Advanced Piezoelectric Single Crystal Based Actuators

Transcription

1 Advanced Piezoelectric Single Crystal Based Actuators Xiaoning Jiang, 1* Paul W. Rehrig, 1 Wesley S. Hackenberger, 1 Edward Smith, 2 Shuxiang Dong, 3 Dwight Viehland 3 Jim Moore, 4 Brian Patrick 4 1 TRS Technologies, Inc., 282 East College Ave, State College, PA D Hammond Bldg, The Pennsylvania State University, University Park, PA 3 Dept. of Materials Science and Engineering, Blacksburg VA SRS Technologies, 5 Discovery Drive, Huntsville, AL 3586 ABSTRACT TRS is developing new actuators based on single crystal piezoelectric materials such as Pb(Zn 1/3 Nb 2/3 ) 1-x Ti x O 3 (PZN-PT) and Pb(Mg 1/3 Nb 2/3 ) x-1 Ti x O 3 (PMN-PT) which exhibit very high piezoelectric coefficients (d 33 = pc/n) and electromechanical coupling factors (k 33 >.9), respectively, for a variety of applications, including active vibration damping, active flow control, high precision positioning, ultrasonic motors, deformable mirrors, and adaptive optics. The d 32 cut crystal plate actuators showed d 32 ~ -16 pc/n, inter-digital electroded (IDE) plate actuators showed effective d 33 ~ 11 pc/n. Single crystal stack actuators with stroke of 1 µm - 1 µm were developed and tested at both room temperature and cryogenic temperatures. Flextensional single crystal piezoelectric actuators with either stack driver or plate driver were developed with stroke 7 µm - > 25 µm. For large stroke cryogenic actuation (> 1mm), a single crystal piezomotor was developed and tested at temperature of 77 K - 3K and stroke of > 1mm and step resolution of 2 nm were achieved. In order to demonstrate the significance of developed single crystal actuators, modeling on single crystal piezoelectric deformable mirrors and helicopter flap control using single crystal actuators were conducted and the modeling results show that more than 2 wavelength wavefront error could be corrected by using the single crystal deformable mirrors and +/- 5.8 flap deflection will be obtained for a 36 flap using single crystal stack actuators. Keywords: single crystal piezoelectrics, piezoelectric actuator, ultrasonic motor, cryogenic actuator, cryomotor, flextensional actuator, deformable mirror, flap actuator. 1. INTRODUCTION Piezoelectric actuations have been used extensively in a broad range of applications including precision positioning control, active damping control, adaptive optics, adaptive structures for flap control, etc. [1-5]. There are many different type of piezoelectric actuators have been developed, mainly include in-plane actuators (or plate actuators), stack actuators, benders, flextensional actuators, and piezo-motors. In-plane actuation mostly adopts 31 mode to generate motion in plane, enhancement has been of interest by utilizing inter-digital electrodes since the low transverse piezoelectric coefficient (d 31 ) compared to the longitudinal counterpart (d 33 /d 31 ~ 2.4 for most piezos). Stack actuators are featured with amplified stroke (each layer stroke times the number of layers) and high blocking force. Benders and flextensional actuators such as moonie, cymbal, etc. are well known with low profile, large stroke because of the compliance amplification mechanism. Pizomotors on the basis of difference of static and dynamic friction or vibration modes are used for both rotary and linear motion control with high resolution and function of position set and held at power-off. All these piezoelectric actuators based on piezoelectric ceramics are available on the market, however, single crystal piezoelectric actuators with higher energy density comparing with their ceramic counterpart are still in the development stage. Single crystal piezoelectrics based on Pb(Zn 1/3 Nb 2/3 ) 1-x Ti x O 3 (PZN-PT) or Pb(Mg 1/3 Nb 2/3 ) 1-x Ti x O 3 (PMN-PT) exhibit large increases in strain over conventional piezoelectric ceramics (Figure 1) due to the ability to orient the crystals along a preferred high strain crystallographic direction. Due to a unique ferroelectric domain configuration, the crystals piezoelectric strain remains nearly hysteresis free up to levels of ~.5 to.6% depending on the crystal composition [6]. Furthermore, the crystals have been found to retain appreciable piezoactivity to temperatures as low as 2K (Figure 2), an important attribute for many space telescope applications [7]. In this paper, a couple of single crystal * xiaoning@trstechnologies.com; phone ; fax: ; trstechnologies.com Smart Structures and Materials 25: Active Materials: Behavior and Mechanics, edited by William D. Armstrong, Proceedings of SPIE Vol (SPIE, Bellingham, WA, 25) X/5/$15 doi: /

2 piezoelectric actuators developed at TRS recently are presented, and the modeling on using actuator performances for deformable mirror and rotorcraft flap control are also reported. Figure 1. Piezoelectric strain response from single PZN-PT single crystal material compared to piezoelectric and electrostrictive ceramics. The crystal strain remains nearly hysteresis free up to ~.6%. At high fields very large strains > 1% can be achieved but with an increase in hysteresis PZN-8%PT PZT-5H 1 d 33 (pc/n) d 31 (pc/n) d 33 = 393 3K 4 2 Value for PZT Ceramic at Room Temperature Temperature (K) Temperature (K) Figure 2. Cryogenic piezoelectric properties of Single crystal piezoelectrics. d 33 vs. temperature for a crystal of PZN-8%PT (from stack actuator strain measurement). d 31 vs. temperature (from resonance measurements) for a crystal of PZN-8%PT compared to Type VI (PZT-5H) piezoelectric ceramics 2. SINGLE CRYSTAL PIEZOELECTRIC ACTUATORS DESIGN AND FABRICATION 2.1 In-plane single crystal actuators A d 32 cut single crystal plate and IDE single crystal actuators are the two in-plane single crystal piezoelectric actuators (Figure 3). The d 32 cut plate actuator utilizes the promising transverse effect, where d 32 is about -16 pc/n. The displacement occurs largest along <11> edge (x direction in Figure 3 ), which is an in-plane actuation. Single crystal plates with patterned electrodes were prepared as IDE actuators (Figure 3), details on design and fabrication of IDE actuators please refer to [8]. Leadwires were soldered to the two electrodes and endcaps may be bonded to the two ends as interface. z y z y <11> x <1> x <11> Figure 3. Planar piezoelectric actuation. 31 mode; IDE actuation. 254 Proc. of SPIE Vol. 5761

3 2.2 Single crystal stack actuators For stack actuators, <1> PMN-PT crystal plates were diced from the PMN-PT crystal boule and cut into final dimensions, and followed by 5 Å Cr/2 Å Au sputtering on both sides of crystal plates to form electrodes. The electroded plates were then poled at 1KV/cm, and the properties such as dielectric constant, dielectric loss and d 33 were then recorded. The complete stack was composed of crystal plates, metal shim, endcaps and epoxy bonding layers, which were assembled as schematically shown in Figure 4. Figure 4 shows the picture of assembled single crystal stack actuators. Endcap Metal shim Tab for soldering Electroded Single Crystal Epoxy Figure 4. Single crystal stack actuators. schematic view of a stack actuator; (c) Photograph of single crystal stack actuators. 2.3 Single crystal flextensional actuators Two different types of flextensional piezoelectric actuators have been designed and fabricated. One is 31 mode flextensional actuator, where a single crystal plate was used as the driver for the thin metal endcaps. Single crystal plates with electrodes were prepared and properties such as dielectric constant, loss and d 33 were measured. Thin metal plates were pressed onto the endcap dies to form endcaps and bonded with single crystal plate using epoxy. Leadwires were soldered to the endcaps and Figure 5 shows an assembled 31 mode flextensional single crystal actuator. Another is called 33 mode flextensional actuator, where a single crystal stack actuator is inserted into a thin metal frame (amplification mechanism). The frame was provided by Adeptronics and is a standard APA6 frame, not a specially designed one. The PMN-PT stack actuator consists of 4 layers of PMN-PT plates with dimension of 5mmx5mmx.5mm. The assembled 33 mode flextensional actuator is shown in Figure 5. Figure 5. Single crystal flextensional actuators. 31 mode flextensional actuator. 33 mode flextensional actuator. 2.4 Single crystal piezomotor Proc. of SPIE Vol

4 Single crystal piezomotor based on wobbling mode with center coupling is schematically shown in Figure 6 (with screw), two single crystal ring stacks were fabricated with segmented electrodes to apply one pair of voltage signals with a π/2 phase difference, a wobbling motion at the center part of the stator was excited with high efficiency [9,1]. The stator was designed to be a cylinder with two single crystal piezoelectric ring stacks at the two ends (as shown in Figure. The PMN-PT single crystal rings were machined with patterned electrode. The rings were poled at 1KV/cm at room temperature at <.1 Hz. To assemble the stack, epoxy was used to bond single crystal discs and metals shims together with endcaps. The assembled stator is shown in Figure 6. A single crystal linear motor is shown in Figure 6(c). Single crystal ring stacks Wobbling motion mode Moving screw Piezo-stator Piezo-stator Frictional block slider Figure 6. Wobbling mode piezomotor. schematic view of a wobbling motor. Photograph of an assembled stator with two single crystal stacks. (c) Assembled single crystal linear motor without screw. 3. ACTUATORS CHARACTERIZATION 3.1 In-plane actuators The in-plane actuators (or plate actuators) were characterized using LVDT system for strain measurement and resonant method as well for impedance and phase test. The displacement vs. driving voltage of a plate actuator is shown in Figure 7. Low hysteresis is found for single crystal plate actuators, e.g. the hysteresis of plate actuator at 4V is about.8. A typical resonance mode of a plate actuator is shown in Figure 7, and the average results are shown in Table 1, indicating efficient actuation by using special cut 31 mode of PMN-PT crystals. Displacement ( m ) Driving Voltage (V) Impedance (Log) (Ohm) -9.E+1.E+ 2.E+4 4.E+4 6.E+4 8.E+4 1.E+5 Figure 7. Plate actuator characterization. Displacement vs. voltage of a <11> PMN-PT plate actuator (15mmx6mmx.5mm ); Impedance and phases vs. frequency of the plate actuator Frequency (Hz) Table 1. Resonance measurement for plate actuators. E S 11 Loss d 31 (pc/n) K 31 (1-11 m 2 /N) E+1 6.E+1 3.E+1.E+ -3.E+1-6.E+1 Phase (Degree) 256 Proc. of SPIE Vol. 5761

5 Displacement (µm) IDE Single Crystal (PZN- 4.5%PT)Actuator (12.1x5.7x.25mm) d33=977/9 pm/v Applied voltage (V) Figure 8. Displacement vs. voltage for IDE actuator. The actuator consist of 9 IDE unit cells so the d 33 is calculated by dividing the slope of the displacementvoltage curve by 9. IDE devices with various designs behaved differently in terms of strain yield under the same driving electric field, indicating an aspect ratio effect must be considered for IDE device design. It should be pointed out that a maximum microstrain of 886 (at 15KV/cm) was achieved with the developed PZN-PT single crystal IDE devices compared with 25 microstrain (at 8KV/cm) from the IDE devices made from PZT-5A [8,11]. Figure 8 shows the displacement vs. driving voltage performance of one IDE actuator, suggesting in-plane effective d 33 of 186 pm/v, lower than the normal crystal d 33, more design work must be done to further improve IDE actuator performance. 3.2 Stack actuators PMN-PT stacks with 2-layer, 3-layer, 36-layer and 4-layer 5mmx5mmx.5mm plates were fabricated and tested [12]. Figure 9 shows displacement vs. electric voltage for a 4- layer stack (~ 21mm long), good linearity between displacement and driving voltage is observed. Over 5 µm stroke is obtained with at driving voltage of 7 V, or the field of 14 KV/cm. Figure 9 shows impedance and phase vs. frequency of a 3-layer stack actuator. The similar measurements were conducted with 2-layer, 3-layer and 36- layer stacks. Clean modes are shown, and the elastic properties (compliance) calculated based on the measurement, as shown in Table 2, are close to the reported crystal properties, menaing the actuator assembly process is reliable. The compliance variation may be caused by the non-uniformity of crystal materials or the bonding thickness variation. The results from resonant measurement suggests the fast response and high efficient actuation of single crystal actuators. Displacement ( m) Driving Voltage (V) Figure 9. Stack actuator characterization. strain vs. voltage of a 4-layer stack; impedance and phase vs. frequency of a 3- layer stack. Table 2. Resonance measurement for stack actuators. F r (KHz) f a (KHz) Coupling coefficient Elastic property (or compliance) (1-11 m 2 /N) layer layer 36- layer Impedance (Log) (Ohm) E+1.E+ 1.E+5 2.E+5 3.E+5 Displacement (µm) Frequency (Hz) E+1 6.E+1 3.E+1.E+ -3.E+1-6.E Driving Voltage (V) Phase (Degree) Proc. of SPIE Vol

6 Figure flextensional actuator characterization. displacement vs. driving voltage. impedance and phase of the actuator varies with frequency (resonant modes). 3.3 Flextensional actuators High quality PMN-PT crystal plate with dimension of 15mmx6mmx.5mm and Cr/Au electrodes was bonded to the two brass endcaps with special design to achieve the large motion amplification. The displacement of a 31 mode flextensional actuator was measured using LVDT system and the displacement vs. electric driving voltage is shown in Figure 1. It is seen that over 66 µm stroke was obtained under 4 V (equivalent to field of 8KV/cm), very promising stroke considering such a low profile actuator. The resonant mode of this actuator is recorded as shown in Figure 1, the 7.5 KHz resonant frequency indicates the fast response of this actuator. The 33 mode single crystal flextensional actuator was characterized by measuring the strain vs. field and the resonant frequency. Figure 11 shows the displacement output of the actuator under various electric driving voltages, >85 µm stroke is achieved under 4 V (or field of 8KV/cm), and larger than 1 µm was obtained under 5 V. The resonant modes showed that the resonant frequency of this actuator is about 7-9 KHz (Figure 11). 4 V 3 V 2 V Displacement (um) Driving Voltage (V) Impedance (Log1) (Ohm) E+ 5.E+3 1.E+4 1.5E+4 2.E+4 Frequency (Hz) Phase (degree) Figure 11. Characterization results for a single crystal 33 mode flextensional actuator. displacement vs. driving voltage; resonant modes: impedance and phase vs. frequency. The cryogenic strain performance of the 33 mode flextensional actuators was measured using LVDT system. The actuator was put in a cryostat chamber with temperature varied from 75K to 3K. The stroke is about 74 µm at 75K under 5V (Figure 12). Figure 12. Cryogenic displacement vs. voltage of the 33 flextensional TRS Flextensional PMN-PT Actuator single crystal actuator. Displacement (um) Single crystal piezomotor -4 The fabricated PMN-PT single crystal ring stacks were tested -6 using LVDT system to measure the section displacement under -8 electric field. For each stack, 1 V was applied between positive leads and the ground. The stroke of each section of the two stacks under 1 V was about +/-.2 µm, which is a little bit low Temperature (K) (effective d 33 is about 1 pc/n.) because of the fact that the driving field is low and stacks were made of low performance PMN-PT crystals, but it turned out to be enough to 258 Proc. of SPIE Vol. 5761

7 demonstrate the cryomotor design concept. The resonant mode of the assembled stator was recorded using an impedance analyzer (as shown in Figure 13), where a clear B3 wobbling mode with resonant frequency of 41.5 KHz was observed Impulse:.8ms Cycle:.8s 78 nm/step Impedance (ohm) Impendance Phase Frequency (Hz) Phase ( O ) Stepping position (nm) second Figure.13. Spectrum of the assembled piezoelectric stator. Figure 14. Stepping motion at 78 nm/step in forward and backward direction. An electric pulse with width of.1 ms 7 ms and cycle period of.5 s-1 s was used to drive the PMN-PT motor and record the step displacement using an optical fiber position sensor. The Main specifications of the linear motor at room temperature were: Working frequency: f = 41.5kHz DC voltage for circuit driver: 6V Driving voltage to piezoelectric stator: 6-7Vpp Traveling speed measured: 5-1mm/sec Driving force measured:.2 N Very quick response time: ~ 2 msec Quick direction reverse. Figure 14 shows linear motion with a step displacement of 8 nm. Vibration noise was observed because of electronic and background vibration noise, which could be eliminated by testing the motor on a table with vibration isolation and driving the motor using a controller with shape input control. The response time of the linear motor was measured by applying a sharp impulse voltage and recording the displacement response, it was promising that the response time of this linear motor was 2 ms, faster than most ultrasonic motors [9]. For the cryogenic linear motor test, the piezoelectric motor was placed into a chamber full of liquid nitrogen, and a thermal-coupler type temperature meter was used to monitor the chamber s temperature. Experimental measurements showed that under -2 ºC, the motor still worked properly with only slightly decreased speed. 4. APPLICATION EXAMPLES OF SINGLE CRYSTAL PIEZOELECTRIC ACTUATORS Performance of deformable mirror and rotorcraft flap control using single crystal actuators were simulated, as examples, to demonstrate the advance of single crystal piezoelectric actuators. 4.1 Single crystal piezoelectric deformable mirror Full aperture modeling for 127 mm (5 inch) mirror with 289 single crystal stack actuators (5mmx5mmx2mm) was investigated. Figure 15 shows mirror surface before and after correction for a DM in the case of 2 µm focus error. The input surface is with a 2 µm focus error with a PV of µm, and RMS error of µm. After correction, the PV is.52 µm, and RMS error is.7 µm, which is very promising due to the fact that no DM has been found for such a large focus error correction (>3 λ for nm wave). The required stroke is from µm to 13.1 µm, and the force is from 1.16 N to 3.49 N, both stroke and force are within the single crystal stack actuator s capability. More results on using single crystal piezoelectric actuators for DM can be found in [12]. Comparing with currently available Proc. of SPIE Vol

8 electrostrictive or piezoceramic DMs, which are used for low stroke wavefront correction (5-1 µm), single crystal piezoelectric DM is promising for large stroke correction and also for cryogenic wavefront correction due to the excellent cryogenic properties of single crystal actuators [7]. Figure 15. A 2 µm Focus error correction case for 127 mm (5 inch) mirror with 289 actuators. uncorrected mirror surface; corrected wavefront. 4.2 Single crystal actuators for rotorcraft flap control Three types of actuators were compared according to their possible application in small or full-scale trailing-edge flap actuation. It was found that the performance of single crystal PMN-PT based actuators exceeds that of PZT ceramics. Figure 16. Free Angular Displacement/Block Force Diagram for Single Crystal and PI 151 Stack Flap Actuators with Optimized Offset Distance (e =.75 in. and 1.4 in. respectively, Rotor in Hover at 4 RPM actuating a 36 in. Flap). The difference between different actuator configurations has been shown in the detailed results below (Table 3) and the different trade-offs for each actuator type are discerned. As an example, comparison between single crystal stack actuators and commercially available PZT stacks (PI 151) was investigated for full scale flap control. Flap displacements increases of 16% are reached compared to those obtained for the 2X-frame actuator using a commercially available ceramic stacked actuator at an applied voltage of 45 Volts (3.5 deg. vs. 3 deg. flap deflection). Single crystal actuators 26 Proc. of SPIE Vol. 5761

9 working in a 2X-frame system should result in higher flap deflections, at equal applied voltage (45 Volts). It must be noticed that both actuators, PI 151 and single crystal stack actuators, could be driven up to 725 Volts. PI 151 stack actuators would provide approximately 5 flap deflection output, while single crystal stack actuators would provide 5.8 (Figure 16). Therefore, when driving both actuators at their maximum capability, the one using single crystal stack actuators will provide flap deflections.8 higher than PI 151. This would allow the actuator system to withstand higher aerodynamic force conditions, or to maintain those with a simpler amplification mechanism. Table 3. Full-Scale Trailing-Edge Flap Actuator Comparison for Maximum Allowed Electric Field and Optimized Off-set Distance. Actuator Max. Strain Energy PZT Shear Tube High: 4 (ft.lb/slug) Single Crystal Shear Tube High: 56 (ft.lb/slug) Voltage 18 Volts 3175 Volts Current (at 4 Hz.) Power p-p (Watts) Actuator Weight RPM /Flap Size Max. Flap Deflection (+/-) Single Crystal Bender High: 44 (ft.lb/slug) AC Bias 2 V. PZT Stack High: 111 (ft.lb/slug) DC Bias 725 V. Single Crystal Stack High: 134 (ft.lb/slug) Dc Bias 725 V. ~.5 (A) ~.5 (A) ~.8 (A) ~ 2.5 (A) ~ 2.58 (A) ~ 9 Watts ~ 158 Watts ~ 16 Watts ~ 1812 Watts ~ 1856 Watts ~.775 (lbs) ~.775 (lbs) ~ 1.5 (lbs) ~ 2.16 (lbs) ~ 2.16 (lbs) 36 4RPM 36 4RPM 36 4RPM 36 4RPM 36 4RPM CONCLUSIONS Several different types of single crystal piezoelectric actuators have been developed for various applications. Single crystal in-plane actuators, stack actuators, flextensional actuators and piezomotor were designed, fabricated and tested at both room temperature and cryogenic temperature. The modeling on single crystal piezoelectric deformable mirror and rotorcraft flap control further demonstrated the advance of single crystal piezoelectric actuators. ACKNOWLEDGMENTS We gratefully acknowledge the assistance of Hua Lei at TRS Technologies. The work was sponsored by DARPA under Contract No. W31P4Q-4-C-R18, MDA/NAVY under contract No. N167-3-C-57, NASA under Grant No. NNC4CA87C and NNC4CA91C. REFERENCES [1] F. Straub, V. Anand, and D. Domzalski, Development of a Piezoelectric Actuator for Trailing Edge Flap Control of Full Scale Rotor Blades, Institute of Physics and Publishing, Smart Materials and Structures, 21. [2] NASA JWST Cryogenic Actuator Activities, Http//ngst.gsfc.nasa.gov/hardware/text/actuatoractivities.html. [3] M.P. Langlois, J.P. R. Angel, M. Lloyd-Hart, S. Miller, and G. Angeli, High order adaptive optics system with a high density spherical membrane deformable mirror, SPIE proceedings [4] S. Yoshikawa, et al., Monolithic Piezoelectric Actuators and Vibration Dampers with Interdigital Electrodes, Proc. SPIE, Vol. 3668, pp , [5] S. Koganezawa and T. Hara, Development of shear-mode piezoelectric microactuator for precise head positioning, Fujitsu Sci. Tech. J., 37, 2, pp , 21. Proc. of SPIE Vol

10 [6] S.E. Park and T.R. Shrout, Relaxor based ferroelectric single crystals for electromechanical actuators, Mat. Res. Innovat., 1, pp.2-25, [7] D. S. Paik, S. E. Park, W. S. Hackenberger, and T. R. Shrout, Dielectric and Piezoelectric Properties of Perovskite Materials at Cryogenic Temperatures, J. Mat. Sci, 34, pp (1998). [8] X.N. Jiang, P.W. Rehrig, W.Hackenberger, and T. Shrout, Large Stroke and Low Profile Single Crystal Piezoelectric Actuators, SPIE Proceeding Vol.553, pp , 23. [9] S. Dong, S. Cagatary, K. Uchino, and D. Viehland, A Center-wobbling Ultrasonic Rotary Motor Using A Metal Tube-Piezoelectric Plate Composite Stator, Journal of Intelligent Material Systems and Structures, pp.1-7, 22. [1] X.N.Jiang, P.W. Rehrig, W.S. Hackenberger, S. Dong and D. Viehland, Single Crystal Ultrasonic Motors for Cryogenic Actuations, IEEE UFFC 24, Montreal, Canada. [11] N. Hagood, et al., Improving Transverse Actuation of Piezoceramics Using Interdigitated Surface Electrodes, Proc. SPIE, Vol.1917, pp , [12] X.N. Jiang, P.W. Rehrig, W.S. Hackenberger, J. Moore, S. Chodimella and B. Patrick, Single crystal piezoelectric actuators for advanced deformable mirrors, ASME Proceedings of Adaptive Materials and Systems, Anaheim, CA Proc. of SPIE Vol. 5761