On the interaction of temperature-sensing, Shape Memory Alloy dynamics, and control design for a bending actuator

Abstract

Temperature based control of Shape Memory Al- loys faces difficulties in the determination of the actual material temperature profiles due to the typically small geometrical characteristics and possible high rates of excitation. Under such operating conditions, the expected hysteretic response can be transformed into a temperature rate-dependent one, with peculiar response characteristics. The intricacies of temperature sensing on a thin-wire SMA-actuator are coupled with a control-relevant model to enable the application of a sliding- mode controller. Experimental studies are used to validate the controlled system's performance. I. INTRODUCTION Shape Memory Alloys (SMAs) are chosen as actuators or passive elements in engineering applications because of their unique characteristics, usually resulting in compact and human-friendly systems. The coupling of the application requirements with the material characteristics, requires an interdisciplinary knowledge, a relationship which has been identified early on in (1). Biocompatibility and superelas- ticity have been exploited in medical applications. SMA- based actuators with high force to volume ratio and 'silent' operation have appeared in robotic applications (2). Their resemblance to actuation mechanisms in nature (tendons and muscles) has yielded multiple SMA based biomimetic prototypes (3). Control methods for SMAs rely on the operation of PWM, bang-bang or simple feedback controllers, although different hysteresis model-based schemes have been proposed utilizing the inversion of Preisach models (4) or neural networks. Model based control though, often assumes direct access to the SMA's temperature or its efficient and accurate estima- tion. On the other hand, higher actuator bandwidth requires the use of SMAs with reduced geometrical characteristics, since the heat transfer process is the limiting factor in the actuator's response. Direct measurement of the material temperature by use of thermocouples attached to SMA wires has been previously reported in the literature ((5)), as an efficient means of mea- surement primarily at lower excitation rates. The introduction of uncertainties in the temperature data though, has also been previously reported (6). Still, their use may be required in applications where a) the small SMA wire diameter does not allow for effective temperature measurement by infrared cameras, and b) actuation under high rates of excitation signal prohibit the use of closed chambers with controllable Throughout this experimental study, it is shown that unless the sensor dynamics is included in the model, the rate- independent temperature based models are unable to provide a sufficient system description (formation of temperature- deformation hysteresis at high rates). The sensor dynamics are identified in order to allow for the implementation of a temperature based controller, yielding a high tracking performance for the bending actuator. The main contributions of the current work are identified as: a) demonstration of the rate-dependent responses produced by the introduction of the exogenous sensor-effect in the recording of the material temperature, b) identification of the sensor dynamics and recovery of the 'true' material temperature, c) formulation of an adaptive temperature based model description and d) implementation of a Sliding Mode Controller using the adaptive temperature model for tracking control. This paper is structured in the following manner: Section II provides the theoretically expected dynamics of the SMA- system and Section III describes the experimental methods investigating the recorded system behaviour. The adjustment of the material model and the formulation of the control law is given in Section IV. The evaluation of the controller's performance is shown in Section V followed by concluding remarks.