DESIGN ASPECTS OF SHAPE MEMORY ALLOY BASED ACTUATORS

1.1.1        Generate Mechanical Work from SMA

During the phase transformation SMAs are able to generate mechanical work. The force and the displacement produced during this temperature changes are two important parameters to be considered when developing SMA actuators. This force and displacement produced are basically depending on shape of the SMA element, chosen thermo-mechanical treatment and applied load [6]. Due to the ability to recover a preformed shape after a variation of temperatures, SMAs can be used as temperature sensors as well [7].

                         Figure 2‑4 Thermo-mechancal hysteresis of an SMA helical spring [8]

1.1.2        Shape Memory Alloys for the Actuators

SMAs can be used to build high performance shape memory actuators. These actuators have sense of high dynamic performance and high positioning accuracy. Basically, SMA material can exhibit one-way memory effect and two-way memory effect.  In one way memory effect,

SMA can be deformed in cold temperature and holds its shape until it gets heated. When it is heated, it returns to its original state. Even though by lowering the temperature again does not have any effect on its shape. In two-way memory effect, the alloy remembers two shapes. So when the alloy is cold, it turns to one shape and when heated it turns to another shape [9].

Both one-way SMA and two-way SMA can be used when developing actuators. Two-way SMA can perform in two directions due to its two-way shape memory mechanism though transformation strain associated with it is normally only half of that in one-way SMA. An alternative solution is to put two one-way SMA based actuators one against another to generate mechanical two-way performance. In this case, heating SMA in one actuator to get forward motion, and heating SMA in another actuator to reverse motion. When considering mechanical two-way actuators, they have higher motion and higher force than that in material two-way actuator. When considering material two-way actuators, they are simpler, compacter and much less elements involved [10].

                             Figure 2‑5 Basic types of SMA actuators using one-way SMAs [11]

Figure 3.2 shows three basic types of Shape memory alloy based actuators using one-way SMAs. Figure 3.2(a) shows a One-Way SMA actuator. In this case, the SMA element is elongated initially at low temperature. And when it is heated, element P moves to the direction of the arrow. Figure 3.2(b) shows a Biased Actuator, which can move the element P both back and forward directions. In this case, the SMA element is deformed at low temperature in case of spring. When it is heated, the force which is generated in SMA element pulls the spring by storing energy in it. When the SMA element is cooled, the energy stored in the spring is released and it cause to the SMA element deforms back. So, likewise by lowering and increasing the temperature of SMA element can complete the cycle. Figure 3.2(c) shows a Two-Way Actuator which includes two SMA elements in both opposing sides.in this case, two SMA elements are used in both opposing sides, instead of the above biased actuator. So in this structure, any motion can be performed by appropriately cooling or heating the two SMA elements.

Two-way SMA-based actuator is similar to Figure 3.2 (a), one-way actuator in shape. Its behaviour is more similar to Figure 3.2 (b), biased actuator, because of two-way effect, the alloy can remembers two shapes [10]. As SMA actuators the SMA elements have been made in the following different shapes according to the requirements.

Table 2‑4 Shapes and conditions of SMA elements according to the requirements [12]

Shapes and conditions	Motion types	Requirements
Straight wires in tension	small linear motion	high force
Helical elements	large linear motion/ large rotation	small force /Small torque
Torsion bar/tube	large rotation	small torque
Cantilever strips	large displacement	small force
Belleville-type discs	small linear motion	high force

According to above details, generally only large deformations or large forces can be obtained from SMA, but not both [12].

1.1.3        Advantages of SMA deployed actuator systems

In recent years, the use of SMA based elements has been proposed as an alternative to electrical motors [13]. SMA can be used to build high performance shape memory actuators having sense of high dynamic performance and high positioning accuracy, which are typical requirements for the actuators of robotics systems [14].The main advantages of SMA-Existing Actuators are,'

1. Mass and volume savings - SMA actuators have capability to achieve best output/weight ratio which cannot be realized by traditional actuators or other conventional motors based systems [14].
2. Avoidance of deployment shock loadings - SMA actuators are always associated with spring-deployed structures. Therefore no more need for dampers.
3. Noiseless operation - SMA actuators remove the vibration disturbances to other payloads that are normally associated with motor driven deployment.
   
4. Sensing capability - Both actuating and sensing functions can be combined by measuring changes in electrical resistance associated with the phase transformation.
5. Mechanical simplicity - No more need for dampers, motors etc.  Hence overall system complexity can be reduced.
6. Higher reliability - Deployment is beginning due to a phase transformation and the speed of deployment is related to the temperature of the SMA element and SMA actuators always avoidance of deployment shock loadings.

1.1.4        Limitations accessed when designing SMA actuators

SMA materials have a significant potential for deployment actuators. So, the number of SMA actuator-based applications in today’s world still quite small. This mainly happen due to following three reasons [15]

1.      The design procedure for SMA-based actuators is different from ordinary design in many aspects. Therefore it required an additional knowledge about innovative design aspect.

2.      SMA is generally very different from other traditional materials. Therefore it required a deeper understanding about their thermo mechanical behaviours for design actuators.

3.      Available constitutive thermo-mechanical models for SMAs are in most cases far from reliable. Therefore the development of a reliable methodology to support the design process is required.

1.1.5        Material Selection

There are two main groups of shape memory alloys are available in the commercial market. They are Copper based alloys (CuZnAl and CuAlNi) and NiTi alloys (Nitinol). NiTi SMAs are more expensive than copper based alloys (5-10 times) and they are more difficult to machine [16].

Ø  Reasons for NiTi positively influence the actuator control performance [17].

·      Considerably larger resistivity (about 10 times higher).

-Smaller currents are to be used in case of electrical resistive heating.

·      The functional properties are far better reproducible for NiTi than for other alloys.

·      Higher mechanical strength and higher working stress.

·      Higher memory strain.

·      Higher work density (delivered work per volume unit) can be obtained.

·      NiTi has a claimed corrosion resistance comparable to stainless steel.

Ø  Reasons for NiTi negatively influence the actuator control performance [17].

·      Lower transformation temperatures.

·      Considerable hysteresis.

1.1.6        Thermal Control Aspect of SMA Actuator

Thermal control mechanism is the key for operating a SMA actuator. Cooling of the shape memory element can be performed as radiation, conduction, and convection. Because of the use of temperatures below 100⁰C all radiation effects are negligible. When the enveloping fluid is not moving, the conduction effect is predominant. Therefore in actuator system convection is the main effect when the fluid is moving.

Heat transfer between an object and ambient is given by:

            P_t = h × A × ∆T          

Where:

P_t            = transmitted power (W)

h          = heat transmission coefficient (W/m².K)

A         = cooling surface (m²)

∆T       = temperature difference between object and ambient (K)

The transformation temperatures are determined by temperature difference between SMA and ambient [18]. Therefore the amount of transmitted power mainly depends on h or A. Element such as heat sink and peltier element can be used for cooling effects. In SMA active elements, good heat transfer between active element and cooling device requires. In practically it is very difficult to achieve these performances due to the elongation and contraction cycle. Therefore, liquid convective cooling is most suitable.

                          Figure 2‑6 Three-dimensional behavior for cooling of a NiTi wire [18]

1.1.7        Shape of the SMA Alloy as an Actuator

Cold machining of NiTi is a difficult process. Therefore the SMA is mainly available in simple shapes like wires, pipe, bars, rings and strips [19]. As alternative machining processes of NiTi, Electro Discharge Machining and laser cutting also can be used, but when starting from an ingot, a good Shape Memory behaviour is difficult to obtain, because this way of processing does not induce any dislocations in the material.

Many existing Shape memory alloy based designs are made using Shape Memory springs.  When using spring, it generates a large macroscopic displacement out of a relatively small microscopic strain. But the stress distribution over the cross-section of the spring is not constant. In this case relatively large amount of material volume is needed and for the same output, a larger material volume has to be heated and cooled. This has a negative effect on the efficacy and the bandwidth of the spring based actuator. Using wires as an active element has the advantage of optimal use of the material. In this case use minimal amount of Shape Memory material for same output [20].

Table 2‑5 Comparison of load cases for SMA actuators

Load case	Efficiency (%)	Energy density (J/Kg)
(Carnot)	9.9	-
Tension	1.3	466
Torsion	0.23	82
Bending	0.013	4.6

1.1.8        Typical SMA Actuator

Generally SMA actuator is made up of several parts. These are:

·         Mechanical system

·         SMA element

·         Bias element to restore the deformed shape of SMA element

·         Electric control unit

·         Set of fixtures used to couple the actuator with the mechanical system

Typical SMA based actuator contains a Shape memory element, comprising one or more wires, coils or varies formed shapes of SMA material. These elements can be stretched easily when cool and contract forcibly when hot. They are typically arranged in active pairs in robotic applications [21]. An element is usually heated by means of Joule heating and cooled by heat transfer to the environment. The limiting factors of the speed of these type of actuator

are the heating and cooling rates of the SMA elements. The cooling rate of this SMA element can be increased by including forced oil or water cooling, air cooling, and using thinner SMA wires element.

When miniaturization of develop actuators that high mechanical behaviours should be in a limited space. For this reason one main requirement is that it need be light and compact in size. In different type of actuators, SMA actuators have the highest power to weight ratio, which means that they have a high potential to miniaturization.

In 1989, Hirose et al have compared the power/weight ratio vs. weight of a particular form of SMA actuator with many other conventional motors. According to the above results, he obtain that SMA actuators have the potential capability to achieve high power/weight ratio than other traditional actuators [22]. According to above results SMA actuators have the highest power to weight ratio among light weight technologies, which means that they have a high potential for miniaturization. Micro actuators are most critical components in developing a successful locomotive mechanism for a micro robot and other micro scale applications, because they have to meet tough design specifications in terms of size, power, force, motion range and safety.