3URFHHGLQJVRIWKH,((( ,QWHUQDWLRQDO&RQIHUHQFHRQ5RERWLFV $XWRPDWLRQ :DVKLQJWRQ'&0D\
Manipulation of micro-objects using adhesion forces and dynamical effects D. Sinan Haliyo, Yves Rollot and St´ephane R´egnier The problem is to overbalance this adhesion for the release, when the object has to be placed on a substrate with a lower surface energy. A solution is the use of dynamic effects such as inertia. In the first part of this paper the theoretical approach will be described to show the constraints of the adhesion based on capture and release of micro-objects. In the second part the design of the experimental setup and finite-elements studies aimed to define the dynamic behavior of the sub-parts of the system will be discussed. Simulations of the dynamic model will show the pertinence of obtained results. Then, first experiments will prove the dynamic capabilities of the microgripper. Finally, a complete experimentation consisting of the manipulation of glass micro-spheres will be presented as conclusion, proving the efficiency of the proposed solution based on adhesion forces and dynamic compensation.
Abstract — This paper describes a dynamical strategy for releasing micro objects picked-up by means of adhesion forces. While sticking effects are used in order to capture an object by adequately choosing a high surface energy constitutive material for the end-effector, these same effects handicap considerably the release. We propose to take advantage of the inertial effects of both the end-effector and the manipulated object to overbalance adhesion forces and to achieve the release. Simulations show that for this purpose, accelerations as high as 105 m/s2 are needed. Successful manipulation of a 40µm radius glass sphere is experimented. Keywords— micro-manipulation, adhesion forces, dynamical effects, piezoceramic actuator, high acceleration
I. INTRODUCTION In last years, micro-manipulation has become a rising research field due to the recent developments in biology, micro-electronics and tele-surgery. New industrial and biological applications in need of accurate manipulation methods are more and more numerous[1], [2]. A first approach to this problem adopts the miniaturization method, reproducing macro-scale manipulation systems at micro-scale. Some examples are the chop-sticks gripper of MEL[3] and the 3finger manipulator of LAB. The second approach is to use nano-manipulation methods for micro-manipulation[4]. Finally, the last method consists of using micro-scale specificities to develop dedicated manipulation systems, such as the vacuum tool at EPFL[5] which takes into account adhesion and then uses suction forces higher in magnitude in order to neglect the sticking effects. Since 1996, Sato has opened the way of using surface forces by means of capture of micro-objects[6]. The manipulation taking place under electron microscope in vacuum, its success is due the constrained environment. Rather, our goal is to achieve capture and release of micro-objects using only adhesion forces in a non-constrained environment, i.e in normal laboratory conditions. The capture phase of the manipulation is at present well mastered. A manipulator has been designed in this aim. The gripper is a gold coated piezoresistive silicon micro-beam exhibiting a high surface energy, in order to facilitate the capture.
II. THEORETICAL APPROACH The manipulation task on which the modelling is based consists in picking up a sphere (“object”) initially laying on a planar surface (“substrate”) using a simple tool (“end-effector”), here a rectangular silicon cantilever; then placing it on a selected location on the same substrate. The end-effector is supposed to have a higher surface energy than the substrate, exhibiting a stronger Van der Waals potential. Considering this simple task, the process can be decomposed into 4 basic phases. First, the end-effector is brought in contact with the object (approach for capture). The capture is accomplished by taking advantage of stronger adhesion forces at the object/end-effector interface (capture). The object is then brought in contact with the substrate at the release location (approach for release). To achieve the release, the adhesion between the end-effector and the object must be overbalanced. This can be accomplished by sloping the tool by an angle θ, reducing the adhesion force projected in vertical axis by a factor cos θ. However, a sliding or rolling movement of the object in horizontal axis is possible. In case which the object is not at the very tip of the end-effector, some precision would be lost in placing. Also, it is clear that this method is convenient for only spheric and cylindric objects. In case of a planar object, which is most common in
Laboratoire de Robotique de Paris, UPMC, 10-12 Av. de l’Europe, 78140 Velizy, France E-mail:
[email protected]
,(((
1949
Acceleration (1e5 m/s2)
micro-electronics, dynamical effects are the only contribution to release. θ D2 D2 K
Yp
Yp
Yp
D1 D1
J
D1
Fig. 1. Description of the micro-manipulation task
RELEASE
10
0
0.08 0
CAPTURE ees
By writing dynamical equilibriums of the all components of the system, a simplified dynamical model including adhesion forces and inertial effects can be obtained: me Y¨p ¨1 mo D Yp
=
adh Feext − Foe (D2 ) − me g adh adh(D1 ) Foe (D2 ) · cos θ − Fos
= = D1 + 2Ro + D2
− mo g ¨ ¨ D2 = Yp − D¨1
s(
θ (d
egr
)
) mm
diu
70
0.26
Ra
Fig. 2. Acceleration limit for capture/release
needed. A special micro-gripper capable of such dynamical performances has to be designed.
(1) (2) (3)
This study theoretically proves the feasibility of a manipulation mode based solely on the adhesion forces. The pick-up is “guaranteed” by using an endeffector which surface energy is higher than the substrate on which the manipulated object initally rests. This adhesion bond must be broken in order to accomplish the release. This can be achieved by magnifying the dynamical effects, thus controlling the trajectory of the end-effector, and intrinsically its acceleration.
Figure 1 describes the distances used in the model. me , mo are respectively the masses of the end-effector and the object, Feext is the external force applied adh adh to the end-effector and Foe and Fos are respectively the adhesion forces between the object and the end-effector and the object and the substrate, including Van der Waals, electrostatic and capillary forces. These forces are non-linear functions of distances D1,2 . Ro is the radius of the object, which is supposed to be a perfect sphere. A more detailed description of the dynamical model can be found in our earlier publications[7]. In the initial state of the manipulation, regardless that the aimed operation is capture or release, the object lies on the substrate and is in contact with the endeffector. At this stage, due to the adhesion force and given that the surface energy of the end-effector is higher than the surface energy of the substrate, the object is stuck to the end-effector, which allows the capture. If the seeked operation is release, one must be able to withdraw the end-effector without moving the object. Hence the dynamical effects have to be used to overbalance the adhesion. Simulations of the dynamical model have been carried out using Matlab Simulink software. They show the existence of a value of initial acceleration Y¨ of the tool over which inertial effects overbalance the adhesion between the tool and the object, causing the release. This acceleration depends on both the mass of the object and the angle of slope θ of the end-effector, Figure 2 illustrates this acceleration limit between the capture and release domains. A close examination of these simulation results lead to conclude that in order to achieve the release accurately, accelerations ranging from 104 to 106 are
III. DESCRIPTION OF THE EXPERIMENTAL SET-UP A micro-nano manipulation robotic system has been designed to be integrated to an optical microscope in order to accomplish precise displacements of the microgripper. It combines a 3 DOF (x − y − z) serial micro translator used to create large magnitudes motions (25mm) with a resolutioin resolution on vertical (z) axis. An active micro gripper, whose design is discussed below, is mounted on this system. The first system is built by using commercially available products manufactured by Newport Microcontrol. Its control system is based on ESP6000 motion controllers integrating a PID loop. For the second one, we use a nanotranslator controlled using a capacitive sensor, manufactured by PI Gmbh. A LCD camera is mounted on the optical microscope. Magnifications obtained by the optical chain are x200, x400 and x800, depending on the optics. This camera gives a vertical image of the working area. A second camera, equipped with a x50 to x300 zoom, is placed at the left side of the manipulation area, in order to get a lateral view. The whole experimental set-up is shown on figure 3. All this system is controlled from a PC which integrates interface boards. A graphic user interface (fig. 4) has been developed to give access to the different functions of the system, such as coordinated motion of all axes
1950
with controlled speed and acceleration, capture and release motion cycles. Active micro-gripper PZT Ceramics
Body
Piezoresistive beam Microscope lens
Body
Fig. 4. Graphic User Interface
Tool
viding the end-effector and the force sensor. High accelerations required for release and/or capture are here produced by PZT ceramics.
Z nanometric Microscope base
XYZ micrometric
B. Actuators for high acceleration (a)
The actuator of the micro-gripper must respond to 2 criterias: • It must be able to provide high accelerations (ranging from 10 to 105 m/s2 ) • In addition it must also provide a displacement (about 100nm) in order to pull the gripper out of the range of the adhesion forces. These performances can only be achieved by actuators based on active materials. PZT ceramics is an adequate solution. This kind of actuator has been studied by dynamic simulations. The ceramic used in those simulations is a P1-89 PZT 1mm thick and 5x8mm surface. Results indicate that a command signal of 300 V in magnitude and a rising time in the range of 1 to 100µs would produce desired responses. A special signal generator has been developed. It can produce signals ranging from 0 to 300V , with width ranging from 1 to 390µs. We have also experimented the dynamical response of the P1-89 ceramic (d33 240·10−12 m/V ) using heterodyne laser interferometry. Results, which show that accelerations about 105 m/s2 are reached over a displacement of ∼ 60nm, are discussed in section IV. Figure 5 shows the whole active micro-gripper.
micrometric z axis
active micro-gripper xy micrometric
(b)
Front Camera
Lateral camera
Microscope Micrometric Z axis
Nanometric Z Axis
(c)
Micrometric XY axis
C. End-effector selection
Fig. 3. Schematic view of the micro-manipulator (a), Manipulation area (b), Manipulator general view(c)
The chosen component is a piezoresistive Atomic Force Microscopy force sensor. It consists of a monocrystalline silicon beam of 600x140x10 µm dimensions, with a Wheatstone bridge mounted on its base. The voltage of the Wheatstone bridge is directly proportional to the flexion of the beam (fig. 5-a). The sensor cantilever, produced by bulk micro-machining providing a smooth surface, is directly used as the endeffector. It is mounted on an alumina support of 5x8 mm, which also provides contacts for the Wheatstone bridge. Its total weight is 0.1 g. The stiffness k of the beam is approximately k 21 N.m−1 . The beam has been studied by means of a finite elements method. The obtained principal
A. The design of the active micro-gripper The gripper is the most sensitive part of the system. It has to be as small as possible to satisfy integration constraints. The shape and roughness of its contact surface have to be perfectly defined. Indeed, adhesion forces strongly rely on the contact geometry and the Van der Waals forces drastically decrease with roughness. The micro-gripper must also include a force sensor in order to detect the contact and adhesion forces. A piezoresistive silicon micro-beam issued from AFM technology is integrated into the micro-gripper, pro-
1951
Cantilever
Tension/flexion Caracterisation
Connectors
Flexion of the beam (mm) Flexion of the Beam (nm)
500
Tension/Flexion Caracterisation
Wheastone Bridge
0
−500
−1000
−1500
−2000
−2500
−3000
−3500 −3.5
−3
−2.5
−2
−1.5
−1
−0.5
Tension of the Wheaststone Bridge (V)
0.5
0
Tension of the Wheastone Bridge (V)
(a)
Ceramic Cantilever
(a)
Cantilever
8
Fig. 5. Tool for manipulation by adhesion: (a) AFM Cantilever, (b) end-effector with piezoceramic actuator
Vertical displacement (e-8 m)
(b)
Ceramic
$
$ %
#!
$
$ %
0
"
"#!
$
$ %
"
"
0
resonance frequencies are in the range of [33.8kHz, 211.7kHz, 592.6kHz].
5.2e-5
Time (s)
(b)
Fig. 6. Finite elements analysis of the actuator : (a) Analysis results (b) Command signal (V · d33 )
D. Dynamic behavior of the micro-gripper The micro-gripper has been dynamically studied by means of a finite elements analysis software (ANSYS). First, the silicon cantilever of the AFM device has been studied. The vibrational modes of the silicon cantilever is obtained by modal analysis. Results are in good agreement with constructor data and measurements. The piezoresistivity provides information on the vibration frequency of the beam and its amplitude at the bonded extremity. The knowledge of the vibrational modes allows the calculation of the displacement of the free extremity based on the output signal of the wheatstone bridge. The piezoceramic actuator has also been modeled and dynamically simulated. Results show that in case of a control signal in form of a slope of 300V amplitude in 1µseconds, a 70nm displacement with 105 m/s2 instantaneous acceleration is obtained. Figure 6 shows a typical analysis result, in case of a command signal of 200V amplitude. The PZT actuator used in simulations is a singlelayer P1-89. The vibration-free response is obtained by a command signal frequency set to the resonance frequency of the ceramic[8]. Note that using a multilayer actuator instead of this single-layer device, the obtained acceleration would be multiplied by the number of layers. Finally, the whole micro-gripper, including the same ceramic as the actuator and the AFM device as the end-effector has been modeled (fig.7). The displacement obtained at the free extremity is shown in the figure 7-b. As expected, the vibration takes places on the first mode with a magnitude of 0.32µm (A-B). The calculated acceleration is in the
Vertical displacement (e-7 m)
(a) 2.8
B UZ
-1.2
A 0
2.6 Time (e-4 s)
(b)
Fig. 7. The finite elements analysis of the micro-gripper ; (a) Meshing of micro-gripper, (b) free extremity vibrations
range of 105 m/s2 . This result shows that the designed micro-gripper is capable of desired dynamical performances. Before proceeding to manipulations using this system, its real dynamical behavior has been experimentally studied.
1952
IV. FIRST EXPERIMENTS
Wheatstone bridge tension WHEATSTONE BRIDGE VOLTAGE
3.5 3.5
Wheatstone bridge tension WHEATSTONE BRIDGE VOLTAGE
0.6 0.6
INPUT/1000
3
0.4
The dynamic response of both the actuator and the micro-gripper has been inspected experimentally in order to define reachable accelerations in real operating conditions. In the first part, the actuator alone is considered. Measurements are realized using the heterodyne laser interferometry method. Experimentations have taken place in Laboratoire d’Ondes Acoustiques of ESPCI (Paris, France). This method filters the static component of the response, i.e displacements occurring at frequencies lower than 20kHz. However, accelerations can be measured accurately. The overall displacement, calculated by the finite elements method, is trustworthy and estimated to 70nm. Different control signals, generated by the PC based interface, have been used to study the dynamic behavior of the actuator. Figure 8 shows the response of the actuator to a command signal which reaches 300V in 1µs, stays at this value for 100µs then decreases slowly during 50µs. For easy visualization, the command signal is divided by 100. For the displacement measurement output, 10mV corresponds to 0.1nm. The displacement was estimated by simulations around 60nm, which is in good agreement with the experimental results. Calculated instantaneous acceleration is ∼ 6·104 m/s2 . This measurement justifies the choice of this ceramic as actuator. command/100, output=0.1nm/10mV
77
OUTPUT 1V=10nm
Tension (V) Tension (V)
Tension (V) Tension (V)
INPUT/100
0
−1
−1
−2
−2
0
0
0.5
1
1.5
2
2.5 Time (s) (s) Time
(a)
3
3.5
4
4.5
5
5e-4 x 10 −4
-3−3
0.2
0.3
0.4
0.5 Time (s)
Time (s)
0.6
B 0.7
0.8
0.9
1 1e-3 x 10 −3
−0.8 -0.8 0 0
0.5
OUTPUT 1
1.5
2
2.5
3
Time (s) Time (s)
3.5
4 4e-5 x 10 −5
(b)
Fig. 9. Free extremity acceleration measurements of the microgripper:(b) is a zoom of (a), with command signal reduced by 10.
micro-gripper is suitable for producing high enough accelerations in order to increase inertial effects and to overbalance the adhesion between the end-effector and the manipulated object. V. MANIPULATION EXPERIMENTS First manipulations have been carried out to justify static conditions of the adhesion based manipulation. Dynamic properties are not used. Glass spheres of radius 20 ∼ 40µm lying on a polystyrene substrate have been captured by simple contact, using only sticking effects. The release is accomplished by sloping the end effector by 60o , decreasing the adhesion between the end-effector and the object. The success of this experimentation justifies the use of sticking effects for manipulation purposes[9], [10]. Recall that this “sloping startegy” is valid only for spherical objects, which justifies the development of a more general approach based on dynamical effects. This approach has been then experimented. Glass spheres on a low surface energy substrate have been manipulated using dynamical release strategies. Instead of sloping the micro-gripper, the default orientation θ = 0 is kept. The release is then accomplished by the simple use of the piezoceramic actuator. Figure 10 illustrates the manipulation. The plots of figure 11 represent two cases of release experiments. The observation of the wheatstone bridge provides accurate information on the success of the release phase. In both cases, a 1µs wide impulse is sent to the actuator at t = 0.9 · 10−4 s. In 11-a, the magnitude of dirac is 40V . The produced acceleration is not enough to overweight the adhesion. The object is still stuck to the end-effector. Resulting vibrations are at the resonance frequency of the beam+sphere system. In 11-b, the magnitude of the dirac is 120V . The response of the ceramic is sufficient to produce the necessary acceleration. The inertial force of the object added to the adhesion force between the substrate
INPUT/100
A
OUTPUT 0.1
(a)
-3−3
Tension (V) (command/1000) VOLTAGE (V)
Tension (V) (command/100) VOLTAGE (V)
−0.6
-1−10 0
1
0
−0.4
2
1
A
0
0
OUTPUT 1V=10 nm
3
2
0.2
−0.2
−0.5
4
3
1
0.5
5
4
2
B
6
5
INPUT/100
1.5
Zoom 1 (0.1nm/10mV)
77
6
2.5
0
0
0.2
0.4
0.6
0.8 Time (s) Time (s)
1
1.2
1.4
1.4e-4 x 10
−4
(b)
Fig. 8. Laser interferometry measurements of displacement of the ceramic actuator : (b) is a zoom on (a).
The surface area of the AFM beam being very small, the laser interferometry method cannot be used. Instead, the information provided by the Wheatstone bridge can be used to deduce the instantaneous acceleration of the end-effector, based on the knowledge of output tension/flexion coefficient of the AFM sensor and its vibrational modes. Figure 9 shows the response of the end-effector. The command signal is identical to the one described above. The period of the first oscillation matches the third vibrational mode of the AFM beam. This allows to estimate the instantaneous acceleration in the range of ∼ 5·104 m/s2 . Experiments conclude that the
1953
the success of the release.
CANTILEVER
CANTILEVER AND STICKING OBJECT
VI. CONCLUSIONS This paper described the study and the modeling of a manipulation method taking advantage of the adhesion forces, as well as the design and the realization of an experimental set-up. A dynamical release strategy consisting in the use of inertial effects to overweight the adhesion is elaborated. A prototype micro-gripper, able to perform the capture and the release tasks, providing a force sensor and high-acceleration actuator is constructed and experimented. Reachable accelerations are in the range of 104 ∼ 106 m/s2 . The use of multi-layer ceramics can significantly increase this range. Experiments show that the proposed manipulation mode is easily adapted to many cases and leads to very satisfactory results. In short term, our objective is to explore the properties of deformable materials in order to elaborate adequate dynamical models and proper manipulation strategies in different environmental conditions, such as water or biological solutions. References
OBJECT
(a)
(b)
CANTILEVER
RELEASED OBJECT (c) Fig. 10. Manipulation of a 40µm radius glass sphere: release using inertial effects Wheatstone bridge tension WHEATSTONE BRIDGE VOLTAGE
0.4 0.4
Tension (Command/100) Voltage (V)(V) (Command/100)
Tension (V) (Command/1000) Voltage (V) (Command/1000)
[1]
0.1
0
−0.1
2.5
INPUT/100
2
1.5
1
RELEASE
0.5
OUTPUT OUTPUT
−0.2
0.5
1
1.5
2
0
2.5
3
3.5
Time (s) Time (s)
-0.5 −0.5
4 4e-4 x 10 −4
0
0.5
0
1
1.5
2
2.5
3
3.5
Time (s) Time (s)
(a)
4
4e-4 −4 x 10
(b) Réponse de BRIDGE la poutre WHEATSTONE VOLTAGE
0.8 0.8 Tension (entree/1000) Voltage (V)(V)(Command/1000)
F. Arai and T. Fukuda, “Adhesion-type micro endeffector for micromanipulation,” in proc. of IEEE international conference on robotics and automation, 1997, pp. 1472– 1477. [2] G. Yang, J. A. Gaines, and B. J. Nelson, “A flexible experimental workcell for efficient and reliable wafer-level 3d microassembly,” in proc. of IEEE international conference on robotics and automation, 2001, pp. 133–138. [3] T. Tanikawa, T. Arai, and T. Masuda, “Development of micro manipulation system with two-finger micro hand,” in Proc. of the International Conference on Intelligent Robotics Systems, 1996, pp. 850–855. [4] S. Curran, S. Roth, P. Kinlen, D. L. Carroll, Ph. Redlich, M. Ruhle, and P.M. Ajayan, STM manipulation of individual nanotubes in Molecular Nanostructures, pp 423, Edited by H. Kuzmany, J. Fink, M. Mehring and S. Roth, World Scientific, 1997. [5] G. Danuser, I. Pappas, B. Vgeli, W. Zesch, and J. Dual, “Manipulation of microscopic objects with nanometer precision : Potentials and limitations in nano-robot design,” International Journal of Robotics Research, 1997. [6] H. Miyazaki and T. Sato, “Mechanical assembly of three dimensional microstrutures under a scanning electron microscope,” in Proc. of the international conference on micromechatronics for information and precision equipment, 1997, pp. 335–340. [7] Y. Rollot, S. Regnier, and J-C. Guinot, “Simulation of micro-manipulations : Adhesion forces and specific dynamic models,” International Journal of Adhesion & Adhesives, vol. 19, pp. 35–48, 1999. [8] K. Uchino, “Piezoelectric actuator and ultrasonic motors,” Kluwer Academic Publishers, Series Editor : Harry L. Tuller, 1997. [9] Y. Rollot, S. Regnier, and J-C. Guinot, “Dynamical model for the micromanipulation by adhesion : Experimental validations for determined conditions,” International Journal of MicroMechatronics, vol. to be published 2002 [10] Y. Rollot, S. Haliyo, S. R´egnier, L. Buchaillot, J.C. Guinot, and P. Bidaud, “Experimentation on micromanipulation using adhesion forces in unconstrained environment,” in proc. of the IEEE/RSJ International Coference on Intelligent Robots and Systems (IROS2000), Japon, october 2000, 2000, pp. 653–658.
3
0.2
-0.3 −0.3 00
Wheatstone bridge tension WHEATSTONE BRIDGE VOLTAGE
3.5 3.5
INPUT/1000
0.3
0.6
0.4
INPUT/1000
0.2
RELEASE 0
−0.2
OUTPUT -0.4 −0.4 0.8 -0.8
1
1.2
1.4 Temps (s) Time(s)
1.6
1.8
2 2e-4 x 10
−4
(c) Fig. 11. Dynamical release of a 40µm radius glass sphere (Watch the scale change between plot (a) and (b) !)
and the object is higher than the adhesion force between the end-effector and the object, allowing the release.The separation between the gripper and the object occurs approximately 50µs after the impulse is sent to the actuator, as can be seen on the change of the vibrational frequency to 33kHz, the resonance frequency of the AFM beam. This change in vibrational frequency, clearly visible in fig. 11-c, is used to detect
1954
