ICROS-SICE International Joint Conference 2009
August 18-21, 2009, Fukuoka International Congress Center, Japan

Perception of Finger Angle Position in
Grip-type Haptic Display based on Grasping Force
Katsunari Sato1, Naoki Kawakami1 and Susumu Tachi2
1

Graduate School of Information Science and Technology, The University of Tokyo, Tokyo, Japan(Tel : +81-3-5841-6917; E-mail: {Katsunari_Sato, Naoki_Kawakami }@ipc.i.u-tokyo.ac.jp)
2
Graduate School of Media Design, Keio University, Tokyo, Japan(E-mail: tachi@tachilab.org)

Abstract: In this study, we propose a novel grip-type haptic display for controlling a virtual (or robotic) hand (Fig. 1).
This grip-type haptic display enables us to control a virtual hand and perceive haptic information based on a reactive
force and a spatially distributed tactile feedback. Because this proposed display does not need any actuators to produce
the reactive force, a simple haptic display is developed. In a grip-type haptic display, it is difficult to perceive the finger
position because of the fixed position of the user’s fingers. However, we consider that it is possible to perceive thefinger position by means of the effort exerted. Therefore, we performed an experiment to discriminate the applied force
and confirm that the force discrimination threshold was sufficiently small. Then, we constructed a one-fingered
prototype using an electrotactile display. Using this prototype, we qualitatively confirmed that the reactive force withthe spatially distributed tactile feedback can produce dynamically changing haptic information.
Keywords: Haptics, RT System and Integration, Network and Virtual Reality Systems

1. INTRODUCTION

complicated mechanical parts needed to producekinesthetic sensation, the usability of the device is not
good. Not only does the size of the device limit the user
but also it is cumbersome to wear and difficult tocontrol. Meanwhile, CyberTouch [2] does not produce
force feedback and it comprises compact wearable
haptic displays. The usability of the haptic display isimproved; however, its size still limits the user.
Furthermore, gentle grasping is difficult because of thelack of kinesthetic sensation. Another type of haptic
display is a grip-type haptic display. We can easily use
the grip-type haptic display by simply grasping it. It canmeasure the finger movement by means of button orforce input. We have also developed a cylindrical force
sensor called “MeisterGRIP” as a robotic hand control
system [3]. This sensor can measure the grip force ofeach finger in terms of three-dimensional force vectors.
However, haptic feedback is minimally recognized as
only a reactive force. Therefore, it is difficult torecognize contact of an object and distinguish its
properties.

A haptic display for hand is sought for the gentle anddexterous manipulation of a grasping object intelepresence (or telexistence) and virtual reality
technologies. The haptic display presents the following
two types of information: kinesthetic information(through proprioception) and tactile information(through cutaneous sensation). In the kinesthesia,䎃 thesense of position, posture, and movement of a limb, areimportant for controlling the grasping force that applied
to an object in order to perceive its properties such assize, weight, and softness. In the case of tactile
information, the sense of the distribution of pressure andvibration on the skin surface, is mainly perceive us thecontact condition between the skin and the object.
Therefore, tactile information is important to perceive
texture and exact shape of the object.
There are three requirements for the haptic display forhand. First, the haptic display should measure thecomplex motions of the fingers and reflect them into avirtual (or robotic) hand. This enables us to control thevirtual hand as if it was the user’s hand. Second, thehaptic display should produce both kinesthetic andcutaneous sensations to multiple fingers. To achieve agentle and dexterous manipulation, both forms of
feedbacks are important. The third and final requirement
is that it should be highly usable. As a human-machineinterface, it should be easily used by everyone,
regardless of age and sex.
Thus far, a number of haptic displays have beendeveloped, however, they don’t satisfactorily fulfill theabovementioned requirements. For example, a wearable
haptic display that consists of an encounter-type master
hand and an electrotactile display has been developed[1]. This haptic display can measure the motion ofuser’s fingers and present both kinesthetic andelectrotactile sensation. However, because of thegrip-type haptic display

dynamically-changing
haptic information

measurement ofgrasping force

virtual (or robotic) handFig. 1 Conceptual diagram of grip-type haptic device.

- 1089 -PR0002/09/0000-1089 ¥400 © 2009 SICEIn this study, we propose a novel grip-type haptic
display with a tactile feedback for controlling a virtual
hand (Fig. 1). We consider integrating conventional
grip-type haptic display with a tactile display that canpresent spatially distributed pressure sensation. This
integration allows the haptic display to satisfy all theabovementioned requirements. However, it seems
difficult to control the position of a virtual finger basedon the grasping force. Furthermore, it also seems
difficult to produce dynamically changing haptic
feedback. In this study, we investigate the force
discrimination threshold to confirm the possibility ofcontrolling of a virtual finger. Then, using a
one-fingered prototype, we qualitatively examine thepossibility of producing dynamically changing haptic
sensation.

Therefore, the user can also control the grasping forceof the virtual finger.
In contrast, our grip-type haptic display measures theforce applied to the device by the user’s finger andtransmits it to the position and force of the virtual finger.
When a user tries to move the virtual finger, themovement of the user’s finger causes the muscle of theuser’s finger to contract. Then, the grip-type haptic
display measures the force applied by the fingertip. The
measured grasping force is then transmitted to thejoint-angle positions of the virtual finger when thevirtual finger does not touch an object.

θ v = f (fu )In Eq (3), f(·) represents the conversion function fromthe grasping force to the joint-angle positions of thevirtual hand. Each finger of human has three joints andfour DoF (Degree of Freedom). However, the DoF canbe approximated by three [4] [5]. Therefore, we cancontrol the position of virtual finger using
three-dimensional force vector. Meanwhile, when thevirtual finger touches an object, the measured grasping
force is transmitted to the force of virtual finger as in aconventional haptic display (Eq. 2).
In proposed control method, the position of the user’s
fingertip is not displaced. Therefore, it seems difficult tocontrol the joint-angle positions of the virtual finger.
However, we consider that the user can perceive thejoint-angle positions of the virtual finger through his/her
own sense of effort expended to move the finger. Whenthe user tries to move the virtual finger, he/she expends
the force into each joints of the finger. This force
sensation can help perceive the joint-angle position of
the virtual finger.
It should be noted that we use a low range of grasping
force to control the virtual finger. When the user
controls the virtual finger, he/she perceives somereactive force exerted by the device. This sense ofresistance increases when the user applies a large force.
Furthermore, when the virtual finger touches an object,
we have to control the grasping force with the offset
force that is applied to control the position of the virtual
finger. Therefore, a large grasping force is not suitablefor controlling the virtual finger. We must also consider
that the elastic body of the device and the visualfeedback can reduce this sense of resistance. If thedevice has an elastic body, the displacement of its
surface is expected to reduce the resistance.
Furthermore, visual feedback is also considered to play
an important role in haptic perception [6]. Therefore, theresistance will be further reduced when the user watches
the movement of the virtual hand.

2. GRIP-TYPE HAPTIC DISPLAY
In this chapter, we introduce the methods forcontrolling a virtual finger and the haptic feedback of
the grip-type haptic display. Fig. 2 shows the principleof the grip-type haptic display that we propose. This
display measures a user’s grasping force for controllinga virtual hand. Haptic feedback is provided in the formof a reactive force from the device with a spatiallydistributed tactile display.
2.1 Control of a virtual finger
A wearable haptic display measures the user’s finger
joint-angle positions șv to transmit it to virtual finger’s
positions șh when the virtual finger does not touch an
object (Eq. (1)).
(1)θv = θu

Therefore, the user can control the position of the virtual
finger as if it were his/her actual finger. When thevirtual finger touches an object, the user’s applied force
vector fu to the haptic display is reflected to the virtual
finger’s force vector fv (Eq. (2)).
(2)

fv = fuuser
hapticvisionperception
applying

distributed tactile
virtual hand

force
grip-

user’ s hand

type
hapticdisplay
position or force(3)

2.2 Tactile feedback with reactive force
An ideal haptic display presents three-dimensional
force and tactile information to the user when the virtual
finger touches an object. In this case, the user cancontrol the grasping force and easily perceive theproperties of the object.

grasping force(= reactive force)

Fig. 2 Proposed grip-type haptic display.

- 1090 -In contrast, when the user grasps the grip-type haptic
display, he/she sense only the reactive force exerted by
the device. Based on this reactive force, the user cancontrol the grasping force of the virtual hand. However,this reactive force is always transmitted in the same
direction and amplitude, regardless of the object grasped
by the virtual hand. Therefore, the user cannot perceive
the properties of grasped object. Furthermore, the user
cannot recognize contact with an object in the absenceof visual feedback.
To overcome this difficulty, we integrated thespatially distributed tactile feedback with a reactive
exerted by the device. The spatially distributed tactile
information is of fundamental importance to theperception of contact with an object and the sensing its
physical properties. For example, the distribution ofpressure on the skin surface represents the exact shape,
position, posture, and softness of an object [7]. We
consider tactile information compensates for the poor
haptic feedback of conventional grip-type haptic
displays.

comparison forces from each reference force.
Participants then compared these randomly selectedreference and comparison forces. All combinations ofreference and comparison forces were selected 20 times.
Therefore, through the experiment at a joint, theparticipants conducted the comparison 280 times in total.
This experiment was conducted at PIP and MCP joints.
The participants included three adults: two males andone female in their 20’s.

MCP joint

vibration
moter

fingercase

PIP joint

six-axis
force sensor

3. DISCRIMINATION OF FORCE
Fig. 3 Experimental setup.

In the grip-type haptic device, the user recognizes thejoint-angle position of the virtual finger through his/her
sense of effort expended to move the joints of the finger.
Therefore, the performance of force perception of eachjoint is quite important. Especially, to bend the virtual
finger like as human’s finger, we have to recognize theapplied force of PIP (Proximal Interphalangeal) andMCP (Metacarpophalangeal) joints independently.
Therefore, in this experiment, we examined thethreshold of the applied force of PIP and MCP joints.

3.2 Results
Fig. 4 and 5 show the results of the experiment. The
horizontal axes and vertical axes represent thecomparison force and the ratio of responses thatconsidered the comparison force greater than thereference force. The colored points and lines represent
the average ratio of the three participants set along afitting curve. Furthermore, we also summarized the PSE(point of subjective equality) and JND (just noticeable
difference) in Tab. 1 and Tab. 2. SD represents thestandard deviation. Participants could distinguishincrements in the applied force of the PIP and MCP
joint that were approximately 11 í 17 gf and 10 í 16 gf,
respectively. A two-way (participant, reference force)
ANOVA confirmed that neither participant (F(2, 14) =0.15 for PIP and F(2, 14) = 0.43 for MCP) nor reference
force (F(1, 14) = 0.30 for PIP and F(1, 14) = 0.10 for
MCP) was a significant factor.

3.1 Material and Method
Fig. 3 shows the experimental setup. We mount avibration motor (FM34F, Inc.) on a six-axis force sensor(BL. Autotech Inc.). To reduce the effect of pressure
sensation on force discrimination, participants wore aplastic finger case on their index finger when placing iton the vibration motor.
We investigated the force discrimination threshold byconstant method. First, participants straightened theirfinger on the vibrate motor. Then, they tried to bend
their MCP or PIP joint without moving their finger. The
six-axis force sensor measured the force applied by thefingertip. When the applied force exceeded the arbitrary
force, the vibration motor began to vibrate. Participants
repeated this procedure two times. In the first time, themotor vibrated when the applied force exceeded areference force. In the second time, the motor vibrated
when the force exceeded a comparison force. Afterthese two procedures, participants declared which force
felt grater, i.e. the comparison force or the reference one
or not.
For the grip-type haptic display, the virtual finger must
be controlled using a low force range. Therefore, we set
two reference forces at 80 and 160 gf, respectively. We
set í40.0, í20.0, í10.0, 0, 10.0, 20.0, and 40.0 gf as

3.2 Discussion
Our results confirm that the user can control the
joint-angle positions of a virtual finger through his/her
perceived force of each joint. For example, for the joint
angle of virtual finger bent 1Û by 5 gf, we can perceive a
difference in angle of the virtual finger by 2 í 3 Û. Thisvalue is similar to the angle discrimination threshold ofa human’s PIP and MCP, which is approximately 1.7 í
2.7 Û [8].Therefore, we confirmed that the user cancontrol the joint-angle position of the virtual finger bymeans of his/her sense of effort expended to move thefinger, without any change in the joint-angle of thefinger.
Furthermore, the force discrimination threshold was
not depended on the participant and reference force.
This result was convenient for us to construct the- 1091 -conversion function in Eq. (3). However, a large force
range can cause an undesirable sense of resistance.
Therefore, we have to carefully construct the identicalcontrol method of the joint position of the finger using a
law force range.

Table 1 PSE and JND of PIP joint.

KW
MT
KS

Ref. force
PSE
JND (S.D.)PSE
JND (S.D.)PSE
JND (S.D.)

80 [gf]93.7
13.3 (4.4)
85.5
10.8 (4.0)
91.7
13.8 (4.5)

160 [gf]
141.7
11.2 (4.1)
165.1
16.5 (4.9)
145.8
16.7 (5.0)

Table 2 PSE and JND of MCP joint.

KW
MT
KS

Ref. force
PSE
JND (S.D.)PSE
JND (S.D.)PSE
JND (S.D.)

80 [gf]84.5
9.8 (3.8)
95.6
16.0 (4.9)
87.1
10.8 (4.0)

160 [gf]
148.9
11.6 (4.2)
169.3
15.0 (4.7)
143.3
13.2 (4.4)

4. HAPTIC FEEDBACK
In the grip-type haptic device, we integrated thespatially distributed tactile feedback with the reactive
force to produce dynamically-changing haptic
information. We quantitatively evaluated theeffectiveness of this haptic feedback using a
one-fingered prototype of the grip-type haptic device.

Fig. 4 Results of PIP joint. Above: reference force is
80 gf. Below: reference force is 160 gf.

4.1 One-fingered prototype
To construct a one-fingered prototype (Fig. 6), we
used the distributed force vector sensor called“GelForce” [9] and an electrotactile display [10].
GelForce can measure the distribution of
three-dimensional force vector by an optical method. As
the summation of the all force vector, we can calculate aforce vector F = (fx, fy, fz) applied by the fingertip.
Because the sensor body is made from silicon rubber, it
is expected that the sense of resistance is decreased. The
resolution of force magnitude is approximately 30 gf
and the refresh rate is 60 Hz.
An electrotactile display consists of pin electrodesaligned in rectangular grid of 3 pins × 5 pins. The
electric current from each pin electrode activates thenerve fibers in the skin to produce spatially tactile
sensation. The magnitude of electric current represents
the strength of tactile sensation. Furthermore, it isconsidered that the electrotactile display can selectively
activate the nerve fibers connected to different
mechanoreceptors by controlling the pole of the pin
electrodes. Therefore, it is expected that theelectrotactile display can present various tactile
sensation. Another advantage of the electrotactile
display is its compact body. Because the display does
not need any actuators, it is suitable for integration withthe other devices. However, in this prototype, we usedanodic stimulation because it can present clear spatiallydistributed contact information to the user. The pulse

Fig. 5 Results of MCP joint. Above: reference force is
80 gf. Below: reference force is 160 gf.

- 1092 -width of the electric current was 20 ȝA and the refresh
rate was 60 Hz.

virtual hand

4.2 Virtual hand
To control a virtual hand using the one-fingered
prototype and acquire tactile information, weconstructed a virtual environment (Fig. 7) using an ODE(Open Dynamics Engine) [11]. The virtual environment
consists of a humanoid virtual hand and a target object.
The simulation step is 0.01 s.
The user of the one-fingered prototype can control theindex finger of the virtual hand and touch the target
object. The DoF is 4 for index finger, 2 for MCP joint(bend and side flexion) and 1 for the PIP and DIP (distalInterphalangeal) joints. However, the DIP joint moves incoordination with the PIP joint as Eq. (4) [5].

θ DIP = 0 . 46 θ PIP + 0 . 083 θ PIP 2

DIP
PIP

MCP
target object

Fig. 7 Virtual hand.
4.3 Active haptic feedback
We qualitatively evaluated the dynamically-changing
haptic feedback provided by the electro tactile display.
For evaluation, we used two laptop PCs: one PC wasused to control the one-fingered prototype, and the otherPC was used to simulate a virtual environment. Both thePCs were connected by a LAN, and the data of force
vector and pressure distribution was sent by 100 Hz
using UDP protocol. For the target object, weconstructed a box that rotated at a constant speed. Boththe speed and the direction of rotation could bearbitrarily changed. From the dynamically-changing
electrotactile stimulus, the users could feel the rotationof the box when the virtual finger touched it, even
without moving the virtual finger. Furthermore, they
could also feel that their finger was being pushed up by
the box.
Approximately 30 adults (whose ages ranged fromthe 20’s to 40’s) experienced this system. The
participants tried to control the virtual finger based on
the force applied from their index finger to the device.
They could control each joint of the virtual finger
independently and intuitively. Furthermore, most ofthem could perceive haptic sensation that was
dynamically-changing. They could perceive thedifference of edge and surface, contact position, androtation of the box. Therefore, we concluded that the
proposed grip-type haptic display can produce active
haptic feedback without the need for actuators to
produce reactive force. As a next step, we willquantitatively investigate how this dynamicallychanging haptic sensation can be best produced.
It must be noted that many participants pointed outthat the electric sensation decreased their realistic haptic
sensation. This electric sensation is the greatest problemof the electrotactile display. We have to decrease theelectric sensation to produce a more realistic sensationof touch.

(4)

Therefore, we can approximate the DoF of the index
finger to be 3. In this prototype, the joint-angle positions
are controlled by forces that can be expressed by thethree-dimensional force vector F in Eq. (5).

θ PIP = 2 kf z + kf y
θ MCP _ b = 2 kf z − kf y

(5)

θ MCP _ s = kf x
In this equation, șMCP_b and șMCP_s represent a joint angleof MCP for bending and side flexion, respectively. k is a
proportionality coefficient from the force to joint angle.
These equations are determined by a preliminary
experiment to control the PIP and MCP joint
independently and intuitively.
To present a spatially distributed tactile sensation
using the electrotactile display, we calculate a
distribution of pressure applied to the fingertip of virtual
hand. The 15 force sensors that are attached to thefingertip of the index finger calculate pressure applied
to the fingertip by the target object. This pressure is
calculated by a penalty method. The pressure increases
proportionate to the distance that the sensor sinks into
the object. Furthermore, the pressure also increases ininverse proportionate to the size of the contact area.
Force sensor“GelForce”
xzyElectrotactile
display

5. CONCLUSION
In this study, we proposed a grip-type haptic displaythat uses the user’s grasping force and spatiallydistributed tactile feedback to control a virtual finger

Fig. 6 One-fingered prototype.

- 1093 -and produce haptic sensation. To control the virtual
finger, the user has to recognize his/her finger position
the basis of his/her sense of effort expended. Therefore,
it is important to discriminate the force applied by thefinger. We evaluated the threshold at which force could
be distinguished and confirmed that the threshold was
sufficient for controlling the angle position of a virtual
finger. Furthermore, we constructed a one-fingered
prototype using an electrotactile display. Using thisprototype, we qualitatively evaluated the effectivenessof the haptic feedback that integrates both the spatiallydistributed tactile feedback and reactive force.
In future, we intend to quantitatively evaluate theeffectiveness of the haptic feedback of the proposed
device. We expect that the spatially distributed tactile
feedback and the reactive force can present the physicalproperties of an object, such as stiffness, size, and shape.
Furthermore, we also intend to implement a
multi-fingered grip-type haptic display by integrating
MeisterGRIP with the electrotactile display.

ACKUNOREDGEMENT
This work was partly supported by Grant-in-Aid for
SCOPE and JSPS Fellows (20·10009).

REFERENCES[1] K. Sato, K. Minamizawa, N. Kawakami, and S.
Tachi, “Haptic Telexistence” , 34th Int. Conf. On
Computer Graphics and Interactive Techniques (ACM
SIGGRAPH 2007), Emerging Technologies, 2007.
[2] Immersion Technology Inc., CyberTouch,䙼http://www.immersion.com/index.php”[3] K. Sato, S. Komeiji, N. Kawakami, and S. Tachi,“DevelopmentofGrip-type
MasterHand“MeisterGRIP””, Proceedings of ICRA2009, 2009.
[4] M.Bouzid, G.Popescu, G.Burda, and R.Boian, “TheRutgers Master - IIND Force-Feedback Glove”, IEEE
VR2002 Haptics Symposium, 2002J. Clerk Maxwell, A
Treatise on Electricity and Magnetism, 3rd ed., vol. 2,
pp.68–73, 1892.
[5] S. Walairacht, K. Yamada, S. Hasegawa, Y. Koike,and M. Sato, “Two-Handed Multiple-Finger VirtualObject manipulation Environment with Haptic Cues”,
Electronics and Communications in Japan, Part2,
Vol.87, No.9, pp.65-73, 2004.
[6] A. Lecuyer, “Simulating haptic feedback using
vision: a study of research and applications ofpseudo-haptic feedback”, Presence, Vol. 18, No. 1,
pp.39–53, 2009.
[7] M. A. Srinivasan and R. H. LaMotte, “Tactual
Discrimination of Softness: Abilities and Mechanisms”,
Somesthesis and the Neurobiology of the Somatosensory
Cortex, 1996.
[8] H. Z. Tan, M. A. Srinivasan, C. M. Reed, and N. I.
Durlach, “Discrimination and identification of finger
join-angle position using active motion”, ACM
Transactions on Applied Perception, Vol. 4, No. 2,
pp.578–579, 2007.

- 1094 -[9] Open Dynamics Engine, “http://www.ode.org/”[10] K. Kamiyama, K. Vlack, T. Mizota, H. Kajimoto,N. Kawakami, and S. Tachi, “Vision-Based Sensor forReal-Time Measuring of Surface Traction Fields”, IEEE
Computer Graphics & Applications Magazine, Vol.25,
No.1, pp.68-7, 2005.
[11] H. Kajimoto, M. Inami, N. Kawakami, and S. Tachi,“SmartTouch: Electric Skin to Touch the Untouchable”,
IEEE Computer Graphics & Applications Magazine,
Vol.23, No.1, pp.36-43, 2004.