Glossary of Terms

ABBÉ ERROR: A linear positioning error caused by a combination of an angular error in the ways, and an offset between the precision determining element (leadscrew, feedback device, etc.) and the actual point of interest.

ABSOLUTE MOVE: A positioning move in which all moves are referenced to a fixed original position. For example, if a stage is positioned at +500 mm, an absolute move to +300 mm would result in a move of 200 mm towards the origin (in the negative direction).

ACCURACY: The difference in length between a move made by a perfect positioning system, and any physically realizable system. Numerous error sources contribute to degrade accuracy; see Engineering Considerations.

ASCII: Short for American Standard Code for Information Interchange. ASCII provides a one-to-one mapping between alphanumeric characters, and a digital one-byte word.

BACKLASH: The amount of free play between a leadscrew and nut, or worm and worm gear. It is determined by measuring the range of angular movement of the driven shaft which results in no motion of the positioning table.

BANDWIDTH: The frequency at which the small signal response of a servo system begins to drop off; stage performance is enhanced by increasing servo bandwidth, although structural resonances typically limit the achievable bandwidth.

BRUSHLESS SERVOMOTOR: An “inside-out” DC motor, with a permanent magnet rotor, and electrical coils in the stator. Commutation of current in the windings is typically achieved via external switching transistors, and Hall-effect detectors. This avoids the limited life of brushes and their radiated EMI.

CANTILEVERED LOAD: Any load not symmetrically mounted on a stage. Such loads exert torque moments upon the ways, and the resulting deformations can degrade accuracy.

CLOSED LOOP POSITIONING: The use of feedback devices (encoders, resolvers, interferometers, etc.) to allow a motor to position a user payload accurately.

COEFFICIENT OF FRICTION: This is defined as the ratio of the force required to move a given load to the magnitude of that load. Typical values for ball and crossed roller slides are 0.001 to 0.005.

FLATNESS OF TRAVEL: Deviation from ideal straight line travel in a vertical plane, also referred to as vertical runout.

G.P.I.B.: A standardized protocol, analogous to RS-232, for transmitting digital information. Unlike RS-232, the GPIB interface (also called IEEE-488) transmits data in parallel, not serial format (one byte at a time).

HALF STEP: An extended 8-cycle current switching sequence for stepping motors that doubles resolution, reduces noise, and improves resonant conditions.

HALL-EFFECT SENSOR: Highly accurate, non-contact limit switch which detects the proximity of a magnet and provides a digital output to assure an accurate position reference. Typical repeatability of the DOVER Hall sensor is ± 1-2 microns.

HOLDING TORQUE: Stepping motors, when energized, hold position via a magnetic field. The holding torque is the maximum torque which can be generated before the rotor slips to the next pole location (7.2 degrees for 200 step/revolution steppers).

HOME SWITCH: Any of a variety of sensors which can be used to establish an accurate initial position. This may consist of a standard end-of-travel Hall sensor; a center position opto-interrupter with half-travel blocking vane; an index signal on a linear encoder; a shaft coupling mounted magnet with Hall sensor; or a once-per-revolution encoder index signal. Once-per-revolution sensors will usually require a logical “or”-ing with a linear signal if a unique home position is required.

INCREMENTAL MOVE: Positioning mode in which moves are referenced from the previously held position.

INDUCTANCE: The tendency of a motor coil to store energy in a magnetic field. High speed stepping motor performance is inversely proportional to motor inductance.

LEAD ERROR: The deviation of a leadscrew from its nominal pitch. The error is often monotonic (linear), although periodic error and thermal expansion set limits to its predictability.

LIMIT SWITCH: Typically Hall-effect, optical, eddy current, or mechanical, this is a sensor that is used to sense the end of travel of a linear motion assembly. In addition to preventing overtravel, it is frequently used to establish a precision reference.

MICROSTEPPING: A technique which, instead of switching phase currents in a stepping motor on and off, sinusoidally varies the current in the two windings. This effectively increases the resolution from 200 steps per revolution to 2,000 (÷ 10) or 10,000 (÷ 50) microsteps per revolution.

NUT STIFFNESS: The stiffness of a leadscrew/nut assembly, typically measured in Newtons per meter. This stiffness, together with the moving mass and duplex bearing stiffness, sets the primary natural frequency of a leadscrew-driven stage.

OPEN LOOP POSITIONING: A positioning technique, typically utilizing stepping motors, in which the controller issues a sequence of commands to the motor without any absolute means of detecting if the move has in fact been made. When the load and move velocity and acceleration are appropriately defined, open loop positioning is capable of extended operation without losing steps.

OPTICAL ENCODER: A linear or angular position feedback device, typically providing incremental two channel information in quadrature format (sine or square waves with a 90 degree phase shift between each channel). Such two channel information allows simple counter circuits to function as absolute position indicators.

ORTHOGONALITY: The degree of perpendicularity, or squareness, between the two axes in an X-Y or X-Z table. This parameter is usually measured in arc-seconds or microradians.

PHASE CURRENT: The rated current which a stepping motor requires to generate its rated holding torque. This value is usually based on unipolar (half-coil) operation. This choice of how the motor is wired has significant impact on performance.

PHASE SEQUENCE: The specific sequence of coil current changes used to advance a stepping motor clockwise and counter-clockwise, in either full or half step modes.

PITCH: a) for leadscrews specified in British units, the number of full rotations required to advance the nut 1″. For example, a 5 pitch leadscrew has a lead of 0.200″. Metric screws are specified by lead only, in millimeters. b) an angular deviation possible in positioning systems, in which the table’s leading edge rises or falls as the table translates along its direction of travel. This represents rotation around a horizontal axis, perpendicular to the direction of travel.

QUASI-CLOSED LOOP POSITIONING: A technique using a stepping motor and encoder in which open loop moves are completed, after which an encoder/counter is checked and, if necessary, a small final move or moves is used to achieve the desired accuracy.

RAMPING: The gradual acceleration and deceleration of a stepping motor, essential if performance beyond the start/stop range is required. The slope of the ramp is a function of screw pitch, load, drive voltage and design, and motor.

REPEATABILITY, UNI-DIRECTIONAL: The difference in absolute position reached when returning to a given position from the same direction. This value may mask significant amounts of backlash.

REPEATABILITY, BI-DIRECTIONAL: The difference in absolute position reached when returning to a given position from the opposing direction. This value is usually larger than the uni-directional repeatability.

RESOLUTION: The distance a stage can be commanded to move in a single step. For servo systems, the basic increment produced by its optical encoder, or other feedback device.

RESONANCE, MIDRANGE: A parasitic oscillation which is endemic to stepping motors, although frictional loads may mask its effect. It typically sets in from 5-15 revolutions per second, and can easily cause a loss of synchronization (stalling). All DOVER high speed microstepping drives effectively suppress this resonance.

RESONANCE, PRIMARY: The rotor inertia of a stepping motor, together with its spring-like holding torque, constitutes a basic spring-mass oscillator. In the absence of sufficient damping, stepping at certain frequencies may excite resonance in this system, or resonate with the stage or load, resulting in loss of synchrony. The addition of system damping, microstepping, or ramping through problem speeds will usually eliminate this resonance.

ROLL: An angular deviation from ideal straight line motion, in which the positioning table rotates around its axis of travel as it translates along that axis.

RS-232C: A popular protocol for transmitting digital data over two lines in a bit-serial format. RS-232C specifies signal levels, data formats, maximum transmission distance, etc.

SERVO MOTOR: A DC motor which produces a torque proportional to current. Precise positioning is achieved by linear or PWM (duty cycle) control of motor current or voltage, together with accurate monitoring of position via an external feedback device.

STALL SPEED: The maximum speed which a stepping motor, properly ramped, can achieve without loss of synchrony. This speed is a function of motor inductance, ramp slope, applied load, and drive voltage and design.

START/STOP SPEED: The maximum step rate which can be applied to a stationary stepping motor and still retain error-free performance. Also, the rate from which a stepping motor may be instantaneously stopped without overshooting. This is a function of the screw pitch, load, drive voltage and design, and motor.

STEPPING MOTOR: A type of motor featuring two or four stator coils and a toothed permanent magnet rotor, which moves through a small angle in response to a specific sequence of coil current changes.

STEP RATE: The frequency of coil current changes, or input pulse train, applied to a stepping motor, in pulses/second or hertz. For 200 step/revolution motors, the full step rate multiplied by 0.3 equals the rotation rate in R.P.M.

STRAIGHTNESS OF TRAVEL: Deviation from straight line motion in a horizontal plane. Also referred to as horizontal runout. This error is usually traceable to an underlying angular error of the ways.

T.I.R.: This stands for Total Indicator Reading, which reflects the total absolute deviation from a mean value (versus a ± value which indicates the deviation from a nominal value).

TORQUE: A radially directed force, typically measured in inch-ounces, foot-lbs. or Newton-meters.

YAW: An angular deviation from ideal straight line motion, in which the positioning table rotates around the Z (vertical) axis as it translates along its travel axis.