Advances in Precision Motorized Positioning Systems: Selection Criteria

Today’s broadening spectrum of industrial and research applications has yielded a similarly wide variety of motion technologies — more than a single article can review comprehensively.  But it means that designers and motion control engineers in scores of industries have access to precision motorized positioning systems that fit or even enable their applications. These systems provide very few limitations on travel, precision, repeatability, and speed. Following is an overview of the more prominent types of motorized precision-positioning systems and some of their news.

A precision linear actuator is defined as a positioning device that creates motion in one degree of freedom, and typically does not include a guiding system for the payload. For this discussion, we are interested in electrically-driven units, though, of course, manual micrometer-driven are common, along with screw-driven, pneumatic and hydraulic variants for lower-precision applications. A number of drive technologies can generate linear motion:

These are typically based on linear shafts driven by rotational electromagnetic motors via lead screws or ballscrews. Rotary motion of the motor is converted to linear displacement. The actuators have a generally cylindrical format. Small versions are used to replace micrometers or precision screws, conferring automated actuation.

Stepper motors can be operated open- or closed-loop, meaning without or with position feedback. A stepper motor can be actuated through any specified number of steps in either direction and offers a high probability of achieving them, though certainty can only be achieved through the addition of a position encoder. Rotary encoders keep track of the position of the rotating motor; linear encoders directly encode the output position of the driven linear shaft, eliminating backlash and other errors that might otherwise accumulate in the drivetrain where a rotary encoder cannot observe them. Linear actuators with linear encoders are uncommon, but offer unbeatable bi-directional repeatability for sensitive applications.

Extremely fine positioning resolution can be achieve by piezoelectric actuators. There are several types:

Piezo stack actuators are exquisite, layered structures of specialized ceramic interleaved with metallic electrodes. The piezo ceramic has the unique property of expanding in a controllable manner with the application of an electrical field. These actuators provide short travel ranges (about 1% of their length), sub-nanometer precisions, high forces, and sub-millisecond response.  These are the mainstay of today’s advanced nanotech applications, both in laboratory research and in industrial applications, such as semiconductor manufacturing and genomic sequencing.  Piezo stack actuators are inherently non-magnetic, solid state, and vacuum-friendly, with no wear processes.

Piezo stack actuators can be integrated in flexure guiding systems with motion amplifiers to provide longer travel ranges and multi-axis motion if required.  Wire EDM cut flexures provide state-of-the-art guiding precision along with virtually friction free motion and zero wear and maintenance.  During life testing for the Mars Mission, NASA achieved 100 billion cycles of motion without failures.

Ultrasonic piezomotors are monolithic piezo ceramic structures that are stimulated at their resonant frequency, typically above 100kHz, causing them to flutter on a submicron scale. A friction tip formed or bonded at a resonant node conveys this fluttering oscillation to a workpiece that rides in bearings. The workpiece thereby experiences a force that drives it one direction or the other. These motors can achieve many millimeters of travel and extraordinary speeds in a very small package. PI’s patented PILine® ultrasonic piezomotors, for example, can provide speeds to half a meter per second and step/settle times of a few milliseconds in some applications. Another key attribute is these motors’ automatic self-locking behavior at rest and even when unpowered. This prevents drift and dither of the driven stage. Ultrasonic piezomotors can provide an application-enabling alternative to classical motors when small dimensions, high speed, and unrivaled energy efficiency are important. Like piezo stack actuators, they are non-magnetic and vacuum-compatible.

By attaching a linear servo motor (which can be thought of as a rotary DC motor sliced lengthwise and laid flat) to linear guidance and an output shaft, direct linear actuation of very high speeds can be achieved. Linear DC motors can have a multitude of North/South magnetic pairs, depending on how much travel is needed. These serve the role of the stator in a rotary motor. Gliding along them to generate force is a three-phase coil assembly. The phases are commutated electronically to generate smooth motion in the desired direction, ensuring long life.

A related type of linear actuator is driven by a voice coil motor: a nested pair of cylindrical electromagnetic coils which attract or repel each other along their mutual axis. These provide travels on the order of 25mm and provide extraordinary speeds and accelerations for small loads.  Such mechanisms are very long lived. Voice coil actuators, like PI’s V-270 series, can offer impressive step/settle times owing to their high responsiveness, and their direct actuation of the motion shaft in its low-friction bearings offers exquisite force control as well when an optional tip-force sensor is incorporated. V-270 series actuators’ sophisticated industrial digital controller offers bumpless switching between a variety of position, velocity, force, and mixed servo modes, which makes them ideal for production applications including calibrated force generation and testing of touch-sensitive devices.

For motion and positioning in multiple axes, individual positioning stages can be combined or parallel-kinematic hexapods with up to 6 degrees of freedom of motion can be used.

Bigger is not always better.

The need for miniaturization in the semiconductor and medical device industry also drives the requirements for smaller motion systems. Smaller also means lower mass and the potential for higher acceleration and throughput, especially when combined with today’s most advanced actuation technologies.

On the other hand, some industrial automation processes, such as flat panel testing, and laser processing, require very long travels past one meter with high speed and low runout errors. Air bearing stages with linear motors have emerged as the gold standard for these applications.

Air bearing stages replace mechanical bearings with a frictionless air film and maximize throughput while providing the ultimate level of precision, especially for multi-axis motion.  Planar designs use one reference base plane on which magnetically or vacuum preloaded pucks are floating for both the X and Y axes. H-bridge, three-motor designs provide the highest precision, and can be further improved with active yaw control when equipped with three linear encoders and advanced motion controllers. The benefit is vastly improved orthogonality and straightness. Air bearing stages are usually driven by magnetic linear or torque motors that provide smooth motion without negative cogging effects.

Linear and torque motors can also be combined with mechanical bearings. This combination is often used in industrial applications when the smoothness and straightness/flatness of motion is not quite as critical as with air bearings. Linear motors provide an excellent combination of reliability, precision, and speed.

Unlike motion systems that are run by rotary stepper and servo motors and lower precision rotary encoders, linear motors require linear positional feedback systems. A linear encoder is a digital position transducer that directly measures linear motion where it occurs as opposed to a rotary encoder mounted at the end of a drive train. The linear encoder reads the actual position as close to the point of interest as possible, and therefore, the resulting accuracy and repeatability of the payload is higher.  Linear encoders contain a linear track and a read head. The linear track can range in length from a few mm to several feet. Most encoders are based on an optical grating, however, lower cost magnetic encoders are still available. While resolution in the sub-nanometer range is common, accuracy is typically limited to 1 micron per 100mm. However, this can be improved significantly with modern controllers if calibrated and compensated for with look up tables or polynomial error correction. Incremental linear encoders are still prevalent, due to their interfacing simplicity and higher possible resolution down to the picometer range if used with electronic interpolators, but absolute position encoders are catching up with nanometer resolution models becoming much more affordable.

Rotation stages consist of a platform that rotates relative to a base. The platform and base are joined by some form of bearing which restricts motion of the platform to rotation about a single axis.

A variety of motors and drive principles can be employed, from stepper-motor driven worm gear designs to direct-drive closed-loop torque motors.  Low profile piezo motor stages provide self-locking capabilities with zero jitter and drift and requiring no holding current at rest.

Precision motorized rotation stages are used in applications such as fiber-optical alignment, semiconductor inspection, bio-medical applications, and X-ray crystallography.

Air bearings use a thin film of pressurized air to provide an exceedingly low friction load-bearing interface between surfaces. The two surfaces do not touch. As they are contact-free, air bearings avoid the traditional bearing-related problems of friction, wear, particulates, and lubricant handling, and offer distinct advantages in precision positioning and in high-speed applications, where the elimination of backlash and static friction are critical.

Recent hexapod designs provide extremely high stiffness and rigidity of their components and all moving parts, such as its bearings, joints, and drive screws.  These characteristics result in high natural frequencies which make these new hexapods capable of extreme accuracy, and an ideal tool for precision machining, photonics and optics alignment, metrology and medical applications.

One such hexapod is the miniature 6-axis Parallel Kinematics Hexapod Nano-Alignment System, from Physik Instrumente, which can deliver more than 10 lbs. of force and motion in all six degrees of freedom.  Resolution is as low as 50 nanometers with actuator encoder resolution of 5nm and travel ranges to 40mm linear and 60 degree (rotation), and a velocity of 10 mm/sec.  The hexapod’s high level of accuracy is a combination of extremely precise parts, precision assembly and testing, and sophisticated algorithms built into its vector motion controller that take into account the exact tolerances of each strut and joint, and provide precise coordinates to each of the six actuators. This unit can be used for manufacturing and part placement, alignment of optical components and lasers, microscopy applications and neuroscience that require high precision.

If minimized dimensions are of the essence, the combination of inertia-type piezo motors with the compact SpaceFab design allows for nanometer precise 6-axis motion that fits in the palm of a hand.

Vacuum applications are increasingly more important for many fields of research and industrial manufacturing. Requirements span from vacuum levels from 10-3 mbar to 10-9 mbar.  While piezo ceramic drive units can easily be modified for extreme vacuum, an increasing number of motorized positioning devices also being incorporated successfully into a widening range of vacuum applications where long travel and high precision motion is needed. Positioning systems specifically developed for vacuum operation must meet a number of criteria. Vacuum chambers only offer limited space and therefore require a compact design.