Micropositioning Terms

Definition of Axes and Angles

X: Linear motion in positioning direction
Y: Linear motion perpendicular to positioning direction
Z: Vertical linear motion
q_X: Angular motion around X (roll)
q_Y: Angular motion around Y (pitch)
q_Z: Angular motion around Z (yaw)

Definition of axes and angles

Glossary

Absolute Accuracy
For any given input this is the maximum difference between the commanded (ideal) position and the actual position. Absolute accuracy is often confused with resolution. For real systems, resolution is usually a great deal higher than absolute accuracy which is affected by backlash, hysteresis, drift, linearity, and repeatability. Absolute accuracy in the range of 1 µm or better over more than a few mm of travel, can usually only be achieved with external measuring systems such as laser interferometers or linear glass scales. Leadscrew mounted encoders or stepper motors cannot achieve this kind of accuracy.

Backlash
Lost motion after reversing direction. This can be due to play in screw/nut fittings, gearheads, bearings, etc. Some manufacturers promote controllers with automatic backlash compensation adding the estimated amount of lost motion upon each reversal. A good solution in theory which is limited in practice because backlash varies with load, leadscrew position, temperature, deceleration, direction, wear etc. Backlash can lead to oscillation in closed loop set ups where the position sensor is directly attached to the part to be controlled. Backlash is not to be confused with hysteresis.

Bi-directional repeatability
The accuracy of returning to a position A from any position, regardless of direction. Effects such as hysteresis and backlash affect bi-directional repeatability. See Unidirectional Repeatability.

Cosine Error
A cumulative error that occurs when a drive system is misaligned in regard to the driven part. The error equals 1 – the cosine of the angle between the ideal drive axis and the actual drive axis times the moved distance.

DC Servo Motor
A direct current motor that is operated in a closed loop system (servo circuit). Vibrationless, smooth running, broad speed range and very good low speed torque are characteristics of DC servo motors. For optimum performance a good motor controller with PID (Proportional, Integral, Derivative) filter settings is mandatory.

Guiding Accuracy
See runout

Hysteresis
Hysteresis occurs when reversing direction. Unlike backlash which is lost motion at the beginning of a reversed motion, hysteresis contributes to position error by the relaxation of elastic forces in the drivetrain components. Hysteresis is highly variable based on different load and acceleration values.