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.
Micropositioning Terms
Glossary
Design Resolution The theoretical minimum movement that can be made based on the selection of the mechanical drive components (drive screw pitch, gear ratio, angular motor resolution etc.). Design resolution is usually higher than the practical position resolution (minimum incremental motion). Examples where design resolution is higher than position resolution are motor/gear box driven systems with very high gear reduction ratio, or microstepped motors. A linear stage driven by such a motor can require an input of 1,000,000 steps (encoder counts) to move a distance of one mm. In this case one step performed and displayed by the motor controller is equal to 1 nanometer. In reality the stage may not be able to respond to motion input less than 50 steps due to play, windup, friction, backlash etc.
Minimum Incremental Motion
The minimum motion that can be repeatedly executed for a given input, which is sometimes referred to as practical or operational resolution. For systems with microstepped motors or motor / gearbox combinations design resolution and practical resolution have to be distinguished. Design resolutions of 1 nm or better can be achieved with many motor, gearbox and leadscrew combinations. In practical applications, however, stiction/ friction, windup, and elastic deformation limit resolution to fractions of a micron.
Repeatable nanometer or sub-nanometer resolution can only be provided by solid state actuators (PZTs) and PZT flexure stages (see "PZT Flexure NanoPositioners" and "PZT Actuators" sections for details). See also Design Resolution.
Orthogonality
The deviation from the ideal 90° angle between the X and Y axis.
Precision
An undefined term used differently by different manufacturers. Precision sometimes refers to absolute accuracy, repeatability, even to resolution.
Resolution
See Design Resolution and Minimum Incremental Motion. Resolution is often confused with accuracy.
Runout (Tracking Accuracy, Guiding Accuracy)
Deviation from a straight line. For linear stages, the runout describes unwanted motion in all 5 degrees of freedom (off-axes) other than the intended linear motion in the commanded direction. For a translation in X, linear runout occurs in Y and Z, tilt occurs in theta X (roll), theta Y (pitch) and theta Z (yaw). Runout is caused by the guiding system itself, by the way the stage is mounted (tension!) and the load conditions.
Stepper motor
An electric motor designed for open loop operation providing motion in discrete steps. Stepper motors provide limited dynamic properties compared to DC servo motors of equal size. They also dissipate more heat at steady state operation and produce noise and vibration during motion. Modern, 5 phase designs have mostly overcome these problems. Stepper controller design (open loop) is simple compared to DC servo controller design (closed loop). Stepper motors yield good results in predictable applications.
Stick/Slip Effect
Limits resolution. Caused by the fact that the coefficient of static friction is greater than the coefficient of dynamic friction. When a drive force is applied to a positioner the start of motion is out of phase with the build up of force. In the beginning no motion occurs and when a threshold is reached a sudden jump occurs. Only frictionless devices such as solid state actuators (piezo actuators) do not exhibit stiction and therefore provide superior resolution to "classical mechanical positioners" in the sub-micron to sub-nanometer realm.
Tracking Accuracy
See runout
Unidirectional Repeatability
The accuracy of returning to a given position from any other position, but always from one direction. Unidirectional repeatability is not affected by backlash. See Bi-directional Repeatability.
