Authors：Po-Er Hsu (2013-01-28);
recommended: Yeh-Liang Hsu (2013-02-10).
Note: This article is the Chapter 2 of Po-Er Hsu’s doctoral thesis
“Development of an intelligent robotic wheelchair as the center of mobility,
health care, and daily living of older adults.”
Chapter 2. Mobility assistance design of the intelligent robotic wheelchair
describes the mobility assistance design of the iRW. Maneuverability is of the highest concern when designing the omni-directional
vehicle of the iRW. The omni-directional
vehicle uses four Mecanum wheels to facilitate movement in all directions, in
particular, the sideways movement which cannot be achieved by general electric wheelchairs.
The efficiency of space utilization is discussed in this research for the iRW with the capability of moving
sideways and zero radius of rotation.
2.1 Omni-directional vehicle of the iRW
The iRW is designed for home or nursing home use, mostly indoor or
short-distance outdoor (such as taking a walk in the garden). Maneuverability
is of the highest importance when designing the omni-directional vehicle, so
that the iRW can be used in crowded
indoor environments. Most electric wheelchairs are two-wheel drive and cannot
move sideways, which was the main reason that Mecanum wheels were chosen for
the omni-directional vehicle of the iRW.
Figure 2-1 shows
the Mecanum wheels, which were invented by Swedish engineer Ilon in 1973 [Ilon, 1973]. Around the wheel’s tread
are several barrel-shaped free rollers arranged at an angle of 45 degrees to
the main axis, which enable omni-directional movement without using a classic
steering mechanism. Mecanum wheels are commonly used where maneuverability is
necessary in tight spaces, as with autonomous forklifts. Mecanum wheels also
see applications in robots [De Villiers
et al., 2012; Tsai et al., 2010]
and electric wheelchairs [Hoyer et al.,
1999]. One disadvantage of the Mecanum wheel is its energy consumption.
Due to the rotation of the exterior rollers, only a component of the force at
the perimeter of the wheel is applied to the ground [Adascalitei et al., 2011]. The other shortcoming is that Mecanum
wheels are susceptible to slippage. As a result, with the same amount of wheel
rotation, lateral travelling distance may differ from longitudinal travelling
distance, according to the ground condition [Nagatani et al., 2000].
Figure 2-1. Free rollers in a Mecanum wheel at 45
degrees to the main axis
omni-directional vehicle of the iRW
uses four Mecanum wheels, one on each corner of the chassis, driven by four DC
motors. Figure 2-2 shows how the components of the force vectors at every wheel
sum up or eliminate each other by rotating either clockwise or anticlockwise to
obtain the desired moving direction.
On the iRW, each Mecanum wheel is driven by one standard 24V DC Motor (3A,
max) with a maximum speed of 4900 rpm. The motor control is realized by a central
microcontroller (Arduino Mega 2560) and one full bridge motor driver (VNH3SP30)
for each motor. The motor driver enables clockwise and anticlockwise rotation
as well as rotational speed control by using high / low and pulse width
modulation (PWM) signals sent from the microcontroller at a constant frequency
of 490 Hz.
Table 2-1 shows the specifications for the
omni-directional vehicle of the iRW. In
contrast to the joystick control of commercial electric wheelchairs, which can
move a vehicle forward / backward or turn it left / right, the joystick of the iRW can also maneuver sideways to the
left / right or rotate clockwise / counterclockwise. When
older adults use manual or power
wheelchairs indoors, there is an inverse relationship between age and preferred
movement speed [Tolerico et al., 2007; Karmarkar et al., 2011]. Tolerico et al.
 found that the speed of manual wheelchair users is 0.79 ± 0.19 m/s, and further Karmarkar et al.
 indicated that the speeds of manual and electric wheelchair users are
0.64 ± 0.13 m/s and 0.7 ± 0.3 m/s. Based
on these results, the maximum forward speed of the joystick mode of the iRW is set at 50 m/min, which is close
to walking speed of older adults, and the maximum backward speed and sideways
speed are set at 40 m/min and 25 m/min, respectively. To foster operating
safety and promote user comfort, teleoperation and indoor navigation modes are
set at the constant speed 15 m/min, which is below the lowest manual operation speed
suggested by Karmarkar et al. .
Figure 2-2. Combined force vectors in a set of
Table 2-1. Specifications of the omni-directional
vehicle of the iRW
50 m/min (Max)
40 m/min (Max)
Sideways movement / Clockwise / Counterclockwise
Teleoperation / Indoor navigation speed
Mecanum wheel diameter
25.4 mm (10 inches)
Voltage / Current of the motor
24V / 3A (Max)
24 V, 9.6 Ah, 3.4 kg,
400×133×72 (L×W×H, mm)
2.2 Mobility assistance design for different
operators of the iRW
Figure 2-3 shows
a prototype of the iRW. Three
“operators”, the wheelchair user, caregivers, and the iRW itself, are considered in the mobility assistance design for
the iRW. Five operation modes are developed:
Figure 2-3. Prototype of the iRW
When the iRW’s ultrasonic sensors detect an
obstacle within a specific distance (10 cm), the iRW stops all motion and alerts the user with a beeping sound. The
user can override the stopping command using joystick mode or handlebar mode.
mounted on the right armrest of the iRW
is the main operation mode used by the wheelchair user. The joystick includes
three variable resistors that detect the movement of the joystick forward / backward
to produce movement in that direction, left / right to produce sideways
movement in that direction, or rotation clockwise / counterclockwise to produce
rotation in that direction. Turning movement can be produced if the variable
resistors detect forward and left / right movements of the joystick
simultaneously. Releasing the joystick will cause the iRW to immediately stop.
The handlebar mode is designed for the
caregiver who intends to push the iRW.
The diameter of the Mecanum wheels makes the iRW more difficult to push than a manual
wheelchair. To enable the caregiver to push the iRW in an intuitive way, three soft pressure-sensing units are implemented
on the handlebar of the iRW, as shown
in Figure 2-3. The caregiver can push the iRW
to move forward, move backward, turn right, or turn left by applying pressure
to the appropriate pressure units. If equal pressure is applied on the right
and left units, the iRW will move
forward; if higher pressure is applied on the right / left units, the iRW will turn right / left. When
pressure is applied on the middle pressure unit, the iRW will move backward. Pressing a button on either side of the
handlebar will cause the iRW to move
sideways in the direction of that button.
mode is designed for the caregiver who intends to operate the iRW from a remote site. In teleoperation
mode, the camera of the tablet mounted on the iRW’s armrest captures its environment, and a remote-control
interface enables a caregiver at a remote site to view it. The caregiver can
then operate the iRW by sending
commands via the Internet to the tablet, which relays the commands to the
central microcontroller via Bluetooth to guide the iRW to move to the desired location.
shows the user interface of teleoperation mode. The teleoperation user
interface has seven operation commands to control the movement of the iRW: moving forward or backward,
sideways to the right or left, rotating clockwise or anticlockwise, and
stopping. Note that there is no command for turning right / left in
teleoperation mode. Turning right / left is achieved by the series of commands “stop,”
“rotate 90°,” then “move forward.”
Figure 2-4. The teleoperation user interface
semi-autonomous indoor navigation mode uses a concept similar to automated guided
vehicles (AGVs) to reduce the operation load of the wheelchair user or care
givers. QR code (Quick Response code) labels are deployed on the ceiling as the
“virtual AGV track.” When the iRW is
steered under the track, the camera on the iRW
will capture and recognize the QR code. Subsequently, the information conveyed
by the QR code can be interpreted so that the iRW follows the virtual AGV track to move to the specific stop,
such as bedroom, living room, or kitchen. The user then takes control to steer
the iRW to the final location. The
user interface and indoor navigation algorithm are implemented as an App in the
tablet. Details will be described in Chapter 3.
collaborative control is reflected in the assignment of three “operator
priorities” (in descending sequence): the wheelchair user, caregivers, and
finally the iRW itself. The functional
test performed in this research compared the operational efficiency of the five
collaborative control of the iRW is
reflected in the assignment of three “operator priorities” (in descending
sequence): the wheelchair user, caregivers, and finally the iRW itself, to avoid different modes’ disrupting
each other and fosters safety. Following this principle, the five operation
modes are assigned the following precedence (in descending order): obstacle avoidance, joystick mode, handlebar
mode, teleoperation, and indoor navigation. Emergency modes such as
obstacle avoidance might be needed on the shortest notice and therefore were
assigned the highest priority. The manual control modes give the highest
control priority to the wheelchair user (joystick mode), next highest to the
caregiver at the local site (handlebar mode), then to the caregiver at the
remote site (teleoperation mode).
Indoor navigation mode receives the lowest priority. The higher-priority modes
can interrupt the lower-priority ones. Figure 2-5 illustrates the control
scheme of the five operation modes.
Figure 2-5. Flow chart showing priority and processing of the operation modes
2.3 Space utilization assessment
According to the Wheelchair Skills Test (WST), “90° left / right
turn,” “turns 180° in place,” and “maneuvers sideways” are the skills directly related
to changing moving directions of the wheelchair for daily needs [Kirby et al., 2008; Fliess-Douer et al., 2010]. The
turning space is used here to assess and compare space utilization of the iRW with that of general electric wheelchairs.
The specifications for 16 models of electric
wheelchairs commercially available in Taiwan were examined. Their average size was
910×640 mm (L×W) and the turning radius ranged from 500 mm to 800 mm. The size of
the iRW is 800×610mm (L×W) and its turning
radius is 0 mm. Additionally, the iRW
can move sideways, unlike the 16 electric wheelchairs.
Figure 2-6 and
Table 2-2 show the details of the calculation of the turning space required by the electric wheelchairs for “90° left /
right turn” and “turns 180° in place.” For “turns 180° in place,” an electric wheelchair may need a space up to 2m×2m. In contrast, the iRW can make a 180° turn
within a 1m×1m space (actually,
in a circle of diameter 1 m).
Figure 2-6. The turning space of commercial electric
wheelchairs and the iRW
Table 2-2. The calculation of the turning space
Turning radius (mm)
Dimensions of turning
space (L×W, mm)
Area of turning space (mm2)
Commercial electric wheelchairs
Circle of diameter 1m
Commercial electric wheelchairs
Circle of diameter 1m
activities” involve moving to the transfer target and displacing the user’s
body from one surface to another, such as from the wheelchair to a treatment
table, a regular bed, a tub / shower bench, a toilet, or a car seat and vice
versa [Fliess-Douer et al., 2010;
Bromley, 1998]. The capability of the iRW
to move sideways allows it to use much less space for transfer activities than
do general electric wheelchairs.
Figure 2-7 is the enlarged
individual toilet room for barrier-free lavatory recommended by the “Americans
with Disabilities Act Accessibility Guidelines (ADAAG) for buildings and facilities.”
 The turning space is defined as a circle of minimum diameter 60 in. (1,524
mm) for wheelchairs to turn 180°, designated
by the outer dashed circle, which is smaller than the turning space required
by electric wheelchairs in Table 2-2. Figure 2-7, the standard
positions for toilet transfer are position 1 (the side approach) and position 2
(the reverse diagonal approach). Position 3 is for using the sink. The minimum
clear floor space is 1,524×1,422 mm2 for toilet, and 762×1,219 mm2
for sink. As shown in Figure 2-6,
the turning space of the iRW is a
circle of diameter 1 m. It can be easily turned and moved from one position to
the other, for example, sideways directly from position 2 to position 3.
Figure 2-7. The recommended enlarged individual toilet room for barrier-free
Adascalitei F., Doroftei I., 2011. “Practical
applications for mobile robots based on mecanum wheels - a systematic survey,” Proceedings of International Conference On
Innovations, Recent Trends And Challenges In Mechatronics, Mechanical
Engineering And New High-Tech Products Development – MECAHITECH’11, v. 3,
Americans with Disabilities Act Accessibility
Guidelines (ADAAG) for buildings and facilities, 2003. Barrier Free Washroom Planning Guide.
Bromley, 1998. Tetraplegia and paraplegia: a guide for physiotherapists. Churchill
De Villiers M., Tlale N. S., 2012. “Development
of a control model for a four wheel mecanum vehicle,” Journal of dynamic systems measurement and control-transactions of the ASME,
v. 134, pp. 1-6.
Fliess-Douer O., Vanlandewijck Y. C., Manor G.
L., Van Der Woude L. H., 2010. “A systematic review of wheelchair skills tests
for manual wheelchair users with a spinal cord injury: towards a standardized
outcome measure,” Clin Rehabil, v.
24, pp. 867-886.
Hoyer H., Borgolte U., Jochheim A., 1999.
“The OMNI-wheelchair - state of the art. center on diabilities,” Technology and Persons with Disabilities
Conference, Northridge, CA.
Ilon B. E., 1975. “Wheels for a course stable
self propelling vehicle movable in any desired direction on the ground or some
other base,” US Patent and Trademarks
office, Patent 3,876,255.
Karmarkar A. M., Cooper R. A., Wang H.,
Kelleher A., Cooper R., 2011 “Analyzing wheelchair mobility patterns of
community-dwelling older adults,” Journal
of rehabilitation research and development, v. 48, pp. 1077-1086. DOI: 10.1682/JRRD.2009.10.0177
Kirby R. L., Cher S., Kim P., Donald A., Mike
M. A., Paula W. R., François R., 2008. Wheelchair
Skills Training Program (WSTP) Manual version 4.1. Wheelchair Skills
Program, Dalhousie University.
Nagatani K., Tachibana S., Sofne M., Tanaka Y.,
2000. “Improvement of odometry for omnidirectional vehicle using optical flow
information,” IEEE/RSJ International
Conference on Intelligent Robots and Systems.
Tsai C. C., Wu H. L., Lee Y. R., 2010. “Intelligent
adaptive motion controller design for mecanum wheeled omnidirectional robots
with parameter variations,” International
journal of nonlinear sciences and numerical, v. 11, pp. 91-95.
Tolerico M. L., Ding D., Cooper R. A., Spaeth
D. M., Fitzgerald S. G., Cooper R., Kelleher A., Boninger M. L., 2007. “Assessing
mobility characteristics and activity levels of manual wheelchair users,” Journal of rehabilitation research and
development, v. 44, pp. 561-572. [PMID: 18247253] DOI: 10.1682/JRRD.2006.02.0017