had the effect of changing the
signal to noise ratio of the system. This change accounts for part of the
increase in the threshold levels as well as the increase in the variance of
the data. The relatively large jump from the baseline is due to the initial i_
loading of the subject by the tracking task.
CONCLUSIONS
In this paper, an experiment to investigate the effects of the tracking i!!
task workload on the indifference threshold is described. Based on the re- _!
suits and the discussion, the following conclusions can be drawn. !
i) The indifference threshold increases with task loading, i
2) The tilt indifference threshold while performing a tracking task is
approximately .ig.
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1979007417-492
REFERENCES
i. Zacharias, G.: Motion Cue Models for Pilot-Vehicle Analysis. Technical
i Memorandum, Bolt Beranek and Newman Inc., Cambridge, Mass., under
i Contract No. F33615-77-C-0506, September 1977.
2. Hosman, R.J.A.W., van der Vaant, J.C.: Thresholds of Motion Perception
I Measured in a Flight Simulator. Proc. of the Twelfth Annual Conference
on Manual Control, Ames Research Center, Moffett Field, California,
May 1976.
3. Peters, R.A.: Dynamics of the Vestibular System and Their Relation to
Motion Perception, Spatial Disorientation and lllusions, NASA CR-1303, I
1969. I
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Fig. l. Experimental Block Diagram
446
------- 1979007417-429
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Fig. 2. Visual Display
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Fig. 3. Sample Da_a
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1979007417-430
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Fig. 4. Results of the Baseline Data 1
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Fig. 5. Results of the 7 ° R_MSTracking Task
448
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1979007417-431
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OF POOR QUALITY I
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Fig. 6. Results of the 14 ° RMS Tracking Task
21°RM5 INPUT
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Fig. 7. Results of the 21 ° RMS Tracking Task
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1979007417-432
Fig. 8. Data Summary
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The effects of closed loop tracking on a subjective tilt threshold in the roll axis

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