[MOTOR] Piezoelectric Motor Alternative to DC Motors and Solenoid Today Hardware designer would like electronic lock modules to fit within the footprint of existing mechanical lock cylinders. Smaller, lightweight active latches support the industry never-ending drive to reduce aircraft weight. Solenoid assemblies are inexpensive and useful in many applications. However, because solenoids operate using strong magnets to hold the plunger in place, bringing a strong magnetic field into close proximity with the lock can potentially cause them to open. Piezoelectric motors are emerging as an alternative to DC motors and solenoids where miniaturization, low power use, and high reliability are required.

DAN VIGGIANO III & LISA SCHAERTL

C motors and stepper motors include dozens Dof parts, copper w i n d i n g s , permanent magnets, iron cores, gears, and bearings. Micro motors are engineering marvels complex assemblies of incredibly tiny components. However, these increasingly tiny parts are becoming increasingly fragile, which of parts. This simple, robust piezo motor is scalable limits the force they can produce. Their presence in to much smaller sizes than electromagnetic motors, the load path also raises concerns about without significant loss of power efficiency. This robustness, resilience to impact loads, and lifetime. The SQUIGGLE motor is a makes it ideal for portable consumer products such revolutionary linear motor LEAD ZIRCONATE TITANATE that sets a new benchmark Random Domains for small size and high (+) Metal Electrode performance. The patented Domains are ultrasonic motor creates high Aligned force and speed with only a few parts. It replaces complex (-) Metal Electrode electromagnetic gear-head Unpolarised Ceramic Polarised Ceramic motors which have hundreds Material Material Fig. 1. Effect of Electric Field on Piezoelectric Material [MOTOR]

are driven at or near a resonant frequency mode of Axial Dispalcement (+) the crystal shape. Piezoelectric motors control by the exclusive property of piezoelectric ceramic materials, they change shape in response to an applied voltage. V This movement is typically on a micrometer scale for piezo elements with millimeter dimensions. (-) Various motor designs multiply these micrometer- -12 scale movements to deliver many millimeters of Axial Dispalcement=(500X10 m/V) X V continuous motion. One type of design uses Fig. 2. Axial displacement when subjected to applied voltage. piezoelectric elements placed in friction contact as mobile phone cameras and wearable medical with a slider, using the force of the bending piezo devices. The SQUIGGLE motor generates no ceramics to move the slider and push a load. The magnetic fields, it is vacuum compatible, and can piezoelectric ceramics provide off-power hold but be made from non-ferrous metals. This makes it remain in the load path. A more compact design ideal for use in MRI, scanning electron microscopy uses piezoelectric elements to vibrate a nut in a tiny and focused ion microscopy applications. orbital wobbling motion similar to a hula hoop at Piezoelectric ceramic is mass-produced for an ultrasonic frequency matching the first bending applications including underwater transducers, resonant frequency of the nut (approximately 100 point level sensors, medical products, ultrasonic KHz). A mating threaded screw, driven by tangential cleaners, actuators, fish finders, and motors. High- forces from the threads of the nut, rotates and purity lead, zirconate, and titanate powders are translates with a smooth linear motion to push a processed, pressed to shape, fired, electroded, load, as shown in fig. 3. DC motors or solenoids, polarized, and tested. Polarization is achieved the SQUIGGLE motor is self-locking, and the screw using high electric fields (2500 V/ mm) to align is securely held in either position when the power is material domains along a primary axis fig. 1. turned off. This results in lower overall power This process is functionally identical to the way consumption compared to traditional actuators material shapes containing iron are magnetized. because the motor needs to draw power only long Once poled, the material exhibits useful enough to move one stroke to unlatch the door or piezoelectric properties. Piezoelectric motors use cabinet about 0.45 seconds and then again to re- this poled ceramic shape to create motion with the engage the lock after the door closes. The total use of periodic (sinusoidal) electric fields. In fig. 2, duration of power use is about 1.5 seconds. A 200 V charge is applied to a piezoelectric crystal solenoid will revert to its base state as soon as the with a displacement constant of 500 pm/ V. This power is removed. Therefore, it must draw power produces an axial displacement of 0.1 Phase 1 volts microns, a fairly small number that could be g o o d o r b a d depending on the GND Phase 2 volts application. It is very repeatable and lasts a Node lifetime with a properly designed drive circuit, but the motor will not Phase Shift Node travel very far when +90: Forward Motion Opposing -90: Reverse Motion 200 V are applied just Piezoelectric strain bends nut once every second. This Nut Vibration Cause Screw i s t h e r e a s o n to Rotate and translate piezoelectric motors Fig. 3. Piezoelectric Motor Bias [MOTOR] continuously, not only to disengage the latch but convenience, safety, and flexibility in a wide range also to hold the latch open long enough for a of applications from building automation to person to move the door before powering off to re- automobiles. Today, system designers are adding engage the lock. The total duration of power use is smallest size to the requirements list for the at least 4 seconds in most cases, or at least 2-3 electronic actuators at the core of access control times longer than the piezoelectric module. In systems. Some of the applications with high addition, solenoids often draw higher power (up to demand for miniaturization include, 3 W) during use. Bistable solenoids use permanent Door Locks magnets to hold a second position with the power Hardware designers want electronic lock modules off. However, these latching solenoids are larger to fit within the footprint of existing mechanical lock than standard solenoids and could be even more cylinders. This enables easier retrofit and faster, susceptible to opening by an external magnetic more cost-effective mechanical system upgrades, field. They also draw several watts of power when thereby speeding technology adoption. moving. The SQUIGGLE module is controlled by Medication Carts an external miniature drive card or by an ASIC Smaller locks enable smart access storage in controller that can also fit into the lock cylinder with portable carts that caregivers use during patient the module. The electronics are powered by a 3 V care rounds. Electronic access control on these battery, with ASIC versions drawing 500-700 mW mobile carts helps reduce medication errors, during use depending on the drive technique. A eliminates extra trips to secure cabinets for direct linear drive circuit results in the smallest controlled substances such as narcotics, and footprint. A resonant drive circuit is larger but provides electronic records of all transactions. reduces power use by adding inductors, which Airplane Compartments combine with the capacitive piezoelectric ceramics Smaller, lightweight active latches support the to set up a resonant circuit in which power is industrys never-ending drive to reduce aircraft conserved. The motor driver generates two-phase weight and improve fuel economy. Todays fuel costs signals needed to vibrate the piezoelectric elements have increased the urgency for lighter systems. at very small amplitude and ultrasonic frequency. The ASIC driver accepts input using an I2C Automotive A typical passenger vehicle now has more than 100 protocol to define the output voltage (and thereby motors and sensors in devices such as rearview motor speed) and the direction of motion and to cameras and safety interlocks. Designers are stop the motor at the end of its stroke based on challenged to fit in even more automated devices feedback from the limit switches. The block for driver convenience and safety, while staying diagram is shown in Figure 4. within a total power budget for the vehicles electrical system. Applications Industrial Computing Electronic access control enhances security, Electronic interlocks can prevent hardware damage, for example, preventing a hard drive from 2.5V DC being removed while it is operating. Smaller Booster actuators fit in tight spaces, from server racks to 24V DC Circuit laptops. In addition to the demand for smaller-sized Two Phase Signal actuators, many of these applications require low Logic Driver for Motor Drive Circuit power consumption, high speed, and fairly high k

r ASIC c force. While classic actuator solutions, such as DC o a s b n d motors, stepper motors, shape memory alloys, and e

SDA SCL e S e Microprocessor/ F solenoids have failed to meet all of these Microcontroller requirements, new piezoelectric motors are stepping up to the challenge.

Fig. 4. ASIC Driving the Piezoelectric Motor