GelForce
Kazuto Kamiyama†
Hiroyuki Kajimoto†

Kevin Vlack†
Naoki Kawakami†

Terukazu Mizota‡
Susumu Tachi††The University of Tokyo‡Center for Advanced Scientific Technology Incubation
1Introduction

GelForce is a novel type of interface that measures the distributionof both the magnitude and direction of force. The sensor iscomposed of a transparent elastic body, two layers of blue and redmarkers and a CCD camera. Force vectors are calculated from thecaptured movement of the markers.
The history of the force sensor is ancient, yet it has always been
exceedingly difficult to obtain the distribution of force vectors,
because nearly all existing sensing technologies use some sort ofsensing unit for which electric resistance or capacitance depends
on force. For sensors of this type intended to measure thedistribution of force vectors, the wiring becomes extremelycomplex, and the size of the unit expands dramatically as theamount of measured information is increased. On the other hand,the optical system of our developed sensor is simple, and various
applications are therefore possible such as tactile sensation for arobotic fingertip, and a versatile force-based computer interface.

2Fig.1: Video capture of markers in the elastic bodyTechnical Features and Details

Nearly all force sensors fall into two categories. One measuresdirection and magnitude of force at a single point, but cannot
measure its distribution. The other measures the distribution of themagnitude of force, but cannot measure its direction. The force
sensor we have developed integrates these two types to measure
the distribution of force vectors, with both magnitude anddirection. The sensor consists of a transparent elastic body, andtwo layers of colored markers within the body. When a force isapplied on the body's surface, we optically measure the internalstrain of the body through the movement of the markers. Finally,
force vectors are calculated from the strain using elastic theory.
The core innovation is the technique to derive a distribution offorce vectors from the movement of two layers of colored markers.
We wish to obtain 3-dimensional force vectors, but the movementof markers captured by a CCD camera is 2-dimensional. If there is
only one layer of markers, only the magnitude of force can bemeasured with any degree of accuracy, or else two cameras would
be necessary to measure the 3-dimensional movement of markers.
Using two cameras brings about complex camera calibration andoccupies a large space. These complications make downsizing thesensor infeasible in practice. To solve this problem, we use twolayers of markers at different depths. Equipping the two layers isequivalent to measuring 2 times 2 dimensional movement ofmarkers. This provides sufficient measurement depth information
to readily obtain a force vector distribution.
e-mail: {kazuto, vlack, mizota, kaji, kawakami, tachi}@star.t.u-tokyo.ac.jp

Fig.2: Screenshot of measured force vector distribution

3Future Works

One of the goals is to develop a finger shaped sensor to equip on arobotic hand. Herewith, the robot is granted with tactile sensationand can perform fine manipulation tasks just as humans do.
Existing sensors equipped on mechanical hands cannot measuresufficient force information to perform most everyday tasks,
which has severely hampered the development of field robotics.
To develop a sensor with a finger shape, there are two key
technical issues. One is that the sensor needs to be prepared at asmall size and with high density, comparable to the layout of
mechanoreceptors in human skin. The other key issue is that theforce sensor must adapt to the particular form of the fingertip. The
simple structure of this elastic sensor leads to an elegant solution
to these two issues. The sensor does not require a complex sensing
unit, but only markers within a gelatin body, which can bearranged at high density. In addition, because the sensor needs
only to capture the markers, it may be crafted into any shape thatallows it to do so. From these points, we are now developing thefinger shaped sensor.