Motion Basics Terminology & Standards

Cross-coupling: In multi-axis systems cross-coupling refers to a change in one axis resulting from an input to another axis.

Functional Point: Point within the coordinate frame of the positioning system where a measurement and/or process is occurring.

Target Position: Position to which the functional point is programmed to move to.

Actual Position: Measured position reached by the functional point.

Position Deviation: Actual position reached by the functional point minus the target position.

Motion Control Coordinate System: In free space, an object is considered to have six degrees of freedom: three linear, along the x, y, and z-axes and three rotational around those (see Figure 1). All motions described here follow the right-hand coordinate system convention. The cross-product of +X and +Y axes (pointer and middle fingers) is the +Z axis (thumb). Also, if the thumb of the right hand points in the positive direction of an axis, the fingers will wrap around the axis in the direction of positive rotation about that axis. All movements are composed of translations along and/or rotations about the coordinate axes. Generally, the X and Y axes are on the horizontal plane, the direction of travel of the first or bottom stage is aligned with the X axis, and the Z-axis is vertical. For parallel kinematic systems, cardan angles are used.

Resolution: Resolution, also referred to as display or encoder resolution, is the smallest increment that a motion system can be commanded to move and/or detect. It is not the same as Minimum Incremental Motion. A system may or may not be able to consistently make incremental moves equal to the resolution. Factors that can affect a move include friction, load, external forces, system dynamics, controller, vibrations, and inertia.

Minimum Incremental Motion: Minimum Incremental Motion (MIM) is the smallest increment of motion a device is capable of consistently and reliably delivering. It should not be confused with resolution, which is typically based on the smallest controller display value or smallest encoder increment. Resolution can be significantly smaller than the smallest actual motion output, a key distinction, but unfortunately not always well understood. Newport specifies the minimum incremental motion with most of its products. Others refer to it as "practical resolution". Two different test methods are used depending on the capabilities of the stage. In high performance stages that use direct drives and frictionless guiding systems, the MIM is only limited by noise.

Accuracy: Accuracy is a measure of the degree to which a given displacement conforms to an agreed upon standard. The accuracy of a motion system can be highly influenced by the test set up, environmental conditions and the procedure used to measure displacement. In the micron and submicron world, thermal expansion can have a profound impact upon accuracy, particularly when temperatures are not constant or well controlled. Other common parameters that adversely affect accuracy include cosine and Abbe errors. In multi-axis systems undesired motion in any direction produces additional uncertainty.

With the majority of modern controllers, linear error compensation can be easily accomplished by entering a compensation factor into the controller. Newport specifies Accuracy after compensation and provides a control report indicating the compensation value with every stage.

Repeatability: Repeatability is a measure of the positioning system’s ability to sequentially position. It can be unidirectional (when approaching target position always from the same direction) or bidirectional (when approaching the target position from either direction). In a number of applications, the repeatability of a motion system is more important than the accuracy. Systematic errors can be taken into account and compensated, but the repeatability is the ultimate limit that is reached after all compensation.

Reversal Error: Reversal error is the distance between actual positions reached for a given target position, when approached from opposite directions. This value is a combination of backlash and hysteresis.

Backlash: Backlash is the result of relative movement between interacting mechanical parts of a drive system that does not produce output motion. Contributing factors include clearance between mechanical parts such as gear teeth and mechanical deformation. Not all systems have backlash and when they do, it mainly affects bi-directional repeatability. Backlash can be compensated by motion controllers due to its repeatable nature.

Hysteresis: Hysteresis is a component of reversal value that is dependent on the recent history of the system. It is the result of elastic forces in the various components and is observed, when the forces acting on a system reverse direction. Hysteresis affects both bi-directional repeatability and accuracy. It also affects precision alignment and position tracking applications as the output motion is not linear to the input motion, when reversing the direction of the motion. Unlike backlash, hysteresis is present in all mechanical systems although its value may be low.

Runout of a Linear Stage – Straightness / Flatness: Runout is the departure of the moving functional point from the desired ideal straight line motion. It consists of two orthogonal components. In ISO-230 and ASME B5.57 standards, runout is referred to as straightness. However, it is common in the motion industry to refer to it as flatness and straightness. Flatness is the runout in the vertical plane and straightness is the runout in the horizontal plane (see Figure 4).

Angular Runout of a Linear Stage – Pitch / Yaw / Roll: Angular runout is the rotation of the moving functional point. It has three orthogonal components commonly referred to as pitch, roll, and yaw. Assuming a linear stage with a horizontal carriage moving along X, pitch is the rotation around the Y axis, yaw is the rotation around the Z axis and roll is the rotation around the X axis (see Figure 5). A precise assessment of the runout errors for a given application can be complex, especially with multi-axis systems or with applications where the relevant reference point is at a long distance from the stage bearings. In such cases, the linear off axis errors introduced by the angular deviations of the stage and amplified by the distance are the main components of runout (Abbe Errors).

Eccentricity of a Rotary Stage: Eccentricity is the radial (perpendicular to the axis of rotation) deviation of the center of rotation from its mean position as the stage rotates through one revolution (Figure 6). It is also referred to as radial runout. A perfectly centered stage with perfect bearings would have no eccentricity.

Wobble of a Rotary Stage: Wobble is the tilt of the axis of rotation relative to the ideal axis, over one revolution (ref. Figure 6). It is most easily observed as a cyclic tilting of the rotating surface or table top of a stage and can produce Abbe error. Like eccentricity, it is generally the result of imperfect bearings.

Position Stability: Position stability is the ability to maintain a position within a specified position range over a specified time interval. It is the sum of drift and vibrations.

Drift: Drift is the slow deviation from a stable position. It mainly depends on the migration of lubricants and thermal variations.

Vibrations: Vibrations (in position noise) are fast alternative motions of small amplitude generated by the environment (noise from the flow, air fans) and electronics (motor driver).

Load Capacity – Centered / Transverse / Axial: Load Capacity is the maximum allowable force that can be applied to a stage in a specified direction while meeting stage specifications. This maximum force includes static (mass * gravity) and dynamic forces (mass * acceleration). Dynamic forces must include any external forces such as vibrations acting upon the stage. The amount of acceleration a stage can impart to a mass is limited to the accelerating force it can produce without exceeding the load capacity. The torque induced by a load not centered on the carriage can be a significant factor to consider when cantilevered loads are accelerated with linear stages.

Centered Normal Load Capacity: For linear stages, this is the maximum load that can be applied to a stage, with the load center of mass at the center of the carriage, in a direction perpendicular to the axis of motion and the carriage surface (see Figure 7). For rotary stages, it is the maximum load along the axis of rotation. In addition, the rotational moment of inertia must be within limits for rotary stages.

Transverse Load Capacity: Also called side load capacity, is the maximum load that can be applied perpendicular to the axis of motion and along the carriage surface (see Figure 7). This is typically smaller than the normal load capacity.

Axial Load Capacity: Axial Load Capacity is the maximum load along the direction of the drive train (see Figure 7). For linear stages mounted vertically, the specified vertical load capacity is usually limited by the axial load capacity. However, cantilevered loading must also be considered.

Back Drivability: With reversible driving system a force applied to the stage carriage can generate motion, when its motor is not powered. Such stages are called back drivable. Stages with direct drives are back-drivable with a low force: Stages with ball-screw are back-drivable, if the applied force is above a given threshold depending on the ball screw characteristics.

Stages with lead-screw are generally not back-drivable, unless pitch of the screw is very high. If the stage is back drivable and used in vertical applications, a payload-dependent force is constantly applied, resulting in downward motion of the stage carriage at power off. Unless otherwise specified, the back-drivability threshold of Newport stages is above the Axial Load Capacity.

Off-Center Load Capacity: The maximum load capacity of a stage is diminished when the load is not centered (refer to the stage specifications section in the catalog for off centered load equations and/or contact Newport’s Applications Engineers to discuss your specific load conditions).

Maximum Inertia: Inertia is the measure of load’s resistance to change in velocity. The larger the inertia, the greater the force required to accelerate or decelerate the load. If there is a constraint on the amount of force available, then the allowable acceleration and deceleration must be adjusted to an acceptable value. Inertia is a product of mass elements and the square of their distance from the axis of rotation. The maximum inertia specified for rotary stage is a value based on available torque (limitation in acceleration) and bearing stiffness (limitation in natural frequency and associated vibrations).

Speed (Velocity): Speed (Velocity) is the rate of change of position. The maximum speed specification is provided at the stage’s normal load capacity. Higher speeds are possible for lower loads or larger motor drivers. Minimum speeds are highly dependent on a motion system's speed stability.

Speed Stability: Speed stability is a measure of the ability of a motion system to maintain a constant speed within specified limits. It is usually specified as a percentage of the desired speed. Also specified as velocity regulation, this parameter depends upon the stage’s mechanical design, its feedback mechanism, the motion controller, control algorithm, the magnitude of the speed, and the application.

The actual speed of a moving part is usually not measured directly. Instead, it is calculated based on a sample of the position. As a result, the value of speed stability depends a lot on the sampling frequency. In order to be accurately defined, speed stability should be specified in a given bandwidth.

Acceleration: Acceleration is the rate of change of velocity. Unless otherwise specified, Newport sets the acceleration of its stages to a value that enables the stage to reach the maximum velocity in 250 ms, so maximum acceleration = maximum speed * 4/s.

Jerk: Jerk is the rate of change of acceleration.

Mean Time before Failure: Mean Time before Failure (MTBF) is the prediction of product reliability. Tests and statistical analysis of parts and components are performed to predict the rate at which a product will fail. It is one of the most common forms of reliability prediction and is usually based on an established analytical model. Many models exist, and choosing one over another must be based on a broad array of factors specific to a product and its application. In general, MTBF is specified with a duty cycle parameter.

In the case of simple system with a constant failure rate (number of failures/unit time), the fraction of units still working after a time equal to the MTBF is 37%.