Gravity Grabber:
Wearable Haptic Display to present Virtual Mass Sensation
Kouta Minamizawa*1
The University of Tokyo

Souichiro Fukamachi*2
The University of Tokyo

Naoki Kawakami*4
The University of Tokyo

Hiroyuki Kajimoto*3
The University of Electro-Communications
Susumu Tachi*5
The University of Tokyo

Executive Summary
We propose a wearable haptic display to present the weight
sensation of a virtual object, which is based on our novel insightthat the deformation on fingerpads makes a reliable weight
sensation even when the proprioceptive sensation is absent. This
device will provide a new form of ubiquitous haptic interaction.

1. Project Overview
In recent times, there have been a number of computer userinterface devices that have some haptic feedback functions, suchas the DUALSHOCK controller [Sony Computer Entertainment,Inc. 1997] or the Wii Remote [Nintendo Co., Ltd. 2006]. However,their haptic feedback is limited to only the vibration function since
there is no other method to provide haptic feedback that can be
implemented with a small and inexpensive device. However, thereis an increasing demand for realistic haptic feedback; thus, asimple and inexpensive method for a highly realistic haptic
display is required. To meet this requirement, we focused on themass of a virtual object, which contributes to the weight and theinertia mass in haptic interaction. If the virtual mass is presented
by a haptic device, the user can perceive a more realistic sensationof the virtual object by grasping than the vibration feedback.
We propose a wearable haptic display to present the mass of avirtual object as shown in Figure 1. In our previous researches[Minamizawa et al., 2006; Minamizawa et al., 2007], weinvestigated the possibility of realizing a simple haptic display on
the basis of finger deformation. Conventionally, it is believed that
it is necessary to reproduce the proprioceptive sensation in orderto present a weight sensation and thus a large grounded device isrequired. However, we found that the deformation of thefingerpads due to the weight of an object can generate a reliableweight sensation even when the proprioceptive sensations on thewrist and arm are absent. This implies that a simple ungroundeddisplay for presenting the virtual mass can be realized byreproducing the fingerpad deformation. Based on our observations,
we designed the mechanism of the haptic display as shown in
Figure 2, which has a simple structure comprising dual motors.

Real world:holding a empty glass withwearing our proposed devices

Virtual world:
the user can perceive
the weight of the virtual water

Figure 1: Conceptual representation of our proposed device.
By wearing our proposed devices on the index finger and thethumb, the user can feel the augmented weight and inertia of
the water that is virtually filled in the actually empty glass.

Figure 2: Our proposed method for generating
vertical stress (left) and shearing stress (right).

We implemented the prototype device shown in Figure 3 and thenconfirmed the recognition ability of the weight sensation
presented on the user’s finger by this method.
*1e-mail: kouta_minamizawa@ipc.i.u-tokyo.ac.jp
e-mail: souichiro_fukamachi@ipc.i.u-tokyo.ac.jp*3e-mail: kajimoto@hc.uec.ac.jp*4e-mail: kawakami@star.t.u-tokyo.ac.jp*5e-mail: tachi@star.t.u-tokyo.ac.jp*2

Figure 3: Implemented prototype device.

By wearing our proposed devices on the index finger and thethumb, the user can perceive the grip force and the mass of avirtual object. In Figure 4, for example, the grip force, gravity,and inertia mass of a virtual object are presented on the user’s
fingerpads during shaking and rotational motion.

the grip sensation. Yao and Hayward [2006] showed that thedynamics of a rolling object can be displayed by presenting only
the rolling noise and impact. These researches indicate that thedynamics of an object can be presented in a simple manner byreproducing the elements of motion.
In our proposed method, we considered the human perceptual
characteristics that the weight perception is effected by thefingerpad deformation and then designed a simple mechanism fora haptic display. This method can provide sufficient
high-definition haptic sensations (grip force, weight, and inertia)
from a virtual object so that the user can have a highly realistic
perception of the virtual object. In addition, it is easy to put on
and take off this device.

Figure 4: Displayed force directions while grasping:shaking from (a) to (b) and rotation from (c) to (e). Theblue arrows indicate the direction of motion and the redarrows indicate the combined force vectors—grip force,gravity, and inertia— that should be reproduced.

In our proposed haptic display, the grasping experience of avirtual object is more realistic than the conventional vibration
function. In the near future, this device can be downsized andunwired according to the evolution of motors and batteries. The
device can then be used in daily life, for example, as a grasping
controller in entertainment systems or as a force-feedback devicefor operating a virtual reality environment in a computer. This
device can be used in combination with the contemporary
mouse-based user interfaces, since it is small enough to be usedwith a mouse while the device is worn on the operator's fingers.
This device also realizes a ubiquitous teleoperation, since thewearable and wireless device can be used to manipulate a robot
from any location. In this project, we will provide alternative
experiencesofubiquitous
hapticinteractions
andtelecommunications.

2. Context and Vision

3. Principle and Device Design

The ultimate goal of our project is to introduce high realistic
haptic interactions in our daily life.

We introduce mainly two technical innovations in this project.
Firstly, we find that the deformation at the fingertips leads toweight perception even without the proprioceptive sensations on
the arm and wrist. This enables the realization of a simple
wearable haptic display for virtual weight. Secondly, we designed
a mechanism to reproduce the fingerpad deformation using asimple method that employs a pair of motors and a belt. We
describe the abovementioned in detail in the subsequent sections.

Grasping is the most important function of the human hand. There
are several haptic displays to present grasping sensations.
Conventional haptic displays [Immersion Co., Ltd., 2000;
Nakagawara et al., 2003] have been designed to correctly
reproduce the physical phenomenon on the interface between
fingertips and an object while grasping. It induces a highly
realistic sensation, but it tends to require a complex and expensive
mechanism; therefore, these haptic devices are used only in a few
professional fields such as 3D-CAD designing, surgical operations,and education in schools. In some master cockpit systems [Tachiet al., 2003], gravity is presented to the operator’s wrist by amulti-DOF grounded force display. Although the resulting systemis large and complex, the presented gravity sensation is not very
similar to the actual sensation since the stimulus points in thesemethods are different from the actual contact surface between theobject and the finger.
On the other hand, there has been an increasing number of
researches from other perspectives in recent times. These
researches aim to achieve the practical use of human perceptual
characteristics and perceptual illusions in the design of haptic
devices. Some researches have focused on the slippage between
the fingertips and the object as a parameter for weight sensation.
Johansson and Westling [1984] showed that partial slippage plays
an important role in grasping an object, and Maeno et al. [2000]
showed a method for controlling the grip force by detecting thestick-slip distribution on the fingerpad. Inaba and Fujita [2006]
showed that simple constrictive pressures on the fingers replicate

3.1. Weight discrimination ability on a fingerpad
without the proprioception
Weight sensation is perceived as the integration of theproprioceptive sensation on the arm and the tactile sensation onfingerpad. Most conventional haptic displays for presenting
virtual weight are designed to reproduce the proprioceptivesensation, due to which heavy grounded devices are required.
However, how can weight sensation be perceived when only thetactile sensation is presented and the proprioception is not?
The forces that are perceived on the fingerpads can be categorized
into vertical and shearing stresses. In Figure 5, for example, it isassumed that the vertical stress (Fn) is the grip force on the handand the shearing stress (Ft) is equal to the gravitational force
acting on the object. We observed that the realistic gravity
sensation of an object can be presented, even when theproprioceptive sensation on the wrist or arm is absent. This can bedone by reproducing these stresses on the fingerpads, which are
the interfaces between the user and the object. This in turn implies
that an ungrounded device can be realized.

Figure 5: Vertical stress (Fn) and shearing stress (Ft)between the finger and object while grasping. Fn is the grip
force and Ft is equal to the gravitational force acting on theobject.

wrists and the sides of their thumbs and index fingers were fixedin order to ensure that they perceived the gravity of an object onlyby the tactile sensation on the fingerpads. The constant method
was used for this experiment. The subjects stated whether the test
object was “heavy,” “similar,” or “light” in comparison to thestandard object, according to a three-alternative forced-choiceprocedure. Figure 6 shows the average rate of “heavy” responses
obtained in the trials in which the standard stimuli were 100 g.
Table 1 shows the 75 % DL for each standard stimulus under twoconditions. According to these results, it is confirmed that thetactile sensation on the fingerpads provides certain perception todiscriminate the weight without proprioceptive sensation.
3.2. Design of the prototype device using dual motors
On the basis of the observation described in section 3.1, we
designed the mechanism shown in Figure 2; this device has asimple construction and a small size. To present the grip sensation,
the dual motors are driven in opposite directions of rotation sothat they roll up the belt. A vertical stress is then generated on thefingerpad of the operator. On the other hand, to present the gravity
sensation, the motors are driven in the same direction of rotation. ,For example, in Figure 2, the belt in the figure on the right isrolled up on the left side and rolled out on the right side. The
shearing stress is then generated form right to left on thefingerpad.

1.000

0.750

0.500

0.250

0.000
-50-40-30-20-1001020304050

We implemented the prototype device shown in Figure 7. The
device comprises a belt (width = 20 mm), a pair of motors(Maxon Motor Corp., RE 10, 1.5 W, φ  = 10 mm, gear ratio =1:16), brass shafts (φ = 6 mm), and a body made of ABS resin.
The body functions to guide the belt so as to provide a goodtangential force on the fingerpad. This device is fixed on themiddle phalanx of the finger by a Velcro strap. The bottom
surface of the device is flushed with the dorsal side of the fingerby a mold so that the reactive force from the body of the device iswidely distributed and barely perceptible.

Figure 6: Average rate of “heavy” response for a 100-g
standard stimulus. The blue dashed lines and pink dottedlines indicate the PSE and the 75 % correct lines in the
with- and without-proprioception sessions.
Table 1: 75% DL for three kinds of standard stimulus
with/without the proprioceptive sensation.
Standard
stimulus
50 gf
100 gf
200 gf75% DL
withproprioception
8.1 gf
9.3 gf13.9 gf

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

In order to evaluate the limitation of our proposed method, wemeasured the difference limen (DL) of the weight detection on thefingerpads without any proprioceptive sensation. This experiment
was performed under two conditions (with and withoutproprioception). In the with-proprioception session, the subjects
set their forearm on an armrest while their wrist was free. The
subjects could perceive the proprioceptive sensation on their wristand fingers. In the without-proprioception session, the subjects’Figure 7: Detail drawing of the prototype device.

Figure 8: Displaying augmented weight on a light-weight cube.

recognized weight of virtual object [gf]References

450400AMEMIYA, T., ANDO, H. AND MAEDA, T., 2006. Perceptual
Attraction Force: The Sixth Force. In Proceedings of ACM
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2

generated shearing stress [mN/mm ]Figure 9: Perceiving virtual weight in static grasping. The
shearing stress is theoretically calculated from the appliedcurrent values, motor specifications, and device structures.
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 an armrest, attached the prototype
devices on their index finger and thumb, and grasped alight-weight styrofoam cube (2 g, 5 cm side length) to fix theposition of their fingers, as shown in Figure 8. The gravity
sensation was then presented as a shearing stress on the index
finger and the thumb simultaneously. The subjects stated how
much they perceived the weight of the object to be in comparison
to various weights of real objects with a similar appearance.
Figure 9 shows the result that the perceived virtual weight has
good linearity with the generated shearing stress.

4. User Experiences
In SIGGRAPH 2007 Emerging Technologies, we will provide
two kinds of the experience of “Gravity Grabbing”. First is tooperate a virtual reality environment with the augmented mass
sensations by our proposed devices. As shown in Figure 1, forexample, the participant can perceive the ruffle of the water in aglass although he/she actually has an empty glass. Second is avideogame-based entertainment system with the haptic interactionof the virtual mass.

5. Conclusion
We found that the vertical and sharing stresses on the fingerpads
due to the weight of the object can generate a reliable weight
sensation even when the proprioceptive sensation on the wrist orarm is absent. On the basis of this observation, we designed awearable ungrounded haptic display to present the gravity
sensation of a virtual object using dual motors and a belt.
This method is simple and it can be introduced in daily-life. We
then implemented the prototype devices and confirmed that virtual
mass sensations can be presented with good linearity. By wearing
our proposed devices on the index finger and the thumb, a user
can perceive the grip force, gravity, and inertia of a virtual object.

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