Ferromagnetic shape memory alloy actuator enabled for nanometric position control using hysteresis compensation

Atefeh Sadeghzadeh, Estibalitz Asua, Jorge Feuchtwanger, Victor Etxebarria, Alfredo García-Arribas
Sensors and Actuators A: Physical, Volume 182, August 2012

Abstract

Ferromagnetic shape memory alloys (FSMA) are a special type of smart materials that change their shape under an applied magnetic field. They present attractive properties for precise positioning systems because the magnetic-induced response is fast and can produce large movements, with the additional advantage of non-contact operation. Their usability is limited by the poor reproducibility of their response to a given stimulus. This is a consequence of the stochastic microscopic processes that produce their macroscopic deformation and, most of all, of the great hysteresis displayed in cyclic operation. Suitable feedback control can overcome that limitation. In this work we show how a commercial FSMA actuator, manufactured by AdaptaMat Ltd., can be enabled for accurate and efficient nanopositioning by adequate control of the drive signal. First, an exhaustive series of tests are described that thoroughly characterize the open-loop response of the actuator: maximum displacement range, hysteresis and repeatability under different drive current amplitudes and frequencies. Second, different feedback control schemes are tested, based on an ordinary Proportional–Integral–Derivative (PID) controller modified with a gain scheduling strategy that helps accounting for the nonlinearity of the system, adjusting the operation of the controller according to the actuation requirements. The positioning accuracy achieved at this stage is about 25 nm, which is close to the detection limit of the capacitive sensor used for measurement and feedback. Finally, a method to compensate the intrinsic hysteresis of the system is implemented and included in the control strategy. The partial cancellation of the system nonlinearities allows for the best performance, considerably reducing the control effort, and improving the transient time and overshoot.

Ferromagnetic shape memory alloys (FSMA) are a special type of smart materials that change theirshape under an applied magnetic field. They present attractive properties for precise positioning systemsbecause the magnetic-induced response is fast and can produce large movements, with the additionaladvantage of non-contact operation. When a suitable piece of FSMA material is exposed to an external magnetic field, it can experience a deformation up to 6% or even 10%.  Additionally, once the FSMA piece reaches the desired length, it remains in the same state even without maintaining the applied magnetic field, with the concomitant advantage of energy saving.

Their usability is limited by the poor reproducibility of their responseto a given stimulus. This is a consequence of the stochastic microscopic processes that produce theirmacroscopic deformation, and, most of all, of the great hysteresis displayed in cyclic operation. Suitablefeedback control can overcome that limitation.

In our work we show how a commercial FSMA actuator,manufactured by AdaptaMat Ltd., can be enabled for accurate and efficient nanopositioning by adequatecontrol of the drive signal. First, an exhaustive series of tests are described that thoroughly characterizethe open-loop response of the actuator: maximum displacement range, hysteresis and repeatabilityunder different drive current amplitudes and frequencies. Larger driving amplitudes produce larger displacements but also, higher frequencies produce larger amplitudes. Besides, sharp field pulses produce the largest displacements. Second, different feedback control schemesare tested, based on an ordinary Proportional–Integral–Derivative (PID) controller modified with a gainscheduling strategy that helps accounting for the nonlinearity of the system, adjusting the operation ofthe controller according to the actuation requirements. The positioning accuracy achieved at this stageis about 25 nm, which is close to the detection limit of the capacitive sensor used for measurement andfeedback. However, a trade-off between the transient response, overshoot and accuracy is necessary. Finally, a method to compensate the intrinsic hysteresis of the system is implemented andincluded in the control strategy. A simpler but quite efficient technique to compensate the hysteresis of the system is to consider it as a phase lag between the stimulus and the material’s response. The partial cancellation of the system nonlinearities allows for the bestperformance, considerably reducing the control effort, and improving the transient time and overshoot. This benefits the reliability and durability of the actuator.

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