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Motion Control Resources

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by Kristin Lewotsky, Contributing Editor
Motion Control & Motor Association

Advances in controls, encapsulation and distance of travel make piezoelectric actuators good choices for scientific, automotive and even industrial applications.

It’s easy to think of piezoelectrics as space-age materials only used in arcane research, but the reality is that they’re all around us, in everything from our spark-generating barbecue lighters to our vibrating cell phones and beepers. They may also be part of your day-to-day world in a motion control application -- thanks to recent advances in design, controls and encapsulation, the technology is capable of far more than you may suspect.

“They’re definitely not a lab product any more,” says Stefan Vorndran, director of marketing communications at PI (Physik Instrumente) L.P. (www.pi.ws; Auburn, Massachusetts) “They’re currently being used in automobile engines, and that’s basically proof that they’re industrial grade.”

Piezoelectric Actuators 101
Piezoelectric materials generate a voltage when compressed; conversely, applying a voltage to a piezoelectric material causes it to expand, making it useful for motion control. Conventional thinking has always held that piezoelectric actuators are not effective except in the most arcane motion control applications, primarily because of small travel distances, vulnerability to environmental conditions and intrinsic nonlinearities. As a result of recent developments, however, many of those assumptions no longer hold true.

There are four basic types of piezoelectric devices for motion control. The first is a standard piezoelectric actuator, which merely takes advantage of the material’s ability to provide displacement. The frictionless devices typically provide sub-nanometer resolution with sub-millisecond response time. Travel ranges from 0.1 to 0.2% of actuator length -- 10-200 μm for an actuator made of stacks of thin disks of piezoelectric material.

More advanced designs, however, can achieve much longer distances. Multilayer bender actuators -- pre-stressed composite structures of piezoelectric films and other layers -- can provide total linear displacements of more than a millimeter. For applications requiring longer travel and very straight motion, flexure-based designs amplify motion to achieve travel of 2 mm or more, with sub-nanometer resolution and response times ranging from 0.1 to 10 milliseconds. Applications include scanning optical microscopy and nanomanipulation.

Piezoelectric technology also shows up in motion control in the form of ultrasonic motors, both linear and rotary designs. In an ultrasonic friction motor, the piezoelectric vibration of a stator plate “walks” along a friction strip to create motion. Ultrasonic friction motors can achieve response times on the order of tens of milliseconds with travel distances of inches; materials and controller advances are yielding ultrasonic motors capable of speeds as high as 600 to 800 mm/s.

The friction has the negative effect of limiting spatial resolution to around 50 nm, however. Likewise, ultrasonic rotary motors, which convert linear motion to rotary motion via a lead screw, suffer from backlash, friction and elasticity.

The offerings are rounded out by piezo walk motors and piezo inertial motors. As the name suggests, piezo walk motors generate motion based on iterations of small, controllable steps (see figure 1). Though walk motors provide unlimited motion with very high resolutions in the picometer regime and can sustain loads as high as 120 lbs., they cannot achieve the high speeds of ultrasonic motors. Piezo inertial motors offer very compact packages, but only low-precision motion for loads of less than 1 oz.

Figure 1

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Piezoelectric “walk” motors use controlled, repeatable steps to provide motion of up to inches. Photo credit: PI

Taking Control
Achieving high spatial positioning accuracy and stability is only possible if you’ve got position feedback devices that are up to the job. Typically, three different position-sensor technologies are used for piezoelectric actuators: screen gauge, LVDT sensors and capacitive sensors. Strain gauge sensors, the lowest precision approach, consist of a resistive film bonded to the piezoelectric actuator or flexure. When the piezoelectric material stretches, it stretches the film, thus changing the resistance proportionally to displacement. A more demanding application might require the use of linear variable differential transformer (LVDT) position sensors.

For precision applications such as atomic force microscopy or scanning optical microscopy, capacitive sensors provide the best accuracy. Most sensors consist of two electrodes with one side fixed to the frame and one fixed to the moving part of the positioning stage. When the two elements move apart, the capacitance changes.

Assisted by these sensors, new controller technology allows devices to maintain sub-nanometer resolution over millimeters of travel. Indeed, precision capacitive sensors have allowed designers to better cope with the intrinsic nonlinearities of piezoelectric materials. Achieving high positional stability and repeatable performance requires a closed loop system, which is where capacitive sensors come in. In addition, digital controllers are available that can much better deal with the nonlinear characteristics of the piezo material.

Another new advance is in packaging. Humidity has long been the bane of piezoelectric actuators. “Basically, we’re dealing with electric fields of typically 1000 V/mm or higher, and high voltage and humidity just don’t mix,” says Vorndran. “Humidity starts a small leakage current, then you get arcing and dielectric breakdown, which kill the piezoelectric actuator.”

Enter the new ceramic encapsulation methods that protect the material from the environment, increasing lifetime and opening up applications in industrial environments. Such applications include out-of-round metal turning, such as for piston manufacturing or diamond-turned optics, or fast valve control in diesel fuel-injection systems for automobile engines.

And then there are the hybrid systems that combine an electrical motor with a piezoelectric motor. “They provide kind of a macro/micro actuator. They can still provide the long stroke that’s associated with an electromagnetic linear actuator but a small piezo component gives them very high resolution once they get to their approximate final position,” says Murray Johns, vice president of Dynamic Structures and Materials, LLC (www.dynamic-structures.com; Franklin, Tennessee). Such systems aren’t simple; each component requires a separate drive. “There’s quite a bit of software involved to do the control of both devices,” he adds.

Piezoelectric Actuators in Action
With the range of motion control options available, why choose piezoelectric actuators? Spatial resolution of course, but piezoelectric technology offers other important advantages for certain applications, such as speed. Combined with sub-nanometer resolution, the speed of piezoelectric positioners makes them perfect fits for semiconductor manufacturing or for biotech applications such as ultrafast autofocusing on biochip wells in drug discovery. “The height of each sample can vary by a few tens or hundreds of microns, and the field for the microscope is usually only a few microns, so you want to use a very fast autofocus system that can focus within a few milliseconds,” says Vorndran, “That’s where piezoelectrics really shine.”

“They also have the capability of providing much higher bandwidth or dynamic frequency response than more traditional electromagnetic technologies,” says Johns. Applications that make use of these capabilities include optical scanning or beam-steering for inspection systems.

Because piezoelectrics are efficient in their use of electrical power, they don’t tend to heat up as much as electromagnetic systems, allowing them to be used in cryogenic applications. “They can be used in vacuum systems in which other, more traditional electromagnetic technologies have some limitations in terms of their outgassing in vacuum or their heat generation in vacuum,” Johns notes. Ultrasonic piezo motors, in particular, are self-locking in the absence of power.

The ability to operate around high magnetic fields is another benefit. “We’ve had several applications where a customer needed motion control within an MRI environment,” says Johns. “The high magnetic field is not compatible with iron-based materials like stainless steel, so we used a titanium actuator frame with the piezoelectric and created a scanning stage.” Other applications include stage positioning in scanning electron microscopes or electron beam systems.

Echoing a theme that we’ve already explored with servo motors (see Serving Up Better Servos, January 2007), piezoelectric motors can be used to replace traditional hydraulic/pneumatic actuators in aerospace applications for which size and weight are important factors. “They can use a piezoelectric linear actuator to replace those larger and heavier technologies,” Johns says. “We see our components going into aerospace applications as both scientific components and as more of a built-in component as part of an interferometer in a satellite or opening valve in a propulsion system.”

Neither Johns nor Vorndran are saying that piezoelectric actuators and motors will replace servo and DC motors any time soon, but for a broadening range of applications, piezoelectrics can provide the ideal solution.

Thanks to Rob Carter of Piezo Systems Inc. (www.piezo.com; Cambridge, Massachusetts), who also provided input for this article.

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