A Brake-based Assistive Wheelchair Considering a Seat Inclination

Daisuke Chugo

, Ryo Kohara

, Masahiro Iwaki

, Satoshi Muramatsu

Sho Yokota

and Hiroshi Mashimoto

School of Science and Technology, Kwnsei Gakuin University, Sanda, Hyogo, Japan

School of Information Science and Technology, Tokai University, Hiratsuka, Kanagawa, Japan

Faculty of Science and Engineering, Toyo University, Kawagoe, Saitama, Japan

Advanced Institute of Industrial Technology, Shinagawa, Tokyo, Japan

Keywords: Manual Wheelchair Assistance, Passive Robotics, Servo Brake, Gravitation-Negating Control, Seat

Inclination Control.

Abstract: Considering the road inclination conditions, we proposed an upper body posture adjustment system for a

passive-type assistive wheelchair. On an inclined road, there is a high probability that a wheelchair will

move in a direction that is different from that desired by its user. In our previous research, we proposed a

system that estimated a wheelchair user’s intentions and worked passively to complement their intentional

force. This was accomplished by negating the wheel traction that was generated by the road’s inclination

using only the servo brakes on each wheel. However, in some cases, our system failed to assist the driving

motion of the user because it negated only the gravitational force. Therefore, our wheelchair succeeded in

avoiding the unintended movement, but its user was required to row the hand rims with a considerable

amount of force to overcome these braking forces. Consequently, we proposed an upper body posture

adjustment system that adapts to the inclined road conditions and reduces the wheel traction that is

generated by gravity. The proposed system inclined the wheelchair seat and aligned the upper body posture

of its user to the center position of the wheelchair. Using this method, the proposed system maintained the

position of the user’s center of gravity with respect to the center position of the wheelchair. Our

experimental findings suggested that the proposed passive-type assistive wheelchair can complement the

user’s intentional force with smaller brake traction, indicating that the user can drive the wheelchair using

less physical strength.

1 INTRODUCTION

Wheelchairs are widely used by mobility-impaired

people in their daily activities. Recently, a number

of serious wheelchair-related accidents have been

reported in Japan. Interestingly, more than 80% of

these accidents were caused by environmental

hazards (National Consumer Affairs Center of Japan,

2002). For example, the inclination of a sidewalk

poses a potentially high risk for wheelchair users.

The Japanese government permits sidewalks to have

an incline of up to 5° (Japan Institute of

Construction Engineering, 2008). Such an

inclination could potentially lead to a wheelchair

deviating from the sidewalk and into the roadway,

which could be catastrophic. Therefore, a wheelchair

driving assistance system is important for use on an

inclined sidewalk.

In previous research, many assistive technologies

have been developed for wheelchairs. For example,

many disabled people routinely use powered

wheelchairs (Yamaha Motor Co., Ltd., 2014). Many

previous researchers have attempted to develop

assistance functions by adding wheels with actuators

that were controlled using robotic technology, such

as motion control (Miller and Slack, 1995), sensing,

and artificial intelligence (Katevas et al., 1997)

(Murakami et al., 2001). These intelligent

wheelchairs provide several functions, such as

suitable motion, obstacle avoidance, and navigation;

thus, they provide a maneuverable system. However,

many wheelchair users have the upper body strength

and dexterity to operate a manual wheelchair. For

these users, such systems may be considerably

expensive and unnecessary.

Therefore, we developed a passive driving

assistance system for a manual wheelchair, which

136

Chugo, D., Kohara, R., Iwaki, M., Muramatsu, S., Yokota, S. and Mashimoto, H.

A Brake-based Assistive Wheelchair Considering a Seat Inclination.

DOI: 10.5220/0006858001360143

In Proceedings of the 15th International Conference on Informatics in Control, Automation and Robotics (ICINCO 2018) - Volume 2, pages 136-143

ISBN: 978-989-758-321-6

employs servo brakes (Chugo et al., 2013; Chugo et

al., 2015; Chugo et al., 2016). This system

incorporates the concept of passive robotics (Hirata

et al, 2007). The proposed system passively operates

on the basis of the external forces imposed by its

user. No actuators are required in our system. Our

wheelchair uses servo brakes that can control the

braking torque to produce the desired motion

according to the applied force and reference track.

The system developed in our previous research

estimated the intended direction of a manual

wheelchair user by determining the characteristics of

their hand motion and maintaining it as the reference

track (Chugo et al., 2013). Using the estimated results,

the system negated the effects of gravitational force

on the wheelchair moving on the inclined road

(Chugo et al., 2015) and enabled it to drive in a

direction intended by its user (Chugo et al., 2016).

However, in some cases, our assistive wheelchair

requires its user to use greater physical force to drive

it. This is because the does not use actuators, only

servo brakes, for assistance. On an inclined road, our

wheelchair cancels the gravitational force by means

of the braking torque; therefore, the wheelchair does

not move in an unintended direction. However, the

wheelchair does not provide any driving force, and

the user is required to row its hand rims with greater

force, which overcomes the braking traction. This

problem makes it difficult for some users to operate

our assistive wheelchair, especially an elderly person,

who might have limited physical strength.

To resolve this problem, it is important to

minimize the braking traction, which negates the

gravitational force. On an inclined road, the upper

body posture of the user tends to deviate from the

center position of the wheelchair, and the position of

their center of gravity (COG) moves in the direction

of gravity. This causes a rotational force, which

causes the unintended movement generated by the

gravitational force increases. Therefore, an upper

body posture adjustment is required to reduce the

unnecessary braking force.

Therefore, this paper proposes an upper body

posture adjustment system for wheelchair users. The

proposed system inclines the wheelchair seat

according to the inclination of the road. The

proposed device measures the position of the user’s

COG and keeps it at the center position of the

wheelchair. This paper is organized as follows: In

Section 2, we introduce our assistive wheelchair and

its problem specifications; In Section 3, we propose

an upper body posture adjustment system; In Section

4, we show the results of experiments using our

prototype; In Section 5, we present our conclusions.

2 ASSISTIVE WHEELCHAIR

2.1 System Configuration

Fig. 1(a) shows our prototype. The proposed

wheelchair utilizes a powder brake, which is a type

of servo brake. Powder brakes are widely used in

industrial applications, and their cost is low

compared with other servo brakes. The powder

brake in Fig. 1(b) (ZKG-YN50, Mitsubishi Electric

Corp.) generated enough braking torque to stop a

wheelchair moving at 4 km/h, containing a 100-kg

user, in one second.

Our prototype is based on a normal, manual

wheelchair (BM22-42SB, Kawamura Cycle Co.,

Ltd.), and our system is compatible with an ordinary

wheelchair that meets the ISO7193, 7176/5

standards. This means that a user can incorporate our

system into their wheelchair without any special

construction.

(a) Overview (b) Installed Servo Brake

Figure 1: Our Prototype.

For measuring the road inclination, our

wheelchair has two tilt sensors: one for roll angle

[Fig. 2(a)] and one for pitch angle [Fig. 2(b)]. These

sensors are modularized and connected by USB

cables to the control computer (PC). In this study,

our prototype wheelchair has this module installed

on its frame, under the seat.

2.2 Traction Required to Negate the

Gravitational Force

Fig. 2 shows a wheelchair model on an inclined road.

On a slope, the gravitational force pulls the

wheelchair to a lower point on the incline.

A Brake-based Assistive Wheelchair Considering a Seat Inclination

137

, f

(a) pitch angle (b) roll angle

Figure 2: Wheelchair model on an inclined road.

Kinematically, when the pitch angle is

, the

gravitational forces applied to the right wheel

and left wheel

are shown by

sin

sr sl

(1)

where m is the mass of the wheelchair, including the

user’s body weight.

Furthermore, kinematically, when the roll angle

, the gravitational forces applied to the right

wheel

and left wheel

are shown by

sin

tr tl

ymg

=− =

−

(2)

where

(

)

yx ,

is the position of the center of

gravity (COG), and T is the distance between the

wheels.

Thus, the required braking traction on each

wheel (

: right wheel,

: left wheel) to cancel

the gravitational force is

sin

sr tr

ymg

τθ





== +



−



(3)

sin

sl tl

ymg

τθ





== −



−



(4)

2.3 Problem Specifications in the

Uphill Condition

Our assistive wheelchair on an inclined road negates

the gravitational forces on the road, as in Eqs. (3)

and (4). Our wheelchair does not have actuators and

can only use braking traction. Therefore, especially

in an uphill condition, our assistive wheelchair

requires the user to use more physical strength to

drive it.

For example, when the user goes uphill on a road,

as in Fig. 3, the wheelchair moves to a lower

position because of the gravitational forces on the

road. In this condition, without an assistance system,

a manual wheelchair user should drive the left wheel

hard so that

ff >

, as in Fig. 3(b) (where

is the

driving force on the right wheel and

is the

driving force on the left wheel). To negate the

gravitational force, our assistive wheelchair controls

the servo brake according to Eqs. (5) and (6). In this

condition, the wheelchair negates only the rotational

moment generated by the gravitational force because

the servo brake cannot generate a driving force.

, f

The wheelchair tends to

move this direction

because of the gravity.

(a) side view (b) top view (w/o assist) (b) top view (w/ assist)

Figure 3: Braking tractions on an inclined road.

()

cr cr cl cr cl cr

riffffelse

ττ

=− > =

(5)

()

cl cr cl cr cl cl

riffffelse

ττ

=− < =

(6)

In the case of Fig. 3(c), our system generates the

braking traction

on the left wheel to negate the

gravitational force pulling the wheelchair to a lower

position (in the right direction). In this case, our

wheelchair user should drive each wheel equally so

that

grgl

ff =

(where

is the driving force on the

right wheel and

()

gl l cl

ff=+

is the driving force

on the left wheel with our assistance force). This

means that the user can row the wheelchair as if on a

flat road. However, the passive system does not

assist the driving force and the required driving

force increases with the braking force

on the left

wheel. Therefore, the user may feel that the

wheelchair is too heavy, especially in this uphill

situation.

ICINCO 2018 - 15th International Conference on Informatics in Control, Automation and Robotics

138

To reduce the user’s load, the braking traction

to negate the gravitational force should be

minimized. From Eqs. (3) and (4), the position of the

COG,

(

)

yx ,

increases the difference between the

right and left braking tractions. This means that if

the position of the COG locates at the center position

of the wheelchair, the user’s load will decrease. On

an inclined road, the upper body posture of the user

tends to deviate from the center position of the

wheelchair. Therefore, it is important to adjust the

upper body posture to reduce the user’s load.

3 UPPER BODY ADJUSTMENT

SYSTEM

3.1 Proposed Device

Fig. 4 shows an overview of the proposed assistance

system. The system consists of a lifting device with

a urethane cushion designed for the wheelchair and

its controller. We designed the lifting device to be as

thin as possible for easy implementation. Generally,

the distance between the seat position and foot

support is important for increasing usability (Defloor

et al., 1999). The thickness of our lifting device is

only 18 mm, and a user can install it without having

to reconfigure the wheelchair.

To realize a thin design for our lifting device, we

developed a tilt mechanism that is based on the

elasticity of acrylic resin, as shown in Fig. 5. Also,

to realize a thin mechanism, there is no sensor in the

lifting device. The controller uses only pressure

sensors to estimate the position of the COG of the

user’s upper body. The four air cells, each of which

has an air compressor, lift or incline the aluminum

base, as shown in Fig. 4(b). The acrylic resin also

prevents the base from shifting. The cells can be

cycled more than 20,000 times, according to the

manufacturer’s specifications.

(a) normal position (b) inclined position

Figure 4: Proposed upper body posture adjustment system.

390

100

100100

380

A-A

(1)

(1) (1)

(2) (2)

(2)

(3)

Figure 5: Design of a lifting device. (1) aluminium base,

(2) air cell, and (3) acrylic resin.

Fig. 6 shows the controller for our posture

adjustment system. It consists of a control box,

which contains interface modules connected to a

control PC by USB cables and an air compressor

module, which contains air compressors, solenoid-

operated valves, and pressure sensors. Both parts are

small and can be installed within a wheelchair body.

The controller requires a 12-V, 2.8-A power supply

in order to lift a 100-kg user, and the standard

batteries in the servo brake system can supply it.

Therefore, the proposed posture adjustment system

does not require additional batteries for the

controller.

Our posture adjustment system has four air cells,

each of which has an air compressor, solenoid-

operated valve, and pressure sensor. To lift the seat

cushion, the system drives the air compressors. To

lower it, the system stops the compressors and opens

the solenoid-operated valves. The elasticity of the

acrylic resin helps the air cells shrink in the case

where no user is seated in the chair. A sensor on

each air cell measures pressure in real time, and the

controller uses the measured data to position the

COG of the user.

Digital output

(Contec

DIO0808LY-USB)

AD Converter

(Contec AI-0608AY-

USB)

Air Compressor

(OKENSEIKO

PFR32B28)

Relay

Solenoid Operated

Valve (TDL Co.,

TDS-V06A-713)

Relay

Pressure Sensor

(Keyence AP-44)

Air-Cell

Control PC

No.1-4

USB

Control Box

Pressure Sensor

Amplifier (Keyence

AP-C40)

Air-Compressor Module

No.1-4

Air

Human Interface

Figure 6: Overview of our controller for a posture

adjustment system.

18 mm

A Brake-based Assistive Wheelchair Considering a Seat Inclination

139

3.2 Position Estimation of the Center of

Gravity

The distribution of pressure on the sitting surface

inflects the posture of the user’s upper body (Rader

et al., 1999). Therefore, we use the position of the

user’s COG on the wheelchair seat as an index to

estimate their upper body posture.

To estimate the position of the COG on the

wheelchair seat, we use the pressure sensor on each

air cell. This position

()

is derived from (7).

The position and coordination of pressure sensors

are shown in Fig. 7. The pressure values

()

4,,1 =ip

are measured by the sensor on each air

cell.

()()

{}

()()

{}

141 232

121 342

ppx ppx p

yppyppy p



=+ ++







=+ ++







(7)

Sensor1

(Height)

(Pressure)

Sensor2

Sensor3 Sensor4

(Height)

(Pressure)

(Height)

(Pressure)

(Height)

(Pressure)

(a) sensor position (b) kinematical relationship

Figure 7: Kinematics of proposed upper body posture

adjustment system.

3.3 Control Algorithm for the

Proposed Device

The proposed device inclines the wheelchair seat

according to the position of the user’s COG.

However, to realize its thin profile, there are no

sensors in the lifting device, and the control box

contains only the air pressure sensors. Therefore, the

system cannot measure the seat inclination directly.

Instead, the controller estimates the seat inclination

using only its pressure sensors and realizes the

reference inclination, which aligns the position of

the user’s COG to the center position of the

wheelchair. The proposed scheme calculates the

volume of the air cells by integrating the airflow

during lifting.

The lifting height h(T) when the air compressor

works for T seconds is derived by

()

(8)

where v(T) is the volume of an air cell and s is its

cross-sectional area. Also, v(T) is the integration of

the airflow, and the volume of the air cell is changed

according to its pressure. The relationship between

the cell’s volume and pressure is inversely

proportional according to Boyle’s law. Therefore,

v(T) is defined by (9) when its pressure is p(T):

() () ()

()

vT pT u pt ptdt⋅= ⋅



(9)

where u is the airflow of the compressor in the case

of an inflow (

0u >

) or the solenoid-operated valve

in the case of an outflow (

0u <

). Also, u is a

function of the pressure p in the air cell that the

pressure sensor can measure. Meanwhile, T is the

operating time of the air compressor or valve. The

parameters of u are derived based on the

manufacturer’s specifications. In this case, we set u

as in (10):

()

ap b u

−+ >







−− <





(10)

where

6.7 10

−

=×

12.5 10 sec

amkPa

−



=× ⋅



7.66 10

−

=×

, and

3.2 10 [ sec]

−

=×

are

constants. From Eqs. (8) to (10), the estimated lifting

height is derived from (11). Our system integrates

the instantaneous height from the start time to T and

can estimate the lifting height through odometry. We

assume that the temperature is constant.

()

hT apt b pt dt

spT

=+⋅

⋅



(11)

Using the height estimation scheme, we can

estimate the inclination of the lifting device from

(12). The estimated height

of each air cell (

4,,1 =i

) is given by (12), and l (=250 mm) is the

distance between them, as shown in Fig. 7(b). The

inclination value is small, and we use approximate

equations, as in (13):

()

{

}

()

{

}

()

{}

()

{}

12 34

14 23

hh hh



−+ ++





−+ + +





(12)

sin , sin

γγ

θθ

≈≈

(13)

Our system realizes the pitch

and roll

inclinations as follows. From the kinematic

relationship, the height of each air cell should be

fulfilled as follows:

ICINCO 2018 - 15th International Conference on Informatics in Control, Automation and Robotics

140

() ()

34 41

0, 0

if if

γθ







(14)

Using (14), our controller sets the minimum

height of an air cell

from the reference inclination.

and

, our controller sets

hh=

and

derives

using (12) and considering the

restraint condition that the all air cells are all

connected to the same plane. Because the outflow is

larger than the inflow, we set

[

]

10hmm=

improve the response time of our device. If the

controller discharges all the air from the cell, the

charging time will be long. Using these principles,

the controller of the proposed device can realize the

reference inclinations.

The proposed system inclines the wheelchair seat

when the position of the user’s COG does not align

with the center position of the chair: it tries to return

the user to their original position. The inclination

references

()

ref ref

are generated by the simple

PID controller as in (15). Considering the individual

differences, as in (16), the proposed controller uses

the position of the user’s COG when they sit on the

seat in a natural sitting posture as a position control

reference. In many cases, this reference

()

org org

almost the center position of the wheelchair seat

()

0, 0

org org

xy≈≈

, which means the proposed

system adjusts the position of the COG to the center

position of the chair.

ref p e i e d

kx k xdt k

ky k ydt k





=− + +





 







=− + +











(15)

ecorg ecorg

xx y yy=− =−

(16)

3.4 Preliminary Experiment with the

Proposed Device

In this experiment, we set the proposed wheelchair

on two inclined roads (

4, 8

=−

°;

°). On these

roads, our wheelchair could be stopped using the

handbrake. Each of five subjects (subjects A–E in

Table 1) sat in our prototype, which adjusted the

positions of their COGs. To verify the effectiveness,

the subjects tried each tried to navigate the inclined

road without the proposed device.

Fig. 8 shows the experimental results. In each

case, using the proposed device, the positions of the

COGs were located around the center position

compared with the results without the device.

Therefore, the proposed device is effective for

adjusting the position of the COG on the wheelchair

seat.

(a)

=−

° (b)

Figure 8: Position of COG (x-direction) on an inclined

road.

4 EXPERIMENTS

4.1 Experimental Setup

In this experiment, the 14 subjects listed in Table 1

tested our prototype. The subjects moved from side

to side in a figure of eight on a test road with an 8°

incline using our prototype wheelchair with upper

body posture adjustment assistance, as shown in Fig.

9. This course had the typical characteristics of an

inclined sidewalk environment: (A) is a straight

uphill path; (B) is curved uphill path; (C) is a

straight downhill path; and (D) is curved downhill

path. To compare the effectiveness of the proposed

assistance system, the subjects repeated these trials

in wheelchairs without the system. Furthermore, for

accurate verification, the subjects did not know how

the proposed assistive system worked and whether

the trials were with or without the system.

Table 1: Subjects.

Subject

Weight

[kg]

Height

[m]

Gender Age

Hand

edness

A 50 1.66 male 23 left

B 52 1.64 female 22 right

C 42 1.60 female 23 right

D 61 1.74 male 24 right

E 53 1.70 male 21 right

F 67 1.84 male 23 both

G 55 1.65 male 24 right

H 60 1.74 male 24 right

I 57 1.67 male 23 left

J 56 1.72 male 21 right

K 65 1.77 male 24 right

L 60 1.75 male 21 right

M 50 1.54 female 23 left

N 50 1.54 female 22 right

-100

-80

-60

-40

-20

ABCDE

Position [mm]

Subject

No Assist

With Assist

100

ABCDE

Position [mm]

Subject

No Assist

With Assist

A Brake-based Assistive Wheelchair Considering a Seat Inclination

141

8deg

Start/Goal Position

(A)

(B)

0.75m

(C)

(D)

(A)

(B)

(C)

(D)

(a) overview of a test course (b) real environment

Figure 9: Test course on an inclined road.

4.2 Experimental Results

The results show that the subjects could drive in an

intended direction when using our system (Fig. 10).

Fig. 11 shows the running tracks of the wheelchair.

With the proposed assistance system, the subjects

could drive the wheelchair according to the

reference path, compared with the running tracks

without the system. This tendency was the same for

left- and right-handed subjects.

Fig. 12 shows the inclination of the seat surface

as the wheelchair passed section C. From Fig. 12(b),

it can be seen that our assistive system inclined the

seat surface according to the inclination angle of the

road and succeeded in adjusting the upper body

posture of the subject. While adjusting the upper

body posture, the maximum braking traction, which

negates the gravitational force, decreased, as shown

in Fig. 13. Especially in a straight uphill path

(section A), the proposed system reduced the

braking traction by more than 50%, and this means

that the subject could drive the wheelchair using less

physical strength. As a result, the driving velocity

increased and, as shown in Fig. 14, the time required

to pass through each section decreased.

To investigate how the proposed system feels to

its users, we conducted the questionnaire survey in

Table 2. Typically, such a questionnaire has scales

of 1–5 or 1–7. In this experiment, to avoid a

concentration of “neither” responses, as is the

Japanese habit (Takahashi et al., 2013), we used a

scale of 1–6.

(a) passing (A) (b) passing (C)

Figure 10: Test run by subject A with the proposed

controller.

(a) subject A (left handed) (b) subject B (right handed)

Figure 11: Running tracks.

(a) without assistance (b) with assistance

Figure 12: Inclination of seat surface and upper body

posture at passing (C) (subject C).

Figure 13: Maximum braking traction in each section.

Figure 14: Passing time for each section.

Fig. 15 shows the questionnaire results. In the

results from Q1 to Q3, the subject feels that the

driving force is light with our assistance system.

Furthermore, the results from Q4 and Q5 indicate

that the subject felt that it was easy to drive on the

reference path with our system. We can verify this

result from the running tracks in Fig. 11. The results

from Q6 and Q7 indicate that the subject felt that the

driving assistance was better with the proposed

device. The assistance algorithm with the servo

-1

-2 -1 0 1 2

Y [m]

X [m]

Reference

Without Assist

With Assist

-1

-2 -1 0 1 2

Y [m]

X [m]

Reference

Without Assist

With Assist

ABCD

Brake Traction [Nm]

Section

Without Assist

With Assist

ABCD

Time [Sec]

Section

Without Assist

With Assist

ICINCO 2018 - 15th International Conference on Informatics in Control, Automation and Robotics

142

brake was the same; therefore, by adjusting the

upper body posture, the subjects felt that the

assistance effectiveness was better.

Table 2: Questionnaire.

Higher Score

(Maximum is 6)

Lower Score

(Minimum is 1)

I do not feel fatigue in

this trial.

A feel fatigue in this trial.

Q2 A hand rim is light. A hand rim is heavy.

It is easy to strain my

arm.

It is hard to strain my arm.

It is easy to drive on the

straight path.

It is difficult to drive on

the straight path.

It is easy to drive on the

curved path.

It is difficult to drive on

the curved path.

The assistance is

smooth.

The assistance is awkward.

Q7 The assistance is quick. The assistance is slow.

Figure 15: Questionnaire results.

5 CONCLUSIONS

This paper proposed an upper body posture

adjustment system for wheelchair users on an

inclined road. Our system succeeded in maintaining

the user’s body balance. As a result, the gravitational

force caused by road inclination was reduced, and

the required braking traction for wheel driving

assistance was also reduced. Consequently, the user

could use less force with the proposed system.

ACKNOWLEDGEMENTS

This work is supported in part by Hyogo Foundation

for Science and Technology, Kawanishi Memorial

ShinMaywa Education Foundation and Exploratory

Research on Feasibility Study (FS) Stage

(AS242Z00295K) by Adaptable and Seamless

Technology Transfer Program through Target-driven

R&D, Japan Science and Technology Agency (JST).

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Q1 Q2 Q3 Q4 Q5 Q6 Q7

Score

Without Assistance

With Assistance

A Brake-based Assistive Wheelchair Considering a Seat Inclination

143