Authors: Po-Er Hsu, Yeh-Liang Hsu, Jun-Ming Lu, Cheng-Hao Chang (2013-01-21);
recommended: Yeh-Liang Hsu (2013-04-06).
Note: This paper is published at International Journal of Advanced
Robotic Systems, Vol. 10, 168., 2013/01.
Seat adjustment design of an intelligent robotic
wheelchair based on the Stewart platform
user makes direct contact with the seat, which serves as the interface between
the user and the wheelchair, for much of a given day. Seat adjustment design is
thus of crucial importance in providing proper seating posture and comfort.
This paper presents a multiple-DOF seat adjustment mechanism, which is intended
to increase the independence of the wheelchair user, while maintaining a
concise structure, light weight, and intuitive control interface. This
four-axis Stewart platform is capable of the motions of heaving, pitching, and
swaying to provide seat elevation, tilt-in-space, and sideways movement
functions. The geometry and types of joints of this mechanism are carefully
arranged so that only one actuator needs to be controlled, enabling the
wheelchair user to adjust the seat by simply pressing a button. The seat is
also equipped with soft pressure-sensing pads to provide pressure management by
adjusting the seat mechanism once continuous and concentrated pressure is
detected. Finally, by comparing with the manual wheelchair, the proposed
mechanism demonstrated the easier and more convenient operation with less
effort for transfer assistance.
Keywords: seat adjustment mechanism, Stewart platform, robotic wheelchair, sideways
people have mobility impairment that requires them to use a wheelchair. In
addition to providing mobility assistance, seat adjustment design is of crucial
importance in a wheelchair. The wheelchair user makes direct contact with the
seat, which serves as the interface between the user and the wheelchair, for
much of a given day. Tolerico et al.  studied the use of manual
wheelchairs for daily living in the home. They found that wheelchair users
spent 8.3 ± 3.3 hours daily sitting on the wheelchair. Sonenblum et al. 
measured 25 non-ambulatory, full-time power wheelchair users and indicated that
the median of the daily time sitting on the wheelchair was 10.6 hours. Souza et
al.  examined 50 journal articles on mobility assistive technology (MAT)
and concluded that electrical wheelchairs should be considered not only as
providing mobility for advanced stages but as mobility assistive technology
that can be integrated with an adjustable seating system to reduce fatigue.
To enable functional
independence for older adults who use a wheelchair, their posture and comfort
should be considered. Redford  reviewed studies specifically concerned
with seating and wheeled mobility for older adults, and concluded that these
users would benefit from matching of mobility to function, cheaper and more
effective cushions, more modular seating systems, and better lifting and
transfer. Among the considerations in wheelchair seat design, pressure
management received the most attention. Sitting-acquired pressure ulcers are a
significant healthcare problem for wheelchair users. Various seating designs
have been proposed for maintaining tissue viability while the user is in the
wheelchair. For example, air cells with pressure sensors to monitor the pressure
distribution on the buttock are often utilized to provide timely adjustment of
the shape of the seat cushion to relieve concentrated pressure [Henderson et
al., 1994; Burns et al., 1999; Chugo et al., 2011]. “Dynamic seating” on the
wheelchair, in which a power-operated seat adjustment mechanism would allow the
user to maintain proper posture, was also proposed [Graf et al., 1995].
backrest recline, and seat elevation are the most clinician-prescribed seat
adjustment functions of electrical wheelchairs to facilitate posture change
and/or assist activities of daily living (ADL) for users [Paralyzed Veterans of
America, 2000; Trefler et al., 2001]. Ding et al.  surveyed the real-life
usage of powered seating functions among wheelchair users during their daily
activities. Results showed that subjects accessed tilt-in-space, backrest
recline, and seat elevation 19 ± 14 times, 12 ± 8 times, and 4 ± 4 times per
day, respectively. Further, of the time participants spent in a wheelchair, 39.3%
± 36.5% was in the tilted or reclined positions. Lacoste et al. 
interviewed 40 participants at home and reported that 97.5 percent used the
powered tilt-in-space and/or recline function in a given day and that 70
percent used the functions primarily to rest, relax, increase comfort, and
have been conducted to evaluate seat pressures at different angles of the
tilt-in-space function in laboratory settings. Sprigle et al.  found that
the tilt-in-space function reduced static seating pressure significantly, which
was a key factor in pressure ulcer prevention. In addition, many
laboratory-based studies suggested that large tilt-in-space angles can be used
to manage seating pressure to reduce the risk of pressure ulcers [Henderson et
al., 1994; Stinson et al., 2002]. Aissaoui et al.  concluded that the
maximum pressure reduction at ischial tuberosities was at 45° of tilt-in-space.
In addition to
providing seating comfort and pressure management, the seat elevation function
for wheelchair users is considered medically necessary [Cooper et al., 2004; Rehabilitation
engineering and assistive technology society of north America, 2005]. The seat
elevation feature can help wheelchair users accomplish mobility-related ADL,
such as performing transfers and reaching objects at different heights to
preserve upper-limb functions and to achieve eye contact in social situations.
A search of
related patents indicated that most seat adjustment functions for wheelchairs
applied linkage mechanisms [Cerreto et al., 2012; Chuang, 2011; Wu et al., 2009].
Additionally, some linkage mechanisms were implemented with actuators or motors
to provide multiple degrees of freedom (DOF) in seat adjustment. Bae and Moon
 designed a wheelchair seat mechanism using four actuators to achieve the
forward/backward tilting, elevation, and standing motions. The seat mechanism
is composed of four independent adjustment mechanisms, each of which is driven
by an actuator. Wada  used a linear drive unit to develop a chair tilting
system for the climbing wheelchair to maintain the angle between the seat and
the floor. Lee et al.  developed a seat adjustment mechanism that is
integrated with the frame of the wheelchair to increase stability by shifting
the center of gravity. The mechanism also provided a tilt-in-space function.
Koontz et al.  evaluated 109 manual wheelchairs, 89 electrical
wheelchairs, and 15 scooters for maneuverability. They concluded that the
angles produced by the tilt-in-space and recline functions lengthen electrical
wheelchairs and therefore increase the space needed for maneuvering. Weight and
complexity of the wheelchair is also increased with the multiple-DOF seat
activities mainly determine the degree of independence attainable by wheelchair
users in daily living. Among all transfer activities, sitting pivot transfer
(SPT) is the most commonly used [Finley et al., 2005; Perry et al., 1996; Bromley,
1998; Somers, 2001]. The basic steps of SPT are moving the buttocks toward the
edge of the initial surface, placing the feet in a stable position, and leaving
one hand on the initial surface (trailing) while placing the other hand on the
target surface (leading). The arms are used to push up from the initial surface
and pivot the body on the feet, swinging and landing the buttocks onto the
target surface [Koontz et al., 2011]. However, proper transfer technique may
not be sufficient when wheelchair users encounter height differences and gap
crossing between wheelchair and target surface [Forslund et al., 2006].
Few studies have
examined the development of a transfer assist function of a wheelchair.
Khatchadourian et al.  used robotic technologies to develop a mobile
robot molded into a forklift for users to perform transfer activities. The user
uses a beacon to position the mobile robot close to him or her. Then, the user
can apply force to the support arms of the lifting mechanism to relocate
himself or herself into the mobile robot to finish the transfer activities.
Bostelman and Albus  built a “Home Lift, Position and Rehabilitation
(HLPR) Chair” to provide independent mobility and transfer activities assist
for the user in the indoor environment. However, the size (1,450×580×1,780 mm
in mobility configuration, 1,450×580×2,410 mm in full lifted configuration) and
weight (136 kg) of the HLPR chair may make it impractical for the home.
Miller and Slack
 were the first to use the term “robotic wheelchair.” They built two
prototypes of robotic wheelchairs that used various sensing and navigation
technologies that are often used in robotics. These wheelchairs were able to
assist users to pass through narrow paths and avoid obstacles. Simpson 
reviewed the many recent studies on the development of “smart wheelchairs” that
can perform somewhat autonomously.
presents the multiple-DOF seat adjustment mechanism of the “intelligent Robotic
Wheelchair” (iRW). The features of
the iRW include (1) four Mecanum wheels
to facilitate movement in all directions and zero radius of rotation, (2)
mobility assistance functions that can be invoked by the wheelchair user,
caregivers, or the iRW itself, and
(3) an information/communication module that monitors selected health indicators
and provides user access to the internet. The seat adjustment mechanism of the iRW is achieved by a four-axis Stewart
platform. The Stewart platform [Stewart, 1965] is a parallel structure robot
that has greater stiffness, positioning accuracy, and payload-to-weight ratio
than do serial structure robots.
seat adjustment mechanism developed in this research is capable of the motions
of heaving, pitching, and swaying to provide a comfortable sitting posture,
seat elevation adjustment function, and transfer activities assist. Special
consideration is paid to arranging the actuators to reduce the control
complexity of the parallel mechanism, so that the wheelchair user can make the
seat adjustment by simply pressing a button. Equipped with soft
pressure-sensing pads, the seat also provides pressure management by adjusting
the seat mechanism when continuous, concentrated pressure is detected.
The rest of the
paper is organized as follows. Section 2 proposes the design concept of the
multiple-DOF seat adjustment mechanism. Section 3 discusses the simulation of
the multiple-DOF seat adjustment mechanism on the iRW. Section 4 describes the control scheme and test results of the
seat adjustment mechanism. Finally, Section 5 concludes the paper.
Design concept of the
multiple-degrees-of-freedom seat adjustment mechanism
The literature review in
Section 1 indicated that a wheelchair seat adjustment mechanism should meet the
following design requirements:
is important for pressure management and comfortable sitting posture. Aissaoui
et al.  found that an effective weight shift could be achieved when
tilt-in-space angle was at least 15°. Sonenblum et al.  evaluated 10
participants and found that none required an angle of tilt-in-space function as
great as 20°. Ding et al.  concluded from a survey that the most
often-used ranges of tilt-in-space angles were 2.5° to 10°, followed by 10° to
20°, and their frequency and time were 6.6 ± 4.9 times for 272.7 ± 228.7
minutes and 7.3 ± 6.6 times for 157.3 ± 171.8 minutes a day. Based on the
literature above, the range of tilt-in-space angle of the iRW was set to be 20°.
Seat elevation / Sideways
capabilities of adjusting seat elevation and sideways movement by the
wheelchair user enhance transfer activities assist. Seat elevation adjustment
is used to reduce the height difference when performing transfer activities. To
accommodate the height of facilities in the living environment where transfer
activities often occur, e.g., a range in height of a toilet from 279 to 432 mm
and of a bed from 430 to 485 mm [Americans
with disabilities act accessibility guidelines, 2003], the seat
elevation adjustment of the iRW
ranges from 370 to 490 mm. The seat can move sideways a maximum of 140 mm, which is believed sufficient to cover the gap when crossing
between wheelchair and target surface.
the functional requirements, the size, weight, and control complexity of the
adjustment mechanism were considered. The specifications for 16 models of
electrical wheelchairs commercially available in Taiwan were examined, and
their average size was 910×640 mm (L×W). These dimensions were taken as the
upper limit for the iRW. Moreover,
the wheelchair user should be able to make the seat adjustment in all DOFs by
simply pressing a button, and stop the seat movement by releasing the button.
The seat adjustment
mechanism of the iRW is based on the
Stewart platform. The seat adjustment mechanism has three DOFs: heave, sway,
and pitch. These facilitate tilt-in-space, seat elevation, and moving sideways.
platform has been used in flight simulators [Advani wt al., 2002], machine
tools [Ting et al., 2004], a biped locomotion system [Sugahara et al., 2005]
and surgery manipulators [Wapler et al., 2003; Tsai et al., 2007]. The Stewart
platform, illustrated in Figure 1, is composed of a fixed base, a movable
platform, and six linear actuators connecting the fixed base to the movable
platform. This is a six-DOF universal-prismatic-spherical mechanism, which
supports heave, surge, sway, yaw, pitch, and roll. No additional structural
members are needed in the Stewart platform because the actuators also function
as structural members. One drawback of the Stewart platform is the small
workspace. Another is complexity in control, which is due to the need to
control six linear actuators simultaneously in a nonlinear manner and to the
existence of singular positions.
Figure 1. Schematic diagram and degrees of freedom
of Stewart platform
research, the Stewart Platform is converted into a multiple-DOF seat adjustment
mechanism, as shown in Figure 2. The seat acts as the movable platform of the
Stewart platform, and the h-shape
structure of the omni-directional vehicle of the iRW acts as the fixed base of the Stewart platform. The number of
linear actuators is reduced from six to four because only three DOFs (heave,
pitch, and sway) of seat adjustments are required. The seat and actuators are
connected with universal joints, and the omni-directional vehicle and actuators
are coupled with two revolute joints and two universal joints.
Figure 2. The multiple-DOF seat adjustment
mechanism of the iRW
Figure 3 depicts
the multiple-DOF seat adjustment mechanism of the iRW. Actuators 1 and 2 are fixed at an incline of 20° from the y-z
plane. Actuators 1 and 2 extend or retract at the same constant speed to provide
a smooth adjustment. An offset angle (7.4°) between Actuators 1 and 2 and the z-axis provides a horizontal force
component when swaying sideways (i.e., in the +y direction). Actuators 3 and 4 are fixed at an incline of 20° from
the y-z plane. The offset angle (7.4°) and direction (+y direction) of Actuator 4 are the same
as for Actuators 1 and 2. To constrain the DOFs, the offset angle between
Actuator 3 and the z-axis, 10.6°, differs from that of the other actuators.
Table 1 shows
the coordinated actions of actuators when the seat is adjusted. For example,
when raising the seat, Actuators 1 and 2 retract, and Actuators 3 and 4 extend.
When adjusting the seat to the right, Actuators 1, 2, and 4 extend, and
Actuator 3 retracts. In this setup, the initial height of the seat is 414 mm.
The range of height adjustment is from 370 mm to 488 mm, limited by the stroke
of the actuators. The range of tilt-in-space angle is from -15° to 22°. The
-15° tilt-in-space can be used in sit-to-stand assist. The range of moving
sideways varies with the height of the seat. At the initial height of 414 mm,
the range of moving sideways is 140 mm. Table 2 shows the fundamental
specifications of the linear actuators and the iRW.
Figure 3. The multiple-DOF seat adjustment
mechanism in front and side view
Table 1. Coordinated actions of actuators
Specifications of the linear actuator and the iRW
Actuator (HIWIN LAS3-1)
Tensile / Thrust / Self-locking force
1200 / 1200
/ 800 (Max, N)
8 / 12 mm/s
24 V / 2.5 (A, Max)
Initial height of the seat
Range of the seat elevation
370 ~ 488 mm
Range of the tilt-in-space
22° (clockwise) / -15° (counterclockwise)
Range of the transfer activities assist
Varies with height of the seat, 140 mm at
the initial height
Simulation of the
multiple-degrees-of-freedom seat adjustment mechanism
As shown in
Figure 4, Actuators 1, 2, and 4 are designed to extend or retract at the same
constant speed when the wheelchair user pushes a button to perform
tilt-in-space, change elevation, or move sideways. Actuator 3 is the only actuator
that has to be controlled in a nonlinear manner to maintain the seat in a
horizontal orientation when adjusting elevation or moving sideways.
presents the results of using SimWise 4D ver. 8.0.2 to examine how the motion of the seat was affected by the length
of each actuator. This simulation also confirmed the ranges of the seat
adjustment described in Table 2.
Figure 4 shows
the simulation result of adjusting seat elevation from 370 mm to 488 mm in 10
seconds. The origin represents the center of the seat. Figure 4(a) shows that the motion of all actuators appears to be
linear. In particular, the lengths of Actuators 1 and 2 remained equal
throughout, and the change in their length was inverse to the change in length
of Actuator 4. Figure 4(b) shows that the seat slightly moved forward or
backward (i.e., along the x-axis)
when the height of the seat was changed. The range of this moving distance
along the x-axis was 43 mm.
Simulation of seat elevation
Figure 5 shows
the simulation result of the tilt-in-space at the initial height, from -15° (counterclockwise) to 22° (clockwise)
in 10 seconds. Figure 5(a) indicates that the change in length of all actuators appears to be linear. In
particular, the lengths of Actuators 1, 2, and 4 remain equal. Figure 5(b)
shows that the seat maintains a constant horizontal orientation within
the y- and z-axes while undergoing tilt-in-space along the x-axis.
Figure 5. Simulation of tilt-in-space
Figure 6 shows
the simulation result of the seat moving sideways 140 mm from its initial
position. Figure 6(a) indicates that the lengths of Actuators 1, 2, and 4
remain equal throughout, while the length of Actuator 3 varies nonlinearly.
Figure 6(b) shows that the seat itself maintains a constant horizontal
orientation (z- and x-axes) while moving sideways in the +y direction.
Figure 6. Simulation of sideways movement
In all three
simulations, the absolute values of the speeds of Actuators 1, 2, and 4
remained constant. This simplifies the design of the control scheme, which is
discussed in the next section.
Control scheme for the
multiple-degrees-of-freedom seat adjustment mechanism
On the iRW, the user would adjust the seat by
pushing one of three buttons located on the armrest, which correspond to
tilt-in-space, seat elevation adjustment, or moving sideways. Actuator 1, 2,
and 4 extend / retract at a preset constant speed at all time. The extend /
retract speed of Actuator 3 would be calculated by an Arduino microcontroller.
Figure 7 shows
how the speed of Actuator 3 would be derived when the seat moves sideways. The
coordinates of the joints of the four actuators on the fixed base and the
position of the movable platform (the seat) are known. There is an optical
sensor in each actuator that detects its current stroke, i.e., the amount of
extension of the actuator. As shown in Figure 7, the current coordinates of the
joint of Actuator 3 at the movable platform J3,
expressed as (x3, y3, z3), can then be calculated. When the wheelchair user
pushes a button to move the seat sideways, Actuators 1, 2, and 4 start to
extend at the same constant speed. After a small interval of time ∆t, the coordinate of J3
would need to become (x3, y3 + ∆y, z3), which
is calculated from the expected changes of the strokes of Actuator 1, 2, and 4
after this time interval. In the program in the Arduino microprocessor, ∆t = 0.04 second (25 Hz). The expected
change of the stroke of Actuator 3 can then be easily calculated and expressed
as a “speed ratio” of Actuator 3 to Actuators 1, 2 or 4. This “speed ratio” is
then implemented in the pulse-width modulation (PWM) control of the Arduino
microcontroller, in which the motor speed control is digitized on a scale of 0~255.
The movement stops when the wheelchair user releases the button, or when one of
the actuators reaches its limit.
Figure 7. Derive the speed of Actuator 3 when the
seat moves sideways
The seat control
scheme was implemented in a prototype. A test was performed to confirm that the
seat maintains a constant orientation when it is being adjusted. A tri-axial
accelerometer module (KXPA4-2050, Kionix) set up at the center of the seat was
used to detect its orientation.
accelerometer module has a built-in low-pass filter at the cutoff frequency of
50 Hz. The module’s sensing range along each axis is ±2 g, and its output
varies with acceleration at the rate of 660 mV/g. If there is no acceleration
applied along an axis, the output voltage Voff
equals half Vdd (3.3 V).
If acceleration A exists toward the
positive direction, the output voltage increases (Vout > Voff)
and vice versa. Equation (1) shows the relation:
sensing is a common application for low-g accelerometers. Figure 4-8 indicates the tilt angles (pitch and roll) of the seat, where φ, ρ,
and θ represent the tilt angles with
respect to the x-, y-, and z-axes relative to ground. Equation (2) identifies the relation
between the tilt angles and accelerations along each axis. Note that in pitch, θ + φ
= 90°, while in roll, θ +
Figure 8. Pitch and roll represented as changes in
angle along the coordinate axes
In the initial
setup, the height of the seat is 414
mm, and the angle φ is +5°. As shown in Figure 9, two
conditions were considered: the seat was empty (continuous lines) and the seat
was loaded with a 70 kg mass (short dash lines). Figure 9(a) shows the values
of the tilt angles while the seat elevation was raised from 370 mm to 488 mm in
about 8 seconds. When no mass was loaded on the seat, the angle φ obtained from the accelerometer module
maintained at 4.73° ± 0.43° to ground, while angle θ maintained at 85.21° ± 0.44° to ground during the elevation
process. When the seat was loaded with the mass, the angle φ obtained was 4.26° ± 1.70°to ground. Figure 9(b) shows the values
of the tilt angles while the seat’s tilt-in-space changed from -15° to 22° in
about 8 seconds. Note that when θ exceeds
90°, the arctan function in Equation (2) will generate a θ value less than 90°. Therefore both the arctan value and the true
angle θ are displayed in Figure 9(b).
Figure 9(c) shows the values of the tilt angles while the seat, which was
maintained at the initial height of 414 mm, was adjusted sideways from 0 mm to
140 mm in about 8 seconds. The angle φ
maintained at 6.14° ± 0.24° when there was no mass loaded on the seat. When the
seat was loaded, the angle φ maintained
at 5.61° ± 1.84°. The results depicted in Figure 9 indicate that the orientation
of the seat remained stable when using the proposed control scheme, though the
70 kg mass has some effect to the stability.
Figure 9. Resulting values of the tilt angles for
each type of seat movement
Figure 10 depicts the flow
chart for processing a user’s pressing a button to move the seat. To avoid
different functions’ disrupting each other and to foster safety, there are
three basic rules in the control flow:
(1) Only one function
can be performed at a time. If two buttons are pressed at the same time by the
wheelchair user, neither function will operate.
(2) To start seat
elevation adjustment or the tilt-in-space function, the distance of sideways
movement has to be zero. If this distance is not zero, the seat will
automatically move back to the center vertical axis, after which the requested
movement will be carried out.
(3) To start sideways
movement, the angle of tilt-in-space has to be zero. If this angle is not zero,
the seat automatically will move back to the horizontal plane while maintaining
its height, after which the requested movement will be carried out.
The seat also
provides pressure management function by automatically adjusting the
tilt-in-space when continuous, concentrated pressure is detected by the soft
pressure-sensing pads. As shown in the flowchart in Figure 10, when no buttons
are pressed, if the pressure distribution is not changed in 30 minutes, the
tilt-in-space function will start to increase (or decrease) five degrees of
seat angle in 60 seconds automatically. The pressure management function will
be interrupted if the wheelchair user pushes any button during the 60 seconds.
Figure 10. Flow chart for processing a user’s
pressing a button to move the seat
User evaluation of the transfer
assistance function provided by the iRW
As described in
the first section, it has been shown in previous studies that the adjustability
of tilt-in-space angles of a wheelchair can help to relieve seating pressure
and improve seating comfort. However, there are few evidences supporting how
the elevation and sideways movement may benefit wheelchair users in transfer
assistance. Thus, in this study, the usability of the transfer assistance
function provided by the iRW was evaluated and compared with that of a manual
participants (3 males and 3 females, averaged 22.8 years old) were recruited
for user evaluation. Each participant was requested to perform the same
transfer activities with the iRW and
then with a manual wheelchair, by simulating how one moves from either the iRW or the manual wheelchair to a piece
of furniture in the home environment, as well as returning to the iRW or the manual wheelchair. While
using the iRW, the participant may
use the elevation adjustment and sideways movement functions to facilitate the
transfer activities. As for the transfer activities with the manual wheelchair,
the participant can use a transfer board. In order to investigate the
effectiveness of the seat elevation adjustment and sideways movement functions
of the iRW, two conditions of transfer
activities were considered. In the first part of the evaluation, the iRW, manual wheelchair, and the target
plane were at the same height of 45 cm. In other words, the participant
transfers between two planes with the same height, by using the sideways
movement function of the iRW or the
transfer board with the manual wheelchair. In the second part of the
evaluation, the height of the target plane was 50 cm, while the heights of the
seat pans of the iRW and the manual
wheelchair were both 45 cm. Thus, the participant needs to use the seat
elevation adjustment and sideways movement functions of the iRW or the transfer board with the
manual wheelchair to move between two planes with different height levels. The
steps of the evaluation are described as follows.
(1) Spending 5 minutes to become familiar with the operation of the
elevation adjustment and sideways movement functions of the iRW and the use of a transfer board with
the manual wheelchair
(2) Crossing a horizontal gap of 15 cm to transfer from the iRW (with the seat height set at 45 cm)
to the target plane with the height of 45 cm by using the sideways movement
function to move as close as possible to the target plane
(3) Transferring back from the target plane to the seat pan of the iRW and using the sideways movement
function to return the seat to the default position
(4) Repeating step 2 and step 3 for five times
(5) Repeating steps 2 to 4 with the manual wheelchair (with the seat
height of 45 cm) by using the transfer board to cover the gap to perform
(6) Filling the questionnaire to collect the subjective responses toward
the usability of the iRW and the
manual wheelchair for transfer activities
(7) Crossing a horizontal gap of 15 cm to transfer from the iRW (with the seat height set at 45 cm)
to the target plane with the height of 50 cm by using the elevation adjustment
and sideways movement functions to move as close as possible to the target
(8) Transferring back from the target plane to the iRW and using the sideways movement function to return the seat to
the default position
(9) Repeating step 7 and step 8 for five times
(10) Repeating steps 7 to 9 with the manual wheelchair (with the seat
height of 45 cm) by using the transfer board to cover the gap to perform
(11) Filling the questionnaire to collect the subjective responses toward
the usability of the iRW and the
manual wheelchair for transfer activities
time required for the transfer activities with the iRW and the manual wheelchair were measured and compared. Figure 11
shows the average operation time for the different transfer activities with
both mobility aids. While transferring to the target plane of 45 cm (Part I),
participants took a significantly shorter time to perform transfer activities
with the iRW than the manual
wheelchair (p = 0.00). Further, there
was a significant difference in operation time among participants when
transferring from the iRW to the
target plane (p = 0.00). This may be
due to the preference when determining how close is enough for the individual
to move the body. In addition, while transferring from the target plane to the iRW, no significant difference was found
in operation time among participants (p
= 0.20). It is because that when returning to the default position, the seat of
the iRW will move to the destination
automatically once the button is pressed. In other words, user adjustment is
not required, and hence individual difference will be eliminated. Further, no
significant difference was found among the participants in operation time while
using the manual wheelchair to perform the transfer activities in both
directions (to the target plane: p =
0.06; to the wheelchair: p = 0.36).
The results imply that the iRW
provided a better efficiency for transfer assistance than the manual wheelchair
did. In addition, individual difference in operation efficiency was found while
using the sideways movement function of the iRW,
but it was not observed when using a transfer board with a manual wheelchair.
Figure 11. The average operation time (s) for the
transfer activities with the iRW and
the manual wheelchair
When the height
of the target plane is at 50 cm (Part II), i.e. higher than the seat pan of both
mobility aids, the participants also took a significantly shorter time to
transfer to the target plane by using the iRW
than the manual wheelchair (p =
0.004). Besides, when transferring from the iRW
to the target plane, there was also a significant difference in operation time
among participants (p = 0.02 < α =
0.05). In this case, the possible reason is the preference of height and
distance differences between the seat pan and the target plane for the safe
transfer. However, when transferring back to the iRW, there was no significant difference in operation time among
participants (p = 0.06). It is
because that when returning to the default position, the seat of the iRW will move to the destination
automatically once the button is pressed. Moreover, while using the manual
wheelchair to perform the transfer activities, there were significant
differences in operation time among participants in both directions (to the
target plane: p = 0.001; to the
manual wheelchair: p = 0.000). It
might be caused by the height difference between the seat pan of the manual
wheelchair and the target plane. For people with different skills and
experiences, the time required for climbing up or sliding down to another plane
with a different height may differ.
As for the subjective responses, the 5-point scale was first used to evaluate the user satisfaction
(1 = very dissatisfied; 5 = very satisfied) toward the iRW. The participants generally felt satisfied with the safety
(average score = 3.7) and stability (average score = 3.5) while moving the seat
of the iRW up and down and in
sideways. In addition, the 5-point scale was adopted to investigate how the
participants feel whether the transfer assistance provided by the iRW or the manual wheelchair fits their
needs (1 = not at all; 5 = very much). The participants reported that the seat
elevation adjustment and sideways movement functions of the iRW (average score = 4.3) fitted their
needs for transfer activities better than the combination of the transfer board
and the manual wheelchair did (average score = 3.0; p = 0.006), no matter whether there was a height difference between
the two planes.
On the other
hand, the 5-point scale was further used to evaluate the level of easiness (1 =
very difficult; 5 = very easy) and the level of convenience (1 = very
inconvenient; 5 = very convenient) toward the transfer assistance provided by
the iRW and the manual wheelchair.
Besides, the Borg CR-10 scale  was adopted to measure the exercise
intensity levels (0 = nothing at all; 3 = moderate; 10 = very very hard) while
using the iRW and the manual
wheelchair for transfer. The statistics in the two parts of evaluation (“to the
target plane” and “from the target plane”) are presented and discussed as
In the first part of the
evaluation, the participants felt the iRW can make the transfer
activities slightly easier (4.0 for the iRW; 3.3 for the manual
wheelchair; p = 0.07) and significantly more convenient (4.2 for the iRW;
2.5 for the manual wheelchair; p = 0.01) than the manual wheelchair in
both directions, as shown in Table 3. Moreover, the participants reported a
significantly lower exercise intensity while operating the iRW (average
score = 2.5) than the manual wheelchair (average score = 4.5; p = 0.02)
in both directions. The results demonstrate that the iRW can help the
user to perform transfer activities between two planes at the same height with
better convenience and less effort than the manual wheelchair.
Table 3. Statistics of the subjective responses
toward the transfer activities at the same height (Part I)
To the target
From the target
*: significant difference (p < 0.05) between the iRW and the manual wheelchair (MWC)
In the second
part of the evaluation, the iRW
assisted the participants to transfer to the higher target plane significantly
more easily (3.8 for the iRW; 2.6 for
the manual wheelchair; p = 0.02) and
significantly more conveniently (4.3 for the iRW; 2.6 for the manual wheelchair; p = 0.008) than the manual wheelchair, as shown in Table 4. While
transferring back from the target plane, there was no difference in easiness
(3.5 for the iRW; 3.8 for the manual
wheelchair; p = 0.24) and convenience
(3.8 for the iRW; 3.3 for the manual
wheelchair; p = 0.25) between the two
mobility aids. The possible reason is that transferring from the higher target
plane to the manual wheelchair was enabled by sliding down through the transfer
board, which makes it easier and more convenient than climbing up. Regarding the
exercise intensity of the transfer activities, the participants reported a
significantly lower intensity with the iRW
(average score = 3.0) than the manual wheelchair (average score = 5.2; p = 0.03) while transferring to the
target plane. When transferring back from the target plane, the participants
perceived a slightly lower intensity with the iRW (average score = 3.0) than the manual wheelchair (average score
= 4.7; p = 0.05). The results reveal
that the iRW can help the user to
perform transfer activities from a plane to another higher one with more ease,
better convenience, and less effort than the manual wheelchair.
Table 4. Statistics of the subjective responses
toward the transfer activities at different heights (Part II)
To the target plane
From the target plane
*: significant difference (p
< 0.05) between the iRW and the
manual wheelchair (MWC)
design is of crucial importance to a wheelchair user. This paper presents the
development of a multiple-DOF seat adjustment mechanism achieved by a four-axis Stewart Platform, which is capable of adjustment
in tilt-in-space, seat elevation, and sideways movement to support the needs of
the wheelchair user at home, including comfortable sitting posture, pressure management, and
transfer activities assist. The actuators are carefully arranged so that only one actuator needs to be
controlled, enabling the wheelchair user to adjust the seat by simply pressing
a button. Figure 12 shows the prototype of the iRW. The size of the iRW prototype is 800×610×870 mm, and the
total weight is 28 kg, which is practical to be used in the home environment. A
memory foam seat, which helps to relieve body pressure for better comfort,
rests on the multiple-DOF seat
adjustment mechanism. The armrests can be opened outward during transfer
assist. This multiple-DOF seat
adjustment mechanism is intended to increase the independence of the wheelchair
user, while maintaining a concise structure, light weight, and intuitive
control interface. From the orientation confirmed test, the 70 kg mass has
slight effect on the stability. However, the user felt safe and stable while
adjusting the seat of the iRW.
Besides, it was demonstrated that the seat adjustment functions of the iRW can help the user to perform
transfer activities more easily and more conveniently with less effort than the
manual wheelchair. Furthermore, with the flexibility of applying the mechanism proposed in this study, it can be readily
implemented in not only the iRW but
also any other electrical wheelchair.
Figure 12. Prototype of the iRW
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