Technical Note:
Linear Translation Stage Terminology

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 behaviors include: dead zone, stiction, looseness, slop and free play.

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

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.

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.

Maximum Normal Load: The maximum static force that can be applied perpendicularly to the mounting surface. This does not apply to dynamic or impact forces.

Orthogonality: The line of motion of any two axes on a multi-axis stage will not deviate from 90° by more than this amount.

Pitch: Rotation about the transverse, or Y axis. This is also known as elevation, particularly in gimbal-type mounts.

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.

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 bidirectional repeatability. Unfortunately, few manufacturers publicize this much tougher measure of motion performance.

Resolution, Display: The smallest incremental step that can be displayed or read from an actuator. For most micrometer type actuators, the display resolution is the smallest graduation either standard or Vernier. For example, a standard micrometer such as the SM-25 has standard graduation of 10µm but a Vernier graduation of 1µm so the best display resolution would be 1µm. The sensitivity of most micrometers is typically the same as or better than the display resolution or graduation.

Roll: Rotation about the longitudinal, or X, axis of travel.

Sensitivity: With reference to a manually actuated device, the smallest linear or angular movement that can be discriminated. This term is sometimes referred to as resolution and often confused with graduation or display resolution. Since manual dexterity varies from person to person, it is assumed that your fingertips are sensitive enough to be able to make 1° rotations of an adjustment screw and therefore, when you see sensitivity quoted for an adjustment screw, it is the travel associated with a 1° turn of the screw. Sensitivity is limited primarily by stiction.

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.

Straight-Line Accuracy: This is the radius of the smallest cylinder that contains the path traced out by any point on the stage as it moves over the full range of travel. Also known as runout.

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.