Mechatronic Design of a Shape Memory Alloy Actuator for Automotive Tumble Flaps: A Case Study

The continuous advance in mechatronics has long attracted researchers toward the development of new highly integrated actuators for automotive applications where reduced space and low weight are common constraints. In this context, Shape Memory Alloys (SMAs) offer many peculiar characteristics that make this technology very attractive for the construction of miniature mechatronic actuators. This paper presents the design, the prototype fabrication, and the functional testing of a case study, where an SMA binary actuator is used for automotive tumble flaps. The innovative solid-state actuation system is proposed as an alternative to electromagnetic and pneumatic effectors, traditionally used to drive the tumble shaft of an air intake manifold for internal combustion engines. Original features of the linear actuator involve the mechanical architecture and the control structure. On the mechanical side, two contrasting sets of SMA springs are used to actively generate the net actuating force during both out-stroke and in-stroke. On the control side, a current feedback is exploited for sensorless real-time monitoring of the working temperature of the SMA springs during electrical supply. Major shortcomings of the proposed solution are a low response time and a power consumption higher than pneumatic and electromagnetic counterparts.

I. Introduction

Common requirements for new automotive products are high miniaturization and integration, small size and weight, and low cost. Innovative actuators based on the technology of Shape Memory Alloys (SMAs) present all these characteristics that make them particularly attractive [1], [3], [4]. Specifically, they feature very high power/weight ratio with respect to traditional electric or pneumatic machines. Other advantages offered by SMA devices are the following: compliance with hard environment; simplicity of actuation mechanism; clean, silent, and smooth motion; sparkfree operation; distributed actuation system; and autosensing ability. The change of electric resistance can be used to monitor the phase transformation of SMA [5]. Accompanying these advantages are also several disadvantages that can be traced back mainly to their relatively slow response speed and to their inherent nonlinear behavior that makes them difficult to control [6]–[8]. Moreover, they are typically characterized by a low efficiency (less than 10%) [9] in converting electrical into mechanical energy. However, if SMA devices are properly designed, the nuisances produced by these intrinsic weaknesses are kept to a minimum. In summary, SMA technology has great potential for the construction of direct binary miniactuators, where it outperforms traditional electromechanical systems in terms of cost, reliability, robustness, and power to weight ratio.