Actuators for Smart Applications

Actuator manufacturers are developing promising technologies which meet high requirements in performance, weight and power consumption. Conventionally, actuators are characterized by their displacement and load performance. This hides the dynamic aspects of those actuation solutions. Work per weight performed by an actuation mechanism and the time needed to develop this mechanical energy are by far more relevant figures. Based on these figures, a selection process was developed. With time and energy constraints, it highlights the most weight efficient actuators. This process has been applied to the Gurney flap technology used as a morphing concept for rotorblades. Three control schemes were considered and simulations were performed to investigate the mechanical work required. It brought forward piezoelectric stack actuators as the most effective solution in the case of an actively controlled rotorblade. The generic nature of the procedure allows to use it for a wide range of applications. INTRODUCTION Hydraulic actuators have been used for a long time in aeronautic applications for their reliability and performance over a large number of cycles. However, their size and weight limit their integration in applications like smart helicopter blades. Piezoelectric stack actuators used to be a good alternative. Manufacturers such as Physik Instrumente have recently developed new types of actuators achieving high performance per unit mass. These new actuators are linear step actuators and linear ultrasonic actuators [1, 2]. Unlike piezoelectric stack actuators, which provide large forces but limited stroke, these achieve large strokes ∗Address all correspondence to this author. with limited forces. These new technologies use piezoelectric elements to transmit force by mean of friction to a moving rod that leads the final motion. They are of great interest for shape-morphing applications in the scope of the Green Rotorcraft Project from the European Clean Sky Join Technology Initiative [3]. This project aims at improving rotorcraft transportation. The work is carried out on multiple aspects: fuel consumption reduction, increased efficiency, improved global transport quality improvement and rotorcraft noise reduction. In order to wisely use those actuation technologies, a selection process has been developed. It allows to match an actuator type to an actuation strategy and has been applied to a rotorblade with Gurney flaps. ACTUATOR TYPES In this study, the actuator selection has been restrained to the three following types of actuators: linear stepped actuators, linear ultrasonic piezoelectric actuators and piezo electric stack actuators. They are more suitable for light weight and high performance applications. 200 actuator references were used from the following manufacturers: Physik Instrumente, Piezo Mechanik, New Scale Technology and Smart Materials. Linear piezoelectric stepped actuators Linear stepped actuators are using piezoelectric elements to sequentially clamp a moving rod to a main piezoelectric longitudinal element that extends and contracts to communicate motion to the rod as shown in Fig. 1 [4]. Depending on the type of actuator, it is possible to hold the rod against the element and consequently have a holding force due to friction while no power is provided to the actuator anymore. The force provided is constant 1 Copyright c © 2010 by ASME over the full stroke. The stroke can be as long as the rod. The limitation of those actuators is the motion velocity. Therefore, they will not be able to deliver their full stroke at high frequencies. FIGURE 1. SKETCH OF THE WORKING PRINCIPLE OF A PIEZOELECTRIC STEPPED ACTUATOR. Ultrasonic piezoelectric actuators Ultrasonic linear actuators use friction forces generated by a block of piezoelectric material on a moving rod [5]. This piezoelectric block is subjected to orthogonal vibration modes to achieve an elliptical motion of all the surface elements as shown in Fig. 2. A slider is pressed on the piezoelectric block which can move forward and backward depending of the applied modes. Again, as friction forces are applied, there is a residual holding force with no driving power. The stroke can be as long as the slider length. This actuator can achieve large velocities but the forces are less significant than these applied by linear stepped actuators. The force applied is constant over the full stroke of the motion. FIGURE 2. SKETCH OF THE WORKING PRINCIPLE OF A PIEZOELECTRIC ULTRASONIC TRAVELLING WAVE ACTUA-