Why Advancements in Piezoelectric Mechanisms and Motorized Actuators Enable Progress in Critical Industries Relying on Ultra Precision Motion Control

Why Advancements in Piezoelectric Mechanisms and Motorized Actuators Enable Progress in Critical Industries Relying on Ultra Precision Motion Control

Motion system engineers and researchers have relied on precision positioning systems, which utilize ballscrew drives for decades. Over time, these systems have undergone gradual improvements, offering several advantages for positioning heavy loads over long distances and they work well in multi-axis machining centers. Additionally, advancements in motor miniaturization have facilitated the creation of small positioning systems that are suitable for small-footprint motion control applications. However, when it comes to the most demanding applications in fields such as optics, semiconductor, and bio/nanotechnology, the traditional approach often falls short. To meet the needs of new applications such as photonics alignment, super-resolution microscopy and semiconductor related processes requiring more responsive and precise motion control systems, manufacturers have had to delve deeper than ever before to keep up with the pace.

Nanometer Precise Motion Generated with the Piezoelectric Effect

The piezoelectric effect: An electric charge applied to a piezoelectric material causes a dimemensinal change. This change can be used to generate force and motion.
PZT unit cell: Left: Perovskite-type lead zirconate titanate (PZT) unit cell in the symmetric cubic state above the Curie temperature.
Right: Tetragonally distorted unit cell below the Curie temperature.

For decades, piezoelectric motion-based precision mechanisms have been recognized for their exceptional ability to achieve sub-nanometer resolution motion, albeit over short motion ranges. However, recent advancements in the design of piezo ceramic actuators and motors, as well as control technologies, have successfully resolved the challenge of balancing travel distance with precision. As a result, users can now choose from a range of piezo-actuated precision motion and positioning systems that offer long ranges, along with excellent velocity dynamics, high force output, compactness, fieldlessness, and remarkable power-off position-hold stability. Let's explore some motion control applications that have driven the development of these groundbreaking technologies:

Ceramic (non-electromagnetic) Motors Can be Used in Semiconductor, SEM Microscopy, Medical Applications

While electromagnetic drive mechanisms can be used in positioning and alignment systems where external magnetic fields are unwanted, for example in e-beam lithography and SEM, the expense of shielding or positioning them outside of the action is a disadvantage. In contrast, fieldless piezo ceramic motors can be built much smaller and can be placed anywhere within these machines without negative effects. The benefits of piezoelectric-effect-based motion solutions, optimized over many years for semiconductor manufacturing and biotechnology research, can also be extended to medical technology. Piezoelectric transducers have already been used in medical technology for applications such as micro pumps, ultrasonic imaging, nano-dispensing valves, and for laser beam control in eye and skin surgery.

In imaging applications, such as 3-D-optical microscopy, MRI systems, and Optical Coherence Tomography (OCT), piezoelectric nanopositioners and mechanisms offer advantages due to their high-efficiency, direct-acting linear motion, high-resolution, fast response, and non-magnetic characteristics.

High Precision Motion with Nanometer-Scale Resolution is Critical in Scanning Microscopy

In drug discovery applications, analyzing a large number of samples quickly is crucial for success. Fluorescence imaging techniques are commonly used and require precise focusing on small amounts of liquid in multiwell plates.

A voice-coil driven objective nanofocussing stage	A piezo-flexure driven objective nanofocussing stage. The latest closed-loop models provide motion to 800 microns.

While conventional electric motors or voice-coil focusing motors drives can provide the required speed and precision for long range well-to-well positioning, frictionless piezo flexure guided objective focusing stages have an advantage when it comes to the ultimate performa nce with millisecond response and nanometer accurate focusing. Their response times, allow for rapid data acquisition, while minimizing photo bleaching caused by long-term exposure.

Nearfield scanning microscopy also requires motion control with similar speed and resolution. The scanning of small samples, typically ranging from 100x100 microns to 500x500 microns, with nanometer-scale lateral resolution can best be achieved by using flexure guided nanopositioning stages with piezoelectric-based drives. The latest designs utilize a parallel-kinematic motion principle with all actuators acting on one moving central platform, significantly reducing inertia and improving dynamics. Capacitance gauges integrated into the stage measure multi-axis movements against a common fixed reference, providing drift-free positioning with nanometer straightness, a feat not possible with conventional designs or stacked multi-axis stage designs.

Video: Principle of piezo stage in AFM SNOM microscopy

In atomic force microscopes (AFM), achieving superior surface metrology results requires nanometer-scale out-of-plane motion (OOPM) of the XY scanning stage. Traditional bearings are inadequate for this purpose and even many flexure designs have mechanical limitations that cannot meet the requirements. By using 6-degree-of-freedom actively controlled trajectories to compensate for off-axis errors, sub-nanometer OOPM can be achieved. These advancements are critical for next generation semiconductor development, which relies heavily on materials testing and accurate AFM data.

A compact 6-degree-of-freedom closed-loop, sample positioning stage with a parallel-kinematic flexure design and capacitive position feedback for AFM applications.  The XYZ motion range is 12 µm and positioning resolution <0.1 nm.  The 3 rotary degrees of freedom are mainly used for error compensation.

Long Distance Motion Ranges Combined with Nanometer Resolution and Extremely High Dynamics

In modern industrial production and testing, speed and efficiency are critical factors. In silicon photonics, ultra-precise scanning motion with very high speed is required to find the optimum coupling position, often in 6-degrees of freedom.  High speed and resolution are one of the most critical factors for reducing test time and thus increasing throughput and reducing production costs. Disk-drive head/media metrology applications, for example require sub-nanometer precise steps that need be executed and settled to nanometer tolerances within milliseconds. Traditional piezo flexure guided positioning systems are limited to travel ranges typically below one millimeter.  However, hybrid approaches where long distance motion is achieved by an electromagnetic motor/drive system and short distance scanning with nanometer resolution is handled by the piezo-flexure nanopositioner, are also available.

A 2x9 Axis dual photonics alignment system on a silicon photonics wafer prober. Here, 6-DOF hexapod micropositioners with torque motors / ballscrew drives provide the long-distance travel. 3-Axis piezo fl exure nanopositioners take care of the fast scanning action. (Image: Cascade Microtech, a FormFactor company)

High-Force Piezo-Stepping Motors for Optical Alignment and High-Energy Physics

Precision actuators with millimeter travel ranges and nanometer resolution are essential for optical alignment in large optical systems and high energy physics, and PiezoWalk linear motor actuators provide many advantages here.

PiezoWalk actuators employ a number of preloaded piezo elements grouped around a ceramic runner. By incrementally moving the runner, with sub-nanometer step resolution, they can provide high-force and long travel step-mode actuation. Within an individual step, picometer resolution motion with high-bandwidth is available (dither mode). PiezoWalk motors are available with vacuum-compatibility, and the piezoeffect is free from electromagnetic fields Another advantage of PiezoWalk motors is the very high power-off holding force up to 170lbs with nanoscale position stability for months and years.

A high-force PiezoWalk Linear Actuator

Different PiezoWalk designs, based on a V8 arrangement of eight actuators provide higher velocity in a more compact package.

Video: V8 piezo motor working principle

Programmable Piezo Shims

Piezoelectric programmable shims differ from traditional piezoelectric actuators because their length can be changed by a programming device.  After the change, they can be disconnectied from the programming device and will hold their position precisely, without the need for power.

High-Speed, Continuous Motion with Ultrasonic Ceramic Motors for Compact High Precision Motion Systems and Non-Magnetic Applications.

Piezoceramic transducers find another application in ultrasonic piezo motors. Unlike stacked structures, these motors are made of monolithic piezoceramic slabs that are electrically excited to generate nanometric oscillations at high frequencies. The directions and amplitude of the oscillations can be controlled by the drive circuit. The oscillation moves a hardened contact point at a resonant node-point in a quasi-elliptical manner, resulting in linear motion when preloaded against a mechanically guided slide. Travel ranges using this method are only limited by the length of the slide and submicrometer resolution is achievable.

Principle design of an ultrasonic linear motors with mechanic coupling and linear encoder.

A compact, high-speed closed-loop XY stage driven by an ultrasonic linear motor.  
The integrated linear encoder provides 10 nm resolution.

Video: A compact ultrsonic XY linear translation stage in a fast alignment application

Summary of Piezoelectric Drive Technologies

For many years, motion and positioning systems based on the piezoelectric effect have enabled motion systems designers push boundaries beyond what seemed technically possible. Advances in controller performance and mechanical designs continuously improve the performance to keep up the growing demands of the optics, biotech, and semiconductor industries. 

More information on piezoelectric motors and nanopositioning systems is available here.