FlexTorque: Exoskeleton Interface for Haptic Interaction
with the Digital World
Dzmitry Tsetserukou1, Katsunari Sato2, and Susumu Tachi3
1Toyohashi University of Technology, 1-1 Hibarigaoka, Tempaku-cho, Toyohashi, Aichi,441-8580 Japantsetserukou@erc.tut.ac.jp
2
University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo, 113-8656 Japan
Katsunari_Sato@ipc.i.u-tokyo.ac.jp
3
Keio University, 4-1-1 Hiyoshi, Kohoku-ku, Yokohama, 223-8526 Japantachi@tachilab.org

Abstract. We developed a novel haptic interface FlexTorque that enables realistic physical interaction with real (through teleoperation system) and VirtualEnvironments. The idea behind FlexTorque is to reproduce human muscle
structure, which allows us to perform dexterous manipulation and safe interaction with environment in daily life. FlexTorque suggests new possibilities forhighly realistic, very natural physical interaction in virtual environments. There
are no restrictions on the arm movement, and it is not necessary 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, andintuitive, users experience a new level of realism as they interact with virtualenvironments.
Keywords: Exoskeleton, haptic display, haptic interface, force feedback,Virtual Reality, game controller.

1 Introduction
In order to realize haptic interaction (e.g., holding, pushing, and contacting 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
need and interest in haptic displays, which can provide realistic and high fidelity
physical interaction in virtual environment. The aim of our research is to implement awearable haptic display for presentation of realistic feedback (kinesthetic stimulus) tothe human arm. We developed a wearable device FlexTorque that induces forces tothe human arm and does not require holding any additional haptic interfaces in thehuman hand. It is completely new technology for virtual and augmented environmentsthat allows user to explore surroundings freely. The concept of Karate (empty hand)
Haptics proposed by us is opposite to conventional interfaces (e.g., Wii Remote [1],SensAble’s PHANTOM [2]) that require holding haptic interface in the hand, restricting thus the motion of the fingers in midair.
A.M.L. Kappers et al. (Eds.): EuroHaptics 2010, Part II, LNCS 6192, pp. 166–171, 2010.
© Springer-Verlag Berlin Heidelberg 2010

FlexTorque: Exoskeleton Interface for Haptic Interaction with the Digital World167

The powered exoskeleton robots, such as HAL [3] (weight of 23 kg) and Raytheon
Sarcos [4] (weight of about 60 kg) intended for the power amplification of the wearer
can be used for the force presentation as well. However, they are heavy, require highpower consumption, and pose danger for user due to the powerful actuators.
The compact string-based haptic device for bimanual interaction in virtual environment was described in [5]. The users of SPIDAR can intuitively manipulate theobject and experience 6-DOF force feedback. The human-scale SPIDAR allowing
enlargement of working space was designed [6]. However, the wires moving in frontof the user present the obstacle for the human vision. They also restrict the humanarm 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.

2 Development of Haptic Interface FlexTorque
In order to achieve human-friendly and wearable design of haptic display, we analyzed the amount of torque to be presented to the operator arm. Generally, there arethree cases when torque feedback 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 very small torque magnitude (in the range of 0-1.5 Nm). The second
situation takes place when slave robot transports heavy object. Here, the torque values
are much higher than in previous case and torque magnitude depends on the loadweight. However, continuous presentation of high torques to the operator will resultin human muscle fatigue. We argue that downscaled torque indicating direction of theforce 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 fixedobject (as it is often the case) is immediate discontinuation of the operator’s arm
motion. Therefore, the power of torque display must be enough to only fixate theoperator arm.

Text=Fext × dext
OriginTflex=Fflex × dflex

MuscleFext

Fflex
Tload=Fload × dload
dext

dflex

TendonInsertion

Fload

dload
Tnet= Tflex - Text
Fig. 1. Structure and action of a skeletal muscle

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D. Tsetserukou, K. Sato, and S. Tachi

The idea behind novel torque display FlexTorque (haptic display that generates
Flexor and extensor Torque) is to reproduce human muscle structure, that allows us
to perform dexterous manipulation and safe interaction with environment in daily life.
Main functions of the muscles are contraction for locomotion and skeletal movement[7]. Muscle is connected to the periosteum through tendon (connective tissue in the
shape of strap or band). The muscle with tendon in series acts like a rope pulling on alever when pulling tendons to move the skeleton (Fig. 1).
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 byeach muscle at a joint is the product of contractile force (F) and moment arm at thatjoint (d). The net torque Tnet is the sum of the torques produces by each antagonisticmuscle. Movement of human limbs is produced by coordinated work of muscles acting on skeletal joints. The structure of the developed torque display FlexTorque andthe detailed view of the driving unit of haptic display are presented in Fig. 2.

DC Motor

Motor holder
Stopper

Pulleys
Timingbelt
Supporter
Shaft
StopperFig. 2. FlexTorque on the human’s arm surface and 3D exposed view of the driving unit

FlexTorque is made up of two DC motors (muscles) fixedly mounted into plastic
Motor holder unit, Belts (tendons), and two Belt fixators. The operation principle ofthe 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 the Insertion
point to be near to the wrist joint in order to develop large torque at the elbow joint.
Let us consider the calculation procedure of the net torque value. The layout of theforces and torques applied to the forearm during flexion is given in Fig. 3.
The tension force Ft of the belt can be derived from:Ft =Tm i,rwhere Tm is the motor torque, i is the gear ratio, and r is the shaft radius.

(1)

FlexTorque: Exoskeleton Interface for Haptic Interaction with the Digital World169

Tm
Ft
dtlαFtxdfFty

TnFig. 3. Diagram of applied forces and torques

The net torque Tn acting at the elbow joint is:Tn = Fty d f = Ft d f cos (α ) ,(2)where df is the moment arm.
The angle α varies according to the relative position of the forearm and upper arm.
It can be found using the following equation:⎛ l 2 + d t2 − d 2f ⎞⎟,2ld t⎝⎠

α = cos−1 ⎜(3)

where dt is the distance from the pivot to the Origin; l is the length of belt, it can becalculated from the rotation angle of the motor shaft.
The biceps actuator of FlexTorque is capable of producing the net torque at the elbow joint as high as 8.0 Nm (Maxon motor RE 13, Stall torque of 8.52 mNm, Gearhead reduction ratio of 17:1). Each unit is compact and extremely light weight (61 g).
This was achieved 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 (Fig. 4).

Fig. 4. Manufactured driving unit of FlexTorque170

D. Tsetserukou, K. Sato, and S. Tachi

The essential advantage of the structure of FlexTorque device is that heaviest elements (DC motors, shafts, and pulleys) are located on the part of upper arm, which isnearest to the shoulder. Therefore, operator’s arm undergoes very small additionalloading. The rest of components (belts, belt fixators) are light in weight and do notload the operator’s muscles considerably. We propose to use term “Karate (emptyhand) Haptics” to such kind of novel devices because they allow presenting the forces
to the human arm without using additional interfaces in the human hands. 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.

3 Applications
The main features of FlexTorque are: (1) it presents high fidelity kinesthetic sensation
to the user according to the interactive forces; (2) it does not restrict the motion of thehuman 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 a wide range of applications in virtual and augmented reality systems and introduce a new way of gameplaying. A number of games for augmented sport experiences, which provide anatural, realistic, and intuitive feeling of immersion into virtual environment, can beimplemented.
Haptic interface FlexTorque was demonstrated at SIGGRAPH ASIA 2009 [8]. We
designed three games with haptic feedback. We developed the Gun Simulator gamewith the recoil imitation and Teapot Fishing game with haptic presentation of the rod
tug when fish bites. The virtual biceps curl exercise machine was designed. With
Virtual Gym game we can do the strength training exercise at home in a playful manner (Fig. 5). The belt tension creates the resistance force in the direction of the forearm motion. The user can adjust the weight easily.
Wii MotionPlus controller was used in order to capture the complex motion of theuser arm. To maintain the alignment of the extensor belt on the elbow avoiding thus
slippage, user wears specially designed pad equipped with guides. In total more than
100 persons had experienced novel haptic interface FlexTorque. We have a got very

Fig. 5. The Virtual Gym gameFlexTorque: Exoskeleton Interface for Haptic Interaction with the Digital World

171

positive feedback from the users and companies. While discussing the possible useful
applications with visitors, the games for physical sport exercises and rehabilitation
were frequently mentioned. The majority of users reported that this device presented
force feedback in a very realistic manner.

4 Conclusions and Future Research
Novel haptic interface FlexTorque suggests new possibilities for highly realistic, very
natural physical interaction in virtual environments, augmented sport, augmented
game applications, and teleoperation.
A number of new games for sport experiences, which provide a natural, 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, etc.). FlexTorque will also enable the presentation of strong vibrations indriving simulator, muscle stiffness, and collision (contact) with a virtual object.
The future goal is to capture the complex movement and recognize the gesture ofthe user through accelerometers and MEMS gyroscopes integrated into the holder andfixator of the FlexTorque and optical full-body motion capture system. The new version of the FlexTorque (ExoInterface) will take advantages of the Exoskeletons(strong force feedback) and Wii Remote Interface (motion-sensing capabilities andsimplicity of usage).
We expect that FlexTorque will support future interactive techniques in the field ofrobotics, virtual reality, sport simulators, and rehabilitation.

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