Aerospace and underwater applications typically require actuators capable of large displacements, precise positioning, and fast response times. To meet these requirements, several classes of actuators based on low-voltage piezoelectric materials have been developed, and, in the case of the Amplified Piezoelectric Actuators (APA series), space qualified. The APA actuators offer large displacements (up to 1mm), large deformations (up to 3%), and large forces (up to 1kN) at low electrical power. These actuators can withstand large external forces and have successfully passed severe qualification tests such as centrifugal accelerations and vibration forces encountered during space launch. Aerospace applications of APAs include scientific instrumentation, such as telescopes and microscopes, microsatellite propulsion valves, and structural vibration control. Aeronautical applications include active flap control in aircraft wings and helicopter blades. Underwater applications focus on the silencing of ships, the piezodiagnostic (NDE) of structural defects in pipelines and hulls, and guidance systems of unmanned vehicles. This paper reviews the use of piezoelectric actuators, in particular APAs, in such applications. Qualification results, when available, are presented and discussed.
In past years, Amplified Piezoelectric Actuators (APA) have been applied to a variety of industrial applications that take advantage of their quick response and precise positioning capabilities. More recently, APA's have been integrated into valve designs to obtain both rapid and precise proportional flow control. This paper presents the design and measurement results of two gas valves that have recently been developed. The first gas valve uses a small APA that is driven by a switching amplifier to obtain a high frequency modulation. A frequency modulation higher than 400Hz, along with a stroke of 100μm, have been measured. These results show that such a design could be applied to fuel injection systems. The second gas valve is also based on an APA. It includes a linear amplifier and a servo controller to obtain an accurate proportional response dedicated to precise gas flow control. Such valves are interesting for instrumentation and space applications, where they can provide a linear and stable flow control. The low power consumption of the piezoelectric valve in space applications is an additional advantage. A stable flow of dry nitrogen ranging from 0.1sccm to 200sccm has been measured with an inlet pressure of 1bar. A variety of modeling tools has been used to design these valves: finite element modeling for the electro-mechanical aspects and for the contact mechanics between the poppet and the seat, as well as computational fluid dynamics for the flow simulation. These tools make it possible to modify the valve design in order to meet different requirements and serve other applications.
Recent developments in ultrasonic motor design have demonstrated that small size tube-shaped motors could be fabricated at low cost. Motors with diameters between 15 and 2.5mm have been fabricated and tested. The performance evaluation of these motors is still in progress, but have already shown promising results: the smallest ones exhibit no-load speeds in the range of 70rad/s and blocked torques close to 0.9mN•m. In this paper, we review the operating principle of these devices and several implementation examples. Then, we show how the finite element method (ATILA) can be used, in combination with genetic optimization procedures, to design tube-shaped motors in various dimensions and for different performance objectives. Several design examples are presented and discussed.
Ultrasonic motors often use a combination of structural modes to generate the desired elliptical vibration field that ultimately results in the linear or rotary motion of an object. Designing an ultrasonic device that combines structural modes of vibration represents a non-trivial exercise, especially when it is desired to maximize the electromechanical coupling coefficient of the piezoelectric elements, the amplitude of vibration, and the force factor of the device. Other parameters may also be combined and render the exercise even more difficult: targeting a specific frequency, constraining dimensions, electrical constraints, etc. To help designing such ultrasonic structures, we propose to use the genetic optimization method in combination to the finite element method. Although evolutionary methods are not new and have been successfully applied to a variety of problems (including smart devices), they have never been applied, to the best of our knowledge, to the design of ultrasonic motors. In this paper, we review the general aspects of the method utilized, and provide several examples, including experimental verification.
An analytical model of the pseudoelasticity of shape memory alloys (SMA) has been developed and validated using tensile test results performed with various kinds of shape memory alloy samples. However, the analytical approach is limited to conventional samples submitted to an uniaxial stress state. Therefore, a numerical model has been developed, which is more suitable for the adaptive structure modeling. To this end, the SMA behavior laws have been implemented in the finite element code ATILA. The validation of the numerical model has been achieved by comparing the finite element results with the experimental ones.
In several fields (optics, space, aircraft, fluid control, biomedical, and manufacturing) there is a strong need for compact, robust and efficient positioning mechanisms that also offer high precision, short response times, low power consumption, low electromagnetic interference and multiple degrees of freedom. Piezoelectric actuators are generally good candidates for building such mechanisms. The products manufactured by Cedrat Recherche SA are piezoelectric actuators offering compact size, high deformation (up to 1%) and high stiffness. These actuators have successfully passed different qualification tests (air and space qualification, lifetime tests). They can easily be integrated in applications, as shown by examples of mechanisms taken from various fields: a super amplified actuator for a MRI biomedical device, a tip-tilt for mirrors, a chopper for X-ray diffraction, a helicopter flap mechanism and an XYZ stage for the AFM microscope of the MIDAS instrument of the ESA ROSETTA space mission.
Positioning instruments offering a submicrometric accuracy within a restricted mass budget will become indispensable in future planetary exploration missions. Among the technologies known to date, only the combination of piezoelectric actuators with capacitive displacement sensors meets such specifications. This solution has the advantages of providing a rugged, frictionless, solid-state mechanism, which can be finely controlled due to the high signal to noise ratio that can obtained with the drive electronics. This actuator-sensor combination was applied to the design of an XY stage that could offer 100 x 100 micrometer strokes in a total mass budget of 400g. Since the required stroke was too large to be achieved directly with the piezoelectric material, the amplification technique developed at Cedrat Recherche was employed. Their Amplified Piezoelectric Actuators were chosen over other techniques, such as Hertzian pivots, because of the mass requirement on the system. The design of the stage made it necessary to address issues such as the guiding functions, especially important to reduce parasitic degrees of freedom. Finite element analysis was used intensively. The engineering model built includes eight APA50S actuators and two capacitive displacement sensors. The operating performance was tested and shown to be close to the predicted results. The strokes and parasitic degrees of freedom were measured using a laser interferometer. The stage was tested over the temperature range (-20°+50°C), submitted to random vibrations tests, and its lifetime was tested over more than one million strokes. The results of these tests and other parameters, such as piezoelectric drift and gravity effects on the functional performances, are discussed. This paper focuses on the design aspects of the XY stage, the tools used for this design and the lessons learned from its development.
A design consisting of an elliptical metal shell whose long axis is occupied by prestressed multilayer actuators can be applied to a large variety of piezoelectric structures to offer compact, low-voltage actuation at operating frequencies that range from DC to 40 kHz. The useful motion or vibration is recovered in the transverse direction, at the extremities of the short axis. The elliptical shell serves as an amplification mechanism. Its dimensions determine the output characteristics of the device in terms of strain, force and resonance frequency. This paper presents the design of three structures based on this concept. 3D finite element analysis result as well as experimental measurements are presented.
In space and automotive applications, the use of piezoelectric actuators is limited because of the high voltages the actuators usually require (over one thousand volts) and the low displacement levels they can achieve (about ten micrometers). We designed a series of Amplified Piezoelectric Actuators (APAs) that overcome these two drawbacks. These APAs operate with an input voltage less than 200 V because they employ a multilayer technology, and they offer displacement levels ten times greater than Direct Piezoelectric Actuators (DPAs), their direct-actuation counterparts.
This study deals with a micromotor based on the use of magnetostrictive thin films. This motor belongs to the category of the Standing Wave Ultrasonic Motors. The active part of the motor is the rotor, which is a 100 micrometers thick ring vibrating in a flexural mode. Teeth (300 micrometers high) are placed on special positions of the rotor and produce an oblique motion which can induce the relative motion of any object in contact with them. The magnetic excitation field is radial and uses the transverse coupling of the 4 micrometers thick magnetostrictive film. The film, deposited by sputtering on the ring, consists of layers of different rare-earth/iron alloys and was developed during a European Brite-Euram project. The finite element technique was used in order to design a prototype of the motor and to optimize the active rotor and the energizer coil. The prototype we built delivered a speed of 30 turns per minute with a torque of 2 (mu) N.m (without prestress applied on the rotor). Our experimental results show that the performance of this motor could easily be increased by a factor of 5. The main advantage of this motor is the fact that it is remotely powered and controlled. The excitation coil, which provides both power and control, can be placed away from the active rotor. Moreover, the rotor is completely wireless and is not connected to its support or to any other part. It is interesting to note that it would not be possible to build this type of motor using piezoelectric technology. Medical applications of magnetostrictive micromotors could be found for internal microdistributors of medication (the coil staying outside the body). Other applications include remote control micropositioning, micropositioning of optical components, and for the actuation of systems such as valves, electrical switches, and relays.
Piezomotors are an increasingly competitive alternative to electromagnetic stepper motors, especially in applications where large bandwidths and/or precise positioning control are desired. Piezomotors use a combination of electromechanical and frictional forces and, compared to conventional electromagnetic motors, have the advantages that no power supply is required to maintain the motor in position and no lubrication is necessary in the device. The operating principle of these motors relies on the use of an ultrasonic vibration, which is created via the piezoelectric effect (at resonance in most cases), in order to generate vibration forces at the `stator/rotor' contact interface. A mechanical preload is also applied at this contact interface and is responsible for the motor's holding force at rest. To meet the specifications of an aerospace application, we developed a new design of Linear PiezoMotors (LPMs). The first prototype we built shows very promising results, and makes the LPM a serious candidate to replace conventional stepper motors. The LPM features the following characteristics: a standing force of 100 N, a blocked force of 37 N, a maximum actuation speed of 23 mm/s, a maximum run of 10 mm, a mass of 500 g, an electrical power of 2.2 W, and a position accuracy superior to 1 micrometers . To our knowledge, the driving force delivered by the LPM has never before been achieved in resonant devices. This paper describes the physical operating principles of the LPM, as well as the modeling tools and experimental techniques we used for its development. Several implementation schemes are also presented and show the wide range of possible applications offered by the linear piezomotor.
This study presents the design, construction, and test results of an electromagnetic stress sensor for monitoring bridge cables and prestressed concrete structures. The sensor uses the reverse magnetostrictive effect found in high elastic limit steels such as those used in cables and in prestressed concrete. This effect is characterized by the variation in the steel's magnetic permeability as a function of its internal stress. Consequently, the internal stresses in this high elastic limit steels can be found by measuring their permeability. The permeability can be measured indirectly by measuring the inductance of a coil placed around or near the cable. We designed a prototype of the sensor with a finite element program. We also used this program to optimize the sensing coil and the measurement frequency and to design the magnetic shielding around the sensor. We built and tested the prototype in our laboratory. We evaluated the sensitivity, precision, linearity, and reliability of the sensor, and also the influence of external thermal and magnetic perturbations on the sensor measurements. The results were very satisfactory. The major advantages of this sensor are its robustness and its ability operate continuously for several decades even in hostile environments. These types of sensors, embedded in or added to the structure, are used to monitor stresses in cables and in prestressed concrete structures used in bridges and nuclear stations.
Two typical characteristics of direct piezoelectric actuators are displacements of ten micrometers and high stiffnesses. Recently, multilayer actuators have been improved, and they now display strains of approximately 1200 ppm at low excitation levels (less than two hundred volts). Thus, they are well suited to perform precise positioning of optical devices. But for industrial needs, this performance is still insufficient for positioning devices with larger displacements (in the range of several hundred micrometers). Numerous designs of mechanical amplifier devices based on the use of flexural hinges have been proposed. Due to their low stiffness, these devices cannot be used in space applications because they would not survive during takeoff. The amplified piezoelectric actuator which we designed and tested, eliminated the low stiffness drawback and ensures good force transmission. Due to the stiffness of the amplifier, the efficiency of the electromechanical transduction is significantly higher than those of conventional amplifier mechanisms. To design this actuator, we performed a numerical finite element simulation that included the piezoelectric effect. Among other things, this model shows the displacement as a function of the excitation and the electrical admittance. The static and the dynamic behaviors were determined. The main features of the actuator are a no-load displacement of 180 micrometers and stiffness of 5 N/m. These characteristics were experimentally verified using an electromechanical test bench including a laser Doppler interferometer, thus confirming the design method. Technological aspects, like the compressive force applied to the piezoelectric material, were considered. Many applications for this amplified actuator already exist. For example, an active mechanism using this actuator can be used to tilt a mirror. Another application of the amplified actuator is in the field of active damping of structures. In this case, the actuator is connected to a resistive shunt so that electrical damping is obtained through the direct piezoelectric effect. The experimental results show that the actuator is interesting because of its high electromechanical coupling, and, consequently, its ability to perform active damping.