Piezoelectric Actuators With Helical Electrodes And Their Applications

In the dissertation, we performed deep analyses and experimental researches to investigate new types of piezoelectric fiber, bulk, and discal torsional actuators based on helical electrode structures and explored their applications in piezoelectric rotary motor and optical scanner.A torsional piezoelectric fiber structure with external helical electrodes was first proposed, which was the smallest piezoelectric torsional actuator in the world. Working principle was explained with strain and stress transforming principles in different coordinate systems. Theoretical models of static deformation and dynamic vibration were built and equivalent circuit of coupled vibrations was also obtained. Influences of primary material and structure parameters on the evaluation criteria of the actuator, such as torsional displacement and resonant frequencies, were analyzed and estimated for the optimization of the structure.With help of finite element analysis software ANSYS, we simulated distribution of electric field in piezoelectric fiber driven by external helical electrodes and pointed out that the actual distribution of electric field was deficient. Influences of these non-uniformities, such as concentration and distortion of the electric field, on the performance of piezoelectric fiber actuator were analyzed. We concluded the changing trends of electric field distributions with different structure parameters of piezoelectric fiber and helical electrodes, and deduced the distribution of strain and stress among the fiber material. A compensation factor k was defined to regulate the expression of torsional displacemet and the trends of the compensation factor k as functions of piezoelectric fiber thickness and electrode helcal angle were investigated.Prototype actuators were manufactured using piezoelectric hollow fibers with an outer diameter of 1 mm and a wall thichness of 0.2 mm and their static and dynamic characteristics were also tested. Experimental results indicated that these actuators have outstanding actuation abilities. Sample with a length of 50 mm and an electrode helical angle of 43°could produce a static torsional angle of 1.7°under a driving voltage of 1000 V. The amplitude of torsional vibration reached up to 2.5°at resonant frequency of 10 kHz under a driving voltage of 500 Vp-p. Through some deliberate comparing experiments, the modified theoretical curve of torsional displacement as a function of electrode helical angle was verified, which indicated that the torsional deformation was maximized with an electrode helical angle around 45°. The trends of the resonant frequencies of torsional and longitudinal vibrations as functions of electrode helical angle and fiber length were also validated in experiment.We developed another two types of piezoelectric torsional actuator, which were piezoelectric bulk actuator with grooved helical electrodes and piezoelectric discal actuator with planar spiral electrodes. Designs of electrode structures and principles of torsional derformations in these two actuators were described respectively. Prototype samples of the two actuators were fabricated and tested. Piezoelectric bulk actuator with a diameter of 8 mm, a length of 10 mm, and electrode pitch of 12 mm could produce a static torsional angle of 0.035°under a driving voltage of 2200 Vp-p. Its first-order torsional and longitudinal resonant frequencies reached up to 31 kHz and 55 kHz, respectively. The amplitudes of these two vibrations were 0.00087°and 43 nm, respectively, under a driving voltage of 5 Vp-p. Piezoelectric discal actuator with an outer diameter of 27 mm and a thichness of 0.2 mm produced a static torsional angle of 0.0052°under a 1000 V voltage. In despite of small torsional deformation, it had a high resonant frequency of 226.5 kHz with a large mechanical Q-factor.The applications of developed piezoelectric actuators in aspects of piezoelectric impact rotary motor and optical scanner were explored. Two types of piezoelectric impact rotary motors based on piezoelectric fiber actuator and bulk actuator were designed, fabricated and tested. Piezoelectric fiber motor could produce a step angle of 0.063°under a saw driving voltage of 600 Vp-p and had a stall torque of 0.08 mNm under a driving voltage of 1000 Vp-p at 3 kHz. Piezoelectric bulk motor could produce a step angle of 0.022°under a driving voltage of 2200 Vp-p and remained a speed of 7.4 rpm with a braking torque of 1.6 mNm under a driving voltage of 1400 Vp-p at 8 kHz. The miniature piezoelectric fiber motor and large-torque bulk motor could satisfy several requirements in the applications of precision angular positioning and actuation. We also designed and fabricated a novel optical scanner based on torsional piezoelectric fiber actuator. Working principle and scanning characteristics of such miniature scanner were described. Sample with a mirror of 1 mm×1 mm could produce a horizontal scanning angle of 17.9°at 6780 Hz and a vertical scanning angle of 2.6°at 10300 Hz under an applied voltage of 400 Vp-p. The two-dimensional optical scanning with large amplitudes and high frequencies indicated its application potentials in laser scanning microscopes, barcode readers, and medical endoscopic imaging systems.