ExoInterfaces: Novel Exosceleton Haptic Interfaces for
Virtual Reality, Augmented Sport and Rehabilitation
Dzmitry Tsetserukou

Katsunari Sato

Susumu Tachi

Toyohashi University of Technology
1-1 Hibarigaoka, Tempaku-cho,
Toyohashi, Aichi,
441-8580 Japan

University of Tokyo
7-3-1 Hongo, Bunkyo-ku, Tokyo,113-8656 Japan

Keio University
4-1-1 Hiyoshi, Kohoku-ku,
Yokohama, 223-8526 Japandzmitry.tsetserukou@erc.
tut.ac.jp

Katsunari_Sato@ ipc.i.
u-tokyo.ac.jp

tachi@tachilab.org

ABSTRACTWe developed novel haptic interfaces, FlexTorque and
FlexTensor that enable realistic physical interaction with real and
Virtual Environments. The idea behind FlexTorque is toreproduce human muscle structure, which allows us to performdexterous manipulation and safe interaction with environment indaily life. FlexTorque suggests new possibilities for highly
realistic, very natural physical interaction in virtual environments.
There are no restrictions on the arm movement, and it is notnecessary to hold a physical object during interaction with objects
in virtual reality. Because the system can generate strong forces,even though it is light-weight, easily wearable, and intuitive, users
experience a new level of realism as they interact with virtualenvironments.

ACM Classification Keywords
H5.2. Information interfaces and presentation: User Interfaces –haptic I/O, interaction styles, prototyping.

General Terms
Design, Experimentation, Performance.

Keywords
Exoskeleton, haptic display, haptic interface, force feedback,
Virtual Reality, augmented sport, augmented games,
rehabilitation, game controller.

1. INTRODUCTION
In order to realize haptic interaction (e.g., holding, pushing, andcontacting the object) in virtual environment and mediated haptic
communication with human beings (e.g., handshaking), the force
feedback is required. Recently there has been a substantial needand interest in haptic displays, which can provide realistic andhigh fidelity physical interaction in virtual environment. The aim
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of our research is to implement a wearable haptic display forpresentation of realistic feedback (kinesthetic stimulus) to thehuman arm. We developed a wearable device FlexTorque thatinduces forces to the human arm and does not require holding any
additional haptic interfaces in the human hand. It is completely
new technology for Virtual and Augmented Environments thatallows user to explore surroundings freely. The concept of Karate(empty hand) Haptics proposed by us is opposite to theconventional interfaces (e.g., Wii Remote [11], SensAble’s
PHANTOM [7]) that require holding haptic interface in the hand,
restricting thus the motion of the fingers in midair.
The powered exoskeleton robots, such as HAL [3] (weight of 23kg) and Raytheon Sarcos [8] (weight of about 60 kg) intended forthe power amplification of the wearer can be used for the force
presentation as well. However, they are heavy, require high power
consumption, and pose danger for user due to the powerfulactuators.
Another class of exoskeletons is aimed at teleoperator systems.
Most of the force feedback master devices are similar in sizes toslave robot and are equipped with powerful actuators. Suchsystems pose dangerousness for human operator and in case of
failure during bilateral control can harm human. In the last years
there have been several attempts to make the force feedback
devices more compact, safe, and wearable.
In [5], an exoskeleton-type master device was designed based on
the kinematic analysis of human arm. Pneumatic actuators
generate torque feedback. The authors succeeded in making thelightweight and compact force reflecting master arm. However,the force-reflection capability of this device is not enough topresent contact forces effectively. An artificial pneumatic muscletype actuator was proposed [4]. Wearable robotic arm with 7 DOF
and high joint torques was developed. Robotic arm uses parallelmechanisms at the shoulder part and at wrist part similarly to themuscular structure of human upper limb. It should be noted,
however, that dynamic characteristics of such pneumatic actuator
possess strong nonlinearity and load-dependency, and, thus, anumber of problems need to be resolved for its successful
application.
The compact string-based haptic device for bimanual interaction
in virtual environment was described in [6]. The users of SPIDAR
can intuitively manipulate the object and experience 6-DOF force
feedback. The human-scale SPIDAR allowing enlargement ofworking space was designed [9]. However, the wires moving infront of the user present the obstacle for the human vision. They
also restrict the human arm motion in several directions and user
has to pay attention to not injure himself. Moreover, user grasps
the ball-shaped grip in such a way that fingers cannot move.
In order to achieve human-friendly and wearable design of haptic
display, we analyzed the amount of torque to be presented to theoperator arm. Generally, there are three cases when torquefeedback is needed. The first case takes place when haptic
communication with remote human needs to be realized. For
example, the person handshakes the slave robot and joint torques
are presented to the operator. Such interaction results in verysmall torque magnitude (in the range of 0-1.5 Nm). The second
situation takes place when a slave robot transports heavy object.
Here, the torque values are much higher than in previous case andtorque magnitude depends on the load weight. However,continuous presentation of high torques to the operator will resultin human muscle fatigue. We argue that downscaled torqueindicating direction of the force would be informative enough.
The third and the worst case of contact state in term of interactive
force magnitude is collision. The result of collision with fixed
object (as it is often the case) is immediate discontinuation of theoperator’s arm motion. Therefore, the power of torque display
must be enough to only fixate the operator arm. For the case ofcollision with movable obstacle, the haptic display should induce
human’s arm motion in the direction of the impact force,
decreasing thus the possible damages.

like a rope pulling on a lever when pulling tendons to move theskeleton (Figure 1).
When we hold a heavy object in a palm, its weight produces
torques in the wrist, elbow, and shoulder joint. Each muscle
generates a torque at a joint that is the product of its contractile
force and its moment arm at that joint to balance gravity force, as
well as inertial forces, and contact forces. Thus, we can feel
object weight.
Because muscles pull but cannot push, hinge joints (e.g., elbow)
require at least two muscles pulling in opposite direction(antagonistic muscles). The torque produced by each muscle at a
joint is the product of contractile force (F) and moment arm atthat joint (d). The net torque Tnet is the sum of the torques
produces by each antagonistic muscle. Movement of human limbs
is produced by coordinated work of muscles acting on skeletal
joints. The structure of the developed torque display FlexTorqueis presented in Figure 2.

2. DEVELOPMENT OF THE HAPTICDISPLAY FlexTorque
The idea behind the novel torque display FlexTorque (haptic
display that generates Flexor and extensor Torque) is toreproduce human muscle structure, that allows us to performdexterous manipulation and safe interaction with environment indaily life.
Text=Fext × dext
OriginTflex=Fflex × dflex

Figure 2. FlexTorque on the human’s arm surface.

MuscleFext

Fflex
Tload=Fload × dload
dext

dflex

TendonInsertion

Fload

dload
Tnet= Tflex - Text
Figure 1. Structure and action of a skeletal muscle.
Main functions of the muscles are contraction for locomotion andskeletal movement. A muscle generally attaches to the skeleton at
both ends. Origin is the muscle attachment point to the morestationary bone. The other muscle attachment point to the bonethat moves as the muscle contracts is Insertion. Muscle isconnected to the periosteum through tendon (connective tissue inthe shape of strap or band). The muscle with tendon in series acts

FlexTorque is made up of two DC motors (muscles) fixedly
mounted into plastic Motor holder unit, Belts (tendons), and two
Belt fixators (Insertions). The operation principle of the haptic
display is as follows. When DC motor is activated, it pulls the belt
and produces force Fflex generating the flexor torque Tflex. The
oppositely placed DC motor generates the extensor torque Text.
Therefore, the couple of antagonistic actuators produce a net
torque at operator elbow joint Tnet. We defined the position of theInsertion point to be near to the wrist joint in order to develop
large torque at the elbow joint.
The position of the operator’s arm, when flexor torque isgenerated, is shown in Figure 3 (where θ stands for angle offorearm rotation in relation to upper arm).

The angle α varies according to the relative position of theforearm and upper arm. It can be found using the following
equation:

Direction of
motor shaft
rotation

Directionof belt
tension⎛ l 2 + d 2f − d t2 ⎞ ,
⎟⎟
⎜
2ld f
⎝
⎠

(3)

α = cos−1 ⎜where dt is the distance from the pivot to the Origin; l is the lengthof belt, it can be calculated from the rotation angle of the motor
shaft.
The detailed view of the FlexTorque is presented in Figure 5.
θ = 55°

θ = 10°θ = 110°

DC Motor

Figure 3. Positions of the human’s arm under flexor torque.
Let us consider the calculation procedure of the net torque value.
The layout of the forces and torques applied to the forearm duringflexion is given in Figure 4.

Motor holder
Stopper

Pulleys
Timing belt

Tm

Supporter
Shaft

Ft
lα

dt

Ftx
df

Stopper

Figure 5. 3D exploded view of the driving unit of FlexTorque.

Fty

Each unit is compact and light in weight (60 grams). This wasachieved due to the use of plastic and duralumin materials in
manufacturing the main components. The Supporter surface has
concave profile to match the curvature of human arm surface(Figure 6).

Tn
Figure 4. Diagram of applied forces and torques.
The tension force Ft of the belt can be derived from:

Ft =

Tmi ,
r

(1)
Figure 6. Driving unit of FlexTorque.

where Tm is the motor torque, i is the gear ratio, and r is the shaft
radius.
The net torque Tn acting at the elbow joint is:Tn = Fty d f = Ft d f cos (α ) ,where df is the moment arm.

(2)

The essential advantage of the structure of FlexTorque device isthat heaviest elements (DC motors, shafts, and pulleys) arelocated on the part of upper arm, which is nearest to the shoulder.
Therefore, operator’s arm undergoes very small additionalloading. The rest of components (belts, belt fixators) are light inweight and do not load the operator’s muscles considerably. We
propose to use term “Karate (empty hand) Haptics” to such kindof novel devices because they allow presenting the forces to thehuman arm without using additional interfaces in the humanhands. The developed apparatus features extremely safe force
presentation to the human’s arm. While overloading, the belt isphysically disconnected from the motor and the safety of thehuman is guaranteed.
The vibration of the human arm (e.g., simulation of driving theheavy truck) can be realized through alternate repeatable jerks oftorque of antagonistic motors. Thus, the operator can perceive theroughness of road surface.
The FlexTorque enables the creation of muscle stiffness. By
contracting belts before the perturbation occur we can increase thejoint stiffness. For example, during collision of human hand withthe moving object in Virtual Environment the tension of the beltof one driving units drops abruptly and the tension of the belt
pulling the forearm in the direction of the impact force increases
quickly.

Figure 7. Augmented Arm Wrestling and Augmented
Collision.

The contact and collision with virtual object can be presented
through FlexTorque as well. In the case of collision, the limb
must be at rest. In such a case, the net torque produced by themuscles is opposed by another equal but opposite torque Tload.
Similarly to the human muscles, the net torque produced by thehaptic display restrains the further movement of the user’s arm.

4. USER STUDY AND FUTURE
RESEARCH

3. APPLICATIONS

We designed three games with haptic feedback. We developed theGun Simulator game with the recoil imitation (Figure 8). Quicksingle jerk of the forearm simulates the recoil force of a gun.
High-frequency series of impulsive forces exerted on the forearm
imitate the shooting by machine gun. In this case upper motor is
supplied with short ramp impulses of current.

The main features of FlexTorque are: (1) it presents high fidelitykinesthetic sensation to the user according to the interactive
forces; (2) it does not restrict the motion of the human arm; (3) it
has wearable design; (4) it is extremely safe in operation; (5) it
does not require a lot of storage space. These advantages allow awide range of applications in virtual and augmented reality
systems and introduce a new way of game playing.

FlexTorque haptic interface was demonstrated at SIGGRAPH
ASIA 2009 [1,2,10]. To maintain the alignment of the extensor
belt on the elbow avoiding thus slippage, user wears speciallydesigned pad equipped with guides.

Here we summarize the possible application of haptic display
FlexTorque:1)

Virtual and Augmented Environments (presentation ofphysical contact to human’s arm, muscle stiffness, object
weight, collision, physical contact, etc.).

2)

Augmented Sport and Games (enhancing the immersive
experience of the sport and games through the force
feedback).

3)

Rehabilitation (user with physical impairments can easily
control the applied torque to the arm/leg/palm duringperforming the therapeutic exercises).

4)

Haptic navigation for blind persons (the obstacle detectedby camera is transferred to force restricting the arm
motion in the direction of the object).

Figure 8. The Gun Simulator game.
In Teapot Fishing game player casts a line by quick flicking therod towards the water (Figure 9).

A number of games for augmented sport experiences, which
provide a natural, realistic, and intuitive feeling of immersion into
virtual environment, can be implemented. The Arm Wrestlinggame that mimics the real physical experience is currently underdevelopment (Figure 7). The user wearing FlexTorque and Headmounted display (HMD) can play either with a virtual characteror a remote friend for more personal experience. The virtual
representation of players’ arms are shown on the HMD. While
playing against a friend, user sees the motion of arms andexperiences the reaction force from rival.
Figure 9. The Teapot Fishing game.

Once the user feels the tug at the forearm (and see float goingdown), he gives fishing rod a quick jerk backward and up. Whenjerk is late, the fish (teapot) gets off the hook. The ramp impulseof the motor torque generates the jerk of the forearm downward
indicating that fish picks up the hook. Such practice can help theuser to get a feel of the real fishing.
With Virtual Gym game we can do the strength training exercise
at home in a playful manner (Figure 10). The virtual biceps curl
exercise machine was designed. The belt tension creates theresistance force in the direction of the forearm motion. The user
can adjust the weight easily.

Tm

OriginFt
Insertion
Upper arm

Forearm

Figure 11. Kinematic diagram of FlexTorque and human arm.
Hands

Origin/Insertion

Ft1

Origin/Insertion

Ft2

T m1

T m2

Left Arm

5. DESIGN OF MULTIPURPOSE HAPTICDISPLAY FlexTensor
The motivation behind the development of the FlexTensor (haptic
display that uses Flexible belt to produce Tension force) was toachieve realistic feedback by using simple and easy to wear haptic
display.
The multipurpose application is realized by means of fixation ofdifferent elements of FlexTensor (i.e. middle of the belt,
Origin/Insertion points) in the particular application. The structureof the FlexTensor is similar to the flexor part of FlexTorque haptic
display. The main differences are: (1) belt connects the movable
points on the human arm; (2) both attachment points of the belt
have embedded DC motors.

Shoulderjoint

Shoulderjoint

Figure 10. The Virtual Gym game.
In total more than 100 persons had experienced novel haptic
interface FlexTorque. We have a got very positive feedback fromthe users and companies. While discussing the possible useful
applications with visitors, the games for physical sport exercises
and rehabilitation were frequently mentioned. The majority ofusers reported that this device presented force feedback in a very
realistic manner.

Right Arm

Figure 12. Kinematic diagram of FlexTensor and human arm.
In the configuration when the middle of the belt is not fixed,
FlexTensor presents the external force resisting the expanding of
the human arms (basic configuration). This action can be used for
simulation of the breaststroke swimming technique, when humansweeps the hands out in water to their widest point (Figure 13).
The configuration, in which the middle of the belt is fixed by user
standing on the band with both (or one) feet, enables presentationof the object weight (Figure 14). The tension of the belt represents
the magnitude of gravity force acting on the human arms. The
fixation of the middle of the belt can be positioned on the humanneck (for simulation of human arm lifting) and on the waist (forsimulation of resistance of environment in the direction of arm
stretching, e.g., in the case of contact with the virtual wall).
FEXT
Breaststroke technique

FEXT
Belt

In the haptic display FlexTorque the function of each attachment
point is predetermined (Figure 11). The configuration of
FlexTensor allows each point to perform the function of Insertionand Origin depending on the purpose of application (Figure 12).
This fact enables to enlarge the area of FlexTensor applications in
Virtual Reality extraordinary.
Figure 13. Application of FlexTensor for swimming training.

7. ACKNOWLEDGMENTS
The research is supported in part by the Japan Science andTechnology Agency (JST) and Japan Society for the Promotion of
Science (JSPS). We would like also to acknowledge and thank
Alena Neviarouskaya for valuable contributions and advices.

Object8. REFERENCES

mg

Motor

[1] FlexTorque. Games presented at SIGGRAPH Asia 2009.
http://www.youtube.com/watch?v=E6a5eCKqQzc

Belt[2] FlexTorque. Innovative Haptic Interface. 2009.
http://www.youtube.com/watch?v=wTZs_iuKG1A&feature=
related
Biceps curl exercise
Middle of a belt is fixed

Figure 14. Application of FlexTensor for the weight
presentation and strength training exercise.
In the case when the palm of one arm is placed on some part ofthe body (e.g., waist, neck), this attachment point becomes Origin.
Such action as unsheathing the sword can be simulated bystretching out the unfixed arm. FlexTensor can interestingly
augment the 3D archery game presenting the tension force
between arms.
The illusion of simultaneous pulling of both hands can be
implemented by exertion of different values of forces Ft1 and Ft2
in the basic configuration (see Figure 12). The illusion of being
pulled to the left side and to the right side can be achieved when
Ft1>Ft2 and Ft1<Ft2, respectively.
The developed apparatus features extremely safe force
presentation to the human’s arm. While overloading, physicaldisconnection of the belt from the motor protects the user from theinjury.

6. CONCLUSIONS
Novel haptic interfaces FlexTorque and FlexTensor suggest new
possibilities for highly realistic, very natural physical interaction
in virtual environments, augmented sport, and augmented gameapplications.
A number of new games for sport experiences, which provide anatural, realistic, and intuitive feeling of physical immersion into
virtual environment, can be implemented (such as skiing, biathlon(skiing with rifle shooting), archery, tennis, sword dueling,
driving simulator, etc.).
The future goal is the integration of the accelerometer and MEMS
gyroscopes into the holder and fixator of the FlexTorque and into
FlexTensor for capturing the complex movement and recognizing
the gesture of the user. The new version of the FlexTorque and
FlexTensor (ExoInterface) will take advantages of the
Exoskeletons (strong force feedback) and Wii Remote Interface(motion-sensing capabilities).
We expect that FlexTorque and FlexTensor will support futureinteractive techniques in the field of robotics, virtual reality, sport
simulators, and rehabilitation.

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