Shape memory alloy actuator for robotic grippers actuated by two pairs of active springs.

Amariei, Daniel ; Vela, Ion ; Gillich, Gilbert-Rainer 等

1. INTRODUCTION

In the recent past period a lot of effort has been lay down to improve the design, the construction and the control of the robotic grippers. Even so, gripper's actuators have still been mostly low in energy density (power-to-weight ratio), inflexible in parameters design, and complex in structure and transmission agent.

In order to overpass these disadvantages of conventional actuators, were used as driving elements for the actuator shape memory alloy (SMA) springs.

Basically, SMAs are functional materials sensing the changes in the ambient temperature, being able to convert their shape to a pre-programmed structure. They are more important for what they do (as an action) than for what they are (as a material). SMAs recover their original induced shape after they exceed a transition temperature (a narrow temperature band, not a single point) between a low-temperature phase and a high- temperature phase.

While NiTi is soft and easily deformable in its lower temperature form (martensite), it resumes its original shape and rigidity when heated to its higher temperature form (austenite). This is called the one-way shape memory effect. The presence of permanent deformation, related to plastic strains or to the residual martensite variants occurring during the material training, allows reversible spontaneous shape change to be obtained during cooling and heating processes without application of any external stress, known as the two-way memory effect.

The occurrence of these unique properties comes from a molecular rearrangement related to a solid state phase variation, the values of these variables being strongly affected by the alloy's composition.

Researches on the electro-thermo-mechanical characteristics of SMA material have already confirmed that SMA actuators have several advantages compared with conventional actuators.

Thus, the recovering force per unit weight of an SMA actuator is higher than that of the conventional robot actuators, while the design of an SMA actuator can be very flexible. The structure of an SMA actuator is fairly simple in comparison with the conventional actuators. Another advantage of SMA actuators is they are easy to be controlled due to the fact that they are heated by electric current device (AC or DC). The first characteristics of SMA wires and springs were experimented on by controlling the electric current (Wang et al., 2003).

[FIGURE 1 OMITTED]

As main disadvantage of SMA actuators is their relative slowness. The differential SMA wires were used to accelerate the speed of response (Epps and Chopra, 1997).

In this study, a differential SMA actuator is employed to accelerate the speed of response and to enlarge the range of motion of the SMA actuator.

So, a number of experiments were performed in order to be able to predict the thermo-mechanical behaviour of shape memory alloys, focused on finding the dependence of actuator's stroke and force function of temperature and a method suitable for the improvement of the cooling time, fact which will lead to augment response in real-time of the actuator.

2. THE ACTIVE NITINOL SPRINGS

The active components of the developed actuator are two pair of Nitinol springs which action in an antagonistic way, heated by electric current. One pair of springs, namely the tension springs are made of 750 [micro]n diameter wire, the coil of the spring being 6 mm. The actuation current (2 A) activates them between 45-55 [degrees]C, each of the spring being able to lift 350 gr. The spring is deformed (fig.1) at a temperature below [M.sub.f] (A to B), followed by unloading (B to C) and again loading with a reaction R (C to D). Shape recovery occurs at an opposing force R during heating to a temperature above [A.sub.f] (D to E), so work is done.

The other pair of springs, respectively the compression springs are made from wire of 950 [micro]m diameter, forming a spring coil of 9 mm. They are activated at 55-65 [degrees]C by a current of 3 A, developing a force exceeding 4 N.

The sample is deformed (A to B) and unloaded (B to C) at a temperature below [M.sub.f]. The residual deformation is restored during heating to a temperature above [A.sub.f] (fig.2).

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

The current research performed in the Centre of Advanced Research, Design and Technology CARDT from Eftimie Murgu University Resita, has as aim to develop an SMA actuator driven by the two pairs of Nitinol active springs and experimentally based on the observation of thermo-mechanical transfer, to examine the feasibility of improving the real-time response of the SMA actuator in grasping processes (Miclosina et al., 2007).

The actuator is able to perform a25 mm maximum stroke, developing a 7 N force. The experimental device is designed and produced in order to give the opportunity to investigate the characteristics of the SMA actuator and then, of the whole gripper. The relations between the characteristics of the gripper and the cooling methods, the heating current and the action frequency are studied experimentally. Furthermore, the position's control of the SMA actuator will be developed.

3. ELECTRICAL COMMAND

When we utilize AC to heat the SMA sprigs, it should be at a frequency significantly higher than the bandwidth of the SMA actuated system to avoid displacement fluctuations. The current I that flow through a SMA element with resistance R due to a certain voltage drop U and the corresponding power P can be found from the following well-known relationships:

I = U/R (1)

P = I x U or P = [I.sup.2] x U (2)

Integrating a plot of power versus time and then dividing by the total time we obtain the average power.

The required average power to achieve actuation temperature can be supplied by a steady or time varying signal. An example of a time varying signal that has been used extensively in electrical actuation is Pulse Width Modulation (PWM). The advantage of this method is that the SMA element suffers a uniform heating. As expected, larger voltages / currents cause much faster actuation, but decrease the lifetime of the active element.

[FIGURE 4 OMITTED]

Thus, it can be easily calculated that the maximum theoretical efficiency of a Carnot cycle between [A.sub.f] and [M.sub.f] has a range of 10 %. In reality, the conversion of heat into mechanical work is much less efficient, resulting that the real efficiency is at least one order smaller than the theoretical Carnot value. This efficiency also depends on factors as the form and the shape of the SMA-actuator.

4. RELIABILITY

The SMA actuators are reliable for at least some thousands of cycles, being in the meantime designed to develop force over a considerable range of motion (Jan, 1999).

The reliability of shape memory devices depends on their global lifetime performance. Time, temperature, stress, strain, strain mode and the amount of cycles are in this respect, important external parameters. Internal parameters that can have a strong influence on the lifetime are: the alloy system, the alloy composition, the heat treatment and the processing. For general purposes, the maximum memory effect, strain and/or stress, will be selected depending on the required amount of cycles. The following table (D. Stockel 1992) can be used as a guideline for standard binary Ni-Ti alloys. It should however be remarked that special treatments and ternary alloys such as Ni-Ti-Cu can yield much higher values of maximum strains and stresses.

5. CONCLUSION

SMA Thermal actuators are much more simple and much easier to be realized that other types of conventional actuators, but have the disadvantage of a reduced operating speed.

The specific actuator designing steps are: dimensioning active elements in function of the imposed source and force; dimensional and structural synthesis of the associated mechanical structure; designing the activation (heating) method and the design the command and control system.

Important to notice is the fact that the design of shape memory applications always require a specific approach, completely different from the classic structural materials actuator's design.

6. REFERENCES

Epps, J.J. & Chopra, I. (2001). "In-flight Tracking of Helicopter Rotor Blades using Shape Memory Alloy Actuators", Smart Materials and Structures, 10 (1), pp 104-111, doi: 10.1088/0964-1726/10/1/310, ISSN 0964-1726

Jan, V.H. (1999). "Non-medical Applications of Shape Memory Alloys", Materials Science and Engineering A, pp 273-275; pp 134-148, ISSN: 0921-5093

Miclosina, C.; Vela, I.; Gillich, G.-R.; Amariei D. & Vela, D. (2007). "On the use of robotic grippers with shape memory alloy actuators in handling light-weight work pieces", Proceedings of the 18th international DAAAAM symposium, 24-27th October 2007, pp 451-452, Katalinic, B. (Ed.), ISBN 3-901509-58-5, ISSN 1726-9679

Stockel, D. (1992). Int. on New Actuators, Actuator, Bremen, pp 79-84

Wang, J.H.; Xu, F.; Yan, S.Z; & Wen, S.Z. (2003). "Electro thermal Driving Mechanism for SMA Spring Actuators" Materials Science Forum, pp 423-424; pp 461-465, ISSN: 0255-5476

Tab. 1. Reliability of SMA devices

Cycles Max. strain Max. stress

1 8 % 500 N/[mm.sup.2]
100 4 % 275 N/[mm.sup.2]
10000 2 % 140 N/[mm.sup.2]
100000+ 1 % 70 N/[mm.sup.2]