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specific points tested to arrive at the best compromise
region were not given.
Motion cues
inadequate
Best compromise
Too much
displacement
1.0
0.8
0.6
0.4
0.2
0.0
1.0 10.0
Motion filter time constant, sec
Motion filter gain
0.1
Figure 3. Suggested lateral translational axis criterion.
Jex et al. expanded their roll study (ref. 41) into the roll
and sway axes (ref. 43), and the effects of the false lateral
translational cue owing to roll attitude were investigated.
A second-order high-pass filter with a damping ratio of 0.7
was inserted into the lateral translational drive path, and a
suggested motion-fidelity criterion of the filter’s gain
versus frequency was proposed as shown in figure 4. A
large region of uncertainty exists because of the limited
range of points tested.
Bray focused on the vertical axis and determined the effects
of motion filter natural frequency on tracking and stabilization
tasks with an idealized helicopter model (ref. 24).
He suggested that the vertical acceleration phase-fidelity
should be accurate down to 1.0–1.5 rad/sec. Fidelity was
somewhat arbitrarily defined as the simulation motion cue
not having a phase error of more than 20° relative to the
model. Moderate decreases in pilot-vehicle crossover
frequency and phase margin were noted if the vertical
motion platform gain was lowered from 1.0 to 0.5. No
other gains were examined.
Sinacori Criteria. Sinacori used a six-degrees-offreedom
helicopter model (ref. 44). Criteria relating the
motion-drive dynamics to motion fidelity were postulated
from a very limited set of data (four test points); they are
shown in figure 5. The criteria suggest that motion
fidelity can be predicted by examining the gain and phase
shift between the math model and the commanded motion
system accelerations at a particular frequency. The phase
shift between these two accelerations is due to the highpass
motion filter placed between the two signals. The
gain and phase of this filter at 1 rad/sec determine the
x and y locations on figure 5, respectively. The amount
by which the commanded motion-system acceleration
phase angle differs from 0° is defined as its phase
distortion. Apparently, a frequency of 1 rad/sec is used,
since that is where the semicircular canals have the
highest gain, as shown in appendix A. The resulting gain
and phase distortions are then located on the appropriate
criterion in figure 5, depending on whether the filter is a
translational or rotational filter.
The criteria show three levels of motion fidelity: high,
medium, and low. The definitions are given at the bottom
of figure 5. As expected, high motion fidelity is associated
with high-gain and low-phase distortion, and low motion
fidelity is associated with low-gain and high-phase
distortion. Sinacori notes that these criteria “. . . have
little or no support other than ‘intuition’” (ref. 44). Still,
these are the most complete criteria proposed to date. The
criteria are either unknown or unused in the simulator
community today, perhaps because they still need to be
validated.
Summary of Criteria. Summarizing the above
criteria, there is apparent agreement that the rotational
gain can be reduced to 0.5 without a fidelity loss, and that
the phase distortion from the high-pass filter should be
minimized at 0.5 rad/sec and above. These requirements
come primarily from studies of roll and of limited pitch.
However, many investigators, somewhat arbitrarily, place
the same requirements on yaw, since they believe the yaw
requirements are natural extensions of the pitch and roll
requirements.
9
Figure 4. Sway motion fidelity criterion (ref. 43).
Low
Specific force
High
Medium Medium
100
80
60
40
20
0
0.6 0.8 1.0
Gain @ 1 rad/sec
0.0 0.2 0.4
Low
Rotational velocity
High
100
80
60
40
20
0
0.6 0.8 1.0
Gain @ 1 rad/sec
Phase distortion @ 1 rad/sec (deg)
0.0 0.2 0.4
High
Medium
Low
Motion sensations are close to those of visual flight
Motion sensation differences are noticeable but not objectionable
Differences are noticeable and objectionable, loss of performance, disorientation
Figure 5. Sinacori motion-fidelity criteria (ref. 44).
10
For the translational motion fidelity, the agreement is less
apparent. The data for the translational requirements are
primarily from lateral translational-axis experiments, with
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Helicopter Flight Simulation Motion Platform Requirements(9)
