Study on Adhesion Force Reduction and State Estimation by
Piezo-transducer
Tetsuyou Watanabe , Makoto Iwasaki, Hidekazu Matsumura, and ZhongWei Jiang

Abstract— Our previous paper presented a method for
reducing adhesion forces by oscillation and showed the adhesion
state can be checked by analyzing the data obtained by laser
displacement meter. However, there are several problems in thismethod. 1)The end-effector must be located at the specific point
where laser displacement meter can measure oscillation. 2)The
adhesion state can not be checked if something blocks thelight/laser or the target leaves the measuring point. 3)The total
system becomes very large. To resolve these problems, thispaper firstly presents a method for checking the adhesion stateby piezo-transducer. Next, to achieve more precise
manipulation, we propose a method to deform the end-effector
by adding DC input to the piezo actuator which is also oscillated
simultaneously to reduce adhesion forces. Furthermore, we findthat the first mode resonance frequency shifts with the increaseof the pushing force applied to the object by the end-effector.
Using the shift amount, we develop a method for checking theadhesion state.

I

I. INTRODUCTION

N a micro range, it is a key issue how to cope withadhesion which arises between an object and tools (seeFig.1). To resolve the problem, many methods have beendeveloped, including the adhesion-type micro end-effector[1], the vacuum gripping tool [2], releasing by slopping andoscillating an end-effector [3]-[5], releasing by control ofelectrostatic force [6], and releasing strategy based on
environment information [7]. However, there are stillunresolved problems such as the difficulty of object control
after the release [1]-[6], the difficulty of practical executionof the method [7] (since it is almost impossible to acquire all
required information).
Recently, we developed a new strategy [8] (see also [9]) tocope with the adhesion force and control the object motion.
By minutely oscillating the end-effector and contacting itwith (bringing it near to) the object on the substrate (table),the adhesion force between the end-effector and object will
become small, comparing with the force between thesubstrate (table) and the object (see Fig.2). Then, it is easy toremove the end-effector from the object while the object
adheres to the substrate. Hereafter, we call this(phenomenon) relaxation of adhesion force. If the object isremoved from the end-effector at any time, we say theT. Watanabe is with Graduate School of Natural Science & Technology,
Kanazawa
University,
Kanazawa,
920-1192,Japan(e-mail:te-watanabe@ieee.org).
M. Iwasaki, H. Matsumura, and Z.W. Jiang are with Graduate School of
Science and Engineering, Yamaguchi University, Ube, 755-8611, Japan.

end-effector

micro object
substrate

Fig. 1 Micro object adhered to end-effector

end-effector

micro object
substrate

Fig. 2 Relaxation of adhesion force

adhesion force is relaxed enough. If it is not and the object
sometimes adheres to the end-effector, we say the adhesion
force is not relaxed enough. Using this strategy, we canaccurately manipulate micro object like macro manipulation.
We also show whether this method is available or not can bechecked by laser displacement meter. However, the use of
the laser displacement meter causes several problems. 1) The
end-effector must be located at the specific point where laser
displacement meter can measure oscillation. 2) The adhesion
state can not be checked if something blocks the light/laser or
the target leaves measuring point. 3) The total system
becomes very large. Furthermore, if required fine motion
such as micro assembly, both adhesion force relaxation andfine tuning of grasping force are required simultaneously. In
our previous paper, we move the end-effector by controlling
XYZ stage. Then, one of the methods to get a finer
end-effector motion is to make the resolution of the XYZstage higher. However, practically, it is difficult to get finer
motion by this way since we can not ignore errors resulted inplay of the clamped end of the end-effector.
Concerning the above, this paper, firstly, presents a method
to check the adhesion state by piezo sensor. By applying
frequency analysis to the voltage of the piezo sensor inducedby oscillation of the end-effector, we check the adhesion state.
Next, we propose the method not only to relax the adhesion
force but also to finely tune the pushing force applied to theobject. By adding DC voltage input to the oscillated piezo
actuator, we will deform the end-effector and move its tip
point with fine step, relaxing the adhesion force
simultaneously. In addition, we find the resonance frequency
shifts with the change of the pushing force applied to theobject by the end-effector. Using the shift, we develop amethod to check adhesion state more accurately. Combining
the above methods, we can finely tune the grasping force,

PC

Oscilloscope
Laser displacement meter

Capture board

Function generator
Amplifier
PZT
End-effector

CCD
Microscope

XYZ stage
Substrate

Fig. 4 End-effector and the coordinate. Fig. 5 View of the experiment set up fromthe microscope sidePC

ObjectFig. 3 Overview of the experimental setup

keeping adhesion force relaxed enough.
II. TARGET SYSTEM
Fig. 3 shows the target system, which consists ofmanipulation
part,
image-capturing
part,
end-effector-oscillating part, and displacement-measuring
part. For the simplicity, we assume that: (1) the manipulation
is done in a planner space and a gravity force doesn't work,(2) the object is a sphere, (3) the end-effector and thesubstrate are made of a same material, (4) the end-effector
and the substrate are grounded for preventing an extra charge
at the initial state.
The manipulation part consists of end-effector, micro
object, and substrate. Fig. 4 shows the overview of theend-effector and the coordinate used in this paper. The
end-effector is a cantilever beams made of copper in size of
3x40x0.3[mm]. The beams are rolled copper, and any surface
treatments such as grinding are not conducted. Young's
modulus of copper is 1.02x1011 [N/m2], Its Poisson's ratio is0.35, and its density is 8900 [kg/m3]. On the end-effector, thepiezocell (Fuji ceramics, Z0.2T50x50x50S-W C6) of
3x3x0.2 [mm] is bonded at the position of 1 [mm] from theclamped end for oscillating the end-effector. The surface of
substrate is a copper cut bonded on an alminum board. The
end-effector is attached on XYZ stage (Surugaseiki,
PMZG413) which can be controlled by PC. The object is a
glass sphere (Union, unibeads) with a radius of 200 [10-6m].
Young's modulus of glass is 7.05x1010 [N/m2]. Its Poisson's
ratio is 0.17, and its density is 2500 [kg/m3].
The image-capturing part consists of microscope (Moritex,
ML-Z07545), CCD camera (IAI, CV-S3200), Capture box(Imperx, VCE-PRO), and PC. The overview of the operation
is captured by the CCD camera through the microscope andthe image is sent to PC.
At the end-effector-oscillating part, the end-effector isoscillated by oscillating the piezocell (bonded on theend-effector) by the function generator (Yokogawa, FG120)
through the power amplifier (NF, 4010).
At the displacement-measuring part, the tip motion of theend-effector is measured by laser displacement meter (Sony

VL10). The measured data is sent to PC through oscilloscope(Yokogawa DL1700).
III. ESTIMATION OF ADHSION STATE BY PZTA. Adhesion force relaxation by oscillation
Here, we introduce the method [8] for relaxing adhesion
force by oscillation. In the target system, the effect of
capillary force among adhesion forces is the largest. Here, we
relax such kind of adhesion force by oscillation. By minutely
oscillating the end-effector bringing it near to the object, andcontacting it with the object on the substrate, the adhesion
force between the end-effector and the object is relaxed.
However, this method can not be always available. If thepushing force is large, the effect of the oscillation decreases,
and the adhesion force is not relaxed enough. Therefore,
adhesion state has to be checked. Here, we show the method
for the check. Fig. 5 shows the experimental set up. We set y
direction so that y can be orthogonal to the long side of theend-effector as shown in Fig. 4. We move the oscillated
end-effector by moving the clamped end by XYZ stage, alongy positive direction with the step of 1 [μm] from y(x=0)=-1 toy(x=0)=4 [μm]. Let y(x=0) (the origin) when the end-effector
firstly contacts with the object be 0. At the initial state(y(x=0)=-1), the end-effector does not contact with the object.
At y(x=0)=0, the end-effector contacts with the object. Aty(x=0) ≥ 0, the end-effector pushes the object. y(x=0)
corresponds to the magnitude of the pushing force. We
measure the oscillation of the end-effector by the laser
displacement meter. The input signal for the oscillation is sine
curve whose amplitude is 10 [V], and whose frequency is 4th
mode frequency (this mode is selected so that enough large
kinetic energy can be got while the amplitude is small enoughnot to disturb the manipulation). The left figures of Fig. 6
shows the data obtained by applying FFT to the measured
oscillation. The horizontal axis denotes the frequency and thevertical axis denotes the power spectrum density. At
y(x=0)=-1 [μm] (before contact), only inputted 4th mode
frequency was observed as shown in Fig. 6 (a). Aty(x=0)=0~3 [μm], not only inputted 4th mode frequency but
also lower mode frequencies (see the round marks at Fig. 6(b)) were observed. Fig. 6 (b) shows the typical result. At thisrange, the adhesion force is thought to be relaxed enoughsince it was observed that the object did not adhere to theend-effector at any time when moving the end-effector along

(a) yx=0 = -1

(b) y

x=0

=2

(c) yx=0 = 4
Fig. 6 Experimental results. The left column shows the results obtained by applying FFT to the measured oscillation by laser displacement meter and the rightcolumn shows the results obtained by applying FFT to the measured oscillation by the PZT sensor.

y negative direction (the direction of getting away from theobject). At y(x=0)=4 [μm], only inputted 4th mode frequency
was observed as shown in Fig. 6 (c). It is thought that thepushing force was large and then the effect of oscillation
decreased. At this range. the adhesion force is not relaxedenough since it was observed that the object adheres to theendeffector sometimes when the end-effector moves along y
negative direction. Summarizing, Fig. 6 indicates that we canestimate whether adhesion force is relaxed enough or not bychecking the excitation of the lower mode frequencies.
B. Estimation of adhesion state by PZT
To resolve the problems in the case when using laser
displacement meter as mentioned above, we propose themethod to estimate adhesion state by PZT.
Fig. 7 shows the proposed system. PZT for sensing(hereafter, we call PZT sensor) is bonded on the end-effector
in addition to PZT for actuating (hereafter, we call PZT
actuator). We bonded PZT sensor on the very opposite side of
PZT actuator so that the influence of entity of the PZT sensor
on the end-effector oscillation can be small. We oscillate theend-effector by oscillating the PZT actuator through thepower amplifier. The voltage at the PZT sensor is induced by
the oscillation. We measure the voltage by oscilloscope. The
measured data is sent to PC.
We did experiment to show the validity of the proposed
measuring system. In order to check whether the adhesion
force is relaxed enough or not, based on the results of theprevious section III.A, we will check the excitation of lower
mode peak frequencies in the oscillation (voltage) induced at
the PZT sensor.
Similarly with the experiment of the previous section III.A,
we moved the oscillated end-effector along y positiveEnd-effector

Fig. 7 Overview of the hybrid actuator and sensor system

direction with the step of 1 [μm] from y(x=0)=-1 to y(x=0)=4[μm] (see Fig.5). The input signal for the oscillation was thesame as the previous case. We measured and analyzed thevoltage of the PZT sensor induced by the oscillation of theend-effector.
The right figures of Fig. 6 show the data obtained byapplying FFT to the induced voltage at the PZT sensor.
Similarly with the previous results, At y(x=0)=-1 [μm]
(before contact), only inputted 4th mode frequency wasobserved. At y(x=0)=0~3 [μm], not only inputted 4th mode
frequency but also lower mode frequencies were observed(Fig. 6 (b) shows the typical result). At this range, theadhesion force was relaxed enough. The difference from theprevious case in section III A is the followings. 1) 3rd mode
frequency (around 2000 [Hz]) was observed at the previous
case, while it wasn’t observed at this case. 2) The magnitudesof power spectrum densities for lower mode peak frequencies
in the previous case were larger than those in this case. These
are thought to be due to the bonded position of the PZT sensor.
The collocation of PZTs causes that the PZT sensor has low
gain in the induced voltage. At y(x=0)=4 [μm], only inputted
4th mode frequency was observed. At this range. the adhesion
force was not relaxed enough. Summarizing, these results

Displacement (μm)

End-effector

3.0
2.5
2.0
1.5
1.0
0.5
0.0
0

Fig. 8 Displacement induced by DC input

10

20

30

40

50

DC input voltage (V)Fig. 9 The relation between DC input voltage and tip displacement(a). V = 0(c). V = 23(b). V = 7(d). V = 24

Fig. 10 Experimental results obtained by applying FFT to the induced voltage at the PZT sensor when adding the combined input of the DC and the AC inputs
to the PZT actuator.

indicate that we can estimate whether adhesion force isrelaxed enough or not by the PZT sensor.
IV. FINE TUNING OF GRASPING FORCE BY PZT AND ASHESIONSTATE CONTROL

To achieve both adhesion force relaxation and fine tuningof grasping force simultaneously, we develop a method of
adding DC input voltage to the PZT actuator. In addition, wedevelop a method for adhesion state estimation, based on theshift of lower mode peak frequency which causes due to thechange of grasping/pushing force.
A. Tuning of grasping force by adding DC voltage
When adding DC voltage to PZT, a constant amount ofdistortion arises in a specified direction. Based on thisphenomenon, we will change the force applied to the object
by adding DC voltage to the PZT actuator as shown in Fig. 8.
When adding DC voltage to the PZT, PZT stretches in x
direction(see Fig. 8). Due to the stretch, the tip position (x=lb)
of the end-effector displaces in y direction. We measure thetip displacement of the end-effector by laser displacement
meter when adding DC voltage to the PZT actuator. Fig. 9shows the results. The horizontal axis denotes the input
voltage and the vertical axis denotes the displacement. From
Fig. 9, it can be seen that the tip position can be changed withthe step of about 0.05 [μm], by increasing the input DC
voltage with the step of 1 [V]. Comparing with the case whenusing XYZ stage to move the end-effector (the resolution is 1[μm] at our experimental set up), we can move theend-effector more precisely and continuously. Also, we do
not have to consider the error due to the play of the clampedend of the end-effector.
Next, we try to achieve both adhesion force relaxation andfine tuning of the pushing force applied to object
simultaneously. We add the combined input of AC voltage
input for oscillation and DC voltage input for fine motion to
the PZT actuator. We did experience to see the validity of thisapproach. We apply FFT to the voltage of the PZT sensor
induced by the oscillation. Using the data of power spectrum
density, we will check adhesion state. The input signal for theoscillation was sine curve whose amplitude was 10 [V], andwhose frequency was 4th mode resonance frequency. Firstly,
we set the input voltage for moving the end-effector was 0[V]. Then, by increasing the voltage with the step of 1 [V], webrought the end-effector near to the object and contacted itwith the object. When the DC input was 7 [V], theend-effector firstly contacted with the object. We increased
the DC input until adhesion force was not relaxed enough.
Fig. 10 shows the results obtained by applying frequency
analysis to the induced voltage at the PZT sensor. The
horizontal axis denotes the frequency and the vertical axis
denotes the power spectrum density. At V=0 (~6) [V] (before
contact), only inputted 4th mode frequency was observed,
similarly with the result of Fig. 6 (a) (see Fig. 10 (a)). At V= 7

~23 [V], not only inputted 4th mode frequency but also lower
mode frequencies were observed, similarly with the result of
Fig. 6 (b) (see Fig. 10 (b) and (c)). Comparing Fig. 10 (b) with
Fig. 10 (c), it can be seen that the peak frequency shifts with
the increase of the DC input voltage. From the section III, thisrange can be regarded as the range where adhesion force isrelaxed enough. Then, we did experiment in which at thisrange, we moved the end-effector along y negative direction(the direction of getting away from the object) 10 times. In allcases, the end-effector was removed from the object while theobject adhered to the object. It indicates that at this range, theadhesion force is relaxed enough. At V=24 [V], only inputted
4th mode frequency was observed, similarly with the result of
Fig. 6 (c) (see Fig. 10 (d)). At this range, relaxing adhesion
force is not enough, and we observed that the object
sometimes adhered to the end-effector when moving theend-effector along y negative direction.
Summarizing, we reveal that 1) we can displace theend-effector tip, relaxing the adhesion force simultaneously,by the combined input of the DC and the AC inputs; 2) The
adhesion state can be estimated by checking the excitation oflower mode frequencies.
B. Adhesion state estimation based on peak frequency shift
From the results shown in Fig. 10 (b)~(d), It can be seenthat the lower mode peak frequencies shift with the change ofpushing force applied to the object by the end-effector. Then,we will judge the adhesion state (whether the adhesion force
is relaxed enough or not) by this shift. Among the lower mode
peak frequencies, we focus on the 1st mode frequency (thelowest peak frequency) since its shift can be most easily
detected.
We did experiment to observe the shift of the peak. We
took the same way as the section IV.A. The input signal forthe oscillation was sine curve whose amplitude was 10 [V],and whose frequency was 4th mode resonance frequency.
Firstly, we set the input voltage for moving the end-effector
was 0 [V]. Then, by increasing the voltage with the step of 1[V], we brought the end-effector near to the object andcontacted it with the object. When the DC input was 7 [V],the end-effector firstly contacted with the object. We
increased the DC input until adhesion force was not relaxedenough. We applied frequency analysis to the measured
voltage of the PZT sensor (induced by oscillation), andobserved the shift of the target peak frequency.
Fig. 11 shows the results. The horizontal axis denotes thefrequency and the vertical axis denotes the power spectrum
density. V=0 is a typical result before contact with the object.
The target peak did not appear. Note that in this case, only
inputted 4th mode frequency was observed. At V= 7 ~23[V],it can be seen that the target peak frequency shifts from 250[Hz] to 350 [Hz] with the increase of the DC input voltage(which corresponds to increase of the pushing force). Notethat in this range, not only inputted 4th mode frequency but
also lower mode frequencies were observed, and the adhesion

Fig. 11 Shift of target peak frequency
Table 1 Difference from reference point

Maximum difference
Minimum difference

Difference (Hz)140
100

force was relaxed enough. At V=24 [V], the target peakfrequency did not appear. Note that in this case, only inputted
4th mode frequency was observed and the adhesion force was
not relaxed enough.
Then, we will find the range of the target peak frequency
where the adhesion force is relaxed enough. First, we set thereference peak frequency is the target peak frequency in thecase when the end-effector firstly contacts with the object.
Next, we calculate the maximum difference between thereference peak frequency and the target peak frequency at therange where the target peak frequency appears. We did theexperiments to calculate the difference 5 times. Table 1
shows the results which show the maximum and minimum
differences. To realize the manipulation in which theend-effector can be removed from the object at any time, let
the minimum difference (100Hz) be the threshold. If thedifference from the reference peak frequency is smaller than
the threshold, the target peak frequency will appear at any
time. It indicates that adhesion force is relaxed enough. If thedifference is larger than the threshold, it cannot be guaranteedthat the target peak frequency will appear. Then, it cannot be
guaranteed that the adhesion force will be relaxed enough. By
controlling the pushing force applied to the object so that thedifference from the reference peak frequency can be smaller
than the threshold, we can keep the adhesion force relaxed.
We did experiments of keeping the difference from thereference peak frequency smaller than the threshold. We
observed that the lower mode frequencies were excited in allcases. It indicates that the adhesion force was relaxed enoughin all cases, and the validity of this method were shown.
We summarize the above method and express it as the flow
chart shown in Fig. 12. Here is the procedure of the method.
1. By adding the combined voltage input of DC and AC
inputs to the PZT actuator, we bring the end-effector nearto the object and contact it with the object.

2.
3.

4.
5.
6.
7.
8.

We apply frequency analysis to the measured voltage of
the PZT sensor.
We judge whether or not the end-effector contacts withthe object by checking whether or not the lower mode
frequencies are excited. If judging the end-effector does
not contact with the object, we go back to 1.
Let the lowest mode peak frequency in the case when theend-effector firstly contacts with the object be thereference frequency.
We control the pushing force applied to the object bycontrolling the DC input voltage.
We apply frequency analysis to the measured voltage of
the PZT sensor.
We calculate the difference between the reference
frequency and the lowest mode peak frequency.
If the difference is smaller than the threshold, we judge
that the end-effector can push the object more. If it is not,we judge that the end-effector cannot push the object any
more. We go back to 5.

Start

Increase pushing force

Frequency analysis

Contact ?

No

Yes
Set reference point

Control pushing force

Yes

Fig. 12 Method to keep adhesion force relaxed enough
[2]
[3]

[4]
[5]

[6]
[7]

[8]
F. Arai, D. Andou, and T. Fukuda, “Micro Manipulation Based on
Physical Phenomena in Micro World (1st Report, The Reduction
Method of Van Der Waals Force)”, Trans. of the Japan Society of Mech.
Eng., Series C, vol.62, no.603, 1996, pp. 4286-4293 (in Japanese).

Derive the difference fromreference point

Is the difference larger No
than threshold ?

REFERENCES[1]Frequency analysis
Can push more

In our previous paper [8], we proposed a method for
reducing adhesion force by oscillation. We also showed thatthe available range of the method can be checked byanalyzing the data obtained by applying FFT to the measured
oscillation by laser displacement meter. However, the use oflaser displacement meter causes several problems. 1) The
end-effector must be located at the specific point where laser
displacement meter can measure oscillation. 2) If something
blocks the light/laser or the target leaves the measuring point,
we can not check adhesion state. 3) The total system becomesvery large. Considering the above, in this paper, we havedeveloped the method to check adhesion state by PZT insteadof laser displacement meter. By the proposed method, we cancheck adhesion state at any time of the operation. The validityof this method has been shown by the experiments.
Next, to pursue to realize more precise manipulation, we
have developed a method to finely control the pushing force
applied to the object by end-effector (grasping force),
simultaneously relaxing the adhesion force relaxed enough. It
has been done by adding the combined input of the DC input
voltage for control of the pushing force and the AC input
voltage for oscillation for relaxing adhesion force. In addition,
we have focused on the shift of the lower mode frequencies
which arises due to the change of the pushing force, anddeveloped a method to check adhesion state more accurately.
By combining these method, we can finely tune the pushingforce applied to the object, simultaneously keeping theadhesion force relaxed enough.

Can not push any more

V. CONCLUSION[9]W. Zesch, M. Brunner, and A.Weber, “Vacuum tool for handling micro
objects with a nano-robot”, Proc. of the IEEE Int. Con. on Robotics andAutomation, 1997, pp.1761-1766.
Y. Rollot, S. Regnier, and J. Guinot, “Dynamical model for the
micromanipulation by adhesion : Experimental validations fordetermined conditions”, J. of Micromechatronics, vol.1, no.4, 2002,
pp.273-297.
D. S. Haliyo, Y. Rollot, and S. Regnier, "Manipulation of micro-objects
using adhesion forces and dynamical effects", Proc. of the IEEE Int.
Conf. on Robotics and Automation, 2002, pp.1949-1954.
D. S. Haliyo and S. Regnier, "Advanced applications using u&& mad, theadhesion based dynamic micro-manipulatior", Proc. of the IEEE/ASME
Int. Conf. on Advanced Intelligent Mechatronics, 2003, pp. 880-885.
S. Saito, H. Himeno, and K. Takahashi, "Electrostatic detachment of anadhering particle from a micromanipulated probe", J. of Applied
Physics, vol. 93, no. 4, 2003, pp. 2219-2224.
S. Saito, H. T. Miyazaki, T. Sato, K. Takahashi, and T. Onzawa,"Analysis of micro-object operation based on the dynamics considering
the adhesion under an sem", Proc. of the IEEE/RSJ Int. Conf. onIntelligent Robots and Systems, 2001, pp. 1349-1357.
T. Watanabe, M. Iwasaki, H. Matsumura, Z. Jiang, “Adhesion Forces
Relaxation by Oscillation and Its Application to Micro Manipulation”,
Trans. of the Japan society of Mech. Eng., series C, Vol.74, No.737,
2008, pp.23-30(in Japanese).
T. Watanabe, Y. Serita, “Adhesion state detection by vision and its
application to automatic micro manipulation”, Proc. of the IEEE/RSJ
Int. Conf. on Intelligent Robots and Systems, 2008, pp.458-463.