Shape memory alloy (SMA) is being widely used to implement smart concepts such as shape control and vibration suppression of aerospace structures. Shape memory alloy wires while undergoing phase transformation from martensite

state to austenite state produce large strains. While doing so they serve as actuators and generate large forces when

constrained while recovering their pre-defined shape, imparting this force to the structural component on which they are mounted which results in its movement. While designing the electronic system to energise an aerospace structure with shape memory alloy based actuators the amount of current and time for which this current is passed through the shape memory alloy wire are important considerations. It is equally important to design very efficient and miniature power sources (constant voltage as well as constant current) and control system to achieve fine and steady positioning of the structural component. This paper discusses different issues involved, as well as design and development of the electronics and control system for actuating and controlling a typical aerospace structural component with SMA actuators

Balasubramanian, R

Dayananda, GN

Kumar, Senthil P

Rao, Subba M

Shankar, V

English

National Aerospace Laboratories Institutional Repository

Proceedings of ISSS-SPIE 2002International Conference on Smart Materials Structures and SystemsDecember 12-14, 2002, Indian Institute of Science, Bangalore, IndiaISSS2002/SA-458Development of Electronic actuation system for Shape Memory Alloybased Aerospace StructuresV.Shankar1, G.N.Dayananda, P.Senthil Kumar, M.Subba Rao, R.Balasubramaniam1Corresponding Author, Email: vshankar@css.cmmacs.ernet.inScientists, National Aerospace Laboratories, Bangalore-560 017, INDIAABSTRACTShape memory alloy (SMA) is being widely used to implement smart concepts such as shape control and vibrationsuppression of aerospace structures.  Shape memory alloy wires while undergoing phase transformation from martensitestate to austenite state produce large strains. While doing so they serve as actuators and generate large forces whenconstrained while recovering their pre-defined shape, imparting this force to the structural component on which they aremounted which results in its movement. While designing the electronic system to energise an aerospace structure withshape memory alloy based actuators the amount of current and time for which this current is passed through the shapememory alloy wire are important considerations.  It is equally important to design very efficient and miniature powersources (constant voltage as well as constant current) and control system to achieve fine and steady positioning of thestructural component. This paper discusses different issues involved, as well as design and development of theelectronics and control system for actuating and controlling a typical aerospace structural component with SMAactuators.Key words: Shape Memory Alloy (SMA), Actuator, Power sources, Electronics, and Control system.1. INTRODUCTIONShape memory effect is based on a solid-solid phase transformation of the shape memory alloy that takes place with in aspecific temperature (say from ambient to about 120oC), which produces large strains. There are two states of Shapememory alloy namely martensite (cold shape) and austenite (hot shape). The properties of shape memory alloy vary withits temperature. Forward and reverse transformation occurs at different temperatures, resulting in hysteresis, whichdepends on the thermo-mechanical treatment as well as composition of shape memory alloy. Nickel-Titanium-Coppershape memory alloy commercially known as nitinol has best properties for actuator applications.The required heat energy to shape memory alloy to undergo the phase transformation can be supplied either directly bypassing electrical current or indirectly by placing the shape memory alloy wire in a controlled heating environment suchas oven, immersing it in a hot liquid bath, winding coil around the shape memory alloy and passing current through thiscoil to heat the SMA wire, etc. However, for better control and also for making it amenable for interfacing with thecomputer, the preferred mode of heating the wire is by passing the electrical current directly through the wire.The mechanical and electrical properties determine the behavior of shape memory alloy during phase transformation. So,the system design has to take care of these properties. Since the shape memory alloy shows hysteresis and decrease inresistance with increase in temperature in the forward phase transformation region, the system design should also caterto these requirements.1.1 Shape controlControlled shape change of an aerospace structural component is brought about in order to improve and optimise itsaerodynamic efficiency under varying flight conditions. Shape memory alloy’s in wire form can be effectively integratedinto the structural component and energised to bring about the shape change.1.2 Vibration suppressionShape memory alloy’s can be utilised to suppress steady state vibrations of a structural component. By energising shapememory alloy’s integrated into a structural component the stiffness of the component changes there by shifting itsnatural frequency. By employing proper feed back one can also reduce the amplitude of steady state vibration at aparticular natural frequency of the structure.2. EXPERIMENTAL WORKFigure 1 shows a set up to measure the resistance (in-turn the resistivity) of the shape memory alloy wire during its phasetransformation, which is an important parameter in the design of power sources for actuation1. It can be explained in thefollowing way:0 - 20 mA Constant Current SourceVoltmeterOvenSMA wire1Current SourceFigure 1. Schematic of experimental set up for resistance and resistivity measurement of SMA wireA constant current source was designed, to drive about 10mA through the shape memory alloy actuator wire mountedinside an oven. This is taken as the reference current (I). However, this current is negligible compared to the currentrequired (about 1200mA to 1550mA) to directly heat the shape memory alloy wire to take it through its phasetransformation from martensite to austenite. In this experiment, the phase transformation is brought about by externallyheating the shape memory alloy wire mounted inside the oven from room temperature (30oC) to about 110oC, bychanging the oven temperature. At different oven temperature the voltage (V) across the shape memory alloy wire ismeasured while maintaining a constant current of 10mA through the wire. Then at selected oven temperature values theresistance(R) and in turn resistivity (ρ) value is calculated from the known relations.R=V/I and ρ=R*(A/L)Where, A – the area of the cross section of the shape memory alloy wire.L – the length of the shape memory alloy wire.During the experiment, the change in length (Strain) and temperature of the shape memory alloy wire is acquired using aData Acquisition System (DAS) developed for this purpose. The output from the LVDT, which measures the strain, andthe Thermocouple, which measures the temperature were suitably signal conditioned before feeding to the DAS. Thethermocouple signal conditioner was designed using an analog devices IC – AD595 which is compact, versatile andincludes feature such as cold junction compensation built into its design. The schematic in Figure 2 shows the DASdeveloped for generating the required strain Vs temperature plot. A 16-channel personal computer compatible dataacquisition card was used to acquire strain and temperature data.TerminalBoardPC based DAS 00 .0 50.10 .1 50.20 .2 50.30 .3 50.40 .4 52 0 4 0 6 0 8 0 10 0 12 0Te mpe rature in oC% StrainForward  t ranf ormat ionR everset rans f ormat ion% StrainOvenSMA wireThermocoupleSignal ConditionerLVDT SignalConditionerThermocoupleLVDT% Strain% Strain% Strain% Strain% Strain% StrainFigure 2. Data acquisition system for strain and temperature measurementThe software for acquiring the above data and plot the resulting hysteresis curve was developed using ‘C’ language. Theacquired data is plotted in Figure 3. The plot shows hysteresis behaviour during heating & cooling, which is typical of ashape memory alloy wire2. This type of data generated about the SMA wire behaviour provides valuable information fordesign of power sources and feedback controller.From some of the basic experiments carried out on shape memory alloy wire actuation by electrical method it has beenobserved that both the value of the current and the time of passage of current through the shape memory alloy wire areimportant parameters in any actuator application.00.511.522.533.544.520 40 60 80 100 120Temperature in oC% StrainForward tranformationReversetransformationFigure 3. Plot of Temperature Vs Strain of a SMA wireThe phase transformation of the shape memory alloy wire is almost instantaneous due to passage of current resulting inlarge strain. This in turn produces an angular movement of the structural component on which the shape memory alloywires are mounted. This angular movement is measured by a position sensor unit, which consists of a potentiometermounted at the point of rotation on the structural component, there by providing input to the position controller.3. DESIGN CRITERIAThe requirement of heating current of a shape memory alloy wire depends on its cross sectional area which in turndepends on its diameter i.e. on its current density. For the shape memory alloy wire chosen, the requirement of currentvaried from 1.2A to 1.55A and voltage around 5V. This called for designing and developing voltage and current sources.Both voltage and current sources have been designed and implemented. It has been found that constant current source isreally advantageous when the shape memory alloy elements are remotely located from the source of power as in anaircraft environment. The power sources are made compatible to work from inputs such as 12V / 24V-28V available onthe aircraft power bus. When individual miniature power sources and electrically isolated shape memory alloy actuatorwires are employed, redundancy in the system is improved because the failure of the single source or single shapememory alloy wire does not affect the working of the total system.The output of constant current source works from rated load to short circuit load. This feature is beneficial for thestructural component’s position control. Since the temperature of the shape memory alloy wire is proportional to thesquare of the current through it, it is possible to build a relationship between current and temperature. This helps in thedesign of the controllers, which is discussed later.4. SYSTEM DESIGNThe block diagram of the system configured for shape memory alloy actuated structure is shown in Figure 4. There arethree major building blocks other than structural component. They are programmable power source, control electronicsand position feedback unit.230V,50HzProgrammable powersource (PPS) - Voltageor  current source ControlElectronicsSMA basedStructural ComponentPosition FeedbackRelated ElectronicsDC power sourceAircraft DC power Source  12 / 28 VFigure 4. System configuration for SMA based actuationThe programmable power source is either constant voltage source or constant current source. Shape memory alloy wireof suitable diameter and length was chosen for the structural component. The constant voltage source is simple in designand suitable for open loop operation and also when the shape memory alloy load is located very near to the powersource. On the other hand a constant current source is complex in design but seems to be the right choice for closed loopremotely (about 5 meter away) located load. Figure 5(a) shows the constant voltage source developed and Figure 5(b)shows the same with programmable feature. Figure 6(a) shows the constant current source developed and Figure 6(b)shows the same with programmable feature. Regulator IC   I/ P O/P   A d j   R 2   R 1 SMA  load    Regulator ICRoI/P O/PSMAloadDigital  Inpu tsR1 R2 R4 R5R3R x   Figure 5(a). Miniature voltage source for SMA actuator.      Figure 5(b). Programmable miniature voltage source for SMA actuatorRegulator ICI/PR1SMA loadO/P O/PRegulator ICI/PAdjDigital InputsR1R2R4R5R3SMAloadDigital InputsR1R2R4R5R3Digital InputsR1R2R4R5R3Digital InputsR1R1R2R2R4R4R5R5R3R3Digital InputsR1R2R4R5R3Digital InputsR1R2R4R5R3Digital InputsR1R2R4R5R3Digital InputsR1R1R2R2R4R4R5R5R3R3   Figure 6(a). Miniature current source for SMA actuator.       Figure 6(b). Programmable miniature current source for SMA actuatorThe control electronics element working as interface between the power source and shape memory alloy load consists ofswitching devices and timers required to energise the shape memory alloy for the stipulated time during heat cycle andswitch off the current to the shape memory alloy load during cool cycle.The position feedback unit consists of a potentiometer and related electronics, which provides a voltage outputproportional to the angular position (due to angular movement) of the structural component. For every angular positionthe position sensor provides the corresponding voltage. This voltage is compared with the set voltage (commandvoltage), which represents the desired position of the structural component in an error amplifier and fed back as an errorsignal to the power source to change its voltage there by changing the current flowing in the shape memory alloy wire todrive the component towards the desired position. The scheme developed for actuation of a typical structural componentsuch as rudder of the aircraft is shown in Figure 7.Command input Polarity DetectorRLPPSV to IIPDReference signalΣΣΣSMA actuatorsFigure 7. PID controller for SMA actuatorThe scheme shows a proportional integral derivative (PID) controller implemented to impart desired movement andposition a SMA based structural component. In this scheme the signal processing is carried out in the analog domainwithout resorting to computer (digital). Analog signal processing (ASP) involves calculation of system parameters suchas gain, time constants and component values to ensure stable operation and reliability. Operational amplifiers are thebasic building elements of the ASP design. Design with ASP involves two steps i.e choosing the basic type of operationand formulation of parameters.The PID scheme basically consisting of proportional (gain), integral (integrator) and derivative (differentiator) blocks,works in the following way3.  The command input is dependent on the desired angular position of the structuralcomponent and given in terms of volts. Actual position of the structural component is obtained from the positionfeedback element also in terms of volts. Let the position of the structural component be x1 degrees and the desired angle(position) is x2 degrees. Let V1 and V2 be the voltages corresponding to x1 and x2 degrees. Then the difference betweenV1and V2 will give the error voltage value. This error signal is generated using an operational amplifier. The error signalis fed to I stage of the PID controller, where as the feedback signal is given to the PD stage. Since the feedback signaldetermines the magnitude of the control action it is referred to as control signal.In the structural component considered, SMA wires are mounted on both (left & right) sides of the structure .In thisarrangement, the SMA is biased against itself i.e. when the right side SMA’s are energised (strained) for a specificperiod of time (on time) the other half namely left side SMA’s are not energised (not strained, off time) but still facilitatethe right side movement of the structural component, from its neutral or zero angle position to its final angular position.In the next half of the cycle, right side SMA’s are switched off or de-energised and left side SMA’s are energised(switched on). The structural component moves towards its final left position and reaches it. This way the structuralcomponent does repeatable movementsThe response of the structural component possesses symmetry for right and left side angle control from its mean (center)position. It implies the control action should be made independent of polarity. This is achieved using suitable electronicscircuitry.Figure (8) shows the circuit configuration designed and developed for implementing s PID controller.Figure 8. Circuit diagram of the PID controllerThe output of the control unit is fed to a programmable power supply, which in turn controls the movement and positionof the structural component. The relationship here is that the position of the structural component is dependent on thecurrent passing through the shape memory alloy wire, which is in turn is controlled by the PID controller.6. CONCLUSIONSSome of the issues as related to behavior of shape memory alloy as an electrical load have been brought out. Theconstant voltage as well as constant current actuation of shape memory alloy has been discussed. The design of powersources and their integration into the system configured for actuating an actual structural component have beenexplained. A PID controller scheme to actuate and fine position the aerospace structural component has been configuredand designed. Future work in this direction suggests that there is a need to develop miniature adaptive controllers usingdigital techniques.7. ACKNOWLEDGEMENTSThe authors wish to sincerely acknowledge the support given by the Director, NAL, to carry out this work. Thanks aredue to Head, Advanced Composites Unit (ACU), NAL, and Head, Structures Division, NAL, for their excellent support.The authors appreciate the valuable assistance rendered by Mr.H.N.Ranganatha and Mr.Rakesh Kumar Gautham ofsmart structures Lab, ACU, NAL, in carrying out the experimental work.8. REFERENCES1. G.N. Dayananda and V.Shankar, Testing and Evaluation of Electrical and Related Properties of Shape Memorywires used in Active Shape Control of Aero foils, Submitted to National Seminar on Aerospace Structures (XINASAS) 2002, Bangalore .2. Darel E. Hodgson, Using Shape Memory for Proportional Control, Engineering Aspects of Shape MemoryAlloys, pp 362-366, Butter worth-Heinemann3. Feng-Sheng-Wang, Chi Liang Yeh Yieng and Chiang Wu, ‘PID controller tuning by an interactive multiobjective optimization method’, IEEE transactions on Instrumentation, Measurement & Controls, Vol. 18, No.4,1996.

Development of Electronic actuation system for Shape Memory Alloy

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Development of Electronic actuation system for Shape Memory Alloy

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