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Motion System Configurations
The second-order high-pass motion filter given by
equation (12) was used. The gains, damping ratio, and
natural frequencies evaluated were the same as for Vertical
Experiment I, which are given in table 3.
Procedure
All configurations were tested in blind evaluations, and
they were randomized. Pilots were asked to rate the
motion fidelity of each configuration, using the motionfidelity
definitions given in figure 4. Between each
configuration, in order to calibrate or recalibrate themselves
to the true vehicle model response, pilots flew the
model with full motion (configuration V1) in a visual
scene depicting objects of known size (shown in fig. 35).
All six pilots flew all 10 configurations.
Results
Objective Performance Data
Time-histories and standard deviations of several pertinent
variables for a full-motion case (V1) and a no-motion case
(V10) are shown in figure 54. Both of these runs were
made by the same pilot. This comparison reveals that
when going from full motion to no motion, the target
error, vehicle acceleration, and collective displacement all
increase. Rather than compare time-histories across the
10 cases and six pilots, several pilot-vehicle performance
metrics were determined, and the statistical significance
was evaluated.
First, since the two sum-of-sines inputs were statistically
independent, two effective open-loop pilot-vehicle
describing functions may be determined (ref. 41). One
open-loop describing function applies to the target errors
caused by target motion; it is determined by calculating
the ratios of the Fourier coefficients of h(jw)/e(jw) at the
Figure 54. Compensatory tracking: full motion versus no motion.
43
target input frequencies. The other describing function
applies to target errors caused by the disturbance input; it
is determined from the ratios of the Fourier coefficients of
–dc(jw)/ dctot(jw) at the disturbance input frequencies.
These two describing functions are referred to the “target
following” and “disturbance rejection” describing functions
hereinafter.
From these describing functions, open-loop crossover
frequencies and phase margins were determined by linear
interpolation. Figure 55 shows the magnitude and phase
responses of the disturbance-rejection describing function.
The data for this example are from the full-motion run
(V1) in figure 54. Linear interpolation between the data at
the appropriate frequencies (shown in fig. 55) gives a
crossover frequency of 3.3 rad/sec and a phase margin
of 28°.
The above two measures provide useful information about
the character of the pilot-vehicle response. In particular,
the crossover frequency is a rough measure of how fast
the error is initially zeroed; the higher the crossover
frequency, the faster the initial nulling of the error. The
phase margin is a rough measure of the damping ratio of
the error response; the higher the phase margin, the more
damped the error response.
Each of these open-loop describing functions includes a
different combination of both the pilot’s internal visual
and motion compensation applied to the visual error and
to the acceleration feedback (refs. 26, 41). Although only
the motion cues were changed here, the pilot’s internal
compensation may change in an attempt to account for
degradations in either the visual or motion cues. The
overall effect of these changes on pilot-vehicle
performance and opinion are given next.
Target Following. The target-following crossover
frequencies are given in figure 56 for all of the configurations.
For easy reference, the motion-filter gain K and
natural frequency w, rounded to one decimal place, are
indicated above each bar. The mean value (bar height) and
standard deviation (horizontal line above bar) are shown,
each determined from six values. Each of the six values
corresponds to the individual average for each pilot across
his runs. Upon examining the variance ratio, or F-test
10
-1
10
0
10
1
-20
-10
0
10
20
Magnitude, dB
Phase margin
10
-1
10
0
10
1
-300
-250
-200
-150
-100
Phase, deg
Frequency, rad/sec
Crossover frequency
{
Figure 55. Example disturbance-rejection describing function.
44
V1 V2 V3 V4 V5 V6 V7 V8 V9 V10
0
1
2
3
4
Configuration
Crossover freq., mean and rms, rad/sec
1.0
0.0
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Helicopter Flight Simulation Motion Platform Requirements(30)
