A SPECIFIC LOCOMOTION INTERFACE FOR

VIRTUAL REALITY

Design of a Wheelchair Type Haptic

Cédric Anthierens, Jean-Luc Impagliazzo, Yves Dupuis

LISMMA EA2336, Supmeca Toulon, Maison des Technologies, Place G Pompidou, F83000 Toulon, France.

Eric Richard

ISEN Toulon, Maison des Technologies, Place G Pompidou, F83000 Toulon, France.

Keywords: Virtual environment, disabled persons, multisensorial interfaces.

Abstract: This paper presents a recent advance in Virtual Reality (VR) related to building design or development of

public places. It focuses on the design and implementation of a wheelchair type haptic to simulate ease of

access and displacement of a disabled person in a wheelchair in public places. A VR platform equipped with

this haptic system provides a Virtual Environment (VE) that represents either a street scene or an interior

building scene. This VE is useful for architects who want to efficiently design facilities for disabled access

during the design phase. The first part of this paper deals with the lack of consideration of disabled access in

everyday places and therefore the need to improve facilities, as emphasized by French law. The second part

deals with specifications and expected performances of the VR platform, specifically with regard to the

requirements of the whole Virtual Environment. The third part focuses on the mechatronic design and

explains how each part of the interface works to produce good renderings and a high level of realism with

respect to the main goal defined before. The implementation phase and integration of this specific

behavioural interface into the VR platform is presented in the fourth part. The last part before the conclusion

and perspectives discusses tests and final results obtained from total immersion within two types of

simulated interior environment.

1 INTRODUCTION

For a long time we have observed everyday

difficulties encountered by disabled persons that

could be avoided if the design of private or public

places took their handicap into account. Facilities

satisfying able bodied or disabled persons can

usually be implemented but this solution is not often

chosen. For example, too many public places such as

restaurants, concert halls and trains are not yet

equipped to receive persons who rely on a

wheelchair for their mobility. Accessibility to such

places is therefore reduced or, in some cases, is

impossible.

A recent French law regarding the rights and the

opportunities of disabled persons imposes standards

to improve accessibility for all on both existing and

new buildings. The L.111-7 article of building and

house French code states that public or private

buildings or facilities, whether inside or outside

must be accessible to all and notably to disabled

persons regardless of the type of handicap

considered.

Our study focuses primarily on a physically

disabled wheelchair user. Beyond the simple respect

of the building standards in force, it could be

interesting for architects to estimate the importance

of these standards in use for the end-user. Indeed the

difficulties of moving a wheelchair on a banked

walkway or to cross a small step between two rooms

for example is often not understood or perceived by

designers and therefore they do not change their

design according to these considerations.

This is the reason why we would like to give

architects the possibility to experience a virtual

version of a room or exterior space he/she has

designed in which he/she can move with a

426

Anthierens C., Impagliazzo J., Dupuis Y. and Richard E. (2006).

A SPECIFIC LOCOMOTION INTERFACE FOR VIRTUAL REALITY - Design of a Wheelchair Type Haptic.

In Proceedings of the Third International Conference on Informatics in Control, Automation and Robotics, pages 426-431

DOI: 10.5220/0001210904260431

 SciTePress

wheelchair. This would enable architects and

engineers to better understand the interest to design

differently by taking into account the mobility

difficulties of everybody (Harrison, 2004).

The benefits of this study are numerous because

it can help architects and designers by providing a

virtual validation of their modelling, but it can also

contribute to change standards to be more suitable

for the everyday life of wheelchair users.

These improvements could follow different

criteria according to the end user point of view such

as comfort, security, accessibility. The aim of this is

to provide a VE as a useful tool to help designers

and architects. This is why our VE highlights the

main nuisances and difficulties, which can be felt by

a wheelchair user everyday instead of providing a

simulator to learn how to move with a wheelchair.

Technical requirements are different depending upon

the different goals.

Finally, our VE should simulate if it is easy to

move within a virtual room equipped with a table,

chairs and other potential obstacles. This is a good

method to evaluate the accessibility of objects.

Moreover, an emergency exit procedure could be

simulated and evaluated by criteria such as the

evacuation time for example. Finally, accessibility

of a stair via a ramp would be taken into account to

verify the acceptable slope defined by the building

standards.

Our work offers the option to simulate

locomotion either with a mechanical wheelchair or

electric wheelchair.

2 VIRTUAL ENVIRONMENT

SPECIFICATIONS

According to the goals presented above the VE

should be able to make the user feel subtle

difficulties to move with a wheelchair within the

simulated place. Only one user can be immerged at a

time in the considered virtual place. Indeed to

validate the ease of access of the scene, it is not

necessary to create virtual meeting between several

users.

To satisfy a good level of realism our VE must

be multisensorial, this is why we decided to offer

sight, hearing and touch to the user to interact with

the VE. The level of realism should be particularly

considered to make the user feel the difficulties of

moving in a restrictive room for example. The

textural graphic rendering of non interactive objects

is not a priority in this study. On the other hand we

are interested in locomotion, which is the primary

task we have to simulate here. Thus we pay

particular attention to the type of displacement

(forwards, backwards, rotation, linear), the event

during motion (shock, bounce, freewheel) and the

nature of the ground (soft ground, horizontal or

ramped). The difficulties in simulating these are for

example related to the lack of own force to move

forward on a ramp, the lack of free space to turn or

move in a confined place (lift, end of a corridor,

between tables in a classroom...), the lack of skill to

maitain the desired direction while moving on a

banked walkway and so on.

Manoeuvrability is more difficult in an electric

wheelchair than in a mechanical wheelchair.

However several difficulties depend on the user’s

skill and his/her available arm force, which is why

we will first focus on the mechanical wheelchair, as

this is subject to all types of difficulties.

In its framework our Immersive Environment

(IE) is similar to other dedicated VR locomotion

interfaces such as a walking device, bicycle

simulator and so on. Indeed for this type of platform,

the immerged user should perceive that he/she

moves voluntarily on the spot. The realism, we

decided above to focus on difficulties rendering,

must be judged all the time by the three chosen

senses (sight, hearing and touch). Of the four

behavioural primitives (Fuch, 2003) (observe the

virtual world, move and interact in the virtual world,

and communicate with another in the virtual world)

we decided to consider only the first three.

Our contribution points especially to how to

provide the feel of moving in the VE by taking into

account all the difficulties quoted above. The visual

rendering of the virtual scene will be projected

simply on a large screen to create a feeling of

immersion and to display object with actual size. For

our application, head mounted displays are not

suitable as they are expensive and make data

processing more complex because of the required

head tracking. Elsewhere our VE can be displayed

on a hemispherical screen with stereo glasses (reality

centre type) or in a CAVE (room-sized cubic system

with projections on all walls and floor). This last

system is about 3 meters wide and long, so the

interface design must be as compact as possible and

compatible with these dimensions.

As it has been specified above, we wanted to

make the user feel the difficulty and energy required

to move in constrained environment. It is therefore

necessary to design our behavioural interface as a

sensorial and driving interface like a haptic. That

means the user will interact with the locomotion task

A SPECIFIC LOCOMOTION INTERFACE FOR VIRTUAL REALITY - Design of a Wheelchair Type Haptic

427

through the wheels of our interface. He/she will also

perceive a force feedback according to the

characteristics of where he/she is within the VE.

This interface will be designed as a classical haptic

but its force feedbacks could be high and close to the

reality contrary to other haptics which provide a low

force feedback to simulate the actual force.

In addition to the sensorial feedback supplied via

the independent wheels, we wanted to represent a

sensorial feedback to simulate a change of

orientation (pitch) while moving on a ramp. This

feature is not only to increase the realism level but

also to force the user to change his/her posture. For

example while he/she moves up on a sloping ground,

the wheelchair user leans forwards to balance the

wheelchair and avoid falling backwards. This

constrained position is uncomfortable and more

tiring. Standards specify wheelchair access must

have a slope less than 5% for a 10 m long ramp or

less than 12% for a 50 cm long ramp. We know the

exit walkways from some car parks have slopes

much steeper than specified in the buildings

standards, and that is why it is interesting to enable

architects to try this kind of emergency access !

Shocks should be simulated and felt by user, so

we decided to represent a virtual shock by a real

shock provided by the quick change of pitch with a

significant magnitude. Vibrators fixed on each side

of the wheelchair give information about the side

against which the shock happened. This feature

could be interesting to simulate a scenario where a

user encounters an obstacle in a narrow corridor for

example,. These five actuated subsystems comprise

the touch element of our wheelchair type interface.

Of course a 3D visual and a stereo aural

rendering enhances the realism of spatial location

and events like shocks or other events that are a part

of the considered scene independently of the user’s

motion.

3 MECHATRONICAL DESIGN

We present in this part the design of the mechanical

wheelchair interface which will be upgraded later

with a joystick to simulate an electric wheelchair

with its specific behaviour. The sensorial feedback is

the same for both modes of the interface.

3.1 Framework

In order to keep close to reality we decided to base

our interface on a real wheelchair (Figure 1a), which

has been instrumented and in part redesigned (Figure

1b). The original wheelchair has been fixed on a

tubular framework, which supports it and assures a

good stability. This framework can be easily taken

apart and is then easily transportable. A rotating

joint placed a few centimetres in front of the wheel’s

axle (just below the estimated static centre of

gravity) supports the wheelchair and allows it to lean

forwards and backwards to simulate shocks and

displacements on a sloping ground.

Figure 1a: Original

wheelchair used as base.

Figure 1b: CAD model of

mechanical framework.

An electrical jack fixed between the framework

and a horizontal tube in front of the wheelchair

controls this degree of freedom. It is driven by a DC

motor supplied by a PWM signal to be controlled in

position. An external angular sensor is placed by the

side of this joint to measure the pitch orientation of

the wheelchair.

The actuator has been sized according to the

angular acceleration and velocity experienced by the

wheelchair when it moves on a slope (10% max)

with maximal speed of 10 km/h (Equation

1)(Figure2).

Figure 2: Slope.

With V

moy

, the initial horizontal velocity, α the

angle of the slope, l

the wheelbase, h the length to

control.

.4]

tan

1.[

sin.tan.

)(

−

••

=+−= smm

moy

αα

Equation 1

In addition to slopes, shocks to simulate can be

provided by lateral contact against walls. However

to feel realistic lateral contact should represented by

ICINCO 2006 - ROBOTICS AND AUTOMATION

428

a free motion of the user’s legs. Indeed the user’s

legs behave like seismic mass that undergoes a

lateral acceleration or deceleration. To carry out this

controlled mobility, it would be necessary to add a

yaw degree of freedom, which makes the interface

much more complex. In our application, the fine

perception of lateral shocks close to reality is not

necessary, which is why we decided to use a

metaphor to represent them. Vibrators fixed on tubes

each side of the wheelchair work and complete the

electrical jack to make user feel the side on which

the shock happened. Vibrators are based on small

DC motors that turn an eccentric for 50 ms. During

the implementation phase, vibrators were not judged

as necessary because the visual rendering always

gave enough information about the contact location.

Therefore vibrators were not always used

afterwards.

3.2 Wheels

The wheels are certainly the most delicate part to

design according to our final goal because they are

the main parts for user to interact with and move in

the VE as he/she wants. Indeed these parts work in

driving mode to control the motion in VE and work

also in sensorial to give feedback to user.

As mentioned above, we wanted to make the

user feel a force feedback close to the real one to

understand difficulty and strenuousness at a real

level. The tangential user force applied on the

circular handle of the wheel was estimated

experimentally at about 150 N. In order to supply a

high force (50 N.m torque) during shocks or starts,

we could use either a high power motor or an

irreversible system. Mechanical implementation

(bulk) and control of a high power motor would be

delicate at low speed for example, which is why we

decided to choose a transmission power chain with a

reducing ratio close to the threshold of

irreversibility. A 100 Watts DC Motor with a 1:30

reducer ratio was chosen to drive each wheel

independently. Each of them is supplied by a PWM

signal and drives the gears of the wheel and another

gear to drive an incremental encoder. These sensors

provide the feedback signals in order to control the

direction and velocity of the wheels. Motors and

sensors are placed under the seat to maintain the

compactness of the wheelchair and to make it easy

to link the equipment to the power supply or the

main PC.

To get data about the user interaction with the

wheels, the joins between handrails and wheels have

been instrumented. The force or torque measuring

device is very important to provide information

about the interaction between the user and the

interface. As it is not necessary to measure

accurately the actual user’s force applied to the

handrail, we decided to employ a S-shape force

sensor to obtain an image of the tangential force

vector. Like that, handrails now equipped with

spokes can rotate around the main axle and are fixed

on the wheels thanks to the S-shape force sensor

(figure 3).

1 : Tire 2 : Force sensor support

3 : Handrail 4 : Force sensor

5 : Motor support 6 : Driving gear

7 : Slipring brush 8 : Circular handle spoke

9 : Sensor electronics

Figure 3: Wheel subsystem.

The wires of the force sensor and its conditioner

are placed on the wheel and cross the main axle

which is now drilled from tip to tip. The slip-ring

brush on the tip axle assures a good connection

between the rotating parts and fixed part of wires.

The wheel subsystem provides force data to the

Central Unit which computes according to the actual

mechanical parameters of virtual environment the

theoretical value of the velocity applied on each

wheel. This procedure describes the control method

used to drive our wheelchair interface as a driving

and sensorial interface. With its irreversibility

characteristic, this wheel functions as a force

feedback joystick. This mechanical particularity can

supply a high force feedback when a shock happens.

Presently the velocity is controlled by a PID

controller, which dynamic enough to simulate

shocks (Katsura, 2004). The control law is quite

simple because it satisfies well our immersion

criteria (difficulties, tiredness). Moreover shocks

against walls are not very strong and the virtual

wheelchair would not bounce a lot against this kind

of obstacle. Finally during a shock, the electrical

A SPECIFIC LOCOMOTION INTERFACE FOR VIRTUAL REALITY - Design of a Wheelchair Type Haptic

429

jack which control the pitch of the interface will be

preponderant in comparison with other actuators.

3.3 Data Acquisition

Data from all sensors are collected by a NI Data

Acquisition Board PCI 6024E plugged on a

dedicated PC. The latter has to control all input /

output signals related to the sensors and actuators on

the wheelchair.

Figure 4: Global architecture.

Input signals are filtered and shaped by an

electronic rack before supplying the DAQ board.

Figure 4 illustrates the global chosen architecture

composed with 2 PCs. The main PC computes the

virtual world, receives data from the wheelchair

sensors and transmits orders to actuators according

to different control laws. This PC has a wider rule to

treat the virtual environment which will be detailed

below.

Two main power supplies are used to supply

firstly the control part (sensors) and secondly the

powerful devices such as actuators (wheel motors

and electrical jack). For security, an emergency

button is placed near the user’s left hand to power

off all actuators. Those are irreversible and so stop

very quickly when power is shut down.

4 MOTION GENERATION

Motion generation is computed by the Virtual

Environment Simulator (Flatland) which is

presented in the next section.

4.1 Mechanical Behavior

The kinematics and dynamics of our interface are

modelled to provide the interface motion by

including user’s forces, and the reaction of the VE.

Indeed the contact between the wheel and ground

must be well defined to allow a complete immersion

with a good level of realism and so avoid primarily

any disturbances or delays between the visual

rendering and the touch. Note we want to reproduce

the difficulties of motion and this is not limited to

experiencing a scene with obstacles and potential

shocks. Indeed we wanted to highlight that keeping a

desired direction while moving on a banked ground

is particularly difficult and need skill and force from

the wheelchair user.

That is why the user’s forces and effect of the

ground on the wheels have been reduced to a

dynamic torsor applied on the median point of

contact on the ground. Thus by taking into account

the mass distribution, the nature of contact between

wheel and ground, and external actions from user

and ground, we modeled a banking torque which

disturbs the direction of motion. Also the difficulties

involved in a simulating banked ground are rendered

not by roll change of wheelchair interface but by

disturbing the motion direction and via a disturbing

torque applied differently to each wheel. In

freewheel on banked ground, the wheelchair, under

the banking torque action, tends to be oriented in the

direction of the bank and so it can swing around this

direction if it started from an unspecified orientation.

4.2 Joystick Behavior

A joystick driving mode is necessary to simulate the

behavior of an electric wheelchair in a VE. As for

the mechanical model, a dedicated model has been

proposed to simulate the joystick functioning and the

whole electric wheelchair control unit.

Figure 5: Architecture of electric wheelchair.

With φ and θ angles of both joystick’s dof, R and

V respectively the curve radius and the velocity of

the desired trajectory, Crw and Clw respectively

torques on right and left wheels, Frw and Flw

respectively forces on right and left wheels.

The mechanical behavior of the wheelchair is

supplied either by joystick data or user’s force data

depending upon whether the electrical mode or

mechanical mode was chosen (figure 5).

Videoprojecto

Screen

Electronic

rack

board

Main

ICINCO 2006 - ROBOTICS AND AUTOMATION

430

Both modes of control simulate the actual

behaviors well and are implemented in the VE as

described just below.

5 IMPLEMENTATION

The VE runs in Flatland which is a simulator of

virtual world created and developed by LISMMA

laboratory (Yushchenko, 2003). This simulator is

designed to be connected to all kinds of behavioral

interface like haptics or locomotion interfaces for

VR (Dupuis, 2005). Flatland uses Parallel Virtual

Machine libraries to distribute computations on the

PC cluster. The main PC presented above runs

Flatland and controls another PC that tracks eye

movement. This architecture is interesting to

compute heavy tasks such as that which controls the

video rendering.

Figure 6a: A virtual room.

Figure 6b: Electric

wheelchair mode.

Figure 6c: Mechanic

wheelchair mode.

Different types of environment can be

implemented for example like a classroom (figure

6a) or a hall equipped with wheelchair access (figure

6c). All VEs can be explored virtually with any

modes of functioning (figure 6b).

Many experiments have been carried out to

evaluate the degree of realism. Globally, it appears

that the wheelchair interface is very intuitive and

natural. Indeed the user’s action are similar than

those he/she would do in actual reality. Sensorial

feedbacks are immediately understood by the user

and so he/she can focus on the main task, which is to

move as he/she wants.

Furthermore, the difficulty and energy required

to move is simulated well because the motion within

a virtual room is felt as a real effort when the

interface is used in mechanic wheelchair mode.

A virtual corridor is useful to evaluate the user’s

skill and difficulties in building design while the

interface was controlled as an electric wheelchair.

6 CONCLUSION

The work presented here, widely supported by

SMI/MIT students from Supmeca and ISEN

institutes, achieves its original goals in order to help

building designers in their task.

The locomotion interface wheelchair-type we

have designed is effective in highlighting the

difficulties which can be experienced by disabled

wheelchair users. Mechanical and electric

wheelchairs were implemented and both correctly

simulate the actual behaviour. This study illustrates

the great interest of the generic interface feature in

order to connect any kind of locomotion interface to

Flatland.

The next step of this research is to implement a

spatial sound feedback to upgrade our present stereo

rendering. Indeed within a noisy hall or close to a

train in a station for example, noise can easily be

considered as a lack of comfort or more a hindrance

to catch information.

REFERENCES

Harrison C.S, Grant M., Conway B. 2004. A Haptic

Interfaces for Wheelchair Navigation in the Built

Environment. Presence: Teleoperators & Virtual

Environments, Volume 13, Number 5, October 2004,

pp. 520-534(15)

Fuchs, P, Moreau, G 2003. Le traité de la Réalité

Virtuelle. Presses de l'école des mines - 10/2003. 540

p. ISBN: 2-911762-47-9

Dupuis, Y., Anthierens, C., Impagliazzo, J.L.,

Yushchenko, L. 2005. Design of a sensorial and

driving locomotion interface. 16th IFAC world

congress, Pragues, July, 4-8th, 2005.

Yushchenko, L, Impagliazzo, J-L, Anthierens, C. 2003

Design of an advanced scientific simulator equipped

with immersive interfaces. Application to a

micromanipulation task. Mecatronics'2003, Tokyo

Denki university, Hatoyama, Japan, September 9-11,

2003, pp 338-43.

Katsura, S., Ohnishi, K., 2004. Human Cooperative

Wheelchair for Haptic Interaction Based on Dual

Compliance Control. IEEE Transactionc on industrial

electronics, vol. 51, N°.1, February 2004, pp221-228.

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