Advances in Precision Motorized Positioning Systems: Selection Criteria

Inertia drives are another type of piezomotor. These use tiny piezo ceramic elements that are actuated in a sawtooth pattern, driving a shaft or other actuated element via a friction coupling.  The sloped portion of the sawtooth actuation is what provides the motion; the rapid retraction breaks the stiction of the coupling and the actuated element does not retract with the piezo ceramic element. Artful design can achieve silent, virtually stepless operation and long travels together with precision to the nanoscale and self-locking for high stability when stationary.  Examples include PI’s U-Motion series of ultra-compact open- and closed-loop stages, and PIShift actuators.

Piezo ratchet motors are a special form of inertia motors based on the stick-slip effect. A small piezo ceramic actuator embedded in a spring-loaded mechanism drives a precision lead screw. A slow expansion phase results in a small rotation of the screw (see animation). When the piezo element has reached its maximum expansion, a much shorter contraction phase follows – too fast for the screw to follow, because of its inertia.

Open and closed-loop operation is possible. When exact position control is required, an optical encoder integrated into the actuator housing feeds the information back to a closed-loop controller. Resolution in the nanometer rage is feasible. Advantages are the compact size, self-locking design, and high holding forces (100N) compared to the size. Drawbacks are the slow speed – making the drive a good option for motion control applications in confined spaces with low dynamic requirements – such as laser tuning, alignment of optics, and opto-mechanical equipment, and a great choice for replacing mechanical micrometers.

Walking piezomotors are yet another breed. These use four or more piezoceramic fingers which actuate in a stepping sequence to drive a workpiece in a desired direction. Between steps, sub-nanoscale actuation can be achieved. High power-off holding forces and essentially unlimited travel characterize these designs, exemplified by PI’s NEXACT and NEXLINE technologies. The usual non-magnetic and vacuum-friendly attributes apply. These have proven to be enablers in sensitive optical positioning applications where carefully established positions must be maintained with nanometer stability without power for months or years.

Linear Motor Actuators

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.

Linear Translation Stages

A linear or translation stage builds on the principles of a linear actuator, but adds a platform or workpiece for attaching an application load, or for stacking additional stages to form a multi-axis configuration. The stage’s workpiece is a precision component with a linear bearing for guidance.

However, as the drive-technology options have expanded, novel linear stages that incorporate the latest drive technologies have been introduced. These yield advantages of importance to specific applications, such as non-magnetic actuation or package-size benefits.  For example, PI’s U-751/M-686 microscopy stage provides both higher and lower speed capabilities than typical stepper-motor microscopy stages, and incorporates a linear encoder for 100nm-scale bi-directional repeatabilities, all without the bulky overhanging stepper-motor/lead screw assembly that projects from the side of each axis of conventional microscopy stages.

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.

Linear Positioning Stages with Piezomotors

A new generation of piezo ceramic linear motors allows for the construction of matchbox to thumbnail sized linear stages with nanometer resolution and millisecond step / settle times.

The direct drive avoids mechanical components, such as gears and lead screws, making for reliable and high-resolution drives down to a few nanometers. Depending on the drive principle, high velocity, high forces, and/or high resolution are achieved.

Miniature linear positioning stages with high-speed ultrasonic ceramic linear motor and linear encoder feedback are shown below.

Long Travel – Industrial Applications

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 Linear Stages – Planar XY Stages

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.

High-Speed Stages with Linear / Torque Motor Direct Drive

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.

Their high dynamics ensures high throughputs of automated tasks in testing systems, for example, in the semiconductor industry. They also increase efficiency, for example, in electronics production / assembly lines or laser processing.

High-Resolution Linear Encoder Feedback

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

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.

Typically used for the highest precision and smoothness of motion/velocity, air-bearing rotation stages deliver ultra-low runout and wobble, as well as extremely high resolution and repeatability. Pitch, yaw, roll on the order of 1 arc second is feasible. The absence of friction eliminates backlash and gives the air bearing stage ultra-high repeatability.  The durability of air bearings is unlimited if they are calculated, designed, and operated correctly.

Goniometers

A goniometer is an instrument that either measures an angle or allows an object to be rotated to a precise angular position. A positioning goniometer or goniometric stage is a device used to rotate an object precisely about a fixed axis in space.  It is similar to a linear stage, however, rather than moving linearly with respect to its base, the stage platform rotates partially about a fixed axis above the mounting surface of the platform. Positioning goniometers typically use a worm drive with a partial worm wheel fixed to the underside of the stage platform meshing with a worm in the base. The worm is rotated by a motor.

Goniometers are often used in crystallography and in X-ray diffraction to rotate the samples.  They are also useful in (fiber) optic alignment applications.

6 DOF – Parallel Kinematic Systems: Hexapods / PlanarPods

In order to achieve precision at the micron and sub-micron level in multi-axis motion applications, hexapod parallel positioners have become popular in the last two decades. Hexapods effectively reduce the footprint and moving mass of a traditional serial kinematic stacked-stage positioning system while increasing stiffness and responsiveness. This together with the arbitrary, user defined center of rotation and a large clear aperture make them the positioning system of choice in mission-critical applications, including laser processing, photonics alignment and micro-machining in medical devices and other applications.

Hexapods, by definition, are six-legged parallel-kinematic mechanism (PKM) motion systems.  In their most common form consisting of two platforms, a fixed base platform and a second movable platform, which are connected and supported by six independent legs (struts or links) that expand and contract in parallel. A similar 6-axis design, called SpaceFab, is based on a top platform connected to three XY linear stages with three passive struts. It provides similar performance to a hexapod, yet allows for a lower profile and longer XY travel ranges (with reduced angular motion).

Coordinated motion of all struts enables the movable platform, and devices mounted to it, to move in any direction, operating in 6-D relative to the base platform. With 6-DOF, the secondary platform is capable of moving in three linear directions, lateral (X), longitudinal (Y), and vertically (Z), and the three angular directions (pitch, roll and yaw), by the legs. Because hexapods have all six degrees of freedom, they can perform manipulations that encompass total freedom of motion in a relatively compact space, with high stiffness and (when properly designed) without moving/sweeping cables that can break and foul.

Advanced designs include servo-motor-driven systems for moving large optics or mirrors, piezo-based units for nanometer precision control of processes, and non-magnetic and vacuum-compatible versions.

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 Compatible Positioners

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.

The selection of suitable components is crucial for the vacuum-compatibility of a positioning system. The body of the positioning device must be designed for placement in closed compartments to avoid outgassing causing virtual leaks. Holes, as well as screws, need to be vented, and a reduction of the surface is desired. Air pockets, such as in under mountings, must be avoided as they considerably delay pump-down to target pressure or even make generating a stable vacuum impossible.