Wearable Haptic Display to Present Gravity Sensation
Preliminary Observations and Device Design
Kouta Minamizawa*, Hiroyuki Kajimoto†, Naoki Kawakami*, Susumu, Tachi*(*) The University of Tokyo, Japan(†) The University of Electro-Communications, Japan
E-mail: kouta_minamizawa@ipc.i.u-tokyo.ac.jp, kajimoto@hc.uec.ac.jp,
kawakami@star.t.u-tokyo.ac.jp, tachi@star.t.u-tokyo.ac.jp
Abstract
We propose a wearable ungrounded haptic displaythat presents a realistic gravity sensation of a virtual
object. We focused on the shearing stress on thefingerpads due to the weight of the object, and foundthat the deformation of the fingerpads can generate areliable gravity sensation even when proprioceptive
sensation on the wrist or arm is absent. This implies
that an ungrounded gravity display can be realized byreproducing fingerpad deformation. Based on ourobservations, we conducted evaluation tests for thedevice design. We first implemented the prototype
device, which has a simple structure comprising dual
motors, and then evaluated the recognition ability of
the gravity sensation presented on the operator’sfinger by this method.

wearable haptic
display to present thegravity sensation
virtual object

Fig. 1: Conceptual drawing of a wearable haptic
display to present the gravity sensation. The
weight of the virtual object is presented on thefingerpad of the operator.

1. Introduction
The human hand can perceive the shape andgravity of an object that it grasps. In haptic display
design, the discernment of the shape and gravity of an
object is considered to be beneficial from theviewpoints of both safety and operability inteleoperation task. However, most conventional
haptic displays [1]–[3] are designed to present only
the grip forces acting on the fingertips. In somemaster cockpit systems [4], the gravity is presented tothe operator’s wrist by a multi-DOF grounded force
display. Although the resulting system is large andcomplex, the presented gravity sensation is not very
similar to actual sensation, since the stimulus points
in these methods are different from the actual contact
surface between the object and the finger. In agrasping task, the gravity of an object is perceived by
the proprioceptive sensation on the arm, the wrist andthe finger, and the tactile sensation on the fingerpads.

In some researches, the slippage between thefingertips and the object has been focused on as aparameter for weight sensation. Johansson [5]showed that partial slippage plays an important rolein object grasping, and Maeno [6] showed a method
for controlling the grip force by detecting the
stick/slip distribution on the fingerpad. Other
researches present different perspectives. Inaba [7]showed that simple constrictive pressures on thefingers replicate the grip sensation. Yao [8] showedthat the dynamics of a rolling object can be displayedby presenting only the rolling noise and impact.
These researches indicate that the dynamics of an
object can be presented in a simple manner byreproducing the elements of motion.
We aimed to develop a haptic display that canpresent the gravity of objects (Fig. 1). To simplify themechanism, we focused on the shearing stress on thefingerpads caused by the weight of an object.

on
their
wrist
and
fingers.
Inthe
without-proprioception session, the subjects’ wrists
and the sides of their thumbs and index fingers were
fixed, as shown in Figure 3, in order to ensure thatthey perceived the gravity of an object only by thetactile sensation on the fingerpads. Four blindfolded
subjects, aged 21 to 31 years, participated in theseexperiments.
Fig. 2: Vertical stress (Fn) and shearing stress(Ft) between finger and object in grasping. In thisfigure, Fn is the grip force and Ft is the gravity ofthe object.

As shown by Forssberg [9] it is considered thatgravity sensation is perceived as an integration of
proprioceptive and tactile sensations. However, howwould gravity sensation be perceived when only thetactile sensation is presented and the proprioceptive
sensation is not? The forces that are perceived on thefingerpads can be categorized into vertical andshearing stresses. In Figure 2, for example, it isassumed that the vertical stress (Fn) is the grip force
on the hand and the shearing stress (Ft) is the gravityof an object. We observed that a realistic gravity
sensation of an object can be presented, even when
the proprioceptive sensation on the wrist or arm isabsent, by reproducing these stresses on thefingerpads, which are the interfaces between thehuman and the object. This in turn implies that an
ungrounded device can be realized.
In this paper, we propose a wearable ungroundedhaptic display to present the gravity sensation of avirtual object. We conducted experiments to study thedevice design. Further, we also studied the humanweight discrimination ability on fingerpad without
proprioception, and evaluated the reproducibility of
the gravity sensation by the shearing stress in order tosubstantiate our proposal. We will design andimplement the prototype system based on the resultsof the experiments.

2. Experiments to Evaluate Device Design
2.1. Weight discrimination ability on
fingerpad without proprioception
In order to evaluate the limitation of our proposed
method, we measured the difference limen in weightdetection on the fingerpads without proprioceptive
sensation. This experiment was performed under two
conditions (with and without proprioception). In the
with-proprioception session, the subjects set theirforearm on an armrest while their wrist was free. The
subjects could perceive the proprioceptive sensation

Fig. 3: Experimental setup forwith-proprioception sessions.

The procedure for this experiment was theconstant method. The subjects were first asked tograsp one of the standard objects (50, 100, 200 gf) for
2 sec as a standard stimulus. After a 2-sec interval,
the subjects grasped a test object for 2 sec as acomparison stimulus. And more than 5-sec interval
was given between each trial. The subjects then statedwhether the test object was “heavy”, “similar” or“light” in comparison to the standard object,according to a three-alternative forced-choiceprocedure. Each experimental session comprised four
series of trials for each standard stimulus. Twosessions were performed for each condition and aninterval of more than 3 min was provided between
each session.
Figure 4 shows the average rate of “heavy”
responses obtained in trials where the standard
stimuli were 100 g. The blue circles (withproprioception) and red triangles (without
proprioception) represent the average of each of 16
trials for all subjects in two conditions. A blue line
and dotted red line indicate the fitted line withcumulative normal distribution. The 75 percent
difference limen (75 % DL) was derived from thedifference between PSE and the 75 percent
discrimination threshold. Table 1 shows the 75 % DL
for each standard stimulus in two conditions of with/without proprioception. According to this result, it isconfirmed that the tactile sensation on fingerpads
provides certain perception to discriminate the weight
without proprioceptive sensation.

the motors in order to perceive the weight of the test
object; the virtual gravity was observed to be thesame. The result is shown in Figure 7.

1.000

0.750

0.500

0.250

0.000
-50

-40

-30

-20

-10

0

10

20

30

40

50

Fig. 4: Average rate of “heavy" response for a
100-g standard stimulus. The blue dash lines andpink dotted lines indicate the PSE and the 75 %correct lines in with and without proprioception.

Fig. 5: Schematic representation of theexperimental device to generate shearing
stress on the fingerpad. The setup comprises
a belt, a motor with an encoder (Maxon Motor
Corp., RE25, 20 W, gear ratio = 18:1), and a
supporting frame to limit the motion directionof the belt to generate the correct shearing
stress. A sticky disk is placed between thefingerpad and the belt to inhibit slippage.

Table 1: 75% difference limen for three kind of
standard stimulus with / without the proprioceptive
sensation on wrist and fingers

75 % DL
with prop.
8.1 gf
9.3 gf13.9 gf

75 % DL
without prop.
9.3 gf16.5 gf
23.6 gf

2.2. Virtual weight using a grounded setup
In this experiment, we evaluated the
reproducibility of the gravity sensation by theshearing stress on the fingerpads. A groundedexperimental setup was used so as to inhibit the effectof ungrounded condition.
To reproduce the deformation pattern generated by
the weight of an object, we used the experimental
setup shown in Figure 5. The experimental devices
were attached to the subject’s fingers, as shown in
Figure 6. The subjects were asked to grasp test
objects of predetermined weights with the naked
index finger and thumb of their left hand. On theirright hand, a pair of experimental devices was
attached and the shearing stress was generated bybelts that were connected to two motors. The subjects
were asked to adjust the torque strength exerted by

Fig. 6: Displaying gravity sensation to theindex finger and the thumb using a pair ofexperimental devices (described in Fig. 6).
The dorsal sides of the fingers are fixed bymolds so that they do not move. A styrofoam
cube (2 g, 5 cm on one side) is grasped to fix
the position of the fingers.
weight of the real object as standard stumuli [gf]standard
stimulus
50 gf
100 gf
200 gf

450
400
350
300
250
200
150
100
50
0
0

2

4

6

8

10

12

generated shearing range on fingerpads [mm]Fig. 7: Results of the experiment to present
gravity sensation on the fingerpad using agrounded setup shown in figure 6. The blueline is the least-squares-estimated curve.

2.3. Reflexive response to virtual weight120

real object (300g)

grip force (gf)100

In this experiment, we confirmed the reality of thepresented gravity sensation. We examined thereflexive response of the gripping force in a situation
where the weight of the object suddenly increases. If
no significant difference is observed between the
result of the reflexive response to increments in thereal weight and the virtual weight, the gravity
sensation presented by our proposed method can be
described as essentially similar to the actual gravity
sensation.
First, the subjects were asked to grasp a test object
whose
weightwascounterbalanced.
The
counterbalance was then suddenly removed and thesubjects perceived a sudden increase in real weight.
The reflexive change in the grip force was measured
by two force sensors (Nitta Corp., FlexiForce A201)
placed on the padding surface of the index finger andthumb. Second, when the subjects were wearing theexperimental devices shown in Figure 5, a certain
shearing stress was suddenly produced. The
magnitude of this shearing stress was determined so
as to present the same gravity sensation as that of thetest object according to the least-squares-estimatedcurve shown in Figure 7.
Figure 8 shows the result for a case where theweight of the test object is 300 gf. On comparing thetwo graphs in this figure, we observe that theincrements in the grip force in both cases are 80 gf
and they change in 0.4 s. Figure 9 shows the grip
force increments in the reflective responses using test
objects of various weights. This result shows that thereflexive responses to the virtual and real weights
exhibit the same tendency.

80
60
40
20
0
0

100

200

300

400

500

600

real / virtual weight (gf)Fig. 9: Increment of grip force in reflexive
responses by various changes in real (redsquares) and virtual (blue circles) gravity.

2.4. Summary of the experiments
Table 1 shows that the tactile sensation on
fingerpad is sufficient to allow people to perceive thegravity sensation even when the proprioceptive
sensation is absent, although the discriminationthreshold is inferior to integration of tactile andproprioceptive sensation. In Figure 7, we determined
the relation between the gravity sensation and thefingerpad deformation due to the shearing stress. The
result shown in Figure 9 further supports theassumption that the gravity sensation presented byour proposed method is essentially the same as theweight of a real object, though the resolution isinferior. These results indicate that the gravity
sensation can be presented with the shearing stress on
fingerpad without the proprioceptive sensation. In thefollowing section, we will describe the prototype
device design, which is based on these observations.

3. Prototype Device

120

3.1. Design of the prototype device

80
60
40
20
0
0

1

2

3

4

time(s)5

virtual object (300g)

120
100

grip force (gf)grip force(gf)

100

80
60
40
20
0
0

1

2

3

4

time (s)Fig. 8: The grip force rates in the reflective
responses to the real weight (top) and the virtual
gravity presented by the device (bottom).

5

We modified the mechanism proposed by us as a
haptic display for the middle phalanx using dual
motors in [10], and designed the prototype device
shown in Figure 11; this device has a simple
construction and a small size. To present the grip
sensation, the dual motors are driven in opposite
directions of rotation so that they roll up the belt. A
vertical stress is then generated on the fingerpad of
the operator. On the other hand, to present the gravity
sensation, the motors are driven in the same directionof rotation. In the figure on the right in Figure 10, forexample, the belt is rolled up on the left side androlled out on the right side. The shearing stress is thengenerated form right to left on the fingerpad. We
implemented the prototype device shown in Figure 12.

The device comprises a belt (width = 20 mm), a pairof motors (Maxon Motor Corp., RE 10, 1.5 W, φ = 10mm, gear ratio = 1:16) and brass shafts (φ = 6 mm),and a body made of ABS resin. The body functions toguide the belt so as to provide a good tangential force
on the fingerpad. The device is fixed on the middle
phalanx of the finger by a Velcro strap. The bottom
surface of the device is flushed with the dorsal side of
the finger by a mold so that the reactive force fromthe body of the device is widely distributed andbarely perceptible.

styrofoam cube (2 g, 5 cm on a side) to fix theposition of their fingers, as shown in Figure 13. The
gravity sensation was then presented as a shearing
stress on the index finger and the thumbsimultaneously with the same power for 2 sec. The
subjects stated how much they perceived the weightof the object to be in comparison to various weightsof real objects with a similar appearance. Figure 14
shows the result that the perceived virtual weight has
good linearity with the generated shearing stress.

Fig. 10: A method for generating vertical stress(left) and shearing stress (right)

recognized weight of virtual object [gfFig. 13: Displaying augmented weight on alight-weight styrofoam cube.
450
400
350
300
250
200
150
100
50
0
0

Fig. 11: Conceptual drawing of the prototype device

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

2

generated shearing stress [mN/mm ]Fig. 14: Perceiving virtual weight in static
grasping. The shearing stress is theoreticallycalculated from the applied current values,
motor specifications, and device structures.

4. Discussion
Fig. 12: Implemented prototype device

3.2. Evaluation of the prototype device
We evaluated the recognition ability of the virtual
weight presented by the prototype devices in a static
grasping situation. The subjects fixed their arm on anarmrest, attached the prototype devices on their index
finger and thumb, and grasped a light-weight

In order to present virtual gravity sensation in a
virtual reality or teleoperation system, our proposed
method should be extended to present the mass
sensation of a virtual object during active movement.
In situations where the operator moves his/her handactively and the object is set in motion, the force
should be presented according to its acceleration. We
briefly tested the recognition of a virtual weight
during some operations such as shaking and rotating

as shown in Figure 15: we have not, however,conducted quantitative evaluations thus far. During
rotation, for example, the weight of a virtual object ispresented evenly on the index finger and the thumb, a
as shown in Figure 15 (c). As the object is rotatedposition (d), disproportionate weight ins perceived on
the index finger. Then, at position (e), the weight ispresented evenly once again. However, the stressdirection is opposite to that in (c). In these cases, theperception of gravity sensation was clearer than thatin static grasping described in section 3.2. The
operator could perceive certain weights such as 50 g
or 100 g from a light-weight styrofoam cube
according to the change in the vertical and shearing
stresses generated by the devices, although the actual
weight of the cube was just 2 g. According to theseobservations, it is considered that our proposed
method is applicable for presenting not only static
gravity but also the inertia of a virtual object in active
motion.

Fig. 15: Change in the force directions ingrasping during shaking from (a) to (b) androtation from (c) to (e). Blue arrows indicate
the direction of motion and the red arrows
indicate the combined force vectors —grip
force, the gravity, and the inertia— whichshould be reproduced.

5. Conclusions
In this paper, we focused on the shearing stress on
the fingerpads during grasping and proposed awearable ungrounded haptic display to present thegravity sensation of a virtual object. To validate thepossible realization of our proposed method, we
measured the difference limen for weight detection
on the fingerpads without proprioceptive sensation.
The relation between the gravity sensation and theshearing stress was evaluated to design the prototype

device. We then implemented the prototype devices
and confirmed that the presented virtual weight has
good linearity with the generated shearing stress.
This paper presented the evaluation of the
prototype only in a static grasping situation. In our
next study, we will evaluate whether our proposed
method is applicable to only the gravity sensation in
static grasping or to the sensation of inertial force in
active motion as well. In addition, we need to study
the relativity between tactile sensation andproprioceptive sensation in perceiving gravity inorder to investigate the possibility of realizing our
proposed haptic display.

References[1] Y.Yokokohji and T.Yoshikawa: "Maneuverability ofMaster-Slave Telemanipulation Systems", Trans.
SICE, Vol. 26, No. 5, pp. 572–579, 1990.
[2] Immersion Co., "The CyberGrasp: Groundbreaking
haptic interface for the entire hand", (online)
http://www.immersion.com/3d/products/, 2003.
[3] S. Nakagawara, H. Kajimoto, N. Kawakami, S. Tachi,
andI.
Kawabuchi:"An
Encounter-Type
Multi-Fingered Master Hand Using Circuitous Joints",
Proc. IEEE International Conference on Robotics andAutomation (ICRA2005), Barcelona, Spain, 2005.
[4] S. Tachi, K. Komoriya, K. Sawada, T. Nishiyama, T.
Itoko, M. Kobayashi, and K. Inoue: "Telexistence
Cockpit for Humanoid Robot Control", Advanced
Robotics, Vol. 17, No. 3, pp. 199-217, 2003.
[5] S. Johansson and G. Westling: "Roles of Glabrous
Skin Receptors and Sensorimotor Memory in
Automatic Control of Precision Grip When Lifting
Raugher or More Slippery Objects", Experimental
Brain Research, vol. 56, pp. 550-564, 1984.
[6] T. Maeno, S. Hiromitsu and T Kawai: "Control ofGrasping Force by Detecting Stick/Slip Distribution at
the Curbed Surface of an Elastic Finger", Proc. IEEE
International Conference on Robotics and Automation(ICRA2000), San Francisco, USA, 2000.
[7] G. Inaba and K. Fujita: "A Pseudo-Force-Feedback
Device by Fingertip Tightening for Multi-Finger
Object Manipulation", Proc. EuroHaptics Int. Conf.
2006, pp. 275-278, Paris, France, 2006.
[8] H. Y. Yao and V. Hayward: "An Experiment on
Length Perception with a Virtual Rolling Stone", Proc.
EuroHaptics Int. Conf. 2006, pp. 275-278, Paris,
France, 2006.
[9] H. Forssberg, A.C. Eliasson, H. Kinoshita, R.S.
Johannson, and G. Westling: "Development of humanprecision grip I: Basic coordination of force",
Experimental Brain Research, Vol. 85, pp. 451-457,
1991.
[10] K. Minamizawa, K. Tojo, H. Kajimoto, N. Kawakami,
and S. Tachi: "Haptic Interface for Middle Phalanx
Using Dual Motors", Proc. EuroHaptics Int. Conf.
2006, pp. 235-240, Paris, France, Jul. 2006.