On the use of robotic grippers with shape memory alloy actuators in handling light-weight workpieces.

Miclosina, Calin ; Vela, Ion ; Gillich, Gilbert Rainer 等

Abstract: In certain practice situations, a given workpiece is light-weight and easily deformable. In order to handle it with a robotic gripper, the prehension force has to reach relatively low values so as not to deform the workpiece. This paper presents a comparison between a gripper with an electromagnetic actuator and a gripper with a shape memory alloy actuator.

Key words: shape memory alloy, actuator, prehension.

1. INTRODUCTION

In robotic applications it is often necessary to handle light weight workpieces, which do not allow the use of high values of prehension forces.

The handling of the workpieces is made with the help of grippers, which are devices which can grasp an object, in our case the workpiece. Usually the robotic grippers are electric, electromagnetic, hydraulic or pneumatic actuated. It is also possible to use grippers with shape memory alloy actuators if the necessary prehension forces have relatively low values. The control of interaction gripper-workpiece is realized with the help of sensors, which is difficult in case of low values of prehension force.

A shape memory alloy is an alloy which remembers its geometry. Due to this interesting property, shape memory alloys were used in robotic guiding devices or robotic grippers. The shape memory alloys actuators were introduced in different robotic gripper structures (Grant, 1999; Morra et al., 2004). In principle, shape memory alloy strings are moving the mobile part of the actuator versus the fixed one.

Until now in "Eftimie Murgu" University of Resita research has been based on rigid link grippers with electromagnetic actuators. As further research, a robotic gripper with smart memory alloy actuators has to be realized and the prehension force values will be determined.

2. THEORETICAL DETERMINATION OF THE NECESSARY PREHENSION FORCE

In the case in hand we shall consider a flexible ring as workpiece with the following characteristics: inner diameter [D.sub.i] = 80 mm, thickness g = 0,4 mm, height h = 25,8 mm, mass m = 11,87 g and elastic constant k = 0,99 N/mm.

The radial deformation [DELTA]y of the flexible ring is the same with the deformation [DELTA]x on a perpendicular direction, as shown in fig. 1.

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

The radial deformation value [DELTA]x of the workpiece is proportional with the applied force, in this case the prehension force S of a single finger:

S = k x [DELTA]x. (1)

The maximum allowed deformation 2[DELTA][x.sub.max] is considered 2% of the workpiece average diameter [D.sub.med]:

[DELTA][x.sub.max] = 2% x [D.sub.med]/2 = 0,02 x 80,4/2 = 0,804 mm (2)

The maximum value of the prehension force becomes: [S.sub.max] = k x [DELTA][x.sub.max] = 0,99 x 0,804 = 0,796 N (3)

The forces which are acting on the workpiece during handling are: the weight [??], the prehension force [??], the normal [??] and the friction force [[??].sub.f], as shown in fig. 2.

The equilibrium relation for the workpiece is:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (4)

Relation (4) is projected on the x and z axes:

S = N (5)

[F.sub.f] = G/2 = [mu] x N (6)

Replacing N from relation (5) in (6), the minimum value of prehension force for equilibrium is:

[S.sub.min] = G/2 x [mu] = 0,116/2 x 0,3 = 0,194 = N (7)

where [mu] = 0,3 is the friction coefficient between the workpiece and the gripper's finger.

In conclusion, the limit values of the prehension force are: 0,194 N < S < 0,796 N (8)

3. EXPERIMENTAL DETERMINATION OF THE PREHENSION FORCE REALIZED BY AN ELECTROMAGNETIC ACTUATED GRIPPER

The prehension force is measured for different values of the applied voltage to the terminals of the electromagnetic actuator. The kinematical scheme of a gripper mechanism is presented in fig. 3 (Miclosina & Zsarkovetz, 2006).

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

The motion is transmitted from the translational driver [A.sub.0] to the element 1 and then to the intermediate element 2, which drives the articulated parallelogram mechanism 3-4-5, as shown in fig. 3. For the elements 2'-3'-4'-5', the motion is transmitted in a similar way.

For this gripper mechanism, the calculated limits of the actuator drive force F corresponding to the limits of prehension force S, as shown in expression (8), are:

0,265 N < F < 1,087 N (9)

For the designed gripper, the driver [A.sub.0] is an electromagnetic motor.

The experimental stand is presented in fig. 4. The notations have the following meanings: 1--DC electrical power source with output voltages of 75 V, 93 V, 130V, 150 V; 2--switch (circuit breaker); 3--parallel jaw gripper; 4--workpiece; 5--comparator; 6--retainer. On the stand, the displacement of the workpiece is prevented by the retainer 6.

The experimental values of the workpiece deformations and of the prehension force S are shown in table 1.

As the deformation was measured under the action of two jaws, for a single jaw the prehension force was determined dividing by 2 the values from column 4 of the table 1.

The prehension force was determined in a similar way for a gripper with an electromagnetic motor and a rack-gear transmission (Miclosina & Sasec, 2006).

4. SHAPE MEMORY ALLOY ACTUATOR

In practice the driver [A.sub.0] (fig. 3) can be represented by a shape memory actuator. The governing mechanism of shape memory alloys is based on the reversible transformation process between martensite and austenite (Bujoreanu, 2002). It can be initiated either by a change in temperature or by a change in mechanical stress. Most functional properties of SMA are directly related to the mechanism on the level of the shape memory alloys springs, as shown in fig. 5.

Prestrained martensitic SMA springs operate during heating against the elastic stiffness of the host matrix, biasing the strain recovery of the shape memory alloys springs.

[FIGURE 5 OMITTED]

[FIGURE 6 OMITTED]

Advantages of using shape memory alloys actuators are their small size and weight, their high force to weight ratio, and their low cost.

There are also disadvantages, such as relatively small strains, non-linear effects such as hysteresis phenomena and low energy efficiency. Despite these limitations, shape memory alloys actuators have one of the highest payloads to weight ratios among "smart material" based actuators.

In fig. 6 a SMA actuator is presented and notations have the following meanings: 1--fixed end plug; 2--fixing hole; 3--water plug; 4--SMA spring; 5--silicon boot; 6--mobile end plug. The element 4 changes its shape according to the prescribed spring heating.

Precise control of the dependence of temperature gradient permits the calculation of an optimal control for the actuator, in order to obtain an accurate positioning which leads to the desired minimal prehension force necessary to grasp the workpiece, thus allowing only elastic deformation.

For the values determined in relation (9), an actuator with one SMA spring having the following characteristics: compressed length 16 mm, coil 8 mm, wire diameter 0,95 mm can be used. By electrical heating with 3 A, this spring extends to 30 mm and generates a force up to 4 N. Using 4 springs, the elongation can be drastically reduced for the desired drive force.

5. CONCLUSIONS

For the voltages used in the case of the gripper with an electromagnetic actuator, the prehension force has relatively low values. The possibility to realize accurate positioning and force control using electromagnetic actuators is reduced in this case.

Using shape memory alloy actuators, with different geometrical architectures, the gripper's drive is simpler, permitting an accurate positioning and force control. The gripper's weight is also reduced.

6. REFERENCES

Bujoreanu, L.Gh. (2002). Materiale inteligente (Intelligent Materials), Ed. Junimea, ISBN 973-37-0735-X, Iasi, Romania

Miclosina, C. & Zsarkovetz, F. (2006). Parallel Jaw Gripper for Prismatic Workpieces. Robotica & Management, Vol. 11, No. 1, June 2006, page numbers 62-64, ISSN 1453-2069

Miclosina, C. & Sasec, I. (2006). Robotic Gripper for Cilindrical and Spherical Workpieces. Robotica & Management, Vol. 11, No. 2, December 2006, page numbers 27-30, ISSN 1453-2069

Grant, D. (1999). Shape Memory Alloy Actuator, Available from: http://www.cim.mcgill.ca/~grant/sma.html Accessed: 2007-07-16

Morra, F.; Molfino R. & Cepolina, F. (2004). Miniature Gripping Device, Available from: http://smart.tamu.edu/presentations/presentationfiles/ researchpresentations/BrentReport.pdf Accessed: 2007-08-02

Table 1. Experimental values of the workpiece deformations
and of the prehension force S.

Crt. U 2[DELTA]x 2S S
no. [V] [mm] [N] [N]

1 75 0,04 0,04 0,02
2 93 0,15 0,14 0,07
3 130 0,50 0,49 0,245
4 150 0,77 0,76 0,38