Author: Yeh-Liang Hsu, Chong-Fai Wang, Chun-Ming Lien, Chung-Yang
Kao (2000-04-10); Modified: Yeh-Liang Hsu (2003-02-07);
recommended: Yeh-Liang Hsu (2000-04-17).
Note: This paper is published in Journal of the Chinese Institute of
the Industrial Engineers, Vol. 20, No. 1, January, 2003, p. 62~70.
Design and comfort evaluation of
a novel locomotion device for training overhead traveling crane operators in a
of locomotion devices that provide users a sense of walking while their
positions remain localized in the physical world is still one of the major
problems in current virtual reality research. This paper presents the design
and evaluation of a novel locomotion device for training the overhead traveling
crane operators in a virtual environment. The locomotion device employs a
circular platform with hundreds of spherical rollers on it. A hoop combined
with an airbag is used to constrain the position of the user, so that the user
can walk on the rollers in a “step-and-slide” pattern. The user can also walk
and change directions freely without holding on the hoop. When the user steps
on the rollers, the switches underneath the rollers sense the motion of the
feet. Two-dimensional walking data can be acquired successfully, and the user
does not have to wear any special shoes or obstructive sensors other than the
head-mounted display. A subjective comfort evaluation is also performed to
compare the sense of walking on the locomotion device and that of natural
Keywords: virtual reality, operator
training system, locomotion.
of virtual reality (VR) and virtual environment (VE) technologies has attracted
considerable interests from industrial engineers. Wilson  pointed out that VR/VE has the
potential as a tool to support many types of ergonomics contributions,
including assessments of office and workplace layouts, testing consequences for
reach and access, reconfiguring and evaluating of alternative interface
designs, checking the operating or emergency procedures, and training for
industrial and commercial tasks.
virtual environments for training have shown to be a good addition or
alternative to the traditional operator training systems. Tam et al. 
developed a PC-based VR operator training simulator for power-utility personnel.
The same research group also developed a VR-based learn-by-doing system that is
accessible from a Web browser for the training of maintenance workers in the
troubleshooting and inspection of power transformers [Tam et al., 1999]. Arroyo
et al.  presented a low-cost, PC-based, virtual reality application to
train personnel associated with the manual operation of electrical substations
equipments. Matsubara et al.  built an operator training system for
training the control activities of an electric power plant using the conception
of the virtual learning environment. Wilson et al.  developed a virtual
environment combining 3-D factory drawings with 3-D simulation models of crane
and load motion for training overhead crane operators. Virtual environments are
ideal for explorative and self-paced training. Such applications for operator
training can provide real time interaction with virtual devices and the
function to change viewpoint freely, and they also allow employees to train
without taking production equipment out of service.
The function of
“locomotion,” to allow the users to move about the virtual environment, is
required for many training systems. Many VR/VE applications allow the users to
move in a large virtual space in a vehicle. Using the control devices on the
vehicle such as wheels, joysticks, buttons, pedals, etc., the user interacts
with and moves in the virtual space displayed in the head-mounted display (HMD),
while the position of the user (and the vehicle) is fixed.
most intuitive way to move about the real world is to travel on foot. Some
VR/VE applications allow the users to walk in a confined area. The
three-dimensional magnetic sensors attached to the user are often used for
position sensing when the user moves, so that the image of the virtual space
displayed in the HMD changes accordingly. But in this approach, the sensing
range of the sensors and the size of the confined area strictly limit the
movable area of the user. Therefore this approach is not suitable for moving in
a large virtual space. As Iwata  pointed out, the development of
locomotion devices that provide users a sense of walking while their positions
remain localized in the physical world is still one of the major problems in
current virtual reality research.
approach for virtual walking is to use a treadmill. Treadmills for physical
fitness are unidirectional. Therefore the key issue of this approach is how to
redesign the treadmill so that users are free to walk in any direction. Darken
et al.  developed an “omni-directional treadmill” that employs two
perpendicular treadmills and allows its users to walk or jog in any direction
of travel. For similar purposes, Noma and Miyasato  designed a locomotion
interface using a treadmill mounted on a three-axis motion platform. Recently
Iwata  presented the “torus treadmill.” In this system, 12 treadmills are
connected side-by-side, on which users can walk in any direction.
of the locomotion devices using the treadmill approach are usually quite
complicated. Iwata and Fujii  developed the “virtual perambulator” using
a much simpler “sliding surface” approach, as shown in Figure 1. In the virtual
perambulator, a user wears omni-directional roller-skates or low friction
rubber sandals to slide on the floor. A hoop is set around the user’s waist by
which the position of the walker is limited, while the user can walk in any
direction. Novice users can hold the hoop to keep their balance, and trained
users can push their waists against the hoop while walking. Magnetic sensors
measure the motion of the feet of the user, and the image of the virtual space
displayed in the HMD refreshes accordingly. Two hundred and thirty five novice
users tested this system, and 94% of the participants could freely walk around
the virtual space.
Figure 1. The locomotion device using the “sliding
describes a system for training the overhead traveling crane operators in a
virtual environment. The design and evaluation of the locomotion device in this
training system is emphasized. The locomotion device also uses the “sliding
surface” approach. But instead of wearing special shoes, the locomotion device
employs a circular platform consisting of 9 plates. There are 256 spherical
rollers in a array on each
plate to generate the “sliding effect.” The user can walk on the rollers in a
“step-and-slide” pattern. When the user steps on the rollers, the switches
underneath the rollers sense the motion of the feet. A hoop combined with an
airbag is used to constrain the position of the user. The airbag also supports
the waist of the user so that the user can walk freely without holding on the
hoop. In this system, the user does not have to wear any special shoes or
obstructive sensors. The only device the user needs to wear is the HMD.
requirements followed by the design of the locomotion device for training
overhead traveling crane operators are described in this paper. Then a comfort
evaluation is also performed to compare the sense of walking on the locomotion
device and that of natural walking, and conclusions are drawn.
Training system for overhead traveling crane operators
traveling crane is widely used in manufacturing factories to transport
materials and loads from one point to the other. The main structure of an
overhead traveling crane is an I-beam that is driven by an electric motor to
travel on a fixed track. Another motor controls the hoisting mechanism for
lifting loads. The operator holds a crane controller with both hands to hoist
up loads, and to drive the crane to move on the track. Usually the operator
walks with the crane as it travels.
concerning the structural safety, e.g., the fatigue strength of the structure,
of the overhead traveling crane were carried out [Kitsunai et al., 1998; Zai et
al., 1994]. However, a study of local factories in Taiwan shows that, most accidents related
to the overhead traveling crane are not caused by structural failure. Instead,
the major cause of accidents is that the crane is not properly operated.
Operating the crane, to control simultaneous traveling, traversing, and hoisting
motions of the crane and the resulting load swing, requires a lot of skills.
The crane has to be carefully controlled and moved at low speeds to avoid
dangerous load oscillations. The complicated dynamic motion of the overhead
traveling crane also attracts the attention of many researchers in automatic
control [Al-Garni et al., 1995; Lee, 1998]. However, currently overhead
traveling cranes are still controlled by human operators. Therefore, training
of the overhead traveling crane operators is very important.
training of overhead traveling crane operators often takes place in a real
factory where the trainee practices operating a real crane. The VR/VE
technology provides an ideal solution to the training system for the overhead
traveling crane operators. The trainee can practice safely in a virtual
environment. Different factory setups and operating scenarios can be programmed
in the training system to give the trainee more experience before operating a
real crane in the factory. In order to let the user walk in a large “virtual
factory,” a locomotion device becomes essential. The major design requirements
for the locomotion device of the training system of overhead traveling crane
The locomotion device should
allow free, natural two-dimensional walking, while the position of the walker
To provide the sense of
“reality,” the motion of the feet should be detected and transferred to the VR
software rapidly, so that the image of the virtual factory displayed in the HMD
can refresh in real time.
Since the user has to operate
the crane controller with both hands, the user should be able to walk safely in
the locomotion device without holding anything with hands.
Design of the locomotion device
Figure 2 shows
our design of the locomotion device. There are three major components: the main
frame (component 1 in
Figure 2), the roller plate (component 2), and the waist support (components 3,
4, and 5).
Figure 2. The locomotion device
We used the
“sliding surface” approach in this design. To achieve free and natural
two-dimensional walking, we first decided that the user does not have to wear
any special shoes or obstructive sensors other than the HMD. As shown in Figure
3(a), Fruin  defined the sizes of buffer zone areas, including “touch
zone (A),” “no-touch zone (B),” “personal zone (C),” and “circulation zone
(D).” The locomotion device employs a circular platform of the same size as the
“circulation zone,” within which circulation is possible without disturbing others.
The circular platform consists of 9 plates, with 256 rollers in a array on each
plate, as shown in Figure 3(b). The rollers are used to generate the “sliding
effect.” The user walks in a “step-and-slide” pattern on these rollers.
Figure 3(a) Buffer zones [Fruin, 1971] Figure
3(b) The roller plate
Similar to the
virtual perambulator mentioned in the previous section, the user’s waist is
also supported by a hoop while walking on the platform. The height of the hoop
can be adjusted. An airbag attached to a ring at the inner surface of the hoop
is inflated to pressure the waist of the user, so that the user does not have
to intentionally push the waist against the hoop. This design enables the user
to walk naturally and safely without holding anything with the hands. There are
roller bearings between the hoop and the ring that the airbag is attached. The
user can twist the waist freely while the airbag tightly presses the waist.
Note that the supporting plate of the hoop inclines 45 degrees downward, so
that the user can swing both arms freely while walking on the device.
The rollers on
the plates are also used as sensors for the motion of the user’s feet. Figure 4
shows the configuration of a roller. A steel ball (0.75 in. in diameter) is supported by 6 small steel
balls (0.25 in. each in
diameter). A normally closed (N/C) switch is mounted under the steel balls. As
shown in Figure 4, when the user steps on the balls, the spring is compressed
and the switch opens.
Figure 4. The configuration of a roller
Figure 5 shows
how the motion of feet is detected using these switches. Switches on each plate
are arranged into a sequential array circuit (Figure 6), and are scanned
iteratively. Signals obtained from the 9 plates are mapped into a large array
after filtering the noisy signals. Finally, positions of both feet at a given
time are calculated and output to the VR software for further processing.
Combining with signals from the tracker of the HMD, the speed and direction of
the user are computed, the VR software refreshes the image displayed in the HMD
Figure 5. Sensing the motion of the feet
Figure 6. The sequential array circuit
Figure 7 is the
walking data collected when the user walks in one direction. The peaks
represent the positions of the feet. The horizontal axis is the time elapsed.
Obviously, the positions of the user’s left and right foot change
alternatively. This data is used to compute the velocity of the user. Note that
the user follows a “step-and-slide” walking pattern when walking on the
locomotion device. The switches on the platform return signals only during the
“slide” half of the walking pattern. Therefore, the left half of each peak in
Figure 7 is a sudden and discontinuous change. To avoid generating an abrupt
change of velocity, a linear interpolation is used to estimate the position of
the user’s feet during the “step” half of the walking pattern.
Figure 7. Data for unidirectional walking
The update rate of
the image displayed in the HMD, or often called the “frame rate,” is very
crucial to the sense of presence of the user. In order to achieve a
sufficiently high frame rate, only a quarter of the switches are scanned, as
indicated by the solid circles in Figure 3(b). With this resolution, the
smallest movement of the feet that can be detected is about 50 mm.
Figure 8 shows
the positions of both the feet on the x-
and y-axis when the user walks and
gradually turns right. Accumulating the walking distances of each step, a
trajectory of the user is formed in the virtual environment as in Figure 9.
These figures show that two-dimensional walking data can be acquired
(a) Positions of both the feet on the x-axis
(b) Positions of both the feet on the y-axis
Figure 8. Data for two-dimensional walking
Figure 9. Trajectory of the user in
scenes for overhead traveling crane operator training are constructed. The VR
software used in this research is “World Up” by Sense 8 Corporation (http://www.sense8.com/), which provides
real-time functionality in an interactive, object-oriented environment. The HMD
used in this research is “I-glasses X2”
by I-O Display Systems (http://www.i-glasses.com/).
The HMD has 2 full-color 0.7”
LCDs, 360,000 pixels per eye in standard mode, and 180,000 pixels per eye in
3D, which has the capability of true stereoscopic imaging. The HMD weighs about
220g, and has fully
adjustable stereo earphones.
Figure 10 shows
the photo of a complete training system. The user first stands inside the waist
support hoop, then the hoop is lifted to a proper location and the airbag is
inflated. The user wears an HMD, and holds an overhead crane controller with
both the hands. Through the controller, the user interacts with the overhead
traveling crane in the VR scenes, to hoist loads and drive the crane to move on
the track. The user can also walk in any direction as the crane travels.
Figure 10. A photo of the complete system
Subjective comfort evaluation of the new locomotion device
earlier, VR training systems receive more and more attention for operator
training. However, there are limitations to current VR training systems, and
the operating dynamics of a VR working platform will not exactly be the same as
that of the real world. As discussed in previous sections, whether the
locomotion device can provide a natural sense of walking, that is, whether the
user can walk comfortably on the device, is really the key to the success of
the overhead traveling crane operator training system.
level evaluation of a device falls on the observations of body postures and
movements, observations of task performances, and direct subjective ratings of
general comfort. The main action of our locomotion device is “walking.” Since
the change of body postures and the task performances while walking are not
easy to observe and evaluate, a subjective comfort evaluation was performed to
compare the sense of walking on the new locomotion device with that of natural
subjects, fourteen males and four females, volunteered to participate in this
study. The averaged age was 23.6 years, and they all had normal walking
ability. Neither did the subjects participate in the development of this
locomotion device, nor did they have any knowledge about the locomotion device
before the test. Before the first measurement, each subject was informed about
the experimental procedure and agreed to answer the questions sincerely.
experienced three different walking tests: (1) natural walking on the ground;
(2) walking on the locomotion device without any practice; and (3) walking on
the locomotion device with some practice. In the first two tests, the
facilitator guided the testers to walk on the locomotion device or normal
ground by choosing them randomly, without telling the testers what surfaces
they are walking on. The third test was performed after all testers finished
the first two tests. Each walking test lasted for about 3 minutes. Guided by
the fascinator, the testers were asked to walk in one direction for 1 minute,
then to walk freely in any directions for 2 minutes.
subjects should wear the HMD while walking on the locomotion device. But the
scenes displayed in the HMD are controlled by the locomotion device. It is not
feasible to wear HMD and do natural walking on the ground. Furthermore, the
difference between the VR scenes and the surroundings of the real world may
further affect the subjects’ evaluation of the sense of walking on the
locomotion device. Therefore, the subjects’ eyes are covered in all three tests
to avoid the effect from the perception of the eyes, and they were told to
concentrate on the feeling of all parts of the body while walking. The
evaluation here is strictly on whether the locomotion device can provide a
natural, comfortable feeling of walking.
The purpose of
the second test was to investigate users’ response when they use the locomotion
device for the first time. Before the third test, the subjects were
specifically told to practice the “step-and-slide” walking pattern for a few
steps. The purpose of the third test was to investigate whether the users would
adapt themselves to the locomotion device after some practice.
received a 10 minute break between each test. During the break, the subjects
were asked to answer 6 questions subjectively on an eleven-point scale (‘5’ being ‘very comfortable’, ‘0’ being ‘adequate’, and ‘-5’ being ‘very uncomfortable’), and to
describe their experience about the test. The 6 questions were, while walking,
whether the subjects were comfortable (1) around the shoulder; (2) around the
waist; (3) twisting the waist; (4) swinging both arms; (5) around the legs; and
(6) whether the subjects were comfortable with their walking patterns.
Figure 11 shows
the average scores of each question rated by the eighteen testers. While
walking on the locomotion device, the testers felt comfortable around the
shoulder, twisting the waist, swing the arms, and around the legs, though the
scores of the last three areas mentioned above are significantly lower than the
scores of natural walking.
Figure 11. The result of the human description
The testers gave
only an “adequate” rating to the pressure around the waist in tests (2) and
(3). As discussed in the previous section, there is an airbag around the inner
surface of the waist hoop. The airbag is inflated to pressure the waist of the
user tightly, so that the user does not have to hold anything with the hands or
intentionally push against the hoop. However, this pressure around the waist
also gave a strong sense of being constrained, which does not exist in natural
In Iwata and
Fujii’s  “virtual perambulator” discussed in the first section, 94% out
of the 235 novice users could freely walk around the virtual space. Here all
testers could walk freely around the virtual space, but when asked whether they
are comfortable with the “walking pattern” in the questionnaire, almost all
testers gave “less-than-adequate” ratings. Obviously, the sense of the
“step-and-slide” walking pattern is different from that of natural walking. The
users need some practice to get used to this walking pattern, just like
practice to get used to walking on a treadmill. From the results of the second
and third walking tests shown in Figure 11, the testers’ response on walking
pattern did improve after some practice.
The results of
ANOVA analysis in Table 1 shows that the overall comfort rating of “natural
walking on the ground” is significantly higher than those of “walking on the
locomotion device without practice” and “walking on the locomotion device with
practice”（P<0.05）. The difference of overall comfort
rating between “walking on the locomotion device without practice” and “walking
on the locomotion device with practice” is not statistically significant.
Table 1. Results of ANOVA analysis
Natural walking on the ground & Walking on
the locomotion device without practice
Natural walking on the ground & Walking on
the locomotion device with practice
Walking on the locomotion device without practice
& Walking on the locomotion device with practice
of virtual reality and virtual environment technologies provides a powerful
tool for assessments of working environments. Especially in training, VR/VE
devices have the advantage of let the users to explore and practice situations
or behaviors that are dangerous or unavailable using “real devices.” In the
visual aspect, many researches in VR focus on the visual aspect of how to
generate 3D models that the user can interact with intuitively in real time. In
the mechanical aspect, whether VR/VE devices can provide a natural, comfortable
sense of body movement as in the “real devices,” is also very important.
of locomotion devices that provide users a sense of walking while their
positions remain localized in the physical world is still one of the major
problems in current virtual reality research. This paper presents the design
and comfort evaluation of a novel locomotion device for training the overhead
traveling crane operators in a virtual environment. The locomotion device
employs a circular platform with hundreds of spherical rollers on it. A hoop
combined with an airbag is used to constrain the position of the user, and the
user can walk on the rollers in a “step-and-slide” pattern. The airbag supports
the waist of the user so that the user can walk and change directions freely
without holding on the hoop. When the user steps on the rollers, the switches
underneath the rollers sense the motion of the feet. Two-dimensional walking
data can be acquired successfully, and the user does not have to wear any
special shoes or obstructive sensors other than the HMD.
evaluation is performed to compare the sense of walking on the locomotion
device and that of natural walking. The testers felt comfortable with the
locomotion device in general, and they are able to walk freely on the device.
Though the testers are not used to the “step-and-slide” walking pattern, their
response on the walking pattern improved after some practice. The testers also
responded that the waist support of the device was too rigid, which gave them a
strong sense of being constrained.
feedbacks from the testers, some suggestions are made for future development.
Some kind of cushion should be added between the inner surface of the waist
hoop and the airbag ring. This should soften the constrained feeling of the
users and allow the users to lean forward while walking. Note that the surface
of the unidirectional treadmill for physical fitness always inclines backward,
which creates a “climbing effect” that makes the user’s slide motion more
natural. Analogous to the treadmills, another possible improvement for future
development of the locomotion device is to change the roller plate into a
bowl-type surface. This is technically much more difficult but should improve
the sense of the “step-and-slide” walking pattern on the locomotion device.
support from the Institute of Occupational Safety and Health, Taiwan,
ROC is gratefully acknowledged.
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