Piezoelectric motors save power and downsize electronic access control

Industrial Embedded Systems — July 25, 2008

3Designers looking to save power and size are turning to advanced technologies, and motors are no exception. With piezoelectric technology at the heart, a new type of motor is improving small-scale motion systems in a big way.

Electronic access control enhances security, convenience, safety, and flexibility in a wide range of applications from building automation to automobiles. Today, system designers are adding "smallest size" to the requirements list for the electronic actuators at the core of access control systems. Some of the applications with high demand for miniaturization include:

  • Door locks: Hardware designers want electronic lock modules to fit within the footprint of existing mechanical lock cylinders. This enables easier retrofit and faster, more cost-effective mechanical system upgrades, thereby speeding technology adoption.
  • Medication carts: Smaller locks enable smart access storage in portable carts that caregivers use during patient care rounds. Electronic access control on these mobile carts helps reduce medication errors, eliminates extra trips to secure cabinets for controlled substances such as narcotics, and provides electronic records of all transactions.
  • Airplane compartments: Smaller, lightweight active latches support the industryís never-ending drive to reduce aircraft weight and improve fuel economy. Todayís fuel costs have increased the urgency for lighter systems.
  • Automotive: A typical passenger vehicle now has more than 100 motors and sensors in devices such as rearview cameras and safety interlocks. Designers are challenged to fit in even more automated devices for driver convenience and safety, while staying within a total power budget for the vehicleís electrical system.
  • Industrial computing: Electronic interlocks can avert hardware damage, for example, preventing a hard drive from being removed while it is operating. Smaller actuators fit in tight spaces, from server racks to laptops.

In addition to the demand for smaller-sized actuators, many of these applications require low power consumption, high speed, and fairly high force. While classic actuator solutions, such as DC motors, stepper motors, shape memory alloys, and solenoids have failed to meet all of these requirements, new piezoelectric motors are stepping up to the challenge.

Limits of traditional motors and solenoids

Traditional DC motors and stepper motors comprise dozens of parts, including iron cores, permanent magnets, copper windings, gears, and bearings. Micro motors are engineering marvels – complex assemblies of incredibly tiny components, even Microelectromechanical Systems (MEMS) components. However, these increasingly tiny parts are becoming increasingly fragile, which limits the force they can produce. Their presence in the load path also raises concerns about robustness, resilience to impact loads, and lifetime.

Another concern is efficiency, which drops sharply when motor diameter falls below about 10 mm (Figure 1). With smaller parts, an increasing percentage of power is converted to heat instead of motion. Smaller motor diameter requires higher operating speed to produce significant power, thereby requiring greater gear ratio reduction to produce usable torque, which also reduces efficiency. Finally, supplemental mechanical assemblies such as lead screws are needed to translate rotary motion into linear motion for most electronic access control applications, further increasing part count and integration complexity.

Figure 1

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. This limits their effectiveness in some applications, such as electronic access applications where high security is essential.

For these reasons, piezoelectric motors are emerging as an alternative to DC motors and solenoids where miniaturization, low power use, and high reliability are required. Unique design techniques have resulted in robust motors that are half the size of the smallest electromagnetic motors and yet offer greater push force and an overall reduction in power consumption.

Piezo motor background

Piezoelectric motors leverage the unique property of piezoelectric ceramic materials; they change shape in response to an applied voltage. This movement is typically on a micrometer scale for piezo elements with millimeter dimensions. Various motor designs multiply these micrometer-scale movements to deliver many millimeters of continuous motion.

One type of design uses piezoelectric elements placed in friction contact with a slider, using the force of the bending piezo ceramics to move the slider and push a load. The piezoelectric ceramics provide off-power hold but remain in the load path.

A more compact design uses piezoelectric elements to vibrate a nut in a tiny orbital wobbling motion similar to a hula hoop at an ultrasonic frequency matching the first bending resonant frequency of the nut (approximately 100 KHz). A mating threaded screw, driven by tangential forces from the threads of the nut, rotates and translates with a smooth linear motion to push a load, as shown in Figure 2.

Figure 2

This patented direct-drive linear motor, or SQUIGGLE motor, is half the size of other piezo motors, DC motors, and solenoids and yet offers equivalent or better force, stroke, and operating power (see Table 1).

Table 1: Actuator comparison

Inside look: electronic lock module

One microactuator module for electronic locks incorporates an SQL-1.8 SQUIGGLE motor, one of the smallest piezoelectric motors currently on the market, in a polymer housing measuring 4 mm x 4 mm x 14 mm (Figure 3). The tip of the SQUIGGLE motor screw can be used to push a spring-loaded shear pin, causing the pin to engage (or disengage) a locking mechanism or latch. Optical limit switches in the module provide simple position feedback, signaling when the motor screw has reached its forward and reverse motion limits.

Figure 3

Unlike DC motors or solenoids, the SQUIGGLE motor is self-locking, and the screw is securely held in either position when the power is turned off. This results in lower overall power consumption compared to traditional actuators because the motor needs to draw power only long enough to move one stroke to unlatch the door or cabinet – about 0.45 seconds – and then again to re-engage the lock after the door closes. The total duration of power use is about 1.5 seconds.

A solenoid, on the other hand, will revert to its base state as soon as the power is removed. Therefore, it must draw power continuously, not only to disengage the latch but also to hold the latch open long enough for a person to move the door before powering off to re-engage the lock. The total duration of power use is at least 4 seconds in most cases, or at least 2-3x longer than the piezoelectric module. In addition, solenoids often draw higher power (up to 3 W) during use.

Bistable solenoids use permanent magnets to hold a second position with the power off. However, these latching solenoids are larger than standard solenoids and could be even more susceptible to opening by an external magnetic field. They also draw several watts of power when moving.

Driving the motor

The SQUIGGLE module is controlled by an external miniature drive card or by an ASIC controller that can also fit into the lock cylinder with the module. The electronics are powered by a 3 V battery, with ASIC versions drawing 500-700 mW during use depending on the drive technique. A direct linear drive circuit results in the smallest footprint. A resonant drive circuit is larger but reduces power use by adding inductors, which combine with the capacitive piezoelectric ceramics to set up a resonant circuit in which power is conserved.

The motor driver generates two-phase signals needed to vibrate the piezoelectric elements at very small amplitude and ultrasonic frequency. The ASIC driver accepts input using an I2C protocol to define the output voltage (and thereby motor speed) and the direction of motion and to stop the motor at the end of its stroke based on feedback from the limit switches. The block diagram is shown in Figure 4.

Figure 4

More applications possible

Piezoelectric motor modules are also available with high-resolution linear position sensors, rather than the simple limit switches. This allows precise speed control and enables the motor screw tip to be positioned anywhere along its full range of travel.

For example, a tiny autofocus and optical zoom module has two SQUIGGLE motors, two lens assemblies, and two position sensors in an 8 mm x 12 mm x 28 mm package (Figure 5). The lens assemblies move independently to a precise location. Application-specific modules such as this provide closed-loop positioning systems in smaller sizes for a wide range of new applications.

Figure 5

When small size is essential, piezoelectric motors provide an alternative to DC micro motors and solenoids as well as offer much larger travel range and force than the tiny low-force MEMS devices beginning to emerge from research labs.

Todayís piezoelectric motors are robust, strong, light, and easy on power budgets. These motors are found in application-specific motion modules for a wide range of embedded systems, from facility automation to camera phones to medical instruments. A motor this small can open doors to many other types of miniaturized products.

Dan Viggiano III is VP and general manager of New Scale Technologiesí OEM and industrial business based in Victor, New York. He has prior experience as a design engineer and engineering manager at New Scale and Burleigh Instruments (now EXFO). Dan has a BS in Mechanical Engineering and an MBA from the Rochester Institute of Technology.

Lisa Schaertl directs marketing communications for New Scale Technologies. She has 20 years of experience marketing technology products, including design automation software, optical instruments, and electrical and electromechanical components. She has a BS in Electrical Engineering from the South Dakota School of Mines & Technology.

New Scale Technologies, Inc.