A SIMULATION STUDY OF THE NEW CONCEPT

OF A STAIR-CLIMBING WHEELCHAIR

Concept of Construction

Grzegorz Dobrzynski, Wlodzimierz Choromanski and Jerzy Kowara

Warsaw University of Technology, Faculty of Transport, Koszykowa 75, 00-662 Warsaw, Poland

Keywords: CAD, simulation study, stair-climbing wheelchair.

Abstract: The authors present an idea of construction, modelling and simulation studies of a new generation,

mechatronical wheelchair. The wheelchair is meant to drive on various surfaces. Its important feature is the

possibility to overcome obstacles, such as a doorstep of max. 220 mm. The construction model and

simulation studies were carried out in the environment of MBS ADAMS package. An algorithm of

controlling the process of overcoming the doorstep was proposed and an analysis of parametrical sensitivity

of the construction was performed.

1 INTRODUCTION

Modern transport systems should be created with

appropriate observance of the needs of the disabled.

At the design stage, special attention should be paid

to the needs of people with mobility problems. For

these people, important elements of the transport

system include: architectural features, specially

adapted public means of communication, and

individual means of transport (Axelson, 1995),

(Blachowski, 1993). A wheelchair with special

transport features is an important element in the

whole system. The development of the motor

industry led to a dramatic increase of accidents,

which in turn increased the number of people

suffering from permanent disability or impairment

of their movement capabilities. The feedback

received from the disabled clearly suggests that there

are still many barriers that make it difficult for these

people to function individually in society.

Irrespective of the laws in force, the number of

buildings that are not adapted for the disabled is

large. And the adaptation process is long and

expensive. What could help the disabled to

overcome architectural obstacles is a new

wheelchair with special functions, such as moving

up and down the stairs or lifting the disabled person

to such a height that is achievable by a healthy

individual.

This paper presents assumptions and preliminary

simulation tests of a new concept of a wheelchair for

people with motion disabilities. The goal of the

simulation model is first of all to optimize the

wheelchair's parameters in order to achieve the most

desirable safety and ergonomic conditions. The main

advantage of the wheelchair will be its ability to

overcome terrain obstacles, such as thresholds, stairs

or curbs, or lifting the user to heights that are

achievable by healthy people. The wheelchair will

be able to drive into a low-deck bus or tram on its

own. Furthermore, its construction will be simple

and spatially limited.

2 THE CONCEPT OF A

STAIR-CLIMBING

WHEELCHAIR

The concept is presented in the form of a 3D picture

(Fig. 1). The wheelchair can move over flat surfaces,

as well as thresholds, stairs and curbs. The drive

system of the wheelchair consists of two electric

motors (4). The vehicle changes its direction by

means of differentiating the speed between the two

motors. The wheelchair systems are powered by two

service-free gel batteries, 12 V, 2 x 40 Ah. The

batteries are located relatively low to achieve the

most advantageous location of the centre of gravity.

Beneath the seat, there is a seat lifting mechanism

(1). The seat lifting mechanism is connected with the

system to correct the inclination of the seat while

driving through an obstacle. The rear part of the

wheelchair features an arm that can lift the

143

Dobrzynski G., Choromanski W. and Kowara J. (2008).

A SIMULATION STUDY OF THE NEW CONCEPT OF A STAIR-CLIMBING WHEELCHAIR - Concept of Construction.

In Proceedings of the First International Conference on Biomedical Electronics and Devices, pages 143-146

DOI: 10.5220/0001048701430146

 SciTePress

wheelchair over an obstacle (8). It is driven by a

motor module with a gear (6). Distance measuring

systems play an important role when the wheelchair

is moving over an obstacle. There are two such

systems: a front one (5) and a rear one (7). The slide

skid (2) also proves useful in negotiating obstacles.

Figure 1: Wheelchair contraction model – own concept.

The wheelchair features two drive modes: "normal"

and "obstacle". The normal mode is used to drive on

flat or slightly uneven surfaces. The user has two

gears to choose from: I and II. The first gear (I)

allows the wheelchair to drive with the maximum

speed of 3 km/h. It is meant to be used in small

compartments or rooms. One of the benefits of using

the first gear is that it makes steering the wheelchair

much easier. Gear II is to be used for greater

distances. It also requires a greater precision in

handing the steering lever.

The wheelchair also features a lift function. It

lifts the seat to a higher level, which is important for

disabled people.When the lift function is activated,

the wheels of the wheelchair are blocked and driving

is not possible. An audible signal will be introduced

to remind the user that the lift function has been

activated.

3 OPERATIONAL PARAMETERS

AND ALGORITHM FOR

OVERCOMING OBSTACLES

The following operational parameters were assumed

for the initial phase of the project:

• Threshold height (max) 0.22 m

• Threshold depth (min) 0.25 m

• Battery 12V, 2 x 40 Ah,

• Battery weight 24 kg

• Autonomy (working time) 5 hours

• Load bearing capacity 100 kg

• Weight 80 kg

• Min. space required for manoeuvring 1.1x1.1m

• Gear I speed 1 m/s

• Gear II speed 2 m/s

• Mean speed while overcoming stairs 0.1 m/s

• Minimum width of staircase 0.8 m

• Maximum surface inclination 35°

• Seat lift height 0.45 m

The above-mentioned values result from an analysis

of specific features of the wheelchairs currently

available on the market. These values are subject to

minor modifications (Kowara, 2005), (Milanowska,

1997).

The algorithm used to overcome obstacles that is

presented here explains the principles of operation of

the construction described in the above paragraph.

The ascending phases (Fig. 2.) will be carried out in

the same manner if the wheelchair has several stairs

to overcome: when Phase e) is completed, the

system will carry on to Phase a) again. It will

operate through all the phases in a closed loop until

all stairs have been overcome. Descending the stairs

(Fig. 2) will look exactly the same as descending a

singular threshold. It will be carried out in a closed

loop: when Phase g) is completed, the system will

start from Phase c) again.

Ascending an Obstacle. To overcome an obstacle

higher than 5 cm, the obstacle should be approached

backwards, i.e. with the back of the wheelchair.

When the user approaches an obstacle, such as a

curb or stairs, he or she switches the driving mode to

"obstacle". The speed of the main motors is reduced

and the systems detecting distances are activated

(Fig. 2, Phase a). The steering system brings the

wheelchair to the appropriate distance from the edge

of the obstacle and positions the wheelchair

perpendicularly to it. Then, Phase b) starts. Main

drive motors are de-activated and the drive wheels

are blocked. The lift arm starts operating. Moving to

subsequent Phases c) and d), the steering system

controls the seat and keeps it horizontally. Phase e)

starts when the lift arm makes a 180°- turn. The

main drive motors are activated again and, at a

reduced speed, bring the wheelchair onto the

obstacle. The distance measuring systems keep

checking if there is another obstacle for the

wheelchair to overcome (such as another stair). In

case there is another stair, the whole cycle starts

form Phase a). When the last stair has been

overcome, the wheelchair moves backwards at a

reduced speed. During Phase f), the edge of the

obstacle moves over the slide skid and the front

wheels are mounted onto the obstacle. At the end of

Phase g), the user switches the driving mode back to

"normal".

Descending an Obstacle. Preparing to descend an

obstacle, the wheelchair should be driven up to the

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obstacle with the wheelchair front positioned

perpendicularly to the edge of the obstacle. The

"obstacle" mode should be used when the height of

the obstacle (stair or threshold) is higher than 5 cm.

It is the user who decides to switch to this mode of

driving.

Figure 2: Ascending and descending an obstacle–

algorithms of operation.

When the "obstacle" mode is activated (Phase a), the

speed of the main drive motors are reduced and the

distance measuring systems, located between the

front and the rear axes of the wheelchair, start to

operate. Watching the position of the wheelchair

skids, the steering system corrects the direction of

the wheelchair movement (Phase b). Continuing the

descending movement, the distance measuring

system stops the main drive motors at an appropriate

moment (Phase c). Then, Phase d) begins. The drive

wheels are blocked. The lift arm starts to operate.

Moving to subsequent Phases e) and f), the steering

system controls the seat and keeps it horizontally.

Phase g) starts when the lift arm makes a 180°- turn.

The main drive motors are activated again and, at a

reduced speed, bring the wheelchair down the

obstacle. The distance measuring systems keep

checking if there is another obstacle for the

wheelchair to overcome (another stair). In case there

is another stair, the whole cycle starts form Phase d).

When the last stair has been overcome, the

wheelchair moves on at a reduced speed. At the end

of Phase g), the user switches the driving mode back

to "normal".

4 ASSUMPTIONS FOR

BUILDING A SIMULATION

MODEL

There is a huge variety of wheelchairs available on

the market. Designers strive to achieve the most

functional prototype, maintaining the most crucial

features, such as low weight, simple and comfortable

operation, and first of all: low price. The last aspect

is very important in making the wheelchair available

to a wide spectrum of the disabled who, very

frequently, do not belong to the wealthiest groups of

the society. To meet all these requirements, one

must look for new methods of wheelchair designing

and modelling. The literature available suggests that

most research carried out in this field concentrates

on determination the strength and resistance of the

wheelchair construction, so that relevant safety

standards and technical requirements are complied

with. The majority of solutions focus on improving

the form of additional equipment to be installed in

the wheelchairs to overcome surface obstacles. They

are developed without the use of computer

simulation or dynamic tests.

The overview of the literature on wheelchairs able to

overcome such obstacles as stairs or thresholds

(Zabłocki, 2002), as well as simulation studies of

simple constructions carried out in 2D-Working

Model environment, shows the capabilities and

limitations of this kind of vehicles. Due to the fact

that one of the priorities of the model this study

refers to is a simple and compact construction with

due observance of the other requirements, an

analysis of various systems used for overcoming

obstacles of the threshold type led to the

development of the structural concept presented in

Chapter 3. An initial analysis of the systems lets the

authors believe that the construction secures safe

overcoming of obstacles and at the same time is

simple and compact. For the purpose of the

simulation tests, certain simplifications were made:

• The human model is represented by a number of

permanently fixed solids with certain weight

parameters.

• The human model is fixed to the seat.

• The surface on which the wheelchair moves is flat

and horizontal.

• There is no friction between the front wheels and

the front frame on the one hand, and the ground

surface and the obstacle edges on the other hand.

A SIMULATION STUDY OF THE NEW CONCEPT OF A STAIR-CLIMBING WHEELCHAIR - Concept of

Construction

145

• Ascending an obstacle, the wheelchair is

positioned backwards to it.

The position of a human body's centre of gravity in

the sitting position is an important variable in a

simulation model of the construction. A real human

body, as well as its individual elements, is a complex

system with a continuous distribution of mechanical

properties. Therefore, human body modelling as

such is a vast area of science and the subject of a

number of research projects. In this project, the

approximate mass parameters of a human body

model were chosen on the basis of data used in the

research on car seats, and the model developed with

the use of statistical data (Seireg & Arvikar, 1989),

applied to analyze and simulate human walking.

Using the above mentioned data, together with the

recommendations made in PN ISO 7176-11

Standard, "Test dummies", the authors developed a

body model of an average man of 78.5 kg in the

sitting position. Catia system was used for this

purpose.

Geometrical parameters of the wheelchair under this

project were based on the measurements of an

Explorer wheelchair carried out by the authors, as

well as data found in the literature available.

When the wheelchair is in operation, certain

parameters may change. It can be assumed that this

has an impact on the performance of the wheelchair-

user configuration while the wheelchair is in motion.

To verify such a hypothesis, these variables need to

be identified and their impact on the configuration

performance needs to be assessed. Due to a vast

number of various factors having an impact on the

performance of the wheelchair-user configuration,

only certain parameters were selected for further

tests (i.e. those parameters that can significantly

influence the performance of the vehicle while

overcoming an obstacle):

• Friction between the wheels and the surface

• Obstacle height

• Distance of the user's centre of mass from the

back support

• Obstacle approach angle.

The variability of the power intake by the wheelchair during obstacle overcomin

100

200

300

400

500

600

t[s]

powe

[W]

Figure 3: The variability of the power intake by the

wheelchair during obstacle overcoming.

The scope of variability of individual parameters

will be determined during simulation tests.

Examples of simulation results are presented below.

The variability of power consumption while

overcoming a 220 mm obstacle was determined.

5 CONCLUSIONS

The paper presented a constructional concept and

simulative studies of a new generation wheelchair

for motional disabled persons. Taking in

consideration the wide range of studies and analyses

the new construction is submitted to before it can be

introduced into production, it should be

acknowledged that this paper could be very helpful

when creating the real wheelchair construction.

The most important elements of that study include:

• A study of the advanced schema of wheelchair

construction,

• a study of the nominal model of the

construction above,

• a proposal of the control algorithm when

coming across a barrier,

• a construction of the simulative model in the

Adams program.

ACKNOWLEDGEMENTS

The work was covered by a grant of The National

Foundation for the Rehabilitation of the Disabled

for the study on the construction and implementation

series of stabilisers.

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