High-Precision Motion Control with Piezo Actuators and NI Motion 5.2

In recent years, engineers have pushed the limits of motion control, making it more and more precise. Many applications, particularly those in the optoelectronics area, require much higher precision than traditional actuators can deliver. This increase in demand for higher precision motion control has led to many new innovations, including the use of piezoelectric materials to create motion. This article discusses how your can achieve high-precision motion control with piezo actuators and NI Motion 5.2.

Introduction

In recent years, engineers have pushed the limits of motion control, making it more and more precise. Many applications, particularly those in the optoelectronics area, require much higher precision than traditional actuators can deliver. This increase in demand for higher precision motion control has led to many new innovations, including the use of piezoelectric materials to create motion.

Piezoelectric materials generate a voltage when compressed. Conversely, you can cause a piezoelectric material to change shape by sending a voltage into it. Determining the relationship between the input voltage and the change in shape can help you predict the behavior of these materials and make them useful for high precision motion control. Because the change in shape is very small, you may not consider piezoelectric material useful for longer travel motion control. However, by using piezo elements to precisely generate a micro-ellipse and create continuous motion along the length of travel, you now can use piezo elements as motors for moving longer travel distances.

Nanomotion, an NI Motion Partner Program member, now offers high precision piezo-based motors that are used in a variety of high precision stages from companies like Bayside Motion Group and New Focus. Understanding the technology behind these motors can help you choose the best type of motor for your application.

Technology behind Nanomotion Piezo Motors

With Nanomotion's ultrasonic standing wave servo motors, you can meet the demands of a wide range of motion control applications. The standing wave, piezoelectric effect makes these motors ideal for applications from slow-speed constant velocity to high-speed move and settle. The motor technology provides a bridge between high-speed operation and nanometer resolution in a very small area with unlimited travel.

The motor is most often controlled as a closed loop servo motor, similar to a brushless DC motor. As with standard motors that drive either linear or rotary motion, you can use feedback from a linear or rotary encoder to control position.

When the motor elements are preloaded, or compressed, against a bearing structure with an appropriate friction material, the motor works as a friction drive, pushing the mass in a linear, rotary, or spherical manner. The applied voltage on the preloaded motor determines the oscillation amplitude, which in turn determines the range of speed and force that the motor can produce. This range is as wide as 1:250,000.

Static Friction

Static friction, the friction present when the motor is at rest and making contact with the ceramic, exists on the preloaded motor. Because it can brake rapidly and hold position stability while at power off, this intrinsic friction provides a significant advantage for both vertical applications and high-speed move and settle. This motor technology is considered one of the fastest for moves of a few millimeters. Moreover, unlike traditional brush or brushless motors, when the motor is holding in an idle position it consumes no power. Nevertheless, you must manage the same friction through the servo tuning process to generate high-performance motion profiles.

The Nonlinear Voltage to Velocity Profile

In simple terms, when you consider the voltage-to- velocity profile of a brushless DC motor, the only resistance to motion is the friction in the bearing. As motor bearings are typically low-friction ball bearings, the slightest current to the motor generates velocity.

With the Nanomotion motor configuration, the preload of the motor against a work surface (ceramic strip) creates a deadband, or dead-zone, with around -1 V to +1 V of command voltage, where no motion occurs. Once you establish an offset, the profile is linear.

Using NI Motion for Static Friction Compensation

Any servo system with significant static friction, called stiction, requires some type of compensation at the control level. With no compensation for the stiction, the control parameters are disproportionate, with the proportional gain being excessively high. Raising the proportional gain to a level high enough to overcome the initial friction could cause the control system to be unstable and prevent the motor from settling. With the new static compensation feature in NI Motion 5.2, you can use a custom algorithm for the NI 7344 motion controller to compensate for static friction effectively. Specifically, you can preload position error and cause the controller to give a larger burst of torque when it initially starts moving the motor. Static friction compensation also helps when stopping the motor by creating a dead zone in which the motion controller stops sending servo commands to the motor.

After using the new static friction compensation feature to tune your motors, you can use any of the standard NI Motion Control functions just as you would with other motors. For example, you can use NI Motion Assistant to create motion profiles for piezo motors, and generate LabVIEW code just as if you were using it for a simple stepper motor. The combination of NI-Motion 5.2, NI Motion Assistant, and high-precision piezo motors offers easy development as well as very high precision.
Related Links:
Advisors for Motion
Selecting a Stage and Motion Controller for Your Automation Application

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update required
This document should be updated to reflect the improvements for static friction compensation with later versions of NI-Motion. Currently this document refers to NI-Motion 5.2 but the current version is 7.4
- Jochen Klier, National Instruments Germany GmbH. jochen.klier@ni.com - Nov 24, 2006