Haptic Interface for Middle Phalanx Using Dual Motors
Kouta Minamizawa1

Koichi Tojo2

Hiroyuki Kajimoto3

Naoki Kawakami4

Susumu Tachi5

Department of Information Physics & Computing, The University of Tokyo, Japan

ABSTRACT
We propose a new haptic interface for the middle phalanx
that bi-directionally transmits haptic information on “powergrasp.” This is a primary requirement for interactive
tele-operation or telexistence. By using two motors and a belt,
this device can sense and display the haptic information ofthe finger pad of the middle phalanx when grasping an object.
Pressure and weight are the most important sensations withgrasping. In this paper, we first examine the discrimination
ability of the force direction on the middle phalanx to show
validity of our proposal. We then examine gravity sensation
on middle phalanges, and describe the mechanism of the firstprototype.

Fig. 2: Schematic illustration of the haptic interface for middle
phalanx (The device will put on every finger).

Keywords: haptic interface, middle phalanx, grasping,gravity sensation, robot hand, telexistence

1.

INTRODUCTION

Imagine that you are grasping a cylindrical object such as abottle. Typically, you would grasp it using all the parts ofyour hand. In fact, you would not only use the tips of yourfingers, but also the middle phalanges.
As shown in Fig. 1, grasping in robot manipulationconsists of two types—“precision grasp” and “power grasp”[1] [2]. “Precision grasp” implies holding an object with thetips of your fingers, so that you can manipulate it freely anddexterously. However, at the same time, stability is sacrificed
and you might easily drop it. On the other hand, “powergrasp” implies holding an object tightly using all the parts ofyour fingers. You can grasp it stably and safely. This type ofgrasping is appropriate for holding and carrying tasks,
dangerous works, etc.

(a) Precision grasp(b) Power grasp

Fig. 1: Two grasping modes.

1e-mail:e-mail:3e-mail:4e-mail:5e-mail:2

kouta@star.t.u-tokyo.ac.jp
koichi_tojo@ipc.i.u-tokyo.ac.jp
kaji@star.t.u-tokyo.ac.jp
kawakami@star.t.u-tokyo.ac.jp
tachi@star.t.u-tokyo.ac.jp

Today, haptic interfaces have become increasingly
important in tele-operation and telexistence. In particular,
many researches exist on hands and fingers that display
haptic information from slave robot hands, called “masterhand” [3] [4] [5]. Some researches analyze and display theslip sensation on fingertips [6] [7]. Nevertheless, from theviewpoint of grasping type, most of the researches haveattempted to stimulate fingertips (or distal phalanges) offingers. It appears that their aim was to transmit the haptic
information of “precision grasp.” Few researches haveexamined the importance of the haptic sensation on themiddle phalanx.
However, we observed that the “power grasp” situation is acommon occurrence. When we hold up a beverage bottle todrink, or use almost any kind of hand tool such as a screw
driver or a soldering bit, we use the middle phalanges tomanipulate them stably and dexterously. Similarly, whendriving a car, we handle a steering wheel, which is definitely
a “power grasp” task. At the same time, in many “precisiongrasp” situations, not only fingertips but also middle
phalanges are used. Chopsticks, pencils, and scissors are
typical “precision grasp” tools; however, when we lookclosely, the middle and proximal phalanges play an importantrole in handling them stably. The roles of the middle phalanx
are summarized in table 1.
One more important sensation in grasping is gravity. Therealso exist some researches on displaying gravity sensation on
fingers focusing on slippage or deformation of fingertips [8][9]. In addition, recent researches [10] [11] revealed that theslip sensation is used unconsciously to hold an object whose
weight is unknown. In either case, shearing forces on themiddle phalanges, which would be as important as those on
the fingertips, have been largely unnoticed.
In this paper, in order to transmit the grasping and gravity
sensations from a robot hand to an operator with higher
realistic sensation, we propose a bi-directional haptic
interface for the middle phalanx (Fig. 2), which works as asensor and a display at the same time.

Table 1: Roles of the middle phalanx

Fig. 4: Stimulation patterns

2.

SENSITIVITY OF MIDDLE PHALANXInformation of the force direction plays an important role
when you grasp an object. To quantitatively show that themiddle phalanx is as important as the distal phalanx, we
conducted an experiment to examine the threshold ofdifferentiation of force directions on the middle phalanx incomparison with the distal phalanx.
The experiment system is shown in Fig. 3. It is composedof a contact board and two voice-coil type linear actuators(Emic Corp., 511-A) to drive the board horizontally andvertically. By fixing the dorsal side of the finger, we placed
the board on the ventral side of the distal or middle phalanx.
The threshold of differentiation of force directions wasmeasured by the constant method. The first reference
stimulus was a vertical force, and the second comparison
stimulus was a force that has a certain angle θ ranging from–90 to +90 [deg]. Both stimuli continue for 2 s, as shown in
Fig. 4. This was repeated 10 times for every 5 [deg]. The
participant was asked to state the relative change of direction
as either “thumb side” or “little finger side.” On the basis ofthe responses, the number of “thumb side” responses was fit
with the cumulative density function of a normal distribution.
Fig. 5 shows the results obtained from four participants,and Fig. 6 shows the average. From these results, the points
of subjective equality (PSE) and 75% determinationthresholds from PSE (75% DT) were measured (Table 2).

Subject A

Subject B

Subject C

Subject D

Fig. 5: Results of the experiment on the force direction
sensitivity of distal and middle phalanges.
The proportion (vertical axis) of “thumb side” answers were plottedvs. θ (horizontal axis). The red dots/lines represent the distalphalanges. The blue dots/lines represent the middle phalanges.

Fig. 6: Average of the 4 subjects in Fig. 5
Table 2: Points of subjective equality (PSE) and
75% determination thresholds from PSE (75% DT)

Fig. 3: The experiment setup to measurediscrimination ability of force direction.

AB
CDAve.

distal phalanx
PSE
75% DT
–0.2
5.4
+8.6
4.9
+5.2
8.2
+4.5
5.6
+4.5
6.0

middle phalanx
PSE
75% DT+1.1
17.0
+5.3
7.4+10.3
7.3
–0.3
4.0
+4.1
8.9
[deg.]

3.

By fixing dorsal side of the fingers on the mold not tomove, we drove the motor. Then the belt goes down so thatthe finger of examinee dragged through the sticky disk andbecome deformed. A brief question how the examinees felt
when this device dragged their finger showed that theycertainly felt some gravity sensation, as if they were graspingsome weight. The weight displayed in this method wasmeasured by the reconciliation method. First, the examinee
grasped an object of certain weight only with naked middle
phalanges of index finger and thumb. Second, wearing theexperimental devices, the shearing stress was generated andthe examinee adjusted the shearing range on the middle
phalanges so that the gravity sensation is same as the real
object which grasped first. One result of this experiment isshown in Fig. 10.
In this preliminary experiment, we confirmed that themiddle phalanx can discriminate the gravity of objects andthe shearing stress on palmer surface of middle phalanx canreproduce this gravity.

weight of the real object as standard stumuli [gf]This result shows that the PSE are biased to the thumb side,probably due to the asymmetric setup of our experiment(cantilever structure of the contact board). A particularlyinteresting point is that the 75% DT are not very different
between distal and middle phalanges. In some cases, themiddle phalanx shows higher direction sensitivity than thedistal phalanx. This result indicates that there is little
difference between the distal and middle phalanges in thethreshold of differentiation of force directions. In thisexperiment, the stimuli were almost static and SAII receptors
were considered to play a major role in the recognition of theforce direction. It is known that SAII receptors are almost
evenly distributed on a finger [12], which supports ourresults.
According to this result, we proposed a hypothesis that themiddle phalanges discriminates the force direction ofpressure when an object is held using the “power grasp.” On
the basis of this hypothesis, the haptic interface for themiddle phalanx should have the ability to display thedirection of the force as well as determine its magnitude.

DISPLAYING GRAVITY SENSATION

Focusing on the deformation of middle phalanx of thefinger in grasping, we preliminary examined ability of ourproposal in displaying gravity sensation. To reproduce thedeformation caused by the weight of object, we made two
experimental devices shown in Fig. 8 and set them on thelittle finger and the thumb of operator (Fig. 9). This device
composed of a belt, a motor (Maxon Motor Corp., RE25,20W, gear ratio = 18:1), and a supporting frame to limit themotion of the belt in correct shearing direction. There issticky disk between finger and belt so that not slip sensation
but deformation of finger occurs.

450400350
300250200150100500024681012generated shearing range on palmar suface of middle phalanx [mm]Fig. 10: One result of the preliminary experiment on displaying
gravity sensation on middle phalanx.

4.

PROTOTYPE MECHANISM

To transmit the grasping and gravity information with
tele-operation and telexistence, the following features arerequired.

Fig. 8: Construction drawing of the experimental device todisplay gravity sensation on middle phalanx
by generated shearing force on the belt.

- On the remote robot’s side, the power and direction offorce on the palmer side of the middle phalanx must besensed.
- On the operator’s side, the power and direction of force
on the palmer side of the middle phalanx must bedisplayed.
- The device should not hinder the grasping behavior of
the robot or operator.
We designed a prototype (Fig. 11) that fulfills theserequirements on both the remote robot’s side and theoperator’s side. The prototype (Fig. 12) has a pair of geared
EC motors (Maxon Motor Corp., EC6, 1.2W, gear ratio =15:1) with a rotary encoder to activate a belt. The belt
depresses and drags the palmer side of the middle phalanx ofa finger. The two motors work in a complementary manner toobtain a bi-directional force.

Fig. 9: Displaying gravity sensation of virtual object(like a bottle) on middle phalanges of the operator's index fingerand thumb using a pair of experimental devices in Fig. 8.

2) Operator’s side (display) (Fig. 13b)
The measured force information is transmitted to theoperator’s side. We drive the motors to roll up the wires andpresent the same amount of force as that on the remoterobot’s side. The prototype device can generate a force of up
to 6.5 [N]. When compared with “Cyber Grasp (ImmersionCorp.),” which can generate 12 [N] per finger, this force issufficient for a first prototype.

Fig. 11: Construction drawing of the prototype mechanism of
haptic interface for middle phalanx(a) Robot’s side(b) Operator’s side

Fig. 13: Sensing (a) and displaying (b) vertical stress

4.2 SHEARING STRESS(a) Remote robot’s side

When you hold an object such as a bottle, you feel theshearing stress between the bottle and your finger; thisfeeling is very significant and represents the important
information to not drop the bottle.
1) Remote robot’s side (sensing) (Fig. 14a)
Similar to the case of vertical stress, we apply constantcurrents to the motors. When an object touches and drags theventral side of the finger, the belt trails along the object. In
this case, the two motors rotate in the same direction.
2) Operator’s side (display) (Fig. 14b)(b) Operator’s side
Fig. 12: Wearing the first prototype

4.1 VERTICAL STRESSThe measured shearing stress on robot finger is transmitted
to the operator’s side. We drive the motors to roll up one wireand release the other. In this way, the drag sensation can also
be transmitted by these devices. The current prototype has afrequency range of only 0–25 [Hz]. We will improve it up toseveral hundred hertz, so that the transient "slip" sensation
will be presented by combining high-frequency stick-slip andstatic drag.

The most fundamental sensation of grasping is the verticalstress. The prototype transmits this type of force sensation in
the following way.
1) Remote robot’s side (sensing) (Fig. 13a)
An elastic layer is placed between the robot finger and belt.
By applying constant currents to both motors, the wires arepulled with constant tension. When an object touches theventral side of the finger, the elastic layer is squeezed and thebelt is loosened. The motors roll up the belt with constanttension. We acquire the force information by measuring thisrotation.

(a) Robot’s side(b) Operator’s side

Fig. 14: Sensing (a) and displaying (b) shearing stress5. EXPERIMENT ON HAPTICS TRANSMISSION
We tested the prototype in the haptic transmission fortele-operation (Fig. 15). We placed one device on the middle
phalanx of the index finger, and another one on the robot
finger (Tech Experts Inc., Universal Robot Hand). We placed
an elastic layer on the robot finger to measure the pressure, as
discussed in the previous section.
In this master-slave system, an operator’s finger posture
was measured by a bending sensor (Measurand Inc., ShapeSensor). The information is sent to the slave side to operate
the robot finger. When the robot finger touches an object, thehaptic device on the robot side measures the pressure,
whereas that on the operator’s side presents the sensation, as
described in sections 3.1 and 3.2.
In this experiment, the participants were able to clearlyrecognize the pressure and drag sensation when the slave
hand touched an object. The experiment has already revealed
two problems of the prototype. One is that the resolution ofpressure was not sufficient. The other is that we also felt atactile sensation on the dorsal side of the finger. The latterproblem is due to the fact that the device was not kept awayfrom the finger. In our next prototype, we propose that using
a fixation mechanism be used to solve this problem, while
keeping the system compact.

(a) Slave finger(c) Slave finger (touch)(b) Master finger(d) Master finger (display)

Fig. 15: Haptics transmission on middle phalanx
Posture of the operator’s finger (b) is transmitted to the slave robot
finger (a). When the slave finger (c) touches the bottle, the touchsensation is transmitted to the operator’s finger (d)using dual motors on this device.

6.

CONCLUSIONS

We suggested that the haptic information of the middle
phalanx could be as important as that of the distal phalanx. A
haptic interface for the middle phalanx that can sense anddisplay the force direction as well as the force was proposed.
The device is compact and does not hinder the free motion ofthe fingers. By focusing on the deformation of the middle
phalanx of the finger when we grasp an object, we proposed amethod to display the gravity sensation of the object.

The prototype device shown in section 5 has someproblems now. The generating power from the motor is not
so sufficient in exchange for its smallness. The force
direction in displaying shearing stress is not correctly
tangency because we have omitted the supporting frame
shown in Fig. 8. Our future work is to modify the mechanism
so as to sense and display with sufficient temporal /directional resolution.
This paper just showed the concept and first prototypemechanisms. We had examinations on this paper so as tolook into the feasibility of study on the haptic interface for
middle phalanx. These examinations are still too preliminary
and we have to study more on grasping and gravity sensationthat are provided in this method. In near future, we will
mount the next prototype mechanisms on the establishedmaster hand system so as to show availability of haptic
telexistence on the entire finger.

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