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Home  > Motion & Manipulation > Motion

Motion and Manipulation

Glossary


Abbe Error

Linear off-axis error introduced through amplification of tilt and wobble with a long moment arm. This type of error occurs when the point under measurement is at a relatively long distance from the axis of motion.

Accuracy
The maximum expected difference between the actual and a desired position for a given input. Highly dependent on method of actual position measurement.

Accuracy, Absolute
The output of a system versus the commanded or ideal input.

Accuracy, On-Axis
The uncertainty of position after all sources of linear error are eliminated. Linear errors include: cosine error, leadscrew pitch error, abbe error and thermal expansion effects. 

Backlash
The maximum magnitude of an input that produces no measurable output upon reversing direction. Typically the result of poor meshing between drivetrain components as with lead screw threads. 

Display Resolution
The smallest motion detectable by a motion
device’s precision rule, micrometer or motor controls.

Eccentricity
Sometimes called concentricity, eccentricity in a rotary device is the deviation of the center of rotation from its mean position as the device turns.

Error
The difference between an obtained performance parameter and the ideal or desired result. Errors fall into two primary categories, on-axis and off-axis errors.

Friction
Friction is defined as the resistance to motion between surfaces in contact. Friction can be constant or it can vary with speed. Elements contributing to overall friction may be in the form of drag, sliding friction, system wear or lubricant viscosity. 

Friction, Static
The friction that must be overcome to impart motion to a body at rest. Since static friction is higher than sliding friction, the force which must be applied to impart motion is greater than the force required to keep the body in motion. As a result, when a force is initially applied, the body will begin to move with a jump in some unpredictable and unrepeatable manner, producing non-linear, non-repeatable motion.

Gear Ratio, Drive Train
A motion instrument’s drive train gear ratio is the relationship between received input motion and the delivered output motion. Ratios are expressed in the numerical notation a:b, where "a" represents the received motion or device input in revolutions or some other unit, and "b" represents the delivered or resulting output motion in revolutions for rotary devices or 1" of travel in linear motion instruments.

Hysteresis
The difference in the absolute position of an object for a given commanded input when approached from opposite directions. It is due to elastic forces accumulated in various drivetrain components, leadscrew wind-up, for instance. Often confused with backlash.

Load Capacity, Stage
The maximum centered load that can be placed directly on an XYZ motion stage and is typically limited by the load capacity of the bearings.

Load Capacity, Lateral or Moment
Also called side or bending load capacity, it is the maximum load that can be applied perpendicular to a shaft’s axis of motion. 

Load Capacity, Axial
The maximum centered and balanced compressive or tensile load that can be applied to a stage’s or shaft’s longitudinal or parallel axis of motion.

Minimum Incremental Motion
The smallest motion a device is capable of delivering reliably, not the smallest display resolution increment.

Play
Uncontrolled movement due to looseness of mechanical parts. Usually increases with the components age. Play is a contributor to backlash.

Position Stability
The ability to maintain a constant position over time. Variation from stable position is called drift. Contributors to drift include worn parts, migration of lubricant, and thermal variation.

Precision
Also known as repeatability, it is the range of deviations in output position that will occur for 95% of the motion excursions from the same error-free input. Accuracy and precision are not the same.

Repeatability
The ability of a motion instrument to reliably achieve a commanded position over many attempts regardless of the direction from which the position is approached.

Runout
The linear, not angular, portion of off-axis error. It is the deviation between ideal straight line motion and actual measured motion in a translation stage. Runout has two orthogonal components, straightness, a measure of in-plane deviation, and flatness, the out-of-plane deviation.

Sensitivity
The minimum input required to produce output motion or the ratio between output motion and input drive. Applicable particularly to manually actuated motion devices.

Tilt
The angular portion of off-axis error. It is the deviation between ideal straight line motion and actual measured motion in a translation stage. Tilt and wobble have three orthogonal components commonly referred to as roll, pitch, and yaw. 

Torque, Detent 
The torque present in an unenergized stepper motor caused by its magnetic rotor. Detent torque allows stepper motors to hold their position even when unenergized. 

Torque, Holding 
The maximum external force or torque that can be applied to a stopped, energized motor without causing the rotor to rotate continuously.

Wobble
Wobble is the angular deviation of the axis of rotation over one complete revolution.


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