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Development of Haptic Device to Display Frictional Moment

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					                IEEE SMC International Conference on Distributed Human-Machine Systems 2008




        Development of Haptic Device to Display
                 Frictional Moment

          Haruhisa Kawasaki                               Yoshio Ohtuka                                  Tetsuya Mouri
       Department of Human and                       Department of Human and                        Department of Human and
         Information Systems                           Information Systems                            Information Systems
        Faculty of Engineering                        Faculty of Engineering                         Faculty of Engineering
            Gifu University                               Gifu University                                Gifu University
  1-1 Yanagido, Gifu 501-1193, Japan            1-1 Yanagido, Gifu 501-1193, Japan             1-1 Yanagido, Gifu 501-1193, Japan
         h_kawasa@gifu-u.ac.jp                                                                          mouri@gifu-u.ac.jp
   Abstract— This paper presents a haptic device newly                using a five-fingered haptic interface robot called HIRO II [8]
developed to display frictional moment, which is attached to the      and also presented a method for computing the frictional
five-fingered haptic interface robot called HIRO II. The device is    moment [9], which is based on soft-finger contact and
a new finger holder in which a small brushless motor and a disk       calculates both static and/or dynamic states of the frictional
are mounted. Subjects’ perception of the frictional moment
                                                                      moment. However, the effect of displaying frictional moment
while wearing the finger holder and operational feeling during
object manipulation in a virtual reality environment were
                                                                      in a virtual realty environment is not verified experimentally
evaluated experimentally using our developed computational            because there is not a suitable device to display frictional
method of frictional moment.                                          moment. In order to enhance the reality of object manipulation
                                                                      in a virtual reality environment, displays not only of frictional
                       I. INTRODUCTION                                force but also of frictional moment are required.

When an object is manipulated by the operator’s hand in                This paper presents a developed compact haptic device,
virtual space, it is desirable that the object’s spatial movement     which is attached to the HIRO II as a new finger holder to
be based on the laws of physics. Constraint force and friction,       display frictional moment. A small brushless motor and a disk
which are generated at the contact points between the                 are built into the device. Subjects’ perception of the frictional
operator’s fingertips and the object surface, are important           moment while wearing the finger holder and the operational
factors to enhance reality. In general, constraint force is           feeling during object manipulation in a virtual reality
calculated in proportion to the penetration depth of the              environment were evaluated experimentally to show the
fingertip touching the object, and frictional force on a              effectiveness of the combination of the developed device and
tangential plane at the contact point is calculated in proportion     the computational method of frictional moment.
to the constraint force. Physically, there are two friction states
at the contact point: namely, static (without sliding) and
                                                                         II. COMPUTATIONAL METHOD OF FRICTIONAL MOMENT
dynamic (with sliding) friction states. Some research on
friction models for haptic rendering has already been carried         Physical simulation in haptic environments requires that the
out [1]-[6]. For example, W. S. Harwin et al. [3] presented a         manipulated objects have a number of the same physical
method based on so-called god-objects [2] and friction cones,         properties as are present in the real world. For instance, an
which permits the computation of both static and dynamic              object’s dynamic motion must be based on external forces
forces. In this method, the frictional force is calculated in         such as grasping force, gravitational force, frictional force and
proportion to the penetration depth of the fingertip into the         frictional moment. We have presented a computational
virtual object, and the haptic interface point moves to the edge      method of frictional moment [9], which is proportional to the
of the friction cone if the haptic interface point lies outside the   relative torsion angle and the constraint force between the
friction cone. However, the frictional moment, which is a vital       fingertip and the object. An outline of the method is presented
aspect of modeling in multi-point haptic interactions, has not        briefly for understanding of the experimental system.
yet been considered. Both frictional force and frictional
moment models are needed to simulate the dynamics of an               The relative torsion angle is represented as a rotational angle
object. A method based on the contact volume of polygonal             around ni, which is the unit normal vector of the object surface
objects by Hasegawa et. al., [7] computes frictional force and        at the contact point Ci between the i-th fingertip and the object.
moment, but it needs more computational time than the single          Then, let us establish the fingertip coordinate system
contact approach. Our group has presented the effect of
                                                                      Σ i = { x i , y i , z i } at the contact point Ci of the i-th fingertip,
                                                                         B       B     B     B
displaying frictional force in a virtual reality environment
                                                                      where z i  B coincides with the normal direction of the object

                                                                      surface at the contact point, and the xi B yi B plane coincides




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                       IEEE SMC International Conference on Distributed Human-Machine Systems 2008




with a tangential plane at the contact point. At the same time,                  where γ i ( = k d θ i ) is the coefficient of frictional moment at the
the object coordinate system Σ i = { x i , y i , z i } at the contact            dynamic sate, ρ i is the damping coefficient, and k d (< k s ) is a
point Ci of the object is superposed on Σ i B . When the two                     parameter depending on the physical characteristics of the
coordinate systems move spatially, the rotation angle of the x i                 finger and the object in the dynamic state. These relations
axis about the x iB × x i axis is calculated as sin −1 (|| x iB − x i ||) .      show that the frictional moment is proportional to the
                                                                                 constraint force and a relative torsion angle between finger
The relative torsion angle between the fingertip and the object
                                                                                 and object at the contact point. This computation method has
at the i-th contact point is proportional to the direction cosine
                                                                                 no effect on the contact point and the haptic interface point,
of the x iB × x i axis to the ni axis as given by
                                                                                 which means that computations of frictional force and friction
                                            x iB × x i                           moment can be done separately. Object dynamics can be
   θ i = sin −1 (|| x iB − x i ||) niT                                   (1)     simulated by taking the frictional force and frictional moment
                                         || x iB × x i ||
                                                                                 into account.
The static frictional moment is computed by
                                                                                      III. FIVE-FINGERED HAPTIC INTERFACE ROBOT HIRO II
   m i = ζ i || f i n || ni ,                                            (2)
                                                                                 We developed a 5-fingered haptic interface consisting of the
where ς i is the coefficient of frictional moment, which is
                                                                                 arm and fingertips haptic display shown in Fig. 2, named
given by                                                                         HIRO II. Details of the haptic interface can be seen in [8]. Its
  ς i = k sθ i ,                                                         (3)
                                                                                 mechanism is here outlined briefly for understanding of the
                                                                                 experimental system.
where ks is a parameter depending on the physical
characteristics    of       the     fingertip. This   satisfies                  The HIRO II can present force and tactile feeling at the five
0 ≤ ς i ≤ k sθ i where θ i max is the maximum relative torsion                   fingertips of the human hand. It is designed to be completely
                max
angle at the static frictional moment. Now let us consider a                     safe and similar to the human upper limb both in shape and
frictional moment arc with vertex angle θ i max as shown in Fig.                 mobility. Its mechanism consists of a 6 DOF arm and a 15
                                                                                 DOF hand with 5 fingers. Each finger has 3 joints, allowing 3
1. When | θ i |≤ θ i , it is a static frictional moment state, and               DOF. The first joint, relatively to the hand base, allows
                    max
when | θ i | > θ i , it is a dynamic frictional moment state. At                 abduction/adduction. The second joint and the third joint
                     max
the dynamic frictional moment state, the xi*-axis obtained by                    allow flexion/extension. The thumb is almost the same as the
rotating x i around ni with the rotation angle −θ i max can be                   above mentioned fingers except for a reduction gear ratio and
                                                                                 the movable ranges of its joint 1 and joint 2. In order to read
linearly approximated by
                                                                                 the finger loading, a 6-axis force sensor in the second link of
                   θ i max                                                       each finger is installed. To manipulate, the haptic interface
  x i* = x i −             ( x i − x iB )
                     θi                                                  (4)     user has to wear a finger holder on his/her fingertips. The
                                                                                 finger holder has a ball attached to a permanent magnet at the
A new starting point of frictional moment is set by replacing                    force sensor tip and forms a passive spherical joint. We call
xi* with x iB such that x i lies on the edge of the frictional                   this a magnet type spherical joint. This passive spherical joint
moment arc. Then, the dynamic frictional moment is                               has two roles. The first is the adjustment of differences
computed by                                                                      between the human and haptic finger orientations. Each
                                                                                 human finger has 6 DOF and each haptic finger has 3 DOF.
  m i = (γ i || f i n || + ρ iθ i )ni ,                                  (5)     Hence, an additional 3 passive DOF are needed. The second is




                                i max

                                                i
                                                            i min
                                                                    xi
            x iB
                                         x i*
  Fig. 1 Movement of xiB
                                                                                     Fig. 2 Five-fingered haptic Interface robot: HIRO II




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                IEEE SMC International Conference on Distributed Human-Machine Systems 2008




to ensure that the operator can remove his fingers from the         human and haptic fingers orientations and the withdrawal of
haptic interface when it malfunctions. The suction force by the     the operator’s fingers from the haptic interface at need is
permanent magnet is 5 N. By virtue of this mechanism,               possible, which are the same functions of the magnet type
friction can be presented, but frictional moment cannot be          spherical joint.
presented.
                                                                    The suction force and frictional moment between the ball and
                                                                    the support hole are shown in Table 1. The suction force of the
The architecture of the HIRO II system is a distributed
                                                                    non-magnet type joint can be adjusted by changing the ratio of
multithreaded affair on separate PCs [10]. VR simulation and
                                                                    the ball insertion into the support hole, which is given as the
the haptic controller are connected by a high-speed network.
                                                                    ratio of the depth of the ball in the support hole to the diameter
HIRO II is controlled by a redundant force control method in
                                                                    of the ball. Frictional moments of magnet- and non-magnet
which all the joints of the mechanism are force-controlled
                                                                    type spherical joints, which arise from contact between the
simultaneously to present the virtual force. The sampling
cycle of the control is 1 ms.



  IV. FINGER HOLDER TO DISPLAY FRICTIONAL MOMENT

A. Design concept
It is not desirable to make the haptic device to display
frictional moment too complicated. We designed a device
according to the following concepts.
1) In order to display frictional moment at the operator’s            Fig. 3 Developed finger holder to display frictional moment
    fingertip by rotational motion with one degree of freedom, a
    small-sized motor and a disk are built in a finger holder.
2) Frictional moment that more than 80 % operators recognize
    at constraint force 2 N can be displayed [9].
3) In order to simplify the control method and mechanism to                                                      Human’s
   display frictional moment, not angle position control but                                                     finger
   open loop torque control is adopted. Its output motor torque
   is proportional to the frictional moment.
B. Developed device
                                                                                      Disk
We have developed a new finger holder to display frictional                                                  Rigid ball
                                                                                       Motor
moment with almost the same shape as the previous finger
holder [8], as shown in Fig. 3. A small-sized brushless motor
(Namiki Precision Jewry Co., SBL-06H1PG337) is built into             Fig. 4 Structure of developed finger holder
a ball of 8 mm diameter attached to the finger holder as shown
in Fig. 4. Specifications of the motor are given as follows:
outside diameter 2.4 mm, overall length 10.8 mm, mass 0.245
g, gear ratio 337, and maximum torque 0.69 mNm at a rated
voltage of 3 volts. A disk 7 mm in diameter and 1 mm thick is
attached to the end of the motor axis so as to put the operator’s
fingertip in contact with the disk. Frictional moment can be
presented at the fingertip by rotational motion of the motor.
When the ball of the developed finger holder is connected to
the passive spherical joint of HIRO II, the motor does not
work because of the strong magnetic force of the permanent            Fig. 5 Support hole of passive spherical joint without a
magnet in the spherical joint. Therefore, the development of a        magnet
passive spherical joint in which a permanent magnet is not            Table 1 Comparison with magnet and non-magnet type
used is required. Fig. 5 shows the newly designed support hole        spherical joints
of the passive spherical joint without a permanent magnet. We
                                                                         Joint Type          Ratio of ball   Suction force     Frictional
call this a non-magnet type spherical joint. The support hole is                              insertion                      moment at joint
made of MC nylon. The developed finger holder is desorbed                                       [mm]            [Nm]             [mNm]
from the spherical passive joint using the elastic behavior of          Magnet Type              0.31            5.4              2.21
the MC nylon. This structure of the spherical joint realizes
                                                                      Non-magnet type            0.60            3.5              0.45
functions such that the adjustment of differences between the
                                                                                                 0.65            7.5             -------




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                                  IEEE SMC International Conference on Distributed Human-Machine Systems 2008




ball and the support hole, are 2.21 mNm and 0.45 mNm,                             and unclear. Most subjects recognized frictional moment of
respectively. The frictional moment of the non-magnet type                        more than 0.45 mNm with 80% accuracy. This result is very
spherical joint is reduced to one fifth of that of the magnet type                similar to those when a subject puts his index finger directly
spherical joint. Since the frictional moment at the spherical                     on a disk driven by DC motor which is fixed on the load
joint has a negative effect on the display of force sensing, its                  measurement instrument [9], shown in Fig. 7 as the case of no
reduction contributes to improving the accuracy of the force                      finger holder. This means that the existence or nonexistence of
display. However, the movable angle of the non-magnet type                        a finger holder does not have an effect on the recognition of
spherical joint becomes a little bit small because the support                    frictional moment.
hole should cover more than half of the ball to generate
                                                                                  B. Experiment in virtual reality environment
suction force. This point is an issue to be solved in the future.
                                                                                  First, we examined frictional moment states when subjects
                                          V. EXPERIMENTS                          handled a virtual rectangular solid with size 0.06×0.012×0.03
                                                                                  m and mass 2 g using HIRO II as shown in Fig. 8. The subjects
A. Recognition of rotation direction of frictional moment                         wore the newly developed finger holder or the previous finger
Subjects’ perception of frictional moment was examined by                         holder. The small balls in the computer graphics are fingertips.
using a simple experimental system as shown in Fig. 6. The                        In the virtual reality simulation, not only the frictional moment
subject puts his index finger wearing the newly developed                         but also the frictional force were computed. The following
finger holder on a disk with specified pushing force 2 N. This                    numerical values were set to present natural motion of the
force could be observed by the subject through the load                           grasped object: ζ i = 0.3, γ i =0.2, θ i max = 0.02 rad, and ks =1.0
measurement instrument. The frictional moment was                                 and 0.8 at static and dynamic states respectively. The subjects
controlled by a motor current input. The number of subjects                       executed the following actions sequentially as shown in Fig. 9:
(ages 21 to 24) was 10. The directional recognition rate of                       (1) grasp the object firmly at contact points by the thumb and
frictional moment at 2N pushing force (constraint force) is                       the index finger; (2) lift the object and rest with the static sate
shown in Fig. 7. For measurement of the directional                               where the orientation of object is not varied; (3) slide the
recognition rate, the frictional moments are displayed to the
subject’s fingertip as step inputs at random levels between 0.1
and 0.9 mNm, 13 times for each subject. The display time of
the force moment is 3 seconds. Before the experiment,
subjects trained four times to familiarize themselves with the
experimental apparatus. After displaying the frictional
moment, the subject answered whether he felt a frictional
moment and gave its rotational direction: that is, right, left,




                                                                                      Fig. 8 Object manipulation in virtual reality environment




 Fig. 6 Measurement system of human’s perception of
 frictional moment

                          100
                                                                                                 (1) Grasp               (2) Lift and rest
   Recognition rate [%]




                           80
                           60
                           40
                           20                              finger folder
                            0                              no finger folder
                            0.0     0.2       0.4    0.6       0.8        1.0
                                     Frictional moment [mNm]
 Fig. 7 Recognition rate of rotation direction
                                                                                             (3) Slide and rest              (4) Release
                                                                                      Fig. 9 Scene of manipulation of a rectangular solid




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                                                               IEEE SMC International Conference on Distributed Human-Machine Systems 2008




                                                    1.0                                                                                                                                   1.0




                                                                                                                                                                          Status
                                   Status



                                                    0.5                                                                                                                                   0.5
                                                    0.0                                                                                                                                   0.0
                                                          45      46   47           48             49       50       51   52                                                                     45   47               49              51       53   55
                                                                                    Time [s]                                                                                                                            Time [s]

                                                    0.5                                                                                                                                    0.5
                                Normal force [N]




                                                                                                                                                                      Normal force [N]
                                                    0.4                                                                                                                                    0.4
                                                    0.3                                                                                                                                    0.3
                                                                            y                                                                                                                                  y
                                                    0.2                                                                                                                                    0.2
                                                    0.1
                                                                        z                                                                                                                  0.1             z
                                                    0.0                                                                                                                                    0.0
                                                                                x                                                                                                                                  x
                                                    -0.1 45       46   47           48             49       50       51   52                                                              -0.1 45     47               49              51       53   55
                                                                                         Time [s]                                                                                                                           Time [s]
     Frictional moment [Nm]




                                                                                                                                     Frictional moment [Nm]
                                          0.0015                                                                                                                                  0.0015
                                          0.0010                                                                                                                                  0.0010
                                                                                           y                                                                                                                                  y
                                          0.0005                                                                                                                                  0.0005
                                                                                    z                                                                                                                                   z
                                          0.0000                                                                                                                                  0.0000
                                                                                x                                                                                                                                  x
                                                          45      46   47           48             49       50       51   52
                                     -0.0005                                                                                                                                -0.0005 45                47               49              51       53   55
                                                                                    Time [s]                                                                                                                            Time [s]

                                                                                                                                             Frictional angle [rad]                 0.000
     Frictional angle [rad]




                                                  0.000
                                          -0.005 45               46   47           48             49       50       51   52                                                  -0.005 45               47               49              51       53   55
                                          -0.010                                                                                                                              -0.010
                                          -0.015                                                                                                                              -0.015
                                          -0.020                                                                                                                              -0.020
                                          -0.025                                                                                                                              -0.025
                                                                                    Time [s]                                                                                                                            Time [s]

                                                    1.5                                                                                                                                    1.5
                                                                                                        ψ
                              Euler angle [rad]




                                                                                                                                                                      Euler angle [rad]




                                                    1.0                                                                                                                                    1.0                                              ψ
                                                    0.5                                                          θ                                                                         0.5                                                   θ
                                                    0.0                                                                                                                                    0.0
                                                   -0.5 45        46   47           48             49       50       51   52                                                              -0.5 45     47               49         φ    51       53   55
                                                   -1.0                                        φ                                                                                          -1.0
                                                   -1.5                                                                                                                                   -1.5
                                                                                        Time [s]                                                                                                                            Time [s]


 Fig. 10 Experimental results of object manipulation in virtual reality environment

object around the axis connecting the contact points of the                                                                      grades, with 1 being the worst and 7 the best. The number of
thumb and the index finger to create a dynamic frictional                                                                        subjects (ages 21 to 24) was 10. Fig. 11 shows the results.
moment state and rest again; and (4) release the object. Fig.                                                                    Subjects expressed the feeling that wearing the new finger
10 shows the status that indicates the frictional moment state                                                                   holder is not comfortable by comparison to the case of the old
of the index finger, in which the static frictional moment state                                                                 finger holder. This feeling of discomfort is caused by the
is indicated by level 1 and the dynamic state by level 0.5, as                                                                   electric circuit of the brushless motor, which is mounted on the
well as the normal force, the frictional moment, the relative                                                                    finger holder case as shown in Fig. 8. The feeling of the
torsion angle at the contact point of the index finger, and the                                                                  frictional moment and the coincidence between the object
ZYZ-Euler angles that indicate orientation of the object. Fig.                                                                   motion in computer graphics and the display of the frictional
10 (1) is the case of the old finger holder, that is, that without a                                                             moment when wearing the new finger holder were very good
built-in motor, and (2) is the case of the new finger holder. In                                                                 in comparison to the old finger holder. Operationality and the
both cases, a non-magnet type spherical joint is used.                                                                           immersive feeling when wearing the new finger holder were
Transition between static and dynamic frictional moment                                                                          better than those of the old finger holder. According to the
occurs according to the physical phenomena, and the                                                                              significant difference test of performances by t-test, the
frictional moment at the static state is nearly equal to a value                                                                 improvement in operationality had a significance level of
calculated by the geometrical moment of inertia around the                                                                       10 % and that in the immersive feeling had a significance level
contact point. This means that the new finger holder does not                                                                    of 1%. This means that the combination of the newly
disturb the object manipulation in the virtual reality                                                                           developed finger holder and the computational method of the
environment and gives a physical simulation of reality.                                                                          frictional moment greatly enhances the sense of reality in the
                                                                                                                                 virtual environment.
Second, we evaluated subjects’ psychological experience
when wearing each finger holder, using a rating scale with 7




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                   IEEE SMC International Conference on Distributed Human-Machine Systems 2008
                                                          Old finger holder
                                                          New finger holder

  (a)Comfort at wearing
     the device
                                                                                [9]  H. Kawasaki, Y. Ohtuka, M. O. Alhalabi, T. Mouri, Haptic Rendering
  (b) Feeling of moment
                                                                                     and Perception of Frictional Moment, Proc. of EuroHaptics 2006,
                                                                                     pp.201-206
  (c) Consistency of CG and                                                     [10] O. Halabi, V. Daniulatis, H. Kawasaki, T. Mouri and Y. Ohtuka, Future
  display of frictional moment                                                       Haptic Science Encyclopedia: Realistic Stable Haptic Interaction with
                                                                                     Highly Deformable Objects Using HIRO-II, Journal of Robotics and
  (d) Operationality                                                                 Mechatronics, Vol. 18, No.4, pp. 409-417, 2006
                                                                                [11] H. Kawasaki and T. Mouri, Design and Control of Five-Fingered haptic
  (e) Immersive feeling                                                              Interface Opposite to Human Hand, IEEE Transaction On Robotics,
                                                                                     Vol. 23, No.5, pp. 909-918, 2007


                                  1      2     3      4      5     6      7
  Fig. 11 Result of questionnaire



                         VI. CONCLUSION

We have developed a new finger holder to display frictional
moment as a part of a five-fingered haptic interface robot
HIRO II, in which a small brushless motor and a disk are built.
Experiments of subjects’ perception of frictional moment
using the new finger holder were presented. The combination
of the new finger holder and the computation method of
frictional moment, in which the friction moment is computed
in proportion to the constraint force and the relative torsion
angle between fingertip and the virtual object at the contact
point, have enhanced the reality of object manipulation in the
virtual reality environment.

                          ACKNOWLEDGMENT

The authors express gratitude for partial funding and support
from the Ministry of Education, Culture, Sports, Science and
Technology, and the Ministry of Internal Affairs and
Communications in Japan.

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