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DESIGN AND DYNAMICS OF A SHAPE MEMORY ALLOYWIRE BUNDLE ACTUATOR

Michael J. Mosley1, Constantinos Mavroidis2 and Charles Pfeiffer3

Department of Mechanical and Aerospace Engineering,Rutgers University, The State University of New Jersey

98 Brett Rd., Piscataway, NJ 08854-8058mjmosley@jove.rutgers.edu, mavro@jove.rutgers.edu, cpfeiffe@caip.rutgers.edu

Tel: 732 - 445 – 0732, Fax: 732 - 445 - 3124

ABSTRACT

In this paper, the design and dynamics of a new Shape Memory Alloy (SMA) actuator thatpossesses impressive payload lifting capabilities are presented. This actuator consists of 48 SMA wiresmechanically bundled in parallel forming one powerful muscle. The new actuator can lift up to 100 lbs.,which is approximately 300 times its weight. This actuator was tested in open loop experiments withdifferent weights and different inputs, such as step, ramp, sinusoid, and half sinusoid, and its dynamiccharacteristics were evaluated. Of special importance is the observed unpredictability of the actuator'sresponse when low voltages are applied. This characteristic suggests possible chaotic behavior of theactuator, potentially causing control difficulties in fine and high accuracy tasks.

1. Introduction

Robotic systems have been proposed to perform important tasks in the nuclear industry such ashazardous waste removal and decommissioning of nuclear sites [1, 2]. Since these tasks occur in ahighly radioactive environment, robotic and other automated systems are required to reduce workerexposure to radiation. Robots operating in these remote and hazardous conditions must have a highweight lifting capability and be able to cover a large workspace. At the same time, they must belightweight for easy transportation and dexterous enough to move in a cluttered environment. However,existing robotic systems for macro manipulation are characterized by poor payload to weight ratio andare cumbersome and voluminous.

The objective of this research is to develop a new generation of large-scale robotic manipulatorsthat are strong, lightweight, compact, and dexterous. The key methodology in drastically reducing theweight of the manipulator is the use of Shape Memory Alloy (SMA) wires as actuators of themanipulator joints. In this paper, the design of a new SMA actuator is presented that possessesimpressive payload capabilities.

Shape memory alloy wires, such as Nickel-Titanium (Ni-Ti) wires, have the property ofshortening when heated and thus are able to apply forces. This phenomenon, called the "shape memoryeffect,” occurs when the material is heated above a certain transition temperature changing its phasefrom martensite to austenite. Heating, and thus actuation, of an SMA wire is easily accomplished by

1 Graduate Student2 Assistant Professor, Author for Correspondence3 Research Engineer

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applying a voltage drop across the wire causing current to flow through the material, resulting in jouleheating. Ease of actuation is not the only advantage of SMA actuators. Other advantages are theirincredibly small size, volume, and weight, their high force to weight ratio, and their low cost. Theirlimitations include the need for a large length of wire to create significant linear motion, and the smallamount of absolute force obtained from one SMA wire [3].

Since 1983, SMA artificial muscles, have been used in micro-robotics [4]. It was discovered,that artificial muscle motion could be controlled and that their small size and displacement was ideal formicro-robots, i.e. robots smaller than 1mm. In this context, SMA has been used in many differentrobotic systems as micro-actuators [5-8]. Of special importance is the work done by Grant and Haywardwho proposed methods to increase the absolute percent strain and control of the non-linear effects of anSMA micro-actuator [9]. The use of SMA artificial muscles in large robotic systems has been verylimited. The main reasons are the small strains and absolute forces that are produced.

In this paper, the design and dynamics of a SMA based actuator are presented. This actuator canapply very large forces and thus be used to actuate the joints of macro-robotic mechanisms. The conceptof applying large forces is achieved by “bundling” a set of SMA wires and thus increasing the amount offorce obtained. The main constraint in “bundling” the wires is to increase the force capability of theactuator without sacrificing actuation time. A 48 wire SMA Bundle was constructed and actuated by acomputer controlled electrical circuit. This actuator was designed to lift up to 100 lbs. As far as theauthors are aware, this is the first SMA wire bundle actuator with such force capabilities. In this work,Flexinol Ni-Ti wires made by Dynalloy, Inc. are used. Flexinol wires are manufactured such that theyundergo a maximum length contraction of 8% and can apply a considerable amount of force comparedto their weight. For example, based on the manufacturer specifications, a 0.006 in. (150 µm) diameterFlexinol wire is capable of applying 0.728 lbf (3.24 N) [10]. In order to determine the performancecharacteristics of the wire bundled SMA actuator, an instrumented test rig was designed and constructed.This setup was equipped with a load cell, linear displacement sensor, current and voltage sensors, and athermocouple central to the bundle. Open loop experiments were conducted on the SMA bundle withtwo different weights and four different inputs: step, ramp, sinusoid and half sinusoid. In this paper,representative results from the step input tests are presented. Four different parameters wereexperimentally measured during the experiments: SMA bundle voltage drop, SMA bundle current, SMAbundle contraction, and air temperature at the center of the bundle. These tests showed interesting non-linear dynamic characteristics of the actuator.

2. Design

2.1 General Design Considerations

It is clear that using thicker wires or connecting many wires mechanically in parallel willincrease the force capabilities of the SMA actuator. However, the actuator bandwidth and power supplyrequirements may be dramatically affected by such an arrangement and have to be taken into accountwhen designing an SMA bundle actuator.

One of the parameters that greatly affects the bandwidth of a Flexinol wire actuator is thediameter. As an example, a 0.006 in. (150 µm) diameter wire may lift 0.728 lbs (330 gm) and require 2seconds before it is ready to cycle again whereas a 0.012 in. (300 µm) diameter wire may lift 2.76 lbs(1250 gm) and require 8 sec before it is ready to cycle again [10]. The physical reason behind this

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behavior is the change in surface to volume ratio and thus the change in the heat transfer characteristicsof the setup. Since the bandwidth of a SMA wire is largely determined by heat transfer through thesurface of the wire, changing the surface to volume ratio of the SMA actuator drastically changes itsperformance. Completing the simple calculation for the surface area and volume of a cylindrical SMAwire, one finds that the surface to volume ratio is inversely proportional to the radius of the SMA wire.So, in order to improve the force capabilities of a SMA actuator while not sacrificing bandwidth, it ismore beneficial to use many thin wires rather than a single thick wire. Not only must thin wires be used,but they also must remain separated so that the cooling medium, in this case air, can flow freely aroundall surfaces. One can think of this situation as analogous to the cooling tubes in a condenser where alarge number of tubes are used to greatly increase the heat transfer surface area. On the other hand, ifmany small wires were bundled together like the fibers in a rope, the advantages of the increased surfacearea would not be realized. Hence, the best way to increase the force capability without sacrificingactuation time is to connect many thin wires mechanically in parallel with space between the wires.

Initially, one may think that the best circuit design for the bundle itself is a parallel arrangementof the wires similar to how they are mechanically arranged. However, this results in impractical powersupply requirements. If all of the wires are connected electrically in parallel, the resulting effectiveresistance per unit length (linear resistance), EFFR , is given by the following equation:

∑=

=N

n nEFF RR 1

11(1)

where N is the number of wires in the bundle and nR is the linear resistance of one wire. Since all wireshave the same linear resistance,

N

RR n

EFF = . (2)

Therefore, the effective linear resistance of a SMA bundle with all wires in parallel is inverselyproportional to the number of wires that make up the bundle. As an example, a bundle consisting of 50,12 in. (30.5 cm) long, 0.006 in. (150µm) diameter wires ( R = 1.27Ω/in (0.0323Ω/m)) all in a parallelcircuit would have REFF = 0.0254Ω/in (0.01 Ω/cm) and total bundle resistance REFF = 0.305Ω. Given arequired actuation current of 0.4 A through each wire (i.e. 20 A for the bundle due to the entirely parallelarrangement), the resulting voltage drop across the bundle is 6.1 V.

This combination of low voltage and very high amperage can be avoided by creating a circuitwhere the wires are arranged electrically in a combination of series and parallel paths while remainingmechanically connected in parallel. For a combination of wires in series and parallel electricalconnections:

RLP

NIV SMAB ∗∗∗= and SMAB IPI ∗= (3)

where VB and IB are the voltage drop across and current through the bundle, ISMA is the single wireactuation current, N is the number of wires in the bundle, P is the number of parallel paths, L is thebundle length, and R is the single wire linear resistance. Note that the ratio N/P must be equal to aninteger if identical paths are constructed. For the case where there are 48, 0.006 in. (150µm) wires,

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Figure 1 shows a plot of current and voltage requirements in order to achieve actuation current (400mA) in each wire for different numbers of parallel paths.

In 1984, a SMA servo-actuator was designed using four SMA elements mechanically connectedin parallel and electrically connected in series [11]. The benefits of a lower current requirement, highervoltage requirement, and improved heat transfer over a single thicker wire were realized. However, if alarge number of SMA elements, say 48, are connected completely in series (one parallel path), therequired voltage for actuation becomes very large (see Figure 1). Using different numbers of parallelpaths allows the bundle to be tailored to different applications where there may be current or voltagerestrictions.

Figure 1: Bundle Current vs Voltage

Considering the above discussion, SMA Bundle design has four key parameters that determinethe load capability, displacement capability, and current/voltage requirements. These parameters are:diameter of the wire, the number of wires, the length of the bundle, and the number of parallel currentpaths.

2.2 Achieving Large Motions with Small Actuator Displacements

With only 5 to 8% deflection available, a SMA actuator must be cleverly attached to themechanism it operates in order to achieve large motions. For a simple mechanism consisting of amoving link that pivots about a fixed revolute joint, the small linear displacements of a SMA actuatorcan be converted into large angular motions by fixing one end of the actuator and attaching the free endto the moving link close to the center of rotation of the revolute joint. This is very similar to the waybiological muscles move the links that make up the body. Of course, mechanical advantage is lost as thefree end of the actuator approaches the center of rotation. Figure 2 shows a schematic of one way toachieve large angular deflections from a SMA actuator.

If the SMA actuator pulls on a small, flexible cable that wraps around and fastens to a pulleyfixed to the moving link, the relationship between the required pulley radius, Rp, the maximum SMAdeflection, ∆SMA, and the desired angular deflection of the moving link, Θ, is:

Θ∆

=SMA

pR . (4)

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Clearly from this relationship, for large angular deflections, Rp must typically be small comparedto the length of the moving link, M. Considering a static problem, the resulting ratio of requiredactuation force, FSMA, to the load, FL is:

SMAL

SMA M

F

F

∆Θ

= . (5)

Figure 2: Large Angular Deflections from a SMA Actuator

From Equations (4) and (5) it is clearly seen that if SMA actuators are used in macro-roboticsystems with revolute joints, the large angular motion requirement is satisfied by attaching the SMAactuator closer to the revolute joint axis. However, this attachment creates the need for large momentsand hence large linear forces to be applied by the SMA actuators. Therefore, very strong SMA actuatorsare required. This was the major technical motivation for the design and fabrication of the SMA Bundleactuator presented in this paper.

2.3 Specifying Bundle Parameters for this Research

One of the objectives of this research was to design a SMA actuator that was capable of applyinglarge forces. In theory, a SMA bundle actuator can be designed to apply an arbitrarily large force. Forpractical reasons, the goal was established to design an actuator that can apply a maximum of 100 lbf(445 N) over a distance of approximately 0.5 in. (1.27 cm). Given that Flexinol wires can contract 5 to 8% of their original dimension, a bundle length of 12 in. (30.5 cm) was chosen to meet the displacementcriterion. Simple experiments were conducted to determine the weight lifting capability of differentdiameter Flexinol wires. It was found that a single 0.006 in. (150µm) wire could lift over 2 lbs. (0.9 kg)and possessed a sufficiently rapid cycling time for application in a prototype bundle. Considering thatone wire can lift over 2 lbs., in theory, fifty 0.006 in. wires connected mechanically in parallel could liftat least 100 lbs. In the end, 48 wires were used due to the symmetry of arrangement into a cylindricalbundle (discussed in Section 2.4). Additionally, the number 48 is evenly divisible by 10 integers whichmakes it convenient for varying the number of parallel paths. (The experimental setup was designedsuch that the number of parallel paths could be varied, see Section 2.5.)

2.4 Bundle Construction

A SMA Bundle was constructed consisting of 48, 12 in. (30.5 cm) long, 0.006 in. (150 µm)diameter wires (N=48, L=12 in). The 48 wires were connected mechanically in parallel between two0.25 in. (0.635 cm) thick, 1.5 in. (3.81 cm) diameter virgin Teflon end plates. Teflon was selected due

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to its high dielectric strength, temperature resistance, and good mechanical stability. Since all wires arenot at the same voltage (series/parallel arrangement), it was necessary to keep each one electricallyisolated from the others at the bundle end plates. Each wire passed through the Teflon end plate and wasterminated with a 1/32 in. (0.079 cm) copper crimp, providing an excellent mechanical and electricalconnection (see Figure 3). The copper crimps fit tightly into 48 sockets machined into each end plate.The 48 wires were arranged in a hexagonal close packed array (see Figure 4) resulting in the mostcompact bundle given the size of the crimping devices available at the time. With smaller crimpingdevices, the wires could have been much closer together making an even more compact bundle. Each ofthe 96 crimps was connected to a short section of electrical wire and then to four (two for each side) 25pin parallel connectors for easy connection and removal from the rest of the power circuit.

Figure 3: Crimping Schematic Figure 4: Arrangement of 48 Wires on End Plate

2.5 SMA Bundle Experimental Setup

In order to determine the performance characteristics of the wire bundled SMA actuator, anexperimental setup was designed and constructed (see Figure 5). The experimental setup consisted of 4main parts – the test rig, the power supply, the control and instrumentation unit, and a PC.

The key components that make up the test rig are the frame, the SMA Bundle, the load, andelectrical power connections. The test rig was designed to allow convenient installation, adjustment,and removal of all components and sensors mounted to the structure. The frame provides a rigidhanging point for the upper end of the SMA Bundle. The lower/free end of the vertically oriented bundleis connected to the load. A variable weight and/or springs can act as the SMA Bundle load. Theelectrical connections shown in Figure 5 are greatly simplified. In order to vary the number of parallelelectrical paths through the SMA Bundle, a “patch terminal” system was setup. The Patch Terminalsconsist of four 48-pin cannon plugs. One of the four plugs was wired such that there is a one-to-onecorrespondence between each pin in the plug and the end of each SMA wire at the top end plate. Asecond plug was similarly wired for the bottom end plate. A third plug was wired such that all pins wereat the HI (+) supply voltage. The fourth plug was similarly wired for the LO (-) return voltage. A set ofshort patch wires was used to create the desired current path through the bundle.

The Power Supply for bundle actuation was a Tellabs 48 V (nominal), 10 amp DC power source.The 480 Watt supply provided much greater power than was necessary to actuate the bundle. Recalling

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from the discussion in Section 2.1, changing the number of parallel paths changes the voltage andcurrent requirements of the bundle. For a large number of parallel paths, a low voltage and high currentis needed. For a small number of parallel paths, a high voltage and low current is needed. The 48 V, 10Amp DC power supply made it possible to try different numbers of parallel paths as long as the currentand voltage remained within the capability of the power supply.

The control and instrumentation unit contains the necessary circuitry to control the raw voltagefrom the power supply. Additionally, it includes the sensors, meters, and wiring needed to measure anddisplay force, bundle displacement, current, voltage, and air temperature within the bundle. This unitalso provides the required connection ports to receive input signals from and send data to the PC. Theraw voltage from the power supply was controlled using a custom designed operational amplifier circuit.The custom circuitry was based around a Burr Brown OPA 512 Power Operational Amplifier. The gainof the amplifier was set to 10 so that a 0 to 5 V input signal resulted in 0 to 50 V applied to the SMABundle.

Force was sensed using a Transducer Techniques MLP-100 Mini Low Profile Load Cell whichhas a capacity of 100 lbs. (445 N) and an accuracy of ±0.1 lb. (±0.445 N). The load cell acted as theconnection between the SMA Bundle and the load, and thus sensed the applied force. The signal fromthe load cell was amplified, conditioned, and displayed by a Transducer Techniques DPM-3 DigitalPanel Mount Meter with a high-speed microprocessor.

Figure 5: SMA Bundle Experimental Set-Up

Linear displacement was sensed by a Space Age Control Series 150 Analog-Output Ultra-SmallSubminiature Position Transducer that produced an electrical output proportional to its cable extension.The sensor was fastened to an adjustable bracket on the base of the test rig and the extension cable was

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attached to the bottom of the load. Its range of measurement is 0 to 1.5 in (0 to 3.81 cm) with amaximum linearity error of ±0.361% of full range.

Current was sensed using a Hewlett Packard HP34330A 30Amp Current Shunt, which containeda precision 0.001Ω resistor. This precision resistor provided an output signal of 1 mV per Amp. Thecurrent shunt was electrically connected in series with the SMA Bundle in order to measure the totalbundle current. For DC, the accuracy is ±0.3%.

Various voltages, such as voltage drop across the SMA Bundle, current shunt voltage drop, andposition transducer voltage, were measured and displayed using a Lascar DMM 939 Miniature DigitalLCD Auto-ranging Multimeter. This small panel mount multimeter has an accuracy of ±0.3% of theauto-selected range. A selector switch was installed to shift between different voltage readings.

Air temperature at the center of the SMA Bundle was sensed by a 0.005 in. (0.0127 cm) diametercopper/constantan thermocouple and measured using an Omega DP25-TC Programmable DigitalThermocouple Meter with analog output. This setup provide an accuracy of ±0.9°F (±0.5°C) and amaximum service temperature of 400°F (204°C). The response time (for 63.2% of full response) for athermocouple of this size is 1 sec for an 800°F to 100°F step temperature decrease. The resulting timelag for small temperature changes was decided to be acceptable for the application.

The PC, a 350 MHz Dell, served two main purposes. First, it sent the input/control signal to theoperational amplifier circuitry. Second, it was used to record sensor data. To achieve these twopurposes, the PC was equipped with a Datel model PC-412 Analog to Digital and Digital to Analogconverter board. This board has four analog output channels, which have a range of ±5 volts at ±0.05%linearity error for full-scale range and 16 analog input channels with the same voltage and errortolerances. A Visual C++ program was written that interacts with the Datel board and coordinates thesending of input signals to the experimental setup and the retrieval of data. Since the PC is part of aWindows NT network, Win RT software was used to facilitate interaction with the Windows operatingsystem.

3. RESULTS

The SMA Bundle was tested through open loop experiments using the setup described above.The patch terminals were wired for 8 parallel paths with 6 wires in series for each parallel path. Asshown in Figure 1, this results in a theoretical actuation voltage of 36.6 V and a current of 3.2 A forcomplete contraction of the bundle. This was also the best arrangement to prevent exceeding thethermal limits of the operational amplifier. For the open loop experiments described in this paper, avariable weight without springs acted as the load. The specific weights used were 11 lbs. (4.99 kg) and27.5 lbs. (12.5 kg). Several different types of input signals (step, ramp, sinusoid, and half sinusoid) weredefined in the Visual C++ program and then sent to the SMA Bundle. Each experimental run lastedapproximately 24 seconds during which the following data was recorded: SMA bundle voltage drop,current, contraction, and air temperature at the center of the bundle. A 4th order Butterworth filter wasused to smooth noise in the current and temperature data. In this paper, results from the step responsesare presented.

A complete set of data for three different step input signals with an 11 lb. load is shown inFigure 6. These plots show some important information about the performance of the SMA Bundle.

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Examining the bundle current plot, one sees a significant increase in amperage during bundlecontraction. This is expected since the resistance of the Flexinol wires changes as the material changesphase from martensite to austenite. The resistance of the wire drops during this transformation causingthe current to increase.

Figure 6: Step Input Signal, 11 lb. Load

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Looking at the bundle contraction plot, one notices:

a) A dry friction phenomenon is observed as contraction initiates. It takes a finite amount of timefor the Flexinol wire to reach actuation temperature, which results in a delay. Larger stepvoltages cause faster heating and thus a shorter delay before the system responds.

b) An almost linear, first order system rise to a maximum deflection. The slope of the linearresponse changes with the magnitude of the step. Larger magnitudes result in faster responses.

c) A reasonably stable steady state deflection is reached.

d) A saturation phenomenon is observed for large voltages. It is also clear that when operating nearthe maximum deflection, a large increase in the step voltage (and the corresponding current)results in only a very small increase in contraction. Comparing the contraction for the 18 and 25V curves, which generate 2.2 and 3.2 amps respectively, the difference in deflection is only 0.03in. One can conclude that to improve efficiency, the bundle should be operated in a regionsomewhat below the maximum contraction.

e) A very slow decay towards the initial configuration once the step input ends. The decay occursearlier for the lower step voltages. The actuator never reaches its initial position and anadditional restoration force is needed.

The loss of efficiency when operating near maximum contraction is evident in the airtemperature plot. Comparing the temperature curves for the 20 and 25 V step inputs, there is a muchhigher temperature peak for the 25 V curve. However, this higher temperature did not result inincreased contraction. In other words, a significant amount of thermal energy was wasted since it wasnot converted into mechanical work. Although not fully shown in the contraction versus time plot, thecontraction of the bundle does not return to 0 after the input voltage is dropped to zero. It turns out thatthe 11 lb. load does not provide enough restoration force to fully stretch out the bundle as it transformsfrom austenite back into martensite. Instead, the contraction as a function of time approaches anasymptotic positive value. To ensure constant initial conditions for the experiments with the 11 lb. load,a 27.5 lb. restoration force was applied to the bundle prior to each new experimental run.

Similar step input signals were applied to the SMA Bundle with a 27.5 lb. load. The resultingplots had similar characteristics to those where the load was 11 lbs. Comparing contraction plots for thesame input signal and different loads provides some interesting information. Figure 7 shows thecontraction of the bundle due to a 20 Volt step input with the two different loading conditions. (Notethat the voltage was maintained for a shorter period of time for the 11 lb. load. This was done to capturethe decay of the contraction when the input voltage was dropped to 0.) As expected, the contraction isgreater and more rapid for the lighter load, and the contraction decay following removal of the inputsignal is slower for the lighter load.

For the same 20 Volt step input signal, contraction begins earlier when the load is 11 lbs. thanwhen the load is 27.5 lbs. This can be explained by considering the two opposing strains that exist in aSMA wire that is contracting under load. There is a negative strain due to the shape memory effect andthere is a positive strain due to elasticity. Examining the dynamics of the experimental setup, the freeend of the SMA Bundle starts to move when the tension in the bundle is equivalent to the weight of theload. Until this equilibrium point is reached, the elastic strain cancels the strain due to the shape

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memory effect. So, for the larger load, more shape memory strain is needed to cancel the elastic strain.As a result, there is a longer delay before the load starts to move.

Elastic strain also explains the difference between the maximum steady contraction for an 11 and27.5 lb. load. At the maximum contraction, the difference in elastic strains can be calculated in thefollowing manner:

∆LPL

AE=

where P is the load (11 or 27.5 lb.), L is the length of the bundle (12 in. (30.5 cm)), A is the combinedcross sectional area of all 48 wires (1.36e-3 in2 (8.77e-2 cm2)), and E is Young’s Modulus for Ni-Ti(approximately 6.96e6 psi (48 GPa)). Therefore, the theoretical difference between the two elasticstrains is:

( ) ( ) ( . ) . ..∆ ∆L L lbL

AEin27 5 11 27 5 11 0 02− = −

≈

The same calculation in metric units yields approximately 0.05 cm. Figure 7 shows a difference ofapproximately 0.04 in. The additional 0.02 in. strain is most likely due to the test rig not being perfectlyrigid.

Figure 7: Comparison of Response for 11 and 27.5 lb. Loads

While most of the open loop results under step inputs presented here verify similar resultspresented in the literature for other SMA actuators [9], a new phenomenon was observed: an importantunpredictability in the system’s steady state response when step inputs of low magnitude were applied.This phenomenon can have a degrading effect on the performance of controllers proposed for robotic

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systems driven by such SMA bundle actuators where very small amplitudes and very fine high accuracytasks are desired. Obviously, such unpredictability suggests a possible chaotic response of the system.Complete study of this chaotic response is beyond the scope of this paper and will be studied further inour future work.

This unpredictability is illustrated in Figure 8, which shows the bundle contraction due to a rangeof step inputs from 14 to 20 Volts with a 27.5 lb. load. For steps of magnitude less than 14 V, there isno measurable contraction. For step inputs with magnitudes over 20 Volts, the contraction as a functionof time shows the well-behaved response already discussed. In between, the response is quiteunpredictable. For example, the 17 V step input produced considerably less contraction than did the16.5 V input.

Figure 8: Responses Under Several Step Inputs

In order to further investigate this somewhat erratic behavior, a repeatability test was performedfor a 16 V step input with a 27.5 lb. load. A total of 8 runs were performed with exactly the same inputsignal and load. The only variable was the time interval between each successive run. Runs 0 through 5were performed with a one-minute break between each run. The break between runs 5 and 6, and 6 and7 was 15 minutes. The resulting bundle contraction versus time plots are shown in Figure 9. For runs 0through 5, the greatest contraction was achieved during the first run. For runs 1 through 5, themaximum contraction decreased to less than 20% of the first run. The longer pause before runs 6 and 7somewhat restored the response. Although this behavior must be further investigated, it is most likelydue to differences in the exact phase condition of the SMA wire at the initiation of a run. Runsperformed closer together may not allow a complete shift from austenite back to martensite.

27.5 lb. Load

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Figure 9: Repeatability Test 16 Volt Step Input, 27.5 lb. Load

4. Conclusions and Future Research

The design and dynamics of a new Shape Memory Alloy (SMA) actuator, which is composed of48 SMA wires bundled in parallel and able to lift 100 lbs., are presented. Open loop tests with a set ofstep inputs were shown. These tests revealed interesting non-linear characteristics of the actuator suchas: dry friction, saturation, and unpredictability of response for the same input. In the future, we willinvestigate possible chaotic behavior of the system and study closed loop position and force control.

ACKNOWLEDGEMENTS

The authors would like to thank the following organizations for their financial support for thisproject: Johnson and Johnson Discovery Award, Center for Computer Aids for Industrial Productivity,Rutgers University Special Resource Opportunity Allocation Program, New Jersey Space GrantConsortium and an Excellence Fellowship to Michael Mosley by the Rutgers University GraduateSchool.

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