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Hosman and van der Vaart (ref. 19) found that performance
improved with motion in roll for both disturbance
rejection and tracking over that of the no-motion case.
However, the roll motion in this case included the
spurious lateral specific force cues owing to the lack of
simulator translational motion available to account for
coordination.
One of the few studies that has examined the performance
effects of full motion versus no motion was performed for
the roll axis by McMillan et al. (ref. 20). That study also
showed significant improvement in tracking, but little
improvement in a transfer-of-training metric when full
motion was present over the no-motion case. Similar
results were present by Levison et al. (ref. 21).
Boldovici (ref. 8), in his balanced presentation on both
sides of the motion argument, gives a set of reasons for
employing motion platforms: (1) to reduce the incidence
of simulator sickness (note that this argument is used
both for and against motion), (2) users’ and buyers’
acceptance of improved validity, (3) trainees’ motivation,
(4) to learn how to perform time-constrained dangerous
tasks, and (5) to overcome the inability to perform some
tasks without motion.
For difficult control tasks, early studies showed that
motion allows a pilot to form the necessary lead compensation
more readily with acceleration cues than with the
visual displays alone (refs. 18, 22). For stabilization tasks
a pilot will often use this lead compensation to reduce the
open-loop system phase loss and thus allow an increase in
the pilot-vehicle open-loop crossover frequency to a point
higher than that achieved without motion (ref. 23). This
increased crossover frequency, with the same or better
phase margin, yields tracking performance more akin to
that of flight.
Interestingly, the FAA has been a strong supporter of
platform motion. Indeed, if a device is to be called a
simulator by the FAA, it must have motion. If a device
does not have motion, then the FAA terms it “a flight
training device” (refs. 3, 5).
Developing Requirements for Motion
Since instances have clearly arisen in which the addition
of platform motion shows significant benefits, the
question remains “For those instances, what are the
motion requirements?” Defining the necessary requirements
for the quality, or fidelity, of that motion has been
difficult. The fact that requirements are not known is
6
evident from the following quotes from the literature.
“Unfortunately, explicit definitions of ‘valuable’ motion
fidelity, for specific research or training objectives, remain
for the most part undetermined” (ref. 24). “Formal
experiments to determine acceptable attenuation and phase
lag of the force vector are limited in scope . . .”
(ref. 25). “Future research topics in the area of flight
simulation techniques should encompass minimum
essential visual and motion cueing requirements for a
particular flying mission” (ref. 2).
Although definitive answers regarding necessary motion
requirements do not exist, regulators still suggest which
motion degrees of freedom may be useful. These suggestions
depend on the level of simulator sophistication
desired by the user. For instance, the FAA specifies two
levels of motion sophistication for helicopter flight
simulators: full six-degrees-of-freedom motion, and threedegrees-
of-freedom motion (ref. 5). For the latter, the
nominal three degrees of freedom are pitch, roll, and
vertical. If degrees of freedom different from these are
selected by a user, they must be qualified by the FAA on a
case-by-case basis. Although the selection of the pitch,
roll, and vertical degrees of freedom is reasonable, evidence
to support the selection of these axes or any set of axes is
lacking.
Even though existing motion criteria are incomplete,
however, considerable research has been performed. To
divide and conquer the problem, the six degrees of freedom
are often broken into two categories: rotational motion
and translational motion. But even at this high level,
differences in opinion exist on the relative cueing importance
of these two categories. For example Stapleford
et al. (ref. 26) state, for tracking, “Translational motion
cues appear to be generally less important than rotational
ones, although linear motion can be significant in special
situations.” Young states “For most applications, simulation
of vehicle angular motions is more important than
translational simulation” (ref. 18). In contrast, the
　
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