This is the peer reviewed version of the following article: Suzurikawa J, Kinoshita T, Inoue T, Kamo M, Iida N, Iwamoto K, Matsumoto O, "Evaluation ofChanges in Power Wheelchair Maneuver Induced by a Downhill Turning Prevention Control on Cross Sloped Surfaces.", IEEJ Trans. Electr. Electron.
Eng., 7, pp. S184‐S186, 2012, which has been published in final form at http://dx.doi.org/10.1002/tee.21823. This article may be used for
non-commercial purposes in accordance with Wiley Terms and Conditions for Self-Archiving.

Letter
Evaluation of Changes in Power Wheelchair Maneuver Induced by a Downhill
Turning Prevention Control on Cross Sloped Surfaces
Jun Suzurikawa*,**, Member
Takafumi Kinoshita*, Non-member
Takenobu Inoue*, Non-member
Mitsuhiro Kamo***, Non-member
Norikazu Iida***, Non-member
Kakuya Iwata****, Non-member
Osamu Matsumoto****, Non-member
In order to improve direction stability of a power wheelchair on a cross sloped surface, a downhill turning prevention control(DTPC) has been developed by some researchers and wheelchair suppliers. The DTPC-induced effect on wheelchair maneuver,
however, has not been well clarified. In this study we quantitatively assessed DTPC-induced changes in joystick control
strategies during a driving task on a cross sloped test course. Among several evaluation measures calculated from the joystick
inputs during the test trials, the x-axis joystick displacement amount was found to be significantly decreased by the DTPC. This
result suggests that the DTPC can save wheelchair users from the burden of compensation control on cross sloped surfaces.
Keywords : downhill turning tendency, cross slope, smart wheelchair, man-machine interaction, evaluation

1. Introduction

2. Materials and Methods

Downhill turning tendency of wheelchairs on cross sloped
surfaces is one of main problems that hamper stable driving andthus threaten safety of the users [1]. This unintended veer resultsfrom the moment arm of the center of gravity about thedownhill-side wheel, which affects the running direction of front
casters.
In order to overcome this unstable behavior ofwheelchairs, safety systems with a downhill turning prevention
control (DTPC) have been developed by some researchers andwheelchair suppliers [2],[3]. A standard DTPC system isdesigned to automatically detect and compensate for gravity- orterrain-induced behaviors of wheelchairs [2]-[4].
Powerwheelchairs with DTPC are already commercially available like
“G-Trac™” by Invacare®. To our knowledge, however, few
researches have assessed the effect of the DTPC on wheelchairmaneuver.
In this study, in order to obtain objective evidence of theDTPC’s benefit we investigated how the DTPC influences
wheelchair maneuver strategies during a driving task on a crosssloped surface. For objective and quantitative assessment,
joystick control and wheelchair behavior were recorded during thetask performances with and without the DTPC.

Figure 1 shows a test course with an 8-degree cross slope and atest wheelchair. The test wheelchair was a rear-powered one thatmatchedJapaneseIndustrial
Standards(JIST9203,electric-motor-driven manual wheelchair) and was mounted with acustom-made DTPC system. The details of the DTPC were
described in ref. 4. In short, the DTPC system employs
proportional-integral (PI) control, which modulates angularvelocities of drive wheels to offset a difference between an actual
yaw rate measured by a rate gyro and a target yaw rate defined by
the joystick input. Six subjects with physical disabilities (theweights ranging from 47 to 65 kg, the heights ranging from 153 to170 cm) participated in the test drive. All the subjects had over
one-year experience of daily use of a power wheelchair. The
subjects were instructed to keep as straight as possible during atask, driving between the lines spaced at 4 m apart. Test trials
were repeated fifteen times in each control status, i.e. with theDTPC being on/off. Both the subjects and an experiment
instructor (occupational therapist) for ensuring safety were notinformed of the on/off status during the task performance.

* Department of Assistive Technology, Research Institute of National
Rehabilitation Center for Persons with Disabilities
4-1, Namiki, Tokorozawa, Saitama 359-8555, Japan
E-mail address: suzurikawa-jun@rehab.go.jp** PRESTO, Japan Science and Technology Agency
4-1-8, Honcho, Kawaguchi, Saitama 332-0012, Japan*** AISIN SEIKI Co., Ltd.
2-1, Asahi-machi, Kariya, Aichi 448-8650, Japan**** Intelligent Systems Research Institute, National Institute ofAdvanced Industrial Science and Technology
Central 2, 1-1-1, Umezono, Tsukuba, Ibaraki 305-8568, Japan© 200● The Institute of Electrical Engineers of Japan.

Fig. 1. Test course with an 8-degree cross slope. A test
wheelchair with the DTPC is shown in the inset.
1

X-axis (left/right) and y-axis (forward/backward) joystick angulardisplacements, angular velocities of the drive wheels and video
movies were recorded during the trials.
For quantitative analysis, seven evaluation measures werecalculated from the acquired data and averaged for fifteen trials.
The measures include a lateral displacement of the wheelchair
after running through the test surface, time to complete each trial,
x- and y-axis joystick displacement amounts (JDAx, y), x- andy-axis joystick displacement variation amounts (JDVAx, y) and ayaw rate amount (YRA). The lateral displacements of thewheelchair body were measured from the video recordings. TheJDA and JDVA were calculated as follows:
Tgoal

JDAx  Tstart

JDVAx  JS x (t ) dt

Tgoal

Tstart………………………………………(1)dJS x (t ) dt
dt

Fig. 2. Time courses of x-axis joystick angular displacements.
Angular displacements (black, left axis) and the differentials (red,right axis) are shown for typical trials with (top panel) and without(bottom) the DTPC by the same subject. Joystick inputs for thedownward direction of the cross slope (leftward to the travelling
direction in Fig. 1) are defined as positive.

…………………………………(2)where JSx(t) is a time course of x-axis joystick angular input, andTstart and Tgoal are times of starting and finishing each trial. TheJDA is a temporal amount of an absolute joystick displacementfrom the neutral position during the driving task, whereas theJDVA is an amount of the input variation, which quantifies
stability of the joystick input. The YRA also indicates stabilityof wheelchair behavior and was calculated by Eq. (1), JSx(t) of
which was replaced with the yaw rate. The yaw rate wascalculated from the angular velocities of drive wheels.
This experiment was approved by the institutional review board
at National Rehabilitation Center for Persons with Disabilities andall subjects provided their informed consent.

3.

Table 1. Evaluation measures (N = 6).
With theWithout theEvaluation
p valueDTPC
DTPC(t-test)
measureMean SD
Mean SD
Displacement,
7.1
5.8
6.5
3.6
0.829
cm
Time, s
6.9
1.8
7.1
1.2
0.910
JDAx, deg s
9.85
3.30
29.3
8.33
3.45×10-4
JDVAx, deg
45.7
23.2
50.9
25.3
0.718
JDAy, deg s
187
10.5184
17.00.729
JDVAy, deg
49.4
10.3
52.5
10.2
0.617
YRA, deg
0.377 0.113
0.394 0.067 0.747
subjects should also be increased for further detailed analyses.

Results and Discussion

Figure 2 shows time courses of x-axis joystick inputs (JSx) andtheir differentials (d(JSx)/dt = JSx’) during the driving tasks with
the DTPC being on/off. As compared to the trial with the DTPC,
the joystick displacement during the trial without the DTPC
clearly included an offset component for compensation ofgravity-induced downhill turning. The differential time coursesof the x-axis inputs, on the other hand, showed no distinct
difference in maneuver tendencies between the trials with andwithout the DTPC. In Table 1, the seven evaluation measures aresummarized with means, standard deviations (SDs) and p values
between the trials with and without the DTPC (two-sided t-test).
Thanks to the DTPC, the mean JDAx value, or temporalsummation of JSx(t), dropped to one third of that without theDTPC. This means that the DTPC successfully replaced themanual directional compensation by the subjects.
These
observations clearly demonstrate the effectiveness of the DTPC on
wheelchair maneuver. The distinct decrease of the JDAx is likely
to lead to reduction of the driving burden especially for persons
with upper-body physical impairments [5].
The fact that the other evaluation measures than the JDAx
remained unchanged regardless of the DTPC being on/off will be
attributable to superior driving skill of experienced wheelchairusers. That is, the manual compensation of the gravity-induced
veer by the subjects was sophisticated enough not to significantly
affect the measures unrelated to the compensating operations. As
Sorrent and his colleagues indicated [6], however, experimentswith less experienced subjects and/or more challenging test
courses than those in the present study might unveil
DTPC-induced changes in the other measures. The number of4.

Conclusion

In this study we have obtained quantitative evidence supporting
the effectiveness of the DTPC on wheelchair maneuver.
Comparison of joystick controls with the DTPC being on/off
revealed that the DTPC significantly reduced a total amount ofx-axis (left/right) joystick displacements during the driving task
on a cross slope. This reduction of joystick displacement will bebeneficial for persons with upper-body physical impairments.
(Manuscript received Jan. 00, 2000, revised May 00, 2000)
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(2)
(3)

(4)

(5)
(6)

2

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