Piezo Motors - Motion Control Tips

PI expands digital piezo nanopositioning controller family

February 16, 2018 by Miles Budimir Leave a Comment

PI (Physik Instrumente) has expanded its E-709 family of low-cost digital piezo nanopositioning controllers with a new plug-in module for the C-885 PIMotionMaster modular controller system.

The PIMotionMaster system allows the combination of different drive technologies, controlled through one interface via one software package. For example, stepper motor stages, closed-loop servo motor positioners, and fast piezo-driven nano-focus devices can now be operated from one controller. Up to 20 control cards can be plugged into one 19-in. rack.

The latest piezo control module, the E-709.1CC885, supports functions including wave generator, data recorder, auto zero, and I/O triggers. With a sensor update rate of 10 kHz, two notch filters and linearization algorithms based on 5th-order polynomials it provides exceptionally accurate motion with sub-nanometer precision. The E-709 card combines a power amplifier (-30 to 130 V output), signal conditioning circuitry for capacitance sensors and a digital servo. Piezo controllers with digital servo offer the advantage of higher linearity, and easy access to advanced features in comparison to conventional analog piezo controllers. A comprehensive software package is included: drivers for LabVIEW, dynamic libraries for Windows and Linux, MATLAB, MetaMorph, µManager, Andor iQ. Interfaces include USB, SPI, RS-232, and analog.

After the modules are installed in the rack, individual axis configuration and movement is easily set-up using the included PIMikroMove software, a comprehensive Windows GUI that does not require programming knowledge for commissioning or direct operation.

Modules are currently available for PI translation/rotation stages equipped with:

DC motors
3-phase motors
Stepper motors
Piezo flexure positioners
Piezo ultrasonic motors
PiezoWalk® nanopositioning motors
Piezo inertia motors

Fields of applications include precision motion control in automation, nanopositioning, photonics, semiconductor testing, laser machining.

For more information, visit www.pi-usa.us.

What kind of feedback do piezo motors use?

February 9, 2018 by Miles Budimir Leave a Comment

Piezo motors are known for their extremely small size and high size-to-torque ratio, as well as high-resolution movements (micrometer and even nanometer-level resolution.) The high resolution is attributable to the inherent design of piezo motors themselves, but feedback also helps to ensure highly accurate movement on the order of nanometers.

Incremental linear encoders, such as the LIP 481 from Heidenhain (left), are often used for position feedback with digital motion controllers in piezo motor positioning stages. (Image via Heidenhain) An example of a linear motor positioning stage (right) equipped with digital position feedback. (Image via PI)

As a general rule, piezo motors can be operated either open loop or closed loop. The advantage of closed-loop operation is that it offers higher precision and greater repeatability and linearity. Closed-loop feedback also helps eliminate drift and hysteresis, both factors that can negatively impact precision and accuracy.

There are a few common types of feedback used with piezo motors ranging from strain gauges to optical encoders, depending on the application requirements:

- - Optical encoders: Incremental optical encoders are commonly used for piezo motor feedback. However, absolute encoders can be used as well, although they are typically more costly than incremental encoders.
  - Capacitive sensors: Along with strain-gauge sensors, these are the most common as they are relatively inexpensive, compact, and provide good quality nanometer-resolution readings.
  - Laser interferometers: These offer sub-nanometer resolution, but they are also more bulky and expensive than either capacitive based sensors or encoders.

Can piezo motors and electric motors be combined?

January 26, 2018 by Danielle Collins Leave a Comment

Piezo motors and electric motors are employed for very different uses. Piezo motors provide a high power-to-size ratio for very small movements – typically a few millimeters or less. They also have very fast response times and are self-locking, meaning they can hold a load in position during a power-off condition.

Electric motors, on the other hand, are often used to drive ball or lead screws whose travel is at least several hundred millimeters. And the friction that is produced by sliding or rolling motion – as in the case of screw drives – makes it difficult to achieve low- or sub-micron levels of resolution, and often causes the motor to compensate for overshoot or undershoot at the end of the move by “hunting” for the commanded position.

So where does an engineer or designer turn when the application requires both nanometer-level resolution and travel greater than a few millimeters?

Linear motors and voice coil actuators are possible solutions. But while linear motors offer long stroke lengths with excellent positioning accuracy and repeatability, they still suffer from the effects of friction in the linear bearing guides that are used for supporting the load. Voice coil actuators also offer precise positioning along with excellent force control and smooth motion. But their travel is limited to around 100 mm, and they draw current when holding a load at standstill.

The emergence of linear encoders capable of nanometer-level resolution has made it possible to combine piezo motors and electric motors into a hybrid device. The hybrid design provides the positioning accuracy and repeatability of a piezo system, together with the long travel of a motor-driven ball or lead screw. This is accomplished by incorporating a piezo motor (typically a flexure guided piezo actuator) into the screw assembly, with the electric motor responsible for “coarse” motion and the piezo actuator responsible for “fine” motion.

Two examples of how a piezo device can be integrated with a screw and nut assembly.
Image credit: Physik Instrumente

In simple terms, the electric motor and ball screw assembly makes a large, or “coarse,” move, which reaches very close to the desired position, and the piezo actuator makes a very small, or “fine,” move that positions the stage to within just a few nanometers of the target position. The piezo actuator can also improve the smoothness of motion by compensating for deviations in the motor’s speed during travel.

The enabling technology for this performance is a linear encoder with nanometer-level resolution, which is shared by both the electric motor actuator and the piezo actuator. Both actuators are also controlled within the same servo system, but on two separate control loops. The use of both a common feedback device and a common controller allows the system to constantly read the assembly’s position and coordinate the motions of the electric motor and piezo actuator to achieve nanometer-level positioning accuracy over long travel lengths.

7-PI-1-Hybrid-Positioning-Stage-Servo-Piezo

A hybrid positioning stage that uses a servo-driven ball screw assembly to provide “coarse” motion and a flexure guided piezo actuator to provide “fine” motion. The servo motor and the piezo actuator are driven by a common controller and use a linear encoder with nanometer resolution. The result is nanometer-level positioning and consistent velocity over multiple inches of travel.
Image credit: Physik Instrumente

Hybrid piezo-electric systems are useful in semiconductor inspection and metrology applications, where positioning accuracy in the nanometer range is required, but the devices to be inspected or measured are several inches in length or diameter. These requirements are also common in the optical field, where a mirror might need to be moved several inches, but the position must be achieved to within a few nanometers.

Do piezo motors need specialty PID loops or controls?

January 19, 2018 by Miles Budimir Leave a Comment

The simple answer is, not necessarily. Not every piezo motor requires a PID control loop. However, most piezo motors operate closed-loop with some type of feedback, and PID control is only one possibility, but it is common and effective in controlling piezo motors. Some piezo motor applications can and do run open loop, although piezo actuators (different from piezo motors) are more likely to do so.

The graph illustrates the performance comparison of an Advanced Piezo Control (pink) and PID algorithm (blue), showing the improved settling time offered by Advanced Piezo Control, based on space state control. (Image via Physik Instrumente)

Piezo motors have a number of benefits including their small size, high-resolution capability, and high torque developed relative to their small size. They also have low inertia, which makes them more responsive with fast start and stop times. For optimal performance (meaning better accuracy and repeatability) a closed-loop control scheme is usually a part of a piezo motor system. Common feedback devices that provide input for controllers can include capacitive sensors, laser interferometers, as well as optical incremental encoders, depending on the application.

A typical controller for a piezo motor can be a standard off-the-shelf servo controller available from any number of manufacturers, or it can be a specialized PID controller. Another type of control offered by some manufacturers is the so-called space state controller. In this type of controller, the resonance frequency is dampened leading to faster settling times.

New catalog on piezo-mechanic nanopositioning motion systems

January 18, 2018 by Paul Heney Leave a Comment

Motion and nano precision positioning systems manufacturer PI (Physik Instrumente) has released a new 250-page catalog on precision mechatronic actuators and nanopositioning systems based on piezo drive technology and frictionless flexure guiding systems. Products covered in this catalog use PI’s in-house designed and manufactured Mars Rover-tested piezo ceramic actuators as their drive principle.

A special form of electro-ceramics, piezo actuators deliver a unique combination of precision, speed, and force in a small package. Their lack of friction, particle generation, and associated wear makes them ideal for many high tech applications including photonics, super-resolution microscopy, high-speed dispensing and switching, medical engineering, etc. Miniaturized systems are available for easy integration into new or existing industrial, research, and OEM systems. Where a standard system cannot be used, customized solutions are available as outlined in the intro-section of the catalog.

Examples of standard products highlighted in this publication range from low-cost OEM-type flexure actuators to 6-axis integrated nanopositioning systems, ultra-fast laser steering systems, and photonics alignment systems. Along with the piezo mechanic systems, digital controllers and sub-nanometer precise position sensors are part of the solutions PI offers. Many of these state-of-the-art precision motion products will be showcased at Photonics West in San Francisco later this month.

PI’s piezo actuators were tested by NASA/JPL for the Mars mission and passed 100 billion cycles of life testing without failures.

Applications include optical microscopy, AFM, mechanical engineering, industrial manufacturing, quality assurance, scientific instrumentation, nano-dispensing, photonics packaging, astronomy, semiconductor test and manufacturing.

Request a print copy or download the catalog here: http://www.pi-usa.us/pdf/index.php#PiezoSys

Watch a piezo flexure nanopositioning systems video: https://www.youtube.com/embed/BJbYzLJ7kWA?start=182&rel=0

PI
http://www.pi-usa.us

Three piezo motor designs for rotary motion

January 7, 2018 by Danielle Collins Leave a Comment

One of the best technologies for achieving extremely small, precise movements is the piezo motor. Based on the piezo actuator – layers or stacks of piezo elements that produce motion through the reverse piezoelectric effect – the piezo motor incorporates mechanical elements to produce strokes of several hundred millimeters or more. And while most piezo motors are designed to produce linear motion, there are several types that can also produce rotary motion, making them suitable for applications such as optical equipment and measuring devices.

Linear ultrasonic piezo motors incorporate a piezo actuator, a friction component, and a runner. To achieve rotary motion, several piezo actuators are arranged in a ring-shaped formation, sometimes referred to as the “stator,” and the linear runner is replaced with a ring-shaped rotor. When high-frequency AC voltage is applied to the stator, it causes wave-like, ultrasonic vibrations in the piezo actuators. The vibrations of the stator are transmitted to the rotor through the friction element, causing the rotor to turn at high speed.

In an ultrasonic motor, multiple piezo actuators oscillate at high frequency, contacting the ring and causing it to rotate.
Image credit: Physik Instrumente

A variation of this design uses two ultrasonic piezo actuators, mounted and preloaded tangentially on a friction ring. With each oscillation of the actuators, friction tips contact the ring and advance it, causing it to rotate.

In this design, two piezo actuators are mounted tangentially on a ring. As the actuators oscillate, they advance the ring and cause it to rotate.
Image credit: Physik Instrumente

Another piezo technology that can produce rotary motion is the walking type piezo motor. These designs use multiple piezo bending actuators (or a combination of clamping and shear actuators) to rotate a ring. The piezo actuators are synchronized in pairs – often referred to as “legs.” When voltage is applied, the first pair of legs extends, grips the ring, and moves it in the desired direction. At the same time, the second pair of legs retracts away from the ring and moves in the opposite direction. On the next activation, the first pair of legs retracts and moves backwards while the second pair extends to make contact with the ring and move it further along its direction of rotation. Although each “step” is just a few microns, operating at high frequency provides thousands of steps per second, for high rotational speeds.

In a piezo walking motor, pairs of “legs” work together to produce motion in a longitudinal rod. The same principle can be applied to a ring to produce rotary motion.
Image credit: MICROMO

Piezo inertia motors (commonly referred to as “stick-slip” motors) can also be configured to produce rotary motion. While inertia motors don’t produce rotary motion directly, they’re often used to drive very fine pitch screws. In this design, jaws attached to each end of a piezo actuator engage with the screw, 180 degrees apart. As the actuator rapidly expands and contracts, the jaws turn the screw.

Piezo inertia motors incorporate “jaws” that grip a lead screw on either side, turning the screw as the actuator expands and contracts.
Image credit: Thorlabs, Inc.

Which piezo motor types experience wear?

October 5, 2017 by Danielle Collins Leave a Comment

Piezo elements are used in a variety of configurations to provide rotary or linear motion in applications that require short strokes and fast response times. Piezo elements that are used to produce motion through the inverse piezoelectric effect are known as piezo actuators. These devices provide very small displacements with extremely high resolution. When one or more piezo elements are combined, the assembly is commonly referred to as a piezo motor. Piezo motors require more sophisticated drive electronics, but can provide longer travel lengths than piezo actuators.

Depending on the arrangement of the piezo elements and the type of movement they generate, piezo actuators can generally be grouped into four main types: longitudinal, shear, tube, and contracting. Despite their different configurations and uses, these piezo actuators all operate on the same basic principle: movement is directly proportional to the voltage applied. Because they operate solely on the solid-state dynamics of the piezoelectric material, with no mechanical parts, piezo actuators do not experience wear.

A piezo actuator can be integrated with frictionless flexures and levers to form a flexure guided piezo actuator. The flexures and levers multiply and translate the motion of the piezo stack to provide travel up to a few mm, in one or multiple axes. Because the motion of the flexures is based on elastic deformation of the material, there is no friction and no wear.

Flexures and levers amplify the motion of a piezo actuator.
Image credit: Dynamic Structures and Materials, LLC

Unlike piezo actuators, piezo motors typically incorporate mechanical elements to produce motion, which makes them susceptible to wear. Take ultrasonic piezo motors for example. They operate by electrically exciting a piezo actuator to produce high-frequency oscillations. The actuator is preloaded against a runner via a coupling element. Although the motion (oscillation) of the actuator does not create wear, the interface between the coupling and the runner presents a source of friction, and therefore, wear.

Operating principle of an ultrasonic linear piezo motor
Image credit: Physik Instrumente GmbH

Piezo inertia motors – also referred to as “stick-slip” motors – also use a piezo actuator and runner to produce motion. But in this case, the actuator expands slowly and contracts rapidly. Through a coupling element, the runner is able to move along with the actuator during expansion, but during the rapid contraction, the runner slips on the coupling and effectively stays in place. For rotary motion, a variation on this design uses “jaws” that engage with a very fine thread screw and turns the screw as the actuator expands. In either design, the reliance on friction and the “stick-slip” effect makes wear inherent in piezo inertia motors.

In a piezo inertia motor, jaws engage and rotate a fine-pitch screw as the piezo actuator contracts.
Image credit: Thorlabs, Inc.

A piezo motor design with little or no wear is the piezo stepper motor. Piezo steppers consist of multiple piezo actuators and operate by coordinating clamping and shear motions between the actuators and a runner. Unlike other piezo motor designs, because each of the actuators (also referred to as “legs” due to their walking-type motion) in a piezo stepper motor make contact and then lift off of the runner, friction is minimized and wear is virtually eliminated.

This diagram of a piezo stepper motor shows how pairs of “legs” work together to produce motion in a longitudinal rod.
Image credit: MICROMO

Why is resonant frequency important in piezo applications?

September 21, 2017 by Danielle Collins Leave a Comment

Resonance occurs when the resonant frequency (also referred to as the natural frequency) of an object or system is equal or very close to the frequency at which it is being excited. This causes the object or system to vibrate strongly and can result in unexpected – and sometimes catastrophic – behavior.

When one oscillating object or system (a piezo drive or controller) drives another system (a piezo motor or actuator) at or near the natural frequency of the second system, the second system will oscillate at a high amplitude at a specific frequency. When damping effects are small, the resonant frequency of the system is approximately equal to its natural frequency.

Where:

f₀ = resonant frequency (without load) (Hz)

k_T = piezo stiffness (N/m)

m_eff = effective mass (kg)

The resonant frequency of a piezo motor or actuator depends on its material composition, shape, and volume. For example, a thicker piezo element will have a lower resonant frequency than a thinner element of the same shape. In addition, attaching a load to a piezo motor or actuator reduces its resonant frequency – the higher the load, the more the resonant frequency is reduced. In manufacturer specifications, the resonant frequency given for a piezo actuator assumes that it is unloaded and one end is fixed or attached to a mass that is significantly larger than the actuator.

Where:

f₀‘ = resonant frequency with added mass (Hz)

m´_eff = m_eff + additional mass (kg)

In an electrical circuit representing the piezo element, the frequency at which the impedance of the circuit is at a minimum is the series resonant frequency, f_s. Conversely, the parallel resonant frequency, f_p, in the equivalent circuit occurs when impedance in the circuit is theoretically infinite (assuming mechanical losses are ignored). This is also known as the anti-resonant frequency, f_a.

The series and parallel frequencies are suitable approximations of the minimum and maximum impedance frequencies – f_m and f_n, respectively – and therefore are used to determine the parameters of the piezoelectric motor or system. Maximum response of a piezo system occurs between f_m and f_n. Piezo systems with a higher resonant frequency will have a better phase and amplitude response, which means that the operating frequency can be higher.

Minimum impedance (f_m) occurs at the resonant frequency and maximum impedance (f_n) occurs at the anti-resonant frequency.
Image credit: PI Ceramic GmbH

Theoretically, the resonant frequency is the operating frequency at which the piezo material vibrates most readily and converts electrical energy into mechanical energy most efficiently. However, piezo systems (especially actuators used for positioning) are often operated below their resonant frequencies in order to minimize the phase shift between the driving signal and the actuator.

When operated below their resonant frequencies, piezo actuators act like capacitors, with displacement being proportional to the stored charge. With a rapid increase in control voltage, a piezo system can typically reach its nominal displacement in 1/3 the period of its resonant frequency. However, this causes large overshoot, which must be compensated for by the controller.

Where:

t_min = minimum rise time (s)

f₀ = resonant frequency (Hz)

Feature image credit: Stack Exchange Inc.

Scott Jordan Named Head of Photonics by PI

December 7, 2016 by Danielle Collins Leave a Comment

PI (Physik Instrumente), a leading provider of photonics alignment, nanopositioning, and precision motion control systems has appointed Scott Jordan as Head of the Photonics Market Segment.

Scott Jordan (left) and PI’s VP, Sales & Marketing, Stephane Bussa (right)

Jordan, a PI Fellow, holds a Bachelor of Science degree and Master of Science degree, both in Physics, from University of California, Irvine and an MBA in Finance and New Venture Management from University of Southern California. Jordan, who has several patents related to fast alignment techniques, was central in the development of PI’s Multi-Channel Photonics Alignment System, a novel, modular, industrial-class system that automates SiP alignments for packaging and test, operating at high speed with nanometer precision and device safety. The FMPA system recently won the 2016 R&D 100 award as one of the 100 most technologically significant products of the year in the Analytical/Test category.

Jordan’s seminal contributions and extraordinary record of experience and accomplishments in the field of photonics and precision motion control makes him an integral part of PI as it moves into the future.

PI congratulates Scott Jordan!

Physik Instrumente

Alignment engine from PI is R&D 100 Award finalist

September 13, 2016 by Danielle Collins Leave a Comment

A solution is now available for fast, parallel, nanoscale-accurate, multi-degree-of-freedom global optical alignment optimization required in key SiP production steps from planar test to packaging.

The FMPA Fast Multichannel Photonics Alignment Engine is an advanced alignment system based on a highly specialized digital motion controller with embedded advanced alignment and tracking functionality and a hybrid precision scanning and tracking mechanism, combining the advantages of piezoelectric on servo-motorized drives. From motion control and nanopositioning solutions expert PI (Physik Instrumente) LP, the FMPA system is a 2016 R&D 100 Awards Finalist, which honors the most innovative technologies. Now in its 54^th year, often referred to as the “Oscars of Invention”, winners of this prestigious award will be announced on November 3^rd, during the R&D 100 conference, at the black tie gala dinner.

The Silicon Photonics (SiP) Problem

The convergence of silicon-based electronics and photonics promises a leap in data throughput, parallelism, and energy efficiency. Design and materials challenges have by now been addressed. Practicalities of testing and packaging have not.

Test and packaging of silicon photonics elements require nanoscale alignments that cannot be performed using visual or mechanical references. Instead, these optimizations must enhance the optical throughput itself. In addition, SiP designs often incorporate multiple parallel optical paths with multiple, interacting inputs and outputs, all needing optimization. Simple economics, as well as optical realities, dictate that these be optimized simultaneously, yet until now there has been no technology capable of doing this.

The Solution

With more than three decades of experience in ultra-precision motion control for semiconductor manufacturing and metrology, PI has addressed the need for fast, parallel, nanoscale-accurate, multi-degree-of-freedom global optical alignment optimization required in key SiP production steps from planar test to packaging.

This enabling solution integrates PI’s high-throughput piezo nanopositioning technologies and ultraprecision motion control with novel algorithms. Introduced at the SPIE Photonics West 2015 conference, the groundbreaking technology was developed by PI’s team that comprises more than a century of photonics alignment automation experience and includes foundational participants in the field. The system is part of PI’s broad offering of photonics alignment engines ranging from software-driven stage solutions to integrated 6-axis hexapod micro-robots with built-in alignment functionality.

Specifications, datasheet, and more information