Haptic threshold for pulling force feedback onsurgeon’s fingertip in medical robotic systems
Hiroyuki Chinbe, Takeshi Yoneyama, and Tetsuyou Watanabe

Mitsutoshi Nakada

Faculty of Mechanical Engineering
Faculty of Medicine, Institute of Medical,
Institute of Science and Engineering
Pharmaceutical and Health Sciences,
Kanazawa University, Ishikawa, Japan
Kanazawa University, Ishikawa, Japan{zaqxsw@stu & yoneyama@se}.kanazawa-u.ac.jp; te-watanabe@ieee.org

Abstract—The human fingertip has very high density of thereceptor to accept sense of touch stimulation. The corresponding
somatic sensory area in a brain is very large, and considered to bea specialized part for palpation. A lot of haptic display system thenhave been developed with the investigation of human haptic
perception. However, the researches about the human perception
for pulling force at grasping, namely static frictional force arelimited. This paper investigated it, aiming at a future developmentof pulling and grasping force feedback system for neurosurgicalrobotic systems. For the purpose, this paper explored thepossibility of displaying pulling force to an index finger during
grasping. The absolute and difference thresholds for pulling sensewere the targets. The results showed that grasping disturbs thepulling sense, and the sides of index fingertip can be used to display
pulling sense, relatively large force, namely scaled force feedback
is required for the perception. The results provide an important
insight at a hardware and controller design of force feedback
systems.
Keywords— haptic threshold; pulling sense; finger-tip; surgicalrobot; force feedback;I. INTRODUCTION
Robotic surgical systems require only small holes on patientsfor operations, and provide invasive endoscopic operations andshort hospital stays [1-2]. Additionally, surgeons can also haveseveral merits including magnified visual feedback informationand motion modification (large motion is scaled to small motion
and vibration of motions is reduced). However, the forceps ofthe robotic system are controlled indirectly through the system,and surgeons cannot get (real) haptic or force information [3]. It
could reduce the performance of operations. To realize real safeand precise operations, haptic or force feedback systems are
crucial. Many researchers then have developed haptic and force
feedback systems for medical robots [4-14]. Our group also
developed haptic or force feedback systems [15-18] for an
endoscopic neurosurgery. In the fields of endoscopic
neurosurgery, the manipulator with forceps has to go throughnarrow routes and to work at small and narrow surgical fields.
High precise control is obviously required. Our developed
manipulator has 5 D.O.F (degree of freedom), and its diameteris 3 [mm], as shown in Fig. 1. Force sensors are embedded inforceps and grasping and pulling forces can be measured (see*Research partly supported by JSPS KAKENHI Grant Number
16H04298.

Fig. 2). The grasping force is feedbacked to an index finger as akinesthetic sense via motors while pulling force is feedbacked to
an index finger as a shear sensation by a friction roller. In thiscase, surgeons or operators feel the combined senses. However,the validity of this methodology was not investigated. The
preliminary experiment investigating whether grasping andpulling force can be felt in our robotic system showed that all
participants got the sense of grasping while some of participants
could not get the sense of pulling. Therefore, how to feedback
the pulling sense is a problem. The assessment of appropriate
methodologies and devices realizing them are needed. With thisin mind, this paper investigates how pulling sense should be
feedbacked or displayed during grasping.
According to the opinions of neurosurgeons (coauthors),
pulling force is significant to understand the softness of tissue.
Softness of tumors and that of normal tissues are different andthe detection of the softness is very important to detect theboundary between normal tissues and tumors. The humanfingertip has very high density of the receptor to accept sense of
touch stimulation. Its corresponding somatic sensory area in abrain is very large, and considered to be a specialized part for
palpation. A lot of haptic feedback or display systems have beendeveloped with the investigation of haptic mechanisms [19-23].
However, most of the targets were sliding senses, namely afeedback of kinetic frictional forces. To the best of the authors’knowledge, the results concerning with a feedback of static
frictional forces (pulling sense) are limited. From this viewpoint,
the investigation of pulling sense feedback methodologies is an
important issue at force feedback systems. This paper focuseson this point. Concretely, this paper tries to explore thefollowing things.
Possibility of displaying the pulling force to an index
finger during grasping, via the investigation of absoluteand difference thresholds

Are sides of index finger available for force displaying
area?



The necessity and amount of scaling of feedbacked
force to give a pulling sense at an operation device
which does not allow natural grasping styles.

The target force feedback system is for our developed
system shown in Fig.1. The investigation results (are very
important) could be useful to develop force feedback systems
including pulling sense display, although the useable situations
could be limited to the systems similar to our system. The
remaining of this paper is organized as follows. The subsequent
section describes target robotic system. The investigations ofabsolute and difference thresholds for pulling sense were thenpresented. After the results are discussed, summary is presented.
II. TARGET ROBOTIC SYSTEMS FOR NEUROSURGERY
The final goal is the development of grasping and pulling
forces feedback system for our developed robotic system forneurosurgery shown in Fig. 1. The system consists of theoperation device (master system) and the micro manipulator(slave system). The slave manipulator has 5 D. O. F. including
forward-backward motion, bending motion, two rotationalmotions at the root and tip of manipulator, and griping. The
master operation device was designed so that their motions cancorrespond to the motions of the slave manipulators. At theforceps of the slave manipulator, there are strain gauges formeasuring grasping and pulling forces (see Fig. 2). The
measured forces are feedbacked to operators. This paper deals
with this force feedback system, and find out appropriate
methodology for the feedback. Therefore, displaying the forcesat index finger are considered here. The controller was based onforce reflecting servo and virtual impedance controllers. See[15] for the detail of the controller.

III. INVESTIGATION OF ABSOLUTE THRESHOLD OF PULLINGFORCE DURING GRASPINGFirst, absolute threshold of pulling force was assessed whenpulling and grasping force are applied simultaneously.
A. Participants
Eight Japanese volunteers (seven men and one woman, mean
age 22 1 years (from 21 to 23 years)) participated in theexperiments. They were all right-handed, and none of them
reported any difficulties with their hands and haptics. The
experiments were approved by the Medical Ethics Committee ofKanazawa University.
B. Conditions
Fig. 3 shows the examined styles for displaying or
feedbacking pulling force. The measurement of grasping andpulling force is conducted at forceps, and they can correspond to
thumb and index fingers. Therefore, this paper focused on
displaying or feedbaking at thumb or index finger. As wellknown, index finger has the most number of haptic receptors
among fingers and its distribution density is also the largest [2425]. We then placed a weight on assessing the possibility ofdisplaying the grasping and pulling forces at index finger.
Displaying only for index fingers can reduce the size andcomplexity of the total force feedback system, and the way cancorrespond to the structure of our developed systems shown in
Figs. 1 and 2. The detail of the candidates of styles are as
follows;

Fig. 1. Robotic system for neurosurgery [15-18]
Fig. 3. Examined styles for displaying pulling force.

Fig. 3 (a): A thin plate was grasped by thumb and index fingers,then the grasped plate was pulled. The participants felt thepulling force at the pads of thumb and index fingers. The
magnitude of grasping force was determined by eachparticipant.
Fig. 3 (b): The pad and nail of index finger was pinched by ahandmade fixture, and the fixture was pulled. The magnitudeof pinching force was selected by each participant so that the
influence of pitching (such as discomfort) could be
minimized. The participants felt the pulling force at the padand nail of index finger. Grasping force (in addition to thepitching force) was applied to the fixture part corresponding
to the pad of index fingertip in a lateral direction.
Fig. 2. Force sensors attached on the forceps of the slave manipulator[15-18]

Fig. 3 (c): The sides of index finger was pinched by a handmade
fixture, and the fixture was pulled. The participants felt theTABLE I.
Condition Number
1
2
3
4
5
6
7

Notation
T&I
P&N(0N)
P&N(3N)
P&N(0N)+C
P&N(3N)+C
Side(0N)+C
Side(3N) +C

EXPERIMENTAL CONDITIONS

Displaying area for pulling force
The pads of thumb and index fingers
The pad and nail of index fingers
The pad and nail of index fingers
The pad and nail of index fingers
The pad and nail of index fingers
The sides of index fingers
The sides of index fingers

pulling force at the sides of index finger. Grasping force wasapplied to the pad of index fingertip in a lateral direction.
In the medical robotic systems, force feedback is passively
operated, and there is no active intention of operators, especiallywhen slave manipulator encounters unexpected tissues and there
is no active force. Therefore, passive force feedback isappropriate for investigation. Fig. 3 (b) and (c) correspond topassive force feedback. We set up additional several conditions
regarding to Fig. 3 (b) and (c). Experiments were carried out inthe cases that the grasping force was 0 and 3 N. 3 N corresponds
to the maximum displayable grasping force in our robotic
system show in Fig. 1. The effect of existing of cushion inside
the fixture was also investigated in order to see the effect ofsoftness at the fixing area for index fingertip. The cushion wasmade by composing silicone (Shin-Etsu Co. KE-1308) andhardener (Shin-Etsu Co. CAT1300-3).
Fig. 3 (a) means a natural grasping style where pulling force
is passively applied while grasping force is actively applied. The
participants actively applied grasping force with preferable
magnitude, and paid larger attention to the pulling force.
Therefore, the best sensitivity can be expected in the style.
However, the displaying style shown in Fig. 3 (a) can be took
only when a thickness of grasped object is close to that for theplate. When grasping is not performed, this style cannot beapplied. Therefore, the style shown in Fig. 3 (a) is difficult to be
applied in real force feedback systems. Nonetheless, we need toknow the sensitivity in natural style, and we then examined thisstyle as a reference.

Grasping force
Up to participants
0N
3N
0N
3N
0N
3N

Cushion inside fixture
None
None
CushionCushionCushionCushionof individual fingertip. Its range was 0 – 29 mm. Fig. 7 shows
the fabricated plate and fixture. The plate and fixture were pulled
by DC motor (Maxon A-max22) fixed on a vise through a wire.
The output torque of DC motor was controlled by motor
controller (Maxon 4-Q-DC Servo Control LSC 30/2). A
handmade force sensor shown in Fig.8 was attached at theintermediate point of the wire, in order to measure pulling force.
The sampling time for the force sensor was 5ms, and torquecontrol was used to control the DC motor. Note that the time
constant (61.2 s) is so small that the acceleration time can beignored, and the pulling force corresponding to the torque driven
at the DC motor was then directly applied. The dimension andstructure of the force sensor were shown in Fig.8. Force gauge(IMADA DS2-50N) was fixed at a positioning stage to control
grasping force when the style was Fig. 3 (b) and (c). The
grasping force (3N) at the style of Fig. 3 (b) and (c) was
controlled by the participant oneself. The grasping force wasapplied to the fixture part corresponding to the pad of index
finger at the style of Fig. 3 (b) while it was applied to the pad ofindex finger at the style of Fig. 3 (c). It should be noted that theindex finger was pre-compressed by the fixture, and the value ofgrasping force (0 or 3 N) then means the value without taking
the pre-compression into account. A switch was prepared so thatparticipant can inform the point where she/he detect the force or
the change of force.

Table I shows the summarized experimental conditions with
the notation of the condition.
C. Apparatus
Fig. 4 shows the schematic view of experimental set up when
the style was Fig. 3 (b). The other cases were similarly set up.
Each participant gripped a pole to fix the position of wrist so thatthe set up could correspond to our developed system shown in
Fig. 1. At the index finger, the handmade fixture was attached
so that the pad and nail of the index finger could feel pulling
force. The fixture was made from ABS plastic with 3D printer(Stratasys UPrintSE). The structure and dimensions of the plate(which was also made of the same 3D printer) for the style ofFig. 3 (a) are shown in Fig. 5, and those of the fixture for thestyles of Fig. 3 (b) and (c) are shown in Fig. 6. Both the plateand fixture have a hook to display pulling force by pulling them.
The dimension for the plate was determined so that adult humancan have a space for grasping. The dimension for the fixture wasdetermined to have an enough space to fit index fingertip ofadult human in it. The fixture has an adjuster to fit the thickness

Fig. 4. Schematic view of experimental set up.

Fig. 5. Plate for the stlye shown in Fig. 3 (a)Fig. 6. Fixture for the stlyes shown in Fig. 3 (b) and (c)to this reference. In order to evaluate the effects of “displayingarea”, the existing of “grasping force’’, and “cushion”, t-test was
conducted paired t-test for the pairs listed in Table II. No
significant difference was statistically obtained between[P&N(0/3N)+C] (the pulling force was applied to the pad andnail with cushion) and [side (0/3N)+C] (the pulling force wasapplied to the sides with cushion). If focusing on the effect ofgrasping force and comparing the cases when the grasping force
was 0 and 3 N, it can be seen that grasping reduced thesensitivity of absolute threshold (the absolute thresholdincreased). There was a statistical significant difference between[P&N(0N)+C] and [P&N(3N)+C] while no significant
difference between [Side(0N)+C] and [Side(3N)+C]. It
indicates displaying to the sides of finger can reduce theinfluence of grasping force. The insertion of cushion made thesmall increase of absolute threshold, but no significant
difference was detected.

Fig. 7. Fabricated plate (left) and fixtures (right).

Fig. 9. Obtained absolute threhold of pulling force

Fig. 8. Dimension and structure of force sensors for measureing pulling force.

TABLE II.

THRESHOLDS OF PULLING FORCE

Objective of t-test

D. Procedure
The method of limits was used to identify absolute thresholdof pulling force. During the experiment, the participants closed
their eyes to remove the effects of visual information. Initially,
there is no pulling force. We increased the magnitude of pulling
force by the DC motor so that the applied magnitude could be
ascending series with the speed of 0.1N/sec. The stimulus(pulling force) was continuously increased, but the starting time
for giving the stimulus was randomly selected so that theparticipants couldn’t predict the stimulus. Random starting time
to display the stimulus was announced to the participants inadvance. The participants pushed the switch when she/he felt
pulling force at first time, and we recorded the magnitude ofpulling force at the time. Note that the experiment was
conducted one time for practice. After the practice, theexperiment was conducted three times per person. We evaluatedmean value of obtained pulling force as the absolute threshold.
E. Results
Fig. 9 shows the results showing mean absolute threshold of
8 participants with standard deviation. Small threshold indicatesthat the participants could response sensitively. The smallest
absolute threshold was obtained when the plate was grasped bythumb and index fingers (T&I). As described the above, this is
the reference. It can be seen how sensitivity reduced comparing

RESULTS OF PAIRED T-TEST REGARDING ABSOLUTE

Grasping force
Grasping force
Grasping force
Displaying area
Displaying area
CushionCushionTargeted sample
1
P&N(0N)
P&N(0N)+C
Side(0N)+C
P&N(0N)+C
P&N(3N)+C
P&N(0N)
P&N(3N)

Targeted sample
2
P&N(3N)
P&N(3N)+C
Side(3N)+C
Side(0N)+C
Side(3N)+C
P&N(0N)+C
P&N(3N)+C

Result*P <0.05*P <0.05
P = 0.21
P = 0.17
P = 0.24
P = 0.073
P = 0.21

IV. INVESTIGATION OF DIFFERENCE THRESHOLD OF PULLINGFORCE DURING GRASPING

Next, difference threshold was identified. The participants,
the experimental conditions, and the apparatus were the same as
the previous experiments.
A. Procedure
The method of constant stimuli was used to get thedifference threshold. Similarly to the previous experiment, theparticipants closed their eyes during the experiment. Initially,
constant pulling force was applied for 5 seconds. Next, themagnitude of the pulling force randomly changed. The
participants informed whether they could feel the change ofpulling force. If they felt, they informed whether the pulling
force increased or decreased. The difference of change in thepulling force incremented and decremented with the step of 0.1n
N where n denotes a random integer. This procedure wasrepeated until detecting the difference of change where theaccuracy rate of the response got to more than 80 %. We let thedetected difference be the difference threshold of pulling force.
The experiments were conducted at the cases when the initialconstant pulling force was 1 and 2 N.
B. Results
Fig. 10 shows the results showing mean difference thresholdof 8 participants with standard deviation. The yellow bars show
the results when the initial constant pulling force () was 1
N while the brown bars show the results whenwas 2 N. As
it can be seen that, the obtained results are very similar to theresults for the previous experiment. The difference threshold for2 N is larger than that for
1 N at all 7 cases, anda statistical significant difference was obtained as shown in
Table III. The effect of grasping force was similar to theprevious experiment. When the grasping force was required, thedifference threshold increased. Different from the previous
result, there was a statistical significant difference between[Side(0N)+C] and [Side(3N)+C]. On the other hand, there wasno clear differences based on the differences of displaying areas
and existing of cushion.
V. DISCUSSION
Tow experimental results indicate that if displaying pulling
force with more than 1 N, operators can feel pulling even whengrasping is performing, and if increasing the pulling force withincreasing amount of more than 1.5 N, operators can detect thechange of pulling force. These information is very useful inconstruction of force feedback controller; how scaling of forces
should be done.
From the experiment for identifying a difference threshold,it can be seen that the increase of magnitude of initial constantstimulus decreased the difference threshold. The results cancorrespond to Weber–Fechner law [26] that a psychophysicalsensational intensity is proportional to not the strength of
stimulus but the logarithm of that. Note that further experiments
including the cases of
3, 4, ⋯ N are needed for moreaccurate understanding.
If focusing on the effect of grasping, grasping operation
reduced both the absolute and difference thresholds. The
participants had to feel two kinds of sense at the same index
finger at the same time. This might be considered to proceed akind of misunderstanding.
Next, the displaying area for pulling force is focused. If nograsping was operated, the absolute and difference thresholds
when the displaying area was the pad and nail of index finger
was smaller than that when the displaying area was the sides.
However, if grasping was required, their values were very close.
It indicates that the sides of index finger have low sensitivity for
absolute threshold but the effect of grasping is low. They thencould have a sensitivity nearly equal to that at the pad and nailof index fingers during grasping. Another reason is that the area
feeling pulling force was different from the area feeling grasping
force. The results give an important indication for the validity ofthe methodology that pulling force is displayed at the sides ofindex finger.

Fig. 10. Obtained diference threshold of pulling force

TABLE III.

RESULTS OF PAIRED T-TEST REGARDING DIFFERENCE
THRESHOLDS OF PULLING FORCE

Objective oft-testGrasping force
Grasping force
Grasping force
Grasping force
Grasping force
Grasping force
Displaying area
Displaying area
Displaying area
Displaying area
CushionCushionCushionCushion

1N
2N
1N
2N
1N
2N
1N
2N
1N
2N
1N
2N
1N
2N
-

Targetedsample 1
P&N(0N)
P&N(0N)
P&N(0N)+C
P&N(0N)+C
Side(0N)+C
Side(0N)+C
P&N(0N)+C
P&N(0N)+C
P&N(3N)+C
P&N(3N)+C
P&N(0N)
P&N(0N)
P&N(3N)
P&N(3N)All cases=1N)

Targetedsample 2
P&N(3N)
P&N(3N)
P&N(3N)+C
P&N(3N)+C
Side(3N)+C
Side(3N)+C
Side(0N)+C
Side(0N)+C
Side(3N)+C
Side(3N)+C
P&N(0N)+C
P&N(0N)+C
P&N(3N)+C
P&N(3N)+C
All cases=2N)

Result*P <0.01*P <0.01*P <0.01*P <0.05*P <0.01*P<0.05
P = 0.097
P = 0.10
P = 0.78
P = 0.96
P = 0.062
P = 0.62
P = 0.36
P = 0.82*P <0.01

If considering the effect of cushion, there was no clear
difference between the cases when a cushion was inserted andwhen it was not. It indicates that hardness around the fitting
between the fixture (or cushion) and fingertip does notcontribute to the sensitivities related with the absolute anddifference thresholds of pulling force.
As expected, the best sensitivity was obtained when the thinplate was grasped by thumb and index fingers. The pads offingers have the largest number of haptic receptors [25].
Additionally, the participants actively determined grasping force
with preferable (probably optimal) magnitude, and paid larger
attention to the pulling force. These might be the reasons. From
another viewpoint, the results for this case can be regarded as
optimal or lowest absolute and difference thresholds for pulling
force, when grasping was performed at the same time. The large
difference with the other displaying styles or experimental
conditions indicates that grasping and unordinary style reduce
the sensitivity. It can be understood that more than twice
magnitude was required to detect and differentiate pulling forces
with the style shown in Fig. 3 (b) and (c), comparing to the(natural) style shown in Fig. 3 (a).
If considering the development of a (pulling) force feedback
system for our developed neurosurgical robotic system,[Side+C] (see Table I) might be the best solution. Although thepad has a lot of haptic receptors, grasping dramatically reduced
the sensitivity as the experimental results showed. For easinessof design and fabrication, a display system for grasping force
and that for pulling force should be separated. If taking bothdesign and human perception into account, a grasping force
should be displayed at the fingertip pad while a pulling force
should be displayed at the sides of fingertip. If minding thefrequency response of human (about 10 Hz), the frequency
response of 200 Hz (corresponding to the experiment setting inthis paper) might be enough for the (pulling) force feedback
system.
VI. CONCLUSION
This paper investigated sensorial properties of human whenpulling force sensation was displayed, aiming at the future
development of pulling force feedback system for our
neurosurgical robotic systems. The primal target application was
our own system. The obtained results of human perception canbe valid and useful on developing other (pulling) force feedback
systems, although applicable cases were limited to the systems
similar to our system. Additionally the investigation was limited
to the cases when the contact area was rigid, and we ignored theeffect of softness of target objects. The investigation including
the effect of contact softness would be one of our future works.
The main results were that grasping reduced the sensitivity topulling force and more than twice magnitude was required todetect and differentiate pulling forces. The sides of fingertips
can be used for displaying pulling forces. Especially whendisplaying pulling force with grasping force, the sensitivity at
the sides was equal to the one at the pads where grasping andpulling forces were simultaneously displayed. We also
understood that large magnitude of pulling force should be
displayed when grasping and pulling forces were
simultaneously displayed at index finger. The scaling of
feedbacked force is required to get a clear sensation. The results
indicate that the method of displaying pulling force at sides ofindex fingertip might be a good solution. The implementation of
the results on the robotic system might be one of our future
works.
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