Glossary

Abbe Error

Sideways motion due to angular deviation (q below) coupled with a significant mechanical lever-arm. This looks like runout (dx) but unlike true runout can be minimized by reducing the lever arm, to which it is linearly related. A stage placed atop a mounting rod will exhibit less of this sideways motion than when the rod is mounted on the stage and the measurement is repeated at the same optical axis height. Similarly, XYZ stages incorporating an angle bracket between the moving elements will exhibit apparent runout due to the lever-arm this introduces.

Abbe error results in apparent runout which can be reduced by minimizing the lever-arm.

Absolute Accuracy

The output of a system versus the commanded or ideal input; it is more correctly called inaccuracy. When a motion system is commanded to move 10 mm actually moves 9.99 mm as measured by a perfect ruler, the inaccuracy is 0.01 mm. Misalignment of the stage axis versus the ruler's axis will result in a monotonic inaccuracy proportional to the cosine of the misalignment. See cosine error.

Angular deviation

Cone angle which determines the angular range of motion of the stage. This is an important definition because the measured runout will depend on the height at which the measuring device is mounted upon the stage.

Runout is often specified for the motion at the surface of the stage, but you will find that the angular deviation dominates the actual variations in straight line travel of a device mounted at a height above the stage. The angular deviation is specified in terms of roll, pitch, and yaw.

Backlash

Non-responsiveness on reversal of input. For example, a simple motorizer with motor-mounted encoder might exhibit several microns of position display change on reversal before its output position actually begins to change. Other terms frequently used to describe this or similar behavior include dead zone, stiction, looseness, slop and free play. It can be compensated by various controller schemes. The best is when the controller allows the user to specify the measured backlash of a motion assembly; this amount of extra drivetrain input is then added upon each reversal. This can provide submicron repeatabilities without over- or under-shoot. A less-desirable approach is when the controller automatically overshoots reverse motions and re-approaches the desired position so that the target position is approached from a consistent direction. This is often unacceptable in applications like fiber coupling and micro-ablation.

Cosine Error

Cumulative, monotonic inaccuracy due to misalignment of an actuator axis versus a stage's axis or a stage's axis versus an external optical axis such as an interferometer's. This is proportional to the cosine of the misalignment. This effect is very small; even a very bad misalignment of 2° -- easily discerned by the eye -- results in less than 0.1% cumulative inaccuracy. (This is quite a bit less than the 3.5% apparent transverse motion component proportional to the sine of the same misalignment.) It is evident that the inaccuracy introduced by mounting a micrometer-replacement actuator or direct-metrology encoder with reasonable care is negligible.

Cross-Coupling

Amount of motion in one axis due to the adjustment of a different axis in multiple axis devices, such as X-Y stages or kinematic mirror mounts. For example, the amount of X motion when the Y drive is adjusted in an X-Y stage. Also known as cross-talk.

DC Servo Motor

An analog motor designed to be an active element in a servo circuit. A broad range of such motors are used in precision motion systems, from micro-motors the size of a sugar cube to high-duty-cycle, high-torque units bigger than a fist. Very smooth running, broad speed range without resonance, and good stability are characteristics of DC servo motors if reasonably modern controllers are employed. Poor examples abound, however, and are plagued with drift, overshoot and inaccuracies. (Also, some controllers run DC servo motors in a pulsed fashion that can be noisy.)

Being active elements of an analog servo, there are a host of servo parameters and settings that must be correct for a DC servo motor to perform crisply and stably. From a user's perspective, the manner in which these settings are handled can make a huge difference in a controller's ease-of-use. In some controllers, the parameters are set (and even fine-tuned) automatically and transparently to the user. In others, the user must enter a list of parameters appropriate to their motion device, motor and load before it can be used at all, and then the fine-tuning must be done manually for optimum performance.

Direct Output Motion Metrology

Used in closed-loop systems which perform motion control based on drivetrain output -- the stage platform or actuator shaft position. This eliminates drivetrain errors and is reserved for top-of-the-line motion systems.

Eccentricity 

Displacement of the geometric center of the stage from the center of rotation.

Hysteresis

Non-repeatability on reversal of input. For most motion devices, backlash and stiction are the most significant contributors. However, non-recovery of static deflection is possible, with greatest consequence for some submicron applications when inappropriate materials are used in a motion device's design. In piezo devices, hysteresis is a characteristic property of the material.

Interferometer

An instrument which utilizes the interference property of light to measure distances. Resolution to a few nano-meters is achieved by the most advanced units. In addition to many applications in measuring position, they have been incorporated into motion devices for direct-motion-metrology. However, air is the working fluid for the optical path, rendering even a perfectly vibration-isolated interferometer sensitive to air currents, acoustic noise, changes in barometric pressure, humidity and temperature, etc.

Interpolator 

An electrical circuit which divides a periodic analog signal into divisions of much higher period. Very often used in interferometers (to divide fringes) and glass scale encoders (to resolve moirŽ activity). Interpolation allows use of inherently noise-resistant, slowly-varying analog signals. The quality and internal noise level of the interpolator define a lower limit to its resolution and repeatability.

Glass Scale Encoder

A position measuring device upon which a grating has been applied. Various types exist; most utilize a stationary element in optical series with an identical moving element (reticle). As the reticle translates, a moirŽ effect causes a periodic change in the optical throughput. The pitch or spacing of the grating defines the basic resolution of the device; interpolation can greatly multiply this. Holographically-generated gratings with micron-scale pitch are a recent innovation.

Leadscrew Pitch Error

There are two sources: sinusoidal errors, which are periodic variations of the leadscrew pitch from nominal, and overall departures from the specified pitch. Both are of concern only in closed-loop devices in which the motion metrology is performed on the drive-train input via a motor- or leadscrew-mounted encoder or via a stepper-motor pulse-counting scheme. Overall pitch errors can be compensated by some controllers; the measured lead-screw pitch of a specific motion device can be programmed into such controllers. Using this feature, the user can eliminate all but the sinusoidal and other non-monotonic errors. Lookup tables and error modeling are also used.

Minimum Incremental Motion 

The smallest motion a device is capable of delivering -- not to be confused with resolution claims, which are typically based on the smallest display increment and which can be more than an order of magnitude more impressive than the actual motion a system is capable of producing. This is a key specification but, unfortunately, is rarely disclosed.

MTBF

Mean Time Between Failures. This is a prediction of the lifetime between major service of the device. It does not preclude maintenance or adjustment. For precision motion devices, the MTBF ranges from as little as a few hundred hours to over 20,000 hours for industrial-class devices.

Pitch

Rotation about the transverse, or y, axis. This is also known as elevation, particularly in gimbal-type mounts used in astronomy and ranging.

Play

Uncontrolled movement due to looseness of mechanical parts. Very small in a well-built component, it can increase as a component grows older, especially if it is roughly handled or overloaded.

Precision

Range of deviations in output position that will occur for the same error-free input. Precision is also known as repeatability. Although often confused in common parlance, accuracy and precision are not the same. Figure 4 shows graphically the difference between these two parameters.

Repeatability

The ability of a motion system to achieve a commanded position over many attempts. Manufacturers often specify unidirectional repeatability, meaning the ability to repeat a motion increment in one direction. This side-steps issues of backlash, hysteresis, etc., and therefore is fundamentally irrelevant. A much more significant specification is bi-directional repeatability. Unfortunately, few manufacturers publicize this much tougher measure of motion performance.

Resolution, Display

The smallest incremental step which can be displayed or read from an actuator. The display resolution is not necessarily the same as the position resolution. An example of display resolution is the number of digits on the readout of a motor controller. Differences between display and position resolution can be caused by a variety of reasons including friction and backlash in the system.

Resolution, Position

Smallest difference in movements that can be discriminated. Often confused with display resolution. Your finger tips are sensitive enough to be able to distinguish 1° rotations of an adjustment screw. Therefore, when you see a resolution quoted for an AJS adjustment screw, it is the travel associated with a 1° turn of the screw.

Reversal Error

Small forward motions when a drive is reversed, and vice versa. It is caused by drivetrain wind-up in systems with high internal friction.

Roll

Rotation about the longitudinal, or x, axis of travel.

Runout

Motion other than motion in a straight line in a linear stage. Also called straightness of travel (deviations in the plane of travel) and flatness of travel (deviations out of the plane of travel). Cross-coupling refers to orthogonality errors in multiple axis systems. Runout is the deviation from straight line travel for a single axis.

Sensitivity

Ratio of output motion to input drive. Resolution and sensitivity are again terms that are sometimes confused. As an example of the difference between the two, for the 80 thread-per-inch adjustment screw the resolution is better than one micron (using our 1° turn definition, see position resolution), while the sensitivity is 0.0125 inch or 0.318 mm per turn.

Sinusoidal Errors

Non-cumulative periodic inaccuracies frequently found in leadscrew- or worm-gear-driven devices unless direct output motion metrology is employed.

Static Deflection

Bending of a structural component due to loading. This has little or no effect on most devices' performance as long as component design limits are not exceeded. For example, placing a 5 kg load on a steel crossed-roller-bearing stage will cause little or no measurable change in performance, since such stages are often rated to over 70 kg. Similarly, replacing a 100 g micrometer with a 600 g actuator should not seriously affect the performance or longevity of most stages.

Stepper motor

One of several motor types which increment in discrete steps. Continuous motions are performed by rapid sequences of steps. Small motions can be facilitated by dividing the steps into many discrete parts, a technique called mini-stepping.

Full-stepping motor controllers are fairly straightforward, digital devices -- requiring somewhat less of the fuss and bother encountered with certain DC servo-motor implementations -- and are consequently quite popular among controller designers and users alike. Mini-stepper controllers are somewhat more complex. Unfortunately, poorly-designed stepper devices can run hot and have loud resonances at particular speeds. Advanced electrical drive techniques have mitigated the heat problem, and viscous or ferrofluidic dampers have proven valuable in reducing noise and resonance problems.

Many open-loop stepper-motor systems are marketed as though they were closed-loop -- the controllerÕs count of pulses is taken on faith, though no motion metrology is incorporated. In predictable applications, well-engineered open-loop stepper systems can indeed provide faithful, repeatable motion.

Stiction

Occurs because the coefficient of static friction is always greater than the coefficient of moving friction. When a stage is at rest and force is first applied and slowly increased, no motion occurs. At some threshold, motion suddenly begins, so that the first move of the component will be a jump, giving non-linear and non-repeatable motion. This effect is what limits the smallest incremental movement.

Trapezoidal Motion Profile

Graphing an advanced motion deviceÕs velocity versus time or distance results in a trapezoidal plot: first, there is an acceleration phase, terminating at the commanded velocity, then a deceleration phase. Advanced controllers allow user control of acceleration/deceleration -- valuable for positioning items such as optical fibers which can vibrate if motion is too violent. More advanced controllers allow individual setting of acceleration and deceleration. Even more desirable is the ability of a few controllers to specify these parameters separately for long- and short-motion regimes. The latest advance is user-programmable ÒjerkÓ -- the time rate of change of acceleration. This allows vibration-prone loads to be moved gently but with exceptional efficiency.

Wander

Translation of the axis during rotation. Also known as eccentricity.

Wind-Up

Lost motion due to friction and deflections in the drivetrain. Along with backlash and stiction, this is a major cause of the distinction between display resolution and minimum incremental motion: the drivetrain input may apply a force to the drivetrain and imply that motion has occurred, but the drivetrain absorbs the input (or deflects slightly) because of friction, causing no motion to occur. In this manner, drivetrain friction forms a fundamental limit to incremental motion.

Wobble

Tilt of the axis during rotation.

Yaw

In-plane rotation about the vertical, or z, axis. This is also known as azimuth. This term is also used to refer to the rotation of optics in optic mounts.