Actuator 2002, 8th International Conference on New Actuators, Bremen, Germany, June 10-12 2002, pp. 321-324

A NOVEL MICROGRIPPER WITH PARALLEL MOVEMENT OF GRIPPING ARMS R. Keoschkerjan1, H. Wurmus2 1) Fraunhofer-Institute of Reliability and Microintegration, Munich, Germany 2) Technical University of Ilmenau, Department of Microsystem Technology, Ilmenau, Germany e-mail: ruben.keoschkerjan@izm-m.fraunhofer.de Abstract: The paper presents the development of a new microgripper which can realise a parallel movement of the gripping arms with possibility for simultaneous positioning of the gripped object. The piezoelectric actuation principle was used to generate movement of the microgripper compliant mechanism. The developed kinematic structure of the microgripper allows hundredfold amplification of the piezoelectric element movement and its continuous transmission to the gripping arms. The microgripper was fabricated by means of a UV-lithographic process and chemical wet etching technology from microstructurable photosensitive glass. The parallel microgripper was microfabricated and tested successfully. Introduction

The increased demands of the nowadays microtechnical applications lead to increasing complexity of the microdevices. These demands can only be met by realisation of hybrid assemblies. In order to mount different components, it is essential to develop new assembling technologies which enable handling of very small parts. The problems of handling of very small microoptical, microelectronical or micromechanical elements in nano- or micrometer range can be solved using special microgrippers and micropositioners [1-4, 6]. Increasing the functionalities of such systems is one of the important trends in the modern microsystem technology. However, the majority of the microgrippers already developed realise only a rotational movement of the gripping arms [3, 4, 6]. During the gripping process reaction forces act at the contact points between the gripping arms and the gripped object. In the case of microparts with curved or especially with circular surface such as microlenses the reaction forces act perpendicular to the object surface. (Fig. 1a). The X-component of the reaction force holds the gripped object between the gripping arms. The Y-component of the reaction force however acts parallel to the longitudinal axes of the microgripper and pushes the gripped object out of the gripping arms. This negative effect can be avoided if the gripping process is realised without Y-component of the reaction force (Fig. 1b). This can be achieved by using a gripping mechanism with parallel movement of both gripping arms. In this case a reliable gripping process can be guaranteed.

Rx

Ry

gripping arm gripped object

longitudinal axe a)

Rx Rx

gripping arm gripped object

long

itudi

nal a

xe

b)

Fig. 1: Gripping forces at the contact: a) rotational movement of the gripping arms; b) parallel movement of the gripping arms.

Working principle Fig. 2 shows schematically the developed microgripper which realises a parallel movement of the gripping arms. The kinematic structure consists of two parallelogram mechanisms (1) and (2) with flexure hinges (3) which are arranged parallel to

1

2

3

45

6

K1

K2

9

78

Fig. 2: Scheme of the parallel microgripper.

each other and mounted on the base. The piezoactuator (4) serves as a movement generator for the microgripper mechanism. It is mounted on the base (7) and coupled through two force contact points K1 and K2 to the lever mechanisms (5) and (6). These two lever mechanisms (5) and (6) serve for amplifying of the piezoactuator (4) displacement and simultaneously transmission of this movement to the gripping arms (8) and (9). The parallelogram and the lever mechanisms can be designed in such a way that the piezoactuator movement will be amplified up to 100 times. The release of the gripped object can be achieved when the voltage applied to the piezoactuator (4) is reset and the gripping arms (8) and (9) are opened due to elasticity of the flexure hinges (3). Fig. 3 shows a modification of the new microgripper with two piezoactuators integrated in the previous kinematic structure. This microgripper design has an additional positioning ability. The gripping of an object is realised through the closing of the gripping arms (8) and (9) by applying the same voltage to both piezoactuators (4) and (10). For the additional positioning of the gripped object, it is necessary to apply an additional positive voltage to the piezoactuator (10) and the same negative voltage to the piezoactuator (4). This electric control initiates simultaneously an expansion of the piezoactuator (10) and a contraction of the piezoactuator (4).

2

3

6

9

14

5

78

10

Fig. 3: Scheme of the parallel microgripper with

additional positioning ability Such opposite deformation of both piezoactuators leads to the equal parallel displacement of the

gripping arms (8) and (9) and therefore to the positioning of the gripped object (Fig. 4).

Fig. 4: Positioning process of the gripped object.

Design strategy For the design of the developed microgripper the following analytical procedure was applied: - development of the kinematic modell for the

whole compliant structure; - definition of the piezoactuator and the compliant

mechanism stiffnesses; - adjusting of the mechanism elasticity to the

piezoactuator stiffness; - calculation of the microgripper main geometrical

parameters (flexure hinges thickness etc.). For generation of the movement of the microgripper mechanism, the piezoelectric actuating principle is used because of its precision and high efficiency in the micrometer range. The piezoactuators have a planar construction and can be simply integrated into the microgripper after its microfabrication. Because the deformation of the piezoactuators are in the micrometer range linear relations in the compliant mechanism (Hook law) can be assumed. Under this condition the rotation point of the flexure hinge coincides with its symmetry centre during the deformations. In this case the flexure hinges can be approximated with a rotational kinematic pair with additional bending stiffness. The whole microgripper kinematic model is shown in Fig. 5, where the piezoactuator is approximated as a mass-stiffness-damping model. The degree of freedom of a mechanism can be calculated as:

F= 3×N - 2×P5 - P4, (1)

where N is the number of movable mechanism links, the P5 and P4 are the number of the kinematic pairs of the 5th and 4th order respectively. For the given compliant mechanism they are:

N=6, P5=8, P4=1,

and the degree of freedom is:

F=3×6 – 2×8 – 1 = 1.

E

A

D

C´F

K

B

GKP

CP MP

5

4

3

2

1

C

Fig. 5: Kinematic model of the microgripper

This means that the moving function of the output links (gripping arms) is defined through only one input function (deformation of the piezoactuator). In order to guarantee the functionality of the microgripper (deformability) the stiffness of the compliant mechanism KM and the piezoactuator KP have to be adjusted to each other:

1KK

M

P ≥ . (2)

Both stiffnesses can be defined by energetic method. The potential energy of the microgripper compliant mechanism consists of the sum of the single potential energies saved in the flexure hinges:

∑ ϕ= 2p K

21E ii , (3)

where Ki and ϕi are the bending stiffness and the relative bending angle of the single hinge respectively. The bending stiffness of the flexure hinge is a function of its geometrical and mechanical parameters [5]:

5.0

5.2

0 R9htE2K

π= , (4)

where t, h and R are the height, the thickness and the radius and E is the elastic modulus of the hinge. By using the reduction principle of the energetic method the whole potential energy of the compliant mechanism is given by:

2KMp WK

21E = , (5)

where WK is the movement of the reducing point K which is equal to the deformation of the piezoactuator. The piezoactuator stiffness, which can be used also for the calculation of the gripping force, is given by:

LS

HBK E

11P ⋅

⋅= , (6)

where B×H×L are the piezoactuator dimensions and is the elastic module of the piezomaterial. From

equations (4), (5) and (6), the maximal permitted

thickness of the flexure hinge under condition (3) can be defined as:

E11S

5.2

12

AB

FG

AB

FG2FKP

5.0

522tE4

KR9h

−

= +−

π

ll

lll

(7)

This strategy was used to design a microgripper for the given piezoactuator (piezomaterial PK51) with the following parameters: dimensions: 2×18×0.2 mm3,

elastic module: S = 7.8×10E11

10 N/m2.

Dependent on the piezoactuator dimensions some parameters of the microgripper can be fixed. For the flexure hinges and the compliant mechanism they are:

R=500 µm, t=1000 µm, b=1000 µm; and

lAB=0.6 mm, lFK=2.5 mm, lFG=5 mm, lAC=28 mm

respectively. The permitted thickness of the flexure hinges calculated by Eq. 7 is h= 100 µm. The transmission ratio of the whole compliant mechanism is given by:

94AB

AC

FK

FG =×=λll

ll (8)

The maximal possible output movement of the gripping arms can reach the value:

Smax=∆L×λ = 4.5×94= 423µm,

where ∆L is the relative deformation of the piezoactuator at 100 V. FEM simulation The developed microgrippers were investigated numerically by the FEM. The analysis of the FEM -

Fig. 6: FEM simulation of the movement of the

microgripper finger.

results (Fig. 6) has shown a good agreement between the analytically and numerically obtained geometrical parameters. Furthermore, the deformed state of the microgripper proves the fact of the parallel displacement of the gripping fingers. Microfabrication technology As a material for the microfabrication of the new microgrippers, microstructurable photosensitive glass was used. In comparison to silicon, glass allows to simpler realise the circular profiles of the flexure hinges with high precision by etching technologies. Fig. 7 shows the technological process for the microfabrication of the microgripper. An aluminium layer is deposited on the outgoing glass wafer. The following lithographic and etching steps create a mask with the microgripper structure in the Al-layer. The UV exposure and the tempering initiates the so-called anisotropic property in the glass. Finally the glass wafer with the microgripper structures is etched in the 10% HF solution (Fig. 8). After the mounting of the piezoactuators the microgripper structures were tested successfully.

Al deposition

glass wafer

Al etching

UV exposure

tempering

HF etching Fig. 7: Technological steps of glass

microfabrication.

Fig. 8: Test structure of the microgripper.

Conclusions This study has shown that due to modifications of the kinematics in the compliant mechanisms and

integration of several actuators, new properties of the microgrippers can be achieved. The realisation of the elements with micrometer dimensions such as flexure hinges, compliant microlevers and parallelogram micromechanisms (Fig. 9) demonstrates the excellent properties of the glass microfabrication technique for development of new micromechanical components.

a)

b)

Fig. 9: Elements of the microgripper: a) arrangement of the gripping fingers; b) compliant microlever and parallelogram micromechanism.

References [1] German patent application. DE 44 05501 C1,

(1995). [2] Keoschkerjan, R. et al. Piezoelectric X-Y

micropositioner made of photosensitive glass to form one micro-handling unit. ACTUATOR (2000), 296-299.

[3] Salim, R.: Gestaltung und mikrotechnische Realisierung von Mikrogreifern. TU Ilmenau, Ph.D. thesis, (1997).

[4] Hesselbach, J.; Pittschellis, R.: Miniaturgreifer für die Mikromontage. 41. Internationales Wissenschaftliches Kolloquium, Ilmenau (1996).

[5] W. Paros, L. Weisbord. How to design flexure hinges. Machine Design, T-27, (1966), 151-156.

[6] M. Kohl, B. Krevet, E. Just.: SMA microgripper system. Transducers (2001), 710-713.