Natural Heat-Sinking Control Method for High-Speed Actuation of the SMA

Abstract

This paper describes two methodologies for increasing the actuation speed of the shape memory alloy (SMA) actuator in ambient environment. The first method involves the implementation of a simple, light-weight heat sink, which consists only of a combination of an outer metal tube with the silicone grease, but able to cool the heated alloy effectively. The second method describes a high current pulse actuation that actuates the alloy fastly using pulses in the milliseconds order. We hypothesize that a fast actuation of the SMA results in small increase in temperature, due to energy transformation from heat energy to the kinetic energy in the SMA. This new heating method revolutionizes the actuation of the alloy for a significantly faster response.

Keywords

Shape memory alloy (SMA)simple light-weight heat sink high current pulse actuation

1. Introduction

Shape memory alloys (SMAs) are materials that exhibit the characteristics of both the shape memory effect and the super-elasticity [Auricchio, F., 1995]. The shape memory effect involves a temperature change during phase change. Referring to Fig. 1, when SMA is at low temperature, it mostly remains in the form of crystalline structure of martensite (b), which displays an elastic nature and easily deforms (c). When heated, the crystalline structure transforms to the austenite structure (a), which is less elastic thus strain induced to the SMA at the lower temperature martensite phase can be recovered in the austenite phase. The super-elasticity is an effect where phase change from martensite to austenite occurs without the change in temperature but rather in the increase of load and vice versa ((a)-(c)). As unloading takes place, martensite phase is restored, which can be observed in its plasticity behavior [Bhattacharyya et. al, 2001].

Fig. 1.

Thermo-mechanical property of the SMA

The shape memory effect is the main concern of this research. The SMA produces high recovery forces of approximately 50[kgfmm⁻²] which gives a high output force to weight ratio when using the alloy. The SMA wire does not consume much space or weight when used in the development of SMA based actuators.

However, the SMA also exhibits several disadvantages, such as large hysteresis and slow in actuation speed, of approximately 0.2[Hz] [Dono, S., Saito, A. & Kuwata, T., 2003]. The slowness of the actuation speed, which is known as the major hindrance to its wide application prospects, is due to the slowness of heat dissipation from the alloy to the environment. On top of that, when the alloy is applied in an actuator system, inappropriate heating for actuating the SMA also further contributes to the slowness in the heat dissipation process. These factors will then cause the alloy to remain in its less elastic austenite structure, eventually leading to the difficulty in extension and contraction of the alloy.

In recent years, there have been researches into increasing the actuation speed of the SMA from the heat-sinking aspect and actuation aspect of the alloy. [Russell, R.A. & Gorbet, R.B., 1994] proposed the usage of a mobile rotating heat sink, consists of a metal bar attached to a shaft which rotates in the direction of the contraction of the SMA wire on a horizontal plane, which then cools off the heated SMA wire upon coming in contact with it. The heat sink was reported successful in tracking a 0.16[Hz] rectangular wave, but is bulky and heavy in its structure. [Furuya, Y., & Shimada, H., 1990] described a crab-like robot actuated using SMA springs in water immersion for cooling purposes. The crab moves at a speed of 7.5[cms⁻¹] but the power used increases by 10 fold, compared to when operated in ambient environment which consumes a power of 1[kW].

[Kuribayashi, K., 1991] replaces conventional current limiting method with direct measurement of the temperature of SMA using Cu-Constantan thermocouple. Forced air convection is incorporated using an electrical fan, achieving a response of 0.4[Hz]. The electrical fan makes the actuator bulky and not widely applicable.

[Featherstone, R., & Teh, Y.H., 2004] proposed the usage of resistance as a form of temperature measurement and a maximum safe heating current was designed to prevent overheating. This method was incorporated into Grant's two-stage relay controller which showed an improvement of double in its actuation speed. However, the change in resistance of the SMA is small, approximately 10[%] which therefore makes measurement difficult and inaccurate.

[Seldon, B., Cho, K.J. & Asada, H., 2004] showed how the SMA can be heated segmentally using Peltier devices. In their later work [Seldon, B., Cho, K.J. & Asada, H., 2005], using the same experimental setup, they proposed Hysteresis Loop Control (HCL) method, specifying intermediate temperatures to be ‘pulled back’ to after extreme temperatures of cooling and heating have been reached. This methodology is claimed to have shortened latency times significantly and saved power. The actuation speed achieved by four segment coordination was 0.1[Hz], and Peltier devices are large and bulky, resulting in very low engineering prospects.

There are other control methods proposed in conventional research efforts [Eren, Y. et.al, 2002] and [Elahinia, M.H. & Ashrafiuon, H., 2002] which involve heating of the alloy in a gradual and slow manner which is inefficient and results in heating up the alloy excessively. By increasing actuation speed of the alloy, and maintaining the actuator's light weight and simplicity in nature (Fig. 2), a much wider application can be realized and simultaneously many low force actuation problems can be overcome, which are the objective and motivation behind this research.

Fig. 2.

Research targets simplicity in design and high response of the SMA actuator

2. Methodology

This research deals with the slow actuation speed problem from the following perspectives:

i.)
development of a heat-sinking mechanism for the SMA in ambient temperature for maintaining simplicity for wide application prospects of the alloy.
ii.)
development of a high current pulse actuation method for the SMA actuator for optimum heating of the alloy.

In later parts of this paper, we also describe initial efforts in developing a SMA-based actuator for the application as artificial muscles in prosthetics, implementing the methodologies in (i) and (ii) above.
2.1. Improvement of Heat-Sinking

Natural cooling for the SMA is known to be slow and ineffective, therefore heat-sinking is of utmost importance. And however, due to the fact that complex heat-sinking mechanisms reduce the engineering prospects of the SMA actuator as a whole, we propose the application of a simple, light-weight heat sink to facilitate heat-sinking in ambient environment, as most engineering demands are for ambient environment actuated systems.

2.1.1 Proposed Heat Sink

Our proposed heat sink consists of a combination of an outer metal tube together with the silicone grease shown in Fig. 3. The silicone grease is smeared onto the surface of the SMA of diameter ø 0.3[mm] making sure that no area is left uncovered. In this condition, the silicone grease coated SMA is then inserted into the outer metal tube of diameter ø 0.8[mm].

Fig. 3.

(a) Silicone grease (b) outer metal tube and (c) cross sectional diagram of the proposed heat sink with the SMA wire

The concept of our proposed heat sink is based on the Fourier's heat transfer equation [Masahiro, S., 1995] given below:

q = - λ (\frac{\partial T}{\partial n})

(1)

Q = - λ A t (\frac{\partial T}{\partial n})

(2)

where q, T, and Λ, are the rate of heat transferred per unit area [Wm⁻²], temperature [K] and thermal conductivity [Wm⁻¹·K⁻¹] respectively. For simplicity purposes, we consider only the horizontal distance measured from the SMA for a one dimensional analysis. The total rate of heat transferred for the whole object can be obtained by considering the total surface area of the object involved which is given in (2). Next, we consider an infinitesimal heat transfer in one dimension as shown in Fig. 4. The difference between heat flowing into and out from two vertical planes of distance Δx can be expressed in the form of (3).

\begin{aligned} q_{x - \frac{Δ x}{2}} - q_{x + \frac{Δ x}{2}} \\ = - {(λ_{x} \frac{\partial T}{\partial x})}_{x - \frac{Δ x}{2}} + {(λ_{x} \frac{\partial T}{\partial x})}_{x + \frac{Δ x}{2}} \end{aligned}

(3)

Fig. 4.

One dimension infinitesimal heat transfer

However, expanding the right terms using Taylor's expansion for one dimension gives us the equation in (4).

\begin{aligned} - {(λ_{x} \frac{\partial T}{\partial x})}_{x \pm \frac{Δ x}{2}} \\ = - {{(λ_{x} \frac{\partial T}{\partial x})}_{x} \pm \frac{\partial}{\partial x} {(λ_{x} \frac{\partial T}{\partial x})}_{x} \frac{Δ x}{2} + 0 (Δ x^{2})} \end{aligned}

(4)

Applying (4) into (3), the equation can rewritten in the following form.

q_{x - \frac{Δ x}{2}} - q_{x + \frac{Δ x}{2}} = \frac{\partial}{\partial x} {(λ_{x} \frac{\partial T}{\partial x})}_{x} Δ x

(5)

Since heat is a conserved quantity, the difference between heat in and heat out from an infinitesimal block of heat flow can be attributed to the rise in temperature of the medium encapsulated by Δx. By considering the density ϱ and heat capacity c of the medium, its increase in temperature is shown in the following equation.

ρ c Δ x \frac{Δ T}{Δ t} = (q_{x - \frac{Δ x}{2}} - q_{x + \frac{Δ x}{2}})

(6)

when Δt→0, we obtain the following differential equations for one dimensional heat transfer.

\begin{aligned} ρ c \frac{\partial T}{\partial t} = \frac{\partial}{\partial x} (λ_{x} \frac{\partial T}{\partial x}) \\ ρ c \frac{\partial T}{\partial t} = λ \frac{\partial^{2} T}{\partial x^{2}} \end{aligned}

(7)

Considering the simple fact that thermal conductivity ΛSI (for silicone) > ΛAIR (for air) and surface area Asma (for SMA) < Atube (for outer metal tube) (Fig. 5), we can easily predict from (7) that the heat from the SMA spreads to a wider surface area of the outer metal tube through the high thermal conducting silicone grease, providing a faster dissipation of heat from the SMA wire to the environment. The thermal conductivities for the SMA, silicone grease, outer metal tube and air are provided in Table 1. SMA without heat sink has only air as its medium of contact, which is slow in transferring of heat from the heated SMA to the environment.

Fig. 5.

Cross-sectional diagrams and each case's way of heat dissipation

Table 1.

Thermal conductivities for related materials

	SMA	Silicone	Metal	Air
Thermal conductivity	20.9	2.0	315.0	0.0386

2.1.2 Verification Experiments: FEM Simulation

To analyze the heat transfer property of the proposed heat sink, we adopted the finite element method (FEM) to solve the heat transfer equation (7) on a mesh consisting of 100 × 100 nodes. A visual-based computer simulator was constructed as seen in Fig. 6. Upper figures in (a) and (b) represent the meshes for SMA without and SMA with the heat sink respectively. The blue circles in (b) represent the boundaries of the SMA wire, silicone grease and the outer tube respectively.

By taking into account the thermal conductivities of the SMA wire, silicone grease, air and outer metal tube, a simulation of the rate of heat dissipation was carried out. Initial temperature of SMA was set at 26[°C] and was then increased to 66[°C]. In real world, SMA achieving this temperature and above is assumed to have undergone the austenite transformation, recovering a large part of its strain. The model was then let to ‘cool down’. Using this simulation, the rate of dissipation of heat for the SMA with and without heat sink could be observed and compared (Fig. 6).

In the case of the SMA without heat sink (Fig. 6(a)), we can observe the heat builds up around the wire and prevents further diffusion of heat out from the alloy. Due to the low thermal conductivity of air, the relatively fast heat diffusion from the SMA is slowed down by surrounding air, resulting in heat accumulation effect near the wire which can be observed in the diagram Fig. 6(b) below.

Fig. 6.

Heat transfer model based on FEM for SMA without heat sink and SMA with heat sink.

Conversely, when the heat sink is used, heat from the surface of the SMA is spread to a larger surface area of the outer metal tube via the high thermal conducting silicone grease. As a result, heat does not accumulate near the wire but transferred externally to a larger surface area to be disposed to the environment, validating the effectiveness of the proposed heat sink.

Simulation results show that within 4[s], outer metal tube with silicone grease brought about a 7[°C] reduction in temperature compared to natural cooling (Fig. 7). As soon as heating was ‘switched off’ the cooling curve for when both silicone grease and outer metal tube were in used, shows a steep slope, and that cooling was taking place at a rate of 5[°Cs⁻¹], faster than that without heat sink. The reduction in temperature of 7[°C] (when both the outer metal tube with the silicone grease were used) after only 4[s] suggests that large amount of heat from the heated SMA is being channeled out to the environment through the larger surface area of the actuator.

Fig. 7.

Simulation results of the rate of dissipation of heat

2.1.3 Verification Experiments: Real World Actuation

To further investigate the effectiveness of the proposed heat sink, real experiments were conducted using the SMA (manufactured by Nilaco) as an actuator to haul up a weight of 3[kg] periodically (Fig. 8) for both conditions with and without usage of the proposed heat sink. A faster descend of the weight during cooling is expected to be seen when heat sink is in used.

Fig. 8.

SMA wire used to haul up a 3[kg] weight

A SMA wire of 0.3[mm] in diameter and of length 60[cm] was used for the experiments. A position sensor (PSD sensor, GP2D12) was placed below the weight for detection of the contraction of the SMA upon actuation. On the alloy was attached a thermocouple (TCKT0022) for temperature monitoring (to avoid overheating). Due to difficulty in detecting temperature of SMA accurately in the outer tube, temperature comparison could not be shown in this section. The alloy was passed through a current transducer (LTS25–NP) for contact-less currrent detection. The sensory feedback to the PC was done using a 12 bit CONTEC PC card and the heating of the SMA was controlled using the H8/3664 micro-controller. The sensors and control circuits are connected as shown in Fig. 9. A diode (V39A64) was inserted in the top portion of the circuit for regulating one way flow of current.

Fig. 9.

Schematic diagram of the circuitry design for the entire system settings

Experiments were conducted in room temperature of 27[°C]–28[°C] with minimum wind condition. PWM (Pulse Width Modulation) signals were induced to the SMA to haul up the 3[kg] weight periodically. Comparative experiments were done, where rectangular wave of 0.2[Hz], 2.0[A] of duty ratio 0.4 was applied to the SMA in both cases, with and without heat sink.

As we compare the results obtained from the experiments shown in Fig. 10, in the case of the SMA without heat sink, the alloy reached its maximum strain after approximately 1[s] from start. Natural cooling, without any heat sink seems to be ineffective and slow in channeling heat out from the alloy.

This phenomenon could be observed in Fig. 10(a) (a graph which displays the position of the weight against time) which shows that SMA wire without heat sink had no sufficient time in recovering a visible portion of its strain before the subsequent heating. In the case of SMA without heat sink, even after its current supply was cut, the weight remained at its maximum height and started descending only after 2[s] the current was switched off. As a result the weight remained at the position where the alloy achieved its maximum strain, after about 5[s] from start. As time progressed, the oscillation converged to the position of its maximum strain. Conversely, observing the results in Fig. 10(b), when the proposed heat sink was incorporated, the SMA wire cooled off at the point when current was turned off. The wire recovered a visible part of its strain, which could be observed as the response of continuous peaks and valleys. The oscillation width maintained a value of 7.5[mm] even after 25[s] of actuation, compared to only 3[mm] of width achieved by natural cooling. With the usage of heat sink, the weight did not rise as fast as compared to its counterpart, and when voltage source was cut off, it extended at a faster rate than the SMA without heat sink, with only air as its cooling medium. This can be thought of as, a large amount of heat was being dissipated even during the heating process was taking place.

Fig. 10.

Comparative experiments for cases with and without heat sink when 0.2[Hz], 2.0[A] of duty ratio 0.4 rectangular signal was applied to the SMA

The concept of the proposed heat sink is simple and yet effective in cooling off the heated SMA as mentioned in previous section. The silicone grease smeared onto the SMA wire insulates the wire from the outer tube and simultaneously acts as a high thermal conducting material for the heat from the SMA to the outer metal tube. The thermal conductivity of both silicone and air are ΛSI=2.0 and ΛAIR=0.0386 which shows that the transfer of heat from the SMA is done at a rate equivalent to the ratio of the thermal conductivities of both the materials, $\frac{λ_{S I}}{λ_{A I R}} = 51.8$ . It is therefore made sure that the intermediate silicone grease is in contact with the tube and the SMA wire. The outer metal tube then, provides a larger surface area for the dissipation of heat from the SMA wire to the environment (Fig. 11). Let us denote the surface area of SMA and outer metal tube as Asma and Atube respectively, the diameter of SMA and outer metal tube as ΦSMA and ΦTUBE respectively, the dissipation of heat when with and without the heat sink as Qwith_hs and Qwithout_hs respectively. From (2), we can rewrite the relationship of both heat dissipations from the SMA to the environment as the following:

\frac{Q_{W I T H_H S}}{Q_{W I T H O U T_H S}} = \frac{A_{T U B E}}{A_{S M A}}

(8)

Fig. 11.

Outer metal tube

By substituting the surface area $A = π Φ L$ , we can express the heat transfer in terms of the diameters as follows:

\frac{Q_{W I T H_H S}}{Q_{W I T H O U T_H S}} = \frac{Φ_{T U B E}}{Φ_{S M A}}

(9)

which states that the ratio of the amount of heat being dissipated in both conditions can be expressed as the ratio of their effective diameters respectively. Therefore, we can forecast the amount of heat dissipated for different diameters of SMAs and outer metal tubes using (9) as shown above.

2.2 High Current Pulse Actuation Method

As mentioned earlier, conventional actuation methods have a high tendency to heat the SMA wire gradually, hence overheating the SMA wire causing difficulty in cooling in between actuations. We believe that in order to further improve the actuation speed of the SMA, a different approach in heating or actuation method is necessary. Instead of indirect heating using heat conducting materials (eg. Peltier devices) or gradual heating high current in the form of short pulses can be used as an effective replacement.

2.2.1 Hypothesis of Heat Energy Absorption

The concept of the high current pulse actuation method arrives from the hypothesis of the heat energy absorption during SMA actuation, based on the following equation (10):

E_{E l e c t r i c a l} = E_{K i n e t i c} + E_{H e a t}

(10)

Compared to ordinary metals which do not exhibit shape recovery characteristics, no kinetic energy is involved in the energy transformation from electrical energy to heat energy. Therefore, we believe that when dealing with the SMA, the existence of two forms of energies makes it possible for the interchanging of these energies. Hence, we propose two hypotheses for describing this characteristic of the SMA, which are as follow.

Heat is reduced when SMA is actuated

Rate of heat reduction increases as the actuation speed of SMA increases.

To verify these hypotheses, a series of experiments were carried out to compare the temperature increase and rate of temperature increase for the strained SMA and non-strained SMA. The experiment settings for both conditions are shown in Fig. 8, except that for non-strained SMA, the weight 3[kg] was replaced with a weight of 1[g], light enough as not to strain the alloy. Experimental results are presented as temperature curves against time in Fig. 12.

Fig. 12.

Comparing temperature increase and rate of temperature increase when heating strained and non strained SMA

Firstly, we notice that, the strained SMA when used in actuation shows a lower increase in temperature and rate of increase in temperature compared to when SMA not in actuation, when both settings were actuated using three different power inputs of 7.5[W] for a duration 5[s], 12.5[W] for a duration of 5[s] and 26.25[W] for a duration of 2[s].

These results suggest to us that when SMA is actuated, a part of the heat energy is absorbed for the transformation into kinetic energy in the SMA, which is consistent with the first hypothesis above.

Next, we compared the rate of temperature increase for the strained SMA when three input powers were induced into the alloy. When a high power of 26.25[W] was induced, the rate of temperature increase which should be double that of when inducing 12.5[W], showed a lesser value. However, an almost double in ratio could be observed for the gradient of the temperature curves when 12.5[W] and 7.5[W] were induced. As for control-experiment, all three gradients of the temperature curves obtained from the non-strained SMA when induced the three power inputs show consistency with the ratio of the induced power inputs. These results suggest to us that the higher the power is induced (the faster the SMA is actuated), the faster heat reduction takes place, hence a much slower rate of temperature increase when the SMA is actuated.

Using this concept, we propose the pulse actuation method, which applies high current (for fast actuation of the SMA) using short ON time pulses (for fast cooling of the SMA as input is switched off quickly).

2.2.2 Verification of Proposed Method

To investigate the effectiveness of the proposed high current pulse heating have over gradual actuation methods on the response of the alloy, we conducted experiments using settings shown in Fig. 13 for a more accurate data acquisition purpose. The SMA wire of length 85[cm] was used as an actuator on a horizontal plane, pulling a bias spring in a periodic manner.

Fig. 13.

SMA wire used to pull a bias spring periodically

The circuitry system in Fig. 9 was also integrated into this actuator. The position sensor was used to detect the contraction of the SMA towards the sensor therefore the displacement graphs are seen as decreasing distance against time. The thermocouple and current transducer are both for the detection of the temperature and the current flowing in the alloy respectively. A comparative experiment of actuation cycle between conventional gradual actuation method and our proposed high current actuation method was carried out. For gradual actuation, a low current of 0.75[A] of ON time 3[s] in an interval of 3[s] was induced in a periodic manner for a contraction of 12.5[mm]. On the other hand, in the proposed method, a current of 3[A] of ON time 30[ms] in an interval of 3[s] was induced in a periodic manner, achieving a same contraction of 12.5[mm].

Comparing the results obtained for the comparative experiments as shown in Fig. 14, for gradual heating, each actuation induced an amount of 23.6[J] to the SMA whereas the proposed method induced an amount of mere 3.8[J] to the actuation system. As temperature reached 55[°C], the gradual heating method only recorded a mere 4 actuation cycles, whereas the pulse heating method recorded 9 actuation cycles (Table 2).

Fig. 14

(a) Temperature, displacement and current graphs of SMA when heated gradually using 0.75[A], of duration 3[s] in every 3[s] periodically

Table 2.

Results of verification experiments

Heating	Power [W]	Energy [J]	Actuation Cycles When T°C ≈ 55°C
Gradual	7.9	23.6	4
Pulse	126	3.8	9

We notice that actuating the SMA with high current, the rise time can be decreased and if the input current is switched off quickly, the SMA can be cooled quickly before the next heating. We believe that fast actuation of the SMA results in low rate of temperature increase in the alloy.

2.2.3 Temperature Control for Fast SMA Actuation

One of the advantages of pulse actuation method is in the ability to control energy supply, hence the temperature rise of the system by regulating the duty ratio of the signals generated. In the next experiments, by implementing temperature control (Fig. 15), we show that when the SMA oscillation width reaches a maximum or constant value, the temperature of the alloy can be controlled within a certain value which further increase in temperature can be avoided.

Fig. 14

(Continued) (b) Temperature, displace- ment and current graphs of SMA when heated with high voltage of 3[A] of duration 30[ms] in every 3[s] periodi- cally

Duty ratios of the input pulses were regulated in the manner shown in the Table 3 and 4 for actuation of 1.33[Hz] and 2.0[Hz] respectively.

Table 3.

Temperature control by regulation of duty ratio for 1.33[Hz] actuation

SMA temperature [°C]	T <45	45<=T369:2747	47<=T369:2750
Duty ratio [%]	1.33	1.20	1.17
SMA temperature [°C]	T =>50
Duty ratio [%]	1.13

Table 4.

Temperature control by regulation of duty ratio for 2.0[Hz] actuation

SMA temperature [°C]	T <47	47<=T<49	49<=T<51
Duty ratio [%]	1.0	0.95	0.9
SMA temperature [°C]	51<=T<53	T =>53
Duty ratio [%]	0.85	0.8

The duty ratio at this initial stage was regulated in a step function manner, but as for future work, an adaptive control in regulating duty ratio can be constructed for improvement of this method. For example, initial pulses can be longer in duration for faster reaching of its stable oscillating state, only then shorten for temperature and oscillation stability maintenance.

Referring to the results obtained in Fig. 16 for 1.33 [Hz] actuation, a significant improvement in stability of actuation when compare the case of with and without temperature control. Fig. 16(a) shows that the SMA oscillates initially with a width of 1.72[mm] within a temperature range of 45[°C]–54[°C]. The centre of oscillation width increases in the direction of increasing strain. As the temperature increases above 55[°C], a decrease in strain can be observed, which finally converges to a horizontal displacement of 12.8[mm].

Fig. 15.

Regulation of duty ratio

On the contrary, implementing temperature control (Fig. 16(b)) enables the SMA temperature to be regulated within the range where a stable oscillation of 1.72[mm] can be maintained without further heating, A similar result can be observed in Fig. 17 where a stable oscillation of width 1.70[mm] was achieved slightly below the temperature of 54[°C] for 2.0[Hz] actuation. High current pulse actuation can be used for high speed actuation of the SMA, coupled with temperature control by duty ratio regulation the temperature of the SMA can be kept within the range where stability of oscillation can be maintained without further heating.

Fig. 16.

1.33[Hz] actuation of SMA actuator

3. Application

Incorporating the SMA with the metal outer tube enables the increase in the application field of the SMA wire. In the field of robotics where relatively long wires are used to produce large movements (SMA has a 4%–5% recovery strain), the metal outer tube provides an appropriate housing to the SMA. One end of the wire is fixed using a stopper while the other open end of the wire, which we call the ‘force point’ is attached to the object needed to be actuated. The end of the outer metal tube before ‘force point’, we call the ‘pivot point’. This mechanism is explained in Fig. 18.

Fig. 17.

2.0[Hz] SMA actuation with temperature control

Here, the SMA incorporated with the outer metal tube is used in the actuation of a robotic finger, a project currently in the research state in our laboratory. When the SMA strains upon heating by applied current, the stopper and the pivot point stop the contraction of the wire, allowing just the force point to act as the actuation point. The distance between the actuation point and the pivot point is changeable depending on the nature of the object to be actuated.

Based on the pivot-force point concept explained above, a more applicable SMA based actuator has been developed (Fig. 19). A diameter of Φ 0.3[mm], length of 1.0[m] SMA wire is inserted into the outer tube (Φ 0.8[mm]) and both ends of the wire are each fixed to an electrode. An opening of 25[mm] at the centre is made for the movement of another electrode attached to the wire at its centre. The electrodes are the points where current is passed through. Between the end electrodes and the outer metal tube is fixed a plastic stopper which acts as an insulator, and also to stop the ends of the SMA wire from contracting into the outer tube, thus allowing only the middle part of the wire (centre electrode) to be contracted, pulling the PE line in the direction of its contraction. Object to be actuated is attached to the PE line.

Fig. 18.

Application for the SMA wire when incorporated with the metal outer tube in a robotic finger

In order to actuate a degree of freedom (DOF), two actuators are used for the flexion and extension movements. When the finger is in flexion (Fig. 20), the SMA wire in parts AB and EF are in their original lengths while BC and DE are in extension. As the switches for AB and EF circuits are turned off while switches for BC and DE circuits are turned on, BC and DE contract, pulling the wire in parts AB and EF into extension. A torque in the clockwise direction occurs and the finger is extended as shown. A reverse process can be expected for the flexion of the finger when the orders for the switches are altered.

Fig. 19.

Schematic drawing of the SMA based actuator

Fig. 20.

The mechanism of the actuator for actuation of the robotic finger, clockwise torque (top) and counterclockwise torque (bottom)

4. Conclusion

We described two methodologies for increasing the actuation speed of the SMA. Firstly, we introduced a simple heat sink consisting of the silicone grease and outer metal tube. We verified the effectiveness of heat-sink using FEM analysis and real world experiments. Then we introduced a high current pulse actuation method and coupled with temperature control, a response of 2.0[Hz] of 1.70[mm] oscillation width has been achieved. Lastly we described the initial efforts in developing a new SMA based actuator and carried out preliminary trials for its application in prosthetics.

5. Future Work

As for the extension of our work, we would like to investigate the effectiveness of our two methodologies when combined. For that purpose, an adaptive control method to regulate the duty ratio of the pulse control to the SMA incorporated with heat sink is necessary. Mobilization will also be the research attention in our future work in the application of the SMA.

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