Modular Test Bed for Performance Assessment of Piezoelectric Stick-Slip Actuators

Stepping piezoelectric actuators based on the stick-slip effect inherently make use of a friction contact between stator and rotor. This contact defines not only the actuator’s performance but also is prone to wear and tear. For broad use, the actuator has to be able to perform around 1 million strokes. To assess the actuator’s performance in terms of force, speed, mechanical output, electrical input, and long-term stability under different load- and environmental conditions, as well as different friction partners, a dedicated test bed for a LSPA30µXS motor by C-TEC has to be set up. The assessed actuator is the smallest of C-TEC’s LSPA-series. It consists of a piezo stack (2.5 x 2.5 x 5mm³) mounted in a diamond-shaped lever structure (8.6 x 3.9 x 5 mm³) to enlarge its stroke. This is then applied to a rectangular rod of 12 mm length. On this rod a clamp is mounted with a defined pre-load that is able to slide given a certain relative acceleration that determines the step size and the direction of movement. This is achieved by applying a dedicated signal of 100 V amplitude to the stack. The test bed consists of several modules for measurement and counter-force creation based on the requirements of three test modi. Mode S lets the actuator run against a linear spring thus leading to quick measurement of blocking force and no-load velocity. Mode F provides a constant force to derive single points in the actuator’s characteristic curve. Finally, mode D derives the actuator’s durability by letting the actuator perform a certain force/velocity profile. For each mode, a counter-force module is provided. For mode S a set of springs of different stiffness is used. For mode F a weight-lifting module is introduced also leading to constant forces in one direction. The D-mode is accomplished by a voice-coil actuator that is programmed in a way to provide arbitrary force-velocity curves. The test bed is controlled by a dedicated LabView program running on a standard PC. The PC also collects data provided by the different sensors. Using a Keyence, Osaka, JP laser triangulator (type LK-H052), the actual position of the clamp can be determined. Using a two-channel laser vibrometer by Polytec, Waldbronn, DE (type OVF-2502) provides phase-sensitive information of the piezo stack as well as the movement of the rod. The LabView software uses a Type NI PCI-7830R measurement card to collect data of the laser triangulator, the force sensor on the mechanical side and voltage and current applied to the actuator on the electrical side. Thus the efficiency of the actuator can be derived. Especially in mode D arbitrary load curves can be defined and both directions of movement can be assessed. Thus, a function generator is also included in the software. The whole test stand can be placed under an acrylic glass hood allowing the application of temperatures between 5 and 60°C and relative humidity between 50 and 100%. The described test stand has been set up to assess the actuator’s performance under different conditions of load, environment and internal friction parameters. Also, the influence of different friction partners on performance and lifetime can be assessed. The modular concept allows quick assessment of stall force and no-load speed (mode S), the assessment of the complete characteristic curve (mode F) and lifetime-tests under defined conditions (mode D). Using the described test stand, the energy efficiency of the LSPA30µXS actuator will be optimized in the future