A Pseudo-Force-Feedback Device by Fingertip Tighteningfor Multi-Finger Object Manipulation
Go Inaba1

Kinya Fujita2

To k y o U n i v e r s i t y o f
A g r i c u l t u r e a n d Te c h n o l o g y

To k y o U n i v e r s i t y o f
A g r i c u l t u r e a n d Te c h n o l o g yABSTRACTThis paper proposes a multi-finger pseudo-force-feedback
device that induces tactile sense to each fingertip by itstightening. The compact and lightweight device design wasrealized by limiting the display sense to tactile only, in contrast
to the conventional force-feedback devices stimulate both tactile
and proprioceptive receptors by representing the reaction force.
A prototype device that tightens the fingertip using geared motor
was developed. The device was controlled with open-loop
control architecture with high-voltage initial acceleration pulse
in order to reduce the delay. The contact prediction algorithm
was also implemented to reduce the influence of the delay. The
experimental evaluation demonstrated that the influence of thedelay is negligible in natural grasping, and the recognition ispossible in wide range of object size.
Keywords: force feedback device, tactile, pseudo-force display,fingertip tightening
1INTRODUCTION

Handling of an object is a task that heavily depends on thehaptic information related to the status between the fingers andthe object. Obviously, manipulation with fingers becomesdifficult if the reaction force from the object can not beperceived. Numbers of grounded force-feedback devices [1-3]have been developed, and some of them are commerciallyavailable. The manipulator-based grounded force display
devices generally have higher rigidity and some of them havefunction to represent 6 degree-of freedom force.
However, the multi-device coordination of this type of haptic
device is difficult because of the interference between the
manipulators. Therefore, that is not suitable for multi-finger
object operation. There is another type of force-feedback device
using tensile force of strings [4], but it has the similarinterference problem of the strings.
Recently, another kind of force-feedback devices such as adevice using gyroscopic effect, have been proposed [5,6]. They
have an advantage to display the world coordinate force without
mechanical grounding. However, the display of continuous force
is essentially difficult, and it is still too large to wear on eachfingertip.
Exoskeleton-type ungrounded force-feedback devices for
multi-finger direct manipulation have also been developed [710]. The fingertip reaction force is generated by pulling the wireusing a motor or pushing the attachment using a pneumaticactuator. These devices allow users to feel the reaction force
from the grasping virtual object through their fingers. However,the devices require motors or compressors as the power source
and wires or tubes for power transmission. The device
application time is also a problem with this kind of complicatedwear-on device.
On the other hand, there are some vibro-tactile devices that12email: go(at)reality.ei.tuat.ac.jp
email: kfujita(at)cc.tuat.ac.jp

are attached on fingertips or palm [11][12]. These devices allow
users recognize contact or non-contact status by providing avibration. However, the sensation represented by the device is asubstitution and obviously different from the sense induced by
the contact with an actual object. In summary, it is still achallenge to display accurate reaction forces to each finger witha lightweight device in any kind of force-feedback device.
This study noticed the haptic sense consists of tactile andproprioceptive senses. If the tactile sense is properly displayed,
it is expected it may induce the sufficiently similar sense as theactual contact, even if the proprioceptive sense is not displayed.
Based on this hypothesis, a compact multi-finger pseudo-forcefeedback device by fingertip tightening was developed.
2CONCEPT

The mechanical contact to an object induces two kinds ofsensations as shown in figure 1. The first is the proprioceptive
sense that is perceived by muscle spindles or tendon organs. The
proprioceptive sense provides the internal information that isrelated to muscle force or joint position. Another one is thetactile sensation perceived by the cutaneous mechanoreceptors.
The tactile sense is external information because it is induced by
the contact with outer object. Basically, the force-feedback
device induces both two sensations by generating actual force.
In order to provide the proper proprioceptive sensation, thehaptic device is required to counterbalance the muscle
contractile force. Therefore, the higher-powered actuator isneeded, and it prevents the size reduction of the device.
The reaction force, while a user touches an object as shown infigure1, is perceived by the tactile mechanoreceptors as well as
the tendon organs. The tactile mechanoreceptor provides the one
important part of the force sensation. It is expected the tactile
sense, which is provided at proper timing to the user s motion,practically represents the reaction force, even if theproprioceptive sense is not provided. Therefore, this study
discusses the possibility of pseudo-force-display by stimulating
the tactile receptor only using a prototype device. The prototype
pseudo-force-display device was designed as a fingertip
tightening mechanism for multi-finger object handling.

Figure1: The concept of pseudo-force-display by fingertip tightening

33.1

SYSTEM DESIGHNStructure of the Device

The structure of the device is shown in figure 2. The fingertip
is tightened by rolling up the contact belt using a motor. Ageared motor was utilized to reduce the size and to obtain theenough torque. The simple device design allowed the reductionof the size and the weight.
The driving motor starts to rotate the shaft when the virtual
fingertip contacts with a virtual object. The shaft rolls up thecontact belt, and the actual fingertip is tightened. The contact ofthe belt represents the contact of the fingertip with a virtualobject. The shaft is driven in opposite direction when the virtual
fingertip separates from the virtual object, to releases thefingertip from the contact belt.
The prototype device consists of 10mm diameter aluminumroll shaft, cotton-made 0.5mm thick contact belt, 0.5mm thickpolyvinyl chloride contact plate, 1/75 reduction ratio geared
motor (S.T.L. HS-GM21-ALG), and Velcro strap for the device
fixation. The device weight is 18g per finger. The contact andnon-contact statuses are shown in figure 3.

Figure 2: Structure of the device

Figure 4: Block diagram of the controller

To obtain stable contact force, the repeatability of the shaft
stop position after rewind motion is required. Hence, the rewind
driving time was adjusted in proportional to the output force.
3.3

System Configuration

The simplest structure prototype system that has two pseudoforce-feedback devices for the thumb and the index finger wasimplemented to evaluate the feasibility of the proposed method
as shown in figure 5. The distance between two fingertips wasdetected by the rotary potentiometer connected between the twofinger-devices. The system predicts the contact with the virtual
object based on the measured fingertip distance and outputs therequired voltage at proper time. The contact prediction
algorithm is described chapter 5. In this study, the performance
to represent the contact and non-contact statuses were evaluated.

Figure 3: Non-contact and contact statuses
Figure 5: Prototype of pseudo-force-feedback display

3.2

Device Control

In general, a force sensor is required to control the displayed
reaction force accurately. However, the use of force sensor
increases the device weight and adds electric wires. Thus, theadvantage of the pseudo-force-feedback device is canceled.
Another force control method is an electric current control.
The force can be indirectly controlled by controlling the motor
current that is proportional to the motor torque. However, thefriction of the developed device is high and the back-drivability
is low because of the reduction gear. The contact force output
after constant force output is discontinuous. Therefore, linear
contact force control by the feedback control of the electric
current was not applied.
In this study, the open-loop control of the applied voltage wasutilized as shown in figure 4. The driving voltage was calculated
from the desired force using the device property that wasmeasured in advance. The linearity between the driving voltage
and the contact force showed good linearity as seen in figure 7.
Furthermore, in order to decrease the time delay of motor
output, at the switching from non-contact to contact state, higher
voltage pulse was applied to accelerate the shaft rotation. Thepulse driving voltage was 8V that was chosen as the maximum
rating voltage of the motor. The driving pulse width was decided
experimentally as 20ms that provides constant time delay
independent from desired force.

Figure 6: Entire system configuration

44.1

DEVICE SPECIFICATIONSLinearity and Repeatability

The linearity and the repeatability were evaluated in 9 righthanded male subjects from 20 s to 30 s. A set of 5 trials pervoltage was performed in each subject. The contact force
between the fingertip and the contact plate was measured using
a sheet-shaped force sensor, TEKSCAN Flexi-force. The results
are shown in figure 7. As the average force variation was 17%,
the practically sufficient linearity and the trial-by-trial
repeatability was obtained.

0.3

200Time delay (ms)Force (kgf)

0.25
0.2
0.15
0.1
0.05
0123Driving voltage (V)4The variation of the shaft position after the rewinding forfinger release was also evaluated. The rewind time adjustment
reduced the shaft position variation to 10 degrees. The variationof the fingertip contact force by rewinding was negligible.
Precision

The developed system uses the average model for desired
force to driving voltage transform. Thus, there is a certain levelof error by individual variation. The maximum error in 9
subjects was 54% at the representation of the minimum force,0.5kgf. The personal calibration using force sensor willcompensate the individual variation and allows accurate force
control. The range of the pseudo-force that the developed device
can provide was 0.5 to 0.23kgf.
Delay

The most important property of the haptic device is theresponse time. The time delay from the command output to thecontact force rising was evaluated in the state transition from
non-contact to contact state. The experiments were performed in
9 male subjects from 20s to 30s. One subject performed 5 times
per one driving voltage.
The time delays without the high-voltage acceleration pulse
varied from 100 to 600ms as seen in figure 8. The delays withthe acceleration pulse were from 80 to 90ms, which wassignificantly decreased. The fluctuation with the driving voltage
was also decreased in the latter condition compared to theformer condition. The subject-by-subject variation was also
decreased as confirmed in figure 9.

Time delay (ms)700with acceleration
pulse
high
voltage drive
without acceleration
pulse
non-high
voltage drive

600
500400300200
10000500Figure 7: Relation between finger tightening force and driving voltage

4.3

100004.2

150123Driving voltage (V)Figure 8: Relation between time delay and driving voltage

4123

4 5 6Subject

789Figure 9: Time delay variation across subjects

5CONTACT PREDICTION

It is known that several 10s milliseconds delay of forcefeedback seriously influences the perception. There is concernthat the device delay brings the user the inconsistency between
the visual and the haptic sensation, because the device delay was
80 to 90 ms. In the developed prototype system, a contact
prediction algorithm was implemented. The system predicts thecontact time based on the users grasping motion and starts todrive the motor 80 ms before the predicted contact time.
5.1

Quadratic Function Approximation

The velocity of the fingertips change dynamically while
human grasp an object. Therefore, the dynamic changing patternof the fingertip position was approximated using a quadratic
function. The fingertip position was acquired at 500 hz samplingrate. The quadratic function approximation was performed using
the past N data. The position 80ms after the measurement waspredicted by using the approximated function, and the contact
determination was performed based on the predicted position.
The number of the data for the approximation was dynamicallychanged as follows.
5.2

Dynamic Change of Approximation Data Number

The contact prediction using fixed number of data for theapproximation showed less prediction performance. In the caseof fast finger motion, the contact prediction delay increased if
the data number was not adequately reduced. In contrast, when
the fingertip s motion was slow, the prediction error increased
due to the velocity fluctuation and noise. Hence, the data
number for the approximation was dynamically adjusted. Whenthe fingertip velocity was fast, the number was decreased, while
the data number was increased for slow motion.
The simplest data number control is to use the finger velocity.
However, the velocity-based predication was insufficient in thefaster motion due to the finger acceleration. Similar method
using acceleration is seriously influenced by the noise.
Thus, an indirect method was applied. The approximation
data number was adjusted depending on the time T from thegrasping motion onset to the time that the finger velocity
reaches a specified velocity (80mm/s). The data number wasincreased with T. The data number N was adjusted to make theapproximation data length coincides with the half of T. The
maximum and minimum data numbers were set as 100 and 15.

6.1

EVALUATION EXPERIMENTS
Influence of DelayFirstly, a set of experiments were performed to evaluate theinfluence of the force-feedback delay. In the experiment, acomputer-generated graphic image was displayed that issynchronized with the pseudo-force-feedback, as shown infigure 10. The red, darker, colored ball represents the user s
fingertip position, while the yellow, lighter, colored ball moves
cyclically at constant velocity. The subjects were required tograsp the virtual object repeatedly to track the yellow ball using
red ball. After the tracking task, the subjects were asked whether
the haptic and the visual contact sensations were coincided ornot. The target velocity was changed from 100 to 500mm/s at 50mm/s step. The subjects were 8 male adults.

could be caused by the deceleration of the finger motion due tothe human behavior to avoid the contact of fingers. Even thefurther contact prediction algorithm improvement is desired, theprototype
multi-finger
pseudo-force-feedback
devicedemonstrated the performance to represent the wide range ofsize adequately.

Recognized size (mm)6100without
prediction
non-prediction
withprediction
prediction

80
60
40
20
00
Figure 12:

7Figure 10: Computer-generated image used in the delay influence
evaluation experiment

Maximum velocity (mm/s)

The maximum visual and haptic synchronized velocities areshown in figure 11. The sensation synchronized velocity without
prediction was 250mm/s. The synchronized velocity withprediction was improved to over 400mm/s. The average
fingertip velocity is 300 to 400mm/s in human regular grasping
motion. It is confirmed that the practically sufficient response
was attained.
600

Displayed and recognized virtual object size

CONCLUSIONSIn this study, a multi-finger pseudo-force-feedback device by
the fingertip tightening was prototyped. The pseudo-forcefeedback concept attained compact and lightweight design of thedevice. The high-voltage initial acceleration pulse decreased thedevice delay. Furthermore, the contact prediction algorithm
reduced the influence of the delay. The pseudo-force-feedback is
expected to releases users from heavy mechanical devices and toallow easy-to-do multi-finger manipulation.
The finger tracking system that does not obstruct theadvantage of the device and the continuous control of thedisplayed force are remained.

500

REFERENCES

400
300
200
1000
withoutwith
1
prediction prediction

Figure 11: Maximum visual-haptic synchronized velocity

6.2

20406080
Size of virtual object (mm)Recognition of Virtual Object Size

A set of object size recognition experiments were performed
to examine the size representing performance of the system. In
the experiment, a virtual object that has random size wasdisplayed at first, without visual information. After that, thesubject was required to choose an actual object that has the samesize as the virtual object. The size of the virtual objects were 10,25, 40, 55 and 70mm, while the size of the actual objects were
2mm and 5 to 80mm at 5mm step. Each 7 subjects performed 5tasks per condition.
The displayed and recognized size of the virtual objects areillustrated in figure12. It is confirmed that the accuracy of thesize recognition was improved by the contact prediction.
However, in the case the virtual object was 70mm, therecognized size was smaller than the displayed size. This error
appears the prediction delay due to the insufficient data numberfor the prediction.
In the case the displayed object was 10mm, the contact
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