Applications that require micron or sub-micron positioning and extremely smooth motion and control usually require direct drive systems-linear motors, piezoelectric motors, or voice coil actuators. When high force is required when the stroke length is greater than a few hundred millimeters, a linear motor platform is usually used. But for high-precision applications with a travel range from a fraction of a micrometer to a few hundred millimeters, and a very small form factor is required, piezoelectric motors and voice coil actuators are usually the best choices.
Ball screws-when properly applied and used with a suitable linear encoder-can be positioned to a few micrometers, or even nanometers in a well-controlled environment. However, because the recirculating balls pulsate as they enter and leave the load zone, ball screws lack the smoothness of motion normally required in micron and sub-micron positioning applications.
Other direct drive technologies rely on electromagnetics to generate motion, while piezoelectric actuators use the inverse piezoelectric effect, in which the applied voltage creates strain in the piezoelectric ceramic material, causing it to deform (expand or contract).
Piezo motors combine piezoelectric actuators with mechanical amplification (such as flexible or inertial drives) to produce longer strokes. The complete piezoelectric platform also includes a linear guide system—usually a cross roller guide—to support the load. The result is a very small device that can generate strokes up to a few hundred microns, with the smallest incremental movement (the smallest movement the system can make) as low as a few hundred nanometers. The speed and force depend on the piezoelectric technology used-ultrasonic piezoelectric motors, piezoelectric inertial motors, and piezoelectric stepper motors are the most common types of piezoelectric platforms.
The platform using piezoelectric stepper motor technology can generate forces in the range of 10 N, but at a speed of approximately 10 mm/s. Their advantage lies in extremely small motions, with minimum incremental motion (MIM) capabilities in the single nanometer range over long stroke lengths. On the other hand, platforms using ultrasonic piezoelectric motors can usually produce minimal incremental motions of a few hundred nanometers in a stroke of up to about 50 millimeters. But they have the highest speed capability, about 200 mm/sec, and the force generated is a few newtons. Platforms using piezoelectric inertial motors have similar performance characteristics (minimum incremental motion, speed, and force) to platforms using piezoelectric stepper motors, but take up less space—generally at lower cost.
One of the advantages of piezoelectric motor platforms over other direct drive technologies is that they can generate high holding force when the power is off, thereby eliminating the heat generated when the load is held in place. Piezoelectric drive systems also have a very high force-to-size ratio.
Voice coil actuators combine voice coil motors with low-friction rail systems, such as cross-roller rails, linear shafts, or air bearings. The voice coil motor that drives the actuator works on the principle of Lorentz force—that is, when current is applied to a coil in a magnetic field, it generates force. This force is perpendicular to the direction of the current and magnetic field and causes the moving part of the actuator (magnet or coil) to move linearly.
Since the force and stroke are proportional to the applied current, the voice coil actuator has excellent force and position control, and has a very low hysteresis, so the positioning repeatability is very good. Voice coil actuators can also generate continuous forces of up to hundreds of Newtons.
Although they are commonly used in applications that require micron and sub-micron positioning—the smallest incremental motion is usually about 100 microns—the total stroke length of voice coil actuators can be as high as hundreds of millimeters. However, voice coil actuators require current to keep the load stationary, and they generate more heat during operation than piezoelectric motors, so thermal management must be considered.
As a direct drive mechanism, both piezoelectric and voice coil technologies benefit from very low friction, low moving mass and very low inertia, enabling them to achieve very small, precise movements with high acceleration and deceleration rates.
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