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A
Abbι error
Named after Ernst Abbι, a noted optical designer, it
refers to the positioning error resulting from angular
motion and an offset between the measuring device and the
point of interest.
This can be a significant source of error in positioning
applications. In open loop systems (or closed loop systems
employing rotary feedback), the accuracy is nominally
determined by the precision of the leadscrew. Similarly, in
systems with linear encoders or interferometers, it is that
device which determines the accuracy. Leadscrews only tell
the relative position of the nut and screw, and encoders
tell us only the position of the read head relative to the
glass scale. Extrapolating this to include the position of
an item of interest, despite its firm mechanical connection
to the nut or encoder read-head, is ill founded.
Abbι
offset The value
of the offset between the measuring device and the point of
interest.
Absolute move
A move referenced to a known point or datum.
Absolute programming
A positioning coordinate reference where all positions are
specified relative to a reference or home position.
AC brushless servo
A servomotor with stationary windings in the stator
assembly and permanent magnet rotor. AC brushless generally
refers to a sinusoidally wound motor to be commutated via
sinusoidal
current waveform. (see DC
Brushless Servo)
Acceleration
The change in velocity as a function of time.
Accuracy
An absolute measurement defining the difference between
actual and commanded position.
Accuracy grade
In reference to an encoder grating, accuracy grade is the
tolerance of the placement of the graduations on the encoder
scale.
ASCII
American Standard Code for Information Interchange. This
code assigns a number to each numeral
and letter of the alphabet. Information can then be
transmitted between machines as a series of binary
numbers.
Axial runout
Positioning error of the rotary stage in the vertical
direction when the tabletop is oriented in the horizontal
plane. Axial runout is defined as the total indicator
reading on a spherical ball positioned 50mm above the
tabletop and centered on the axis of rotation.
Axis of rotation
A center line about which rotation occurs.
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B
Back emf, Kemf
The voltage generated when a permanent magnet motor is
rotated. This voltage is proportional to motor speed and is
present whether the motor windings are energized or not.
Backlash
A component of bidirectional repeatability, it is the
non-responsiveness of the system load to reversal of input
command.
Ball screw
A precision device for translating rotary motion into
linear motion. A lead screw is a lower cost, lower
performance device performing the same function. Unit
consists of an externally threaded screw and an internally
threaded ball nut.
Ball screw lead
The linear distance a carriage will travel for one
revolution of the ball screw (lead
screw).
Bandwidth
A measurement, expressed in frequency (hertz), of the
range which an amplifier or motor can respond to an input
command from DC to -3dB on a frequency sweep.
Baud rate
The number of bits transmitted per second on a serial
communication channel such as RS-232
or modem.
BCD
Binary Coded Decimal - A number system using four bits to
represent 0-F (15).
Bearing
A support mechanism allowing relative motion between two
surfaces loaded against each other. This can be a rotary
ball bearing, linear slide bearing, or air bearing (zero
friction).
Bidirectional repeatability
See Repeatability.
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C
CAM profile A
technique used to perform nonlinear motion that is
electronically similar to the motion achieved with
mechanical cams.
Cantilevered load
A load not symmetrically mounted on a stage.
Closed loop
A broad term relating to any system where the output is
measured and compared to the input. Output is adjusted to
reach the desired condition.
CNC
Computer Numerical Control. A computer-based motion
control device programmable in numerical word address
format.
Coefficient of friction
Defined as the ratio of the force required to move a given
load to the magnitude of that load.
Cogging
Nonuniform angular/linear velocity. Cogging appears as a
jerkiness, especially at low speeds, and is due to the
magnetic poles attraction to steel laminations.
Commutation
The action of steering currents to the proper motor phases
to produce optimum motor torque/force. In brush-type motors,
commutation is done electromechanically via the brushes and
commutator. A brushless motor is electronically commutated
using a position feedback device such as an encoder or Hall
effect devices. Stepping motors are electronically
commutated without feedback in an open-loop fashion.
Commutation, 6-step
Also referred to as trapezoidal commutation. The process
of switching motor phase current based on three Hall effect
signals spaced 120 electrical degrees beginning 30 degrees
into the electrical cycle. This method is the easiest for
commutation of brushless motors.
Commutation, modified 6-step
Also referred to as modified sine commutation. The process
of switching motor phase current based on three Hall effect
signals spaced 120 electrical degrees beginning at 0
electrical degrees. This method is slightly more difficult
to implement than standard 6-step, but more closely
approximates the motors back emf. The result is smoother
control and less ripple.
Commutation, sinusoidal
The process of switching motor phase current based on
motor position information, usually from an encoder. In this
method, the three phase currents are switched in very small
increments that closely resemble the motors back emf.
Sinusoidal commutation requires digital signal processing to
convert position information into three-phase current values
and, consequently, is most expensive to implement. The
result, however, is the best possible control.
Coordinated motion
Multi-axis motion where the position of each axis is
dependent on the other axis, such that the path and velocity
of a move can be accurately controlled. Drawing a circle
requires coordinated motion.
Critical speed
A term used in the specification of a lead screw or ball
screw indicating the maximum rotation speed before resonance
occurs. This speed limit is a function of the screw
diameter, distance between support bearings, and bearing
rigidity.
Current command
Motor driver or amplifier configuration where the input
signal is commanding motor current directly, which
translates to motor torque/force at the motor output.
Brushless motors can be commutated directly from a
controller that can output current phase A and B commands.
Current, peak
An allowable current to run a motor above its rated load,
usually during starting conditions. Peak current listed on a
data sheet is usually the highest current safely allowed to
the motor.
Current, rms
Root Mean Square. Average of effective currents over an
amount of time. This current is calculated based on the load
and duty cycle of the application.
Cycle
When motion is repeated (move and dwell) such as repetitive
back-and-forth motion.
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D
DC brushless servo
A servomotor with stationary windings in the stator
assembly and permanent magnet rotor. (See
AC Brushless Servo)
Deceleration
The change in velocity as a function of time.
Duty cycle
For a repetitive cycle, the ratio of on time to total
cycle time used to determine a motors rms current and
torque/force.
Dwell time
Time in a cycle at which no motion occurs. Used in the
calculation of rms power.
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E
Efficiency Ratio
of input power vs. output power.
Electronic gearing
Technique used to electrically simulate mechanical
gearing. Causes one closed loop axis to be slaved to another
open or closed loop axis with a variable ratio.
Encoder marker
Once-per-revolution signal provided by some incremental
encoders to accurately specify a reference point within that
revolution. Also known as Zero Reference Signal or Index
Pulse.
Encoder resolution
Measure of the smallest positional change which can be
detected by the encoder. A 1000-line encoder with a
quadrature output will produce 4000 counts per revolution.
Encoder, incremental
Position encoding device in which the output is a series
of pulses relative to the amount of movement.
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F
Feedback Signal
that provides process or loop information such as speed,
torque, and position back to the controller to produce a
closed-loop system.
Flatness (of travel)
Measure of the vertical deviation of a stage as it travels
in a horizontal plane.
Force, continuous
The value of force that a particular motor can produce in
a continuous stall or running (as calculated by the rms
values) condition.
Force, peak
The maximum value of force that a particular motor can
produce. When sizing for a specific application, the peak
force is usually that required during acceleration and
deceleration of the move profile. The peak force is used in
conjunction with the continuous force and duty cycle to
calculate the rms force required by the application.
Friction
The resistance to motion between two surfaces in contact
with each other.
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G
GPIB.
A standard protocol for transmitting digital information.
The GPIB interface (IEEE-488)
transmits data in parallel instead of serial format. (See
IEEE-488).
Gain
Comparison or ratio of the output signal and the input
signal. In general, the higher the system gain, the higher
the response.
Grating period
Actual distance between graduations on an encoder.
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H
Hall effect sensors
Feedback device (HED) used in a
brushless servo system to provide information for the
amplifier to electronically commutate the motor.
HED
Hall Effect Device. (See
hall
effect sensors)
Holding Torque
- The maximum torque the motor can develop before swinging
to a new pole position. Note that an energized motor has no
torque when it is in position; it develops increasing torque
as the motor shaft is displaced from its nominal position.
In general, a higher holding torque translates to greater
available torque at any specific rotation rate, but this is
complicated by additional factors such as the motors
inductance and the drive voltage. High speed performance is
enhanced by the combination of a low motor inductance and a
high drive voltage. Low inductance results in higher motor
currents, however, which may tax the power supply or driver
output capability. Too high a drive voltage, in turn can
result in excessive drive and motor heating, and may
increase radiated electrical noise. The manner in which
leads are wired to a drive can be critical in determining
overall system performance which is why LG Motion offer 4, 6
& 8 lead wiring options.
HMI
Human Machine Interface. Used as a means of getting operator
data into the system. Also referred to as Man-Machine
Interface (MMI).
Home
Reference position for all absolute positioning movements.
Usually defined by a home limit switch and/or encoder
marker.
Home switch
A sensor used to determine an accurate starting position
for the home cycle.
Hysteresis
A component of bidirectional repeatability.
Hysteresis is the deviation between actual and commanded
position and is created by the elastic forces in the drive
systems.
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I
I/O
Input/Output. The reception and transmission of information
between control devices using discrete connection points.
IEEE-488
A set of codes and formats to be used by devices connected
via a parallel bus system. This standard also defines
communication protocols that are necessary for message
exchanges, and further defines common commands and
characteristics. (See
GPIB)
Incremental move
A move referenced from its starting point (relative move).
Inertia
The physical property of an object to resist changes in
velocity when acted upon by an outside force. Inertia is
dependent upon the mass and shape of an object.
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L
Lead error The
deviation of a lead screw or ball screw from its nominal
pitch.
Lead screw
A device for translating rotary motion into linear motion.
Unit consists of an externally threaded screw and an
internally threaded carriage (nut). (See
Ball screw)
Life
The minimum rated lifetime of a stage at maximum payload
while maintaining positioning specifications
Limit switch
A sensor used to determine the end of travel on a linear
motion assembly.
Limits
Sensors called limits that alert the control electronics
that the physical end of travel is being approached and
motion should stop.
Linear motor
A motor consisting of two parts, typically a moving coil
and stationary magnet track. When driven with a standard
servo amplifier, it creates a thrust force along the
longitudinal axis of the magnet track.
Load carrying capability
The maximum recommended payload that does not degrade the
listed specifications for a mechanical stage.
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M
Master-slave
Type of coordinated
motion control where the master axis position is used to
generate one or more slave axis position commands.
MMI
Man Machine Interface used as a means of getting operator
data into the system. (See HMI)
Motion profile
A method of describing a process in terms of velocity,
time, and position.
Motor brush
The conductive element in a DC brush-type motor used to
transfer current to the internal windings.
Motor, brushless
Type of direct current motor that utilizes electronic
commutation rather than brushes to transfer current.
Motor, stepping
Specialised motor that allows discrete positioning without
feedback. Used for no critical, low power applications,
since positional information is easily lost if acceleration
or velocity limits are exceeded.
Closed loop is now available as are new technologies such as
SILENTstep.
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N
NC Numerical
Control. Automated equipment or process used for contouring
or positioning. (See CNC)
NEMA
National Electrical Manufacturers Association. Sets
standards for motors and other industrial electrical
equipment.
Non-volatile memory
Memory in a system that maintains information when power
is removed.
Open collector A signal output that is performed with a
transistor. Open collector output acts like a switch closure
with one end of the switch at circuit common potential and
the other end of the switch accessible.
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O
Open loop
Control circuit that has an input signal only, and thus
cannot make any corrections based on external influences.
Operator interface
Device that allows the operator to communicate with a
machine. A keyboard or thumbwheel is used to enter
instructions into a machine. (See HMI or
MMI)
Optical encoder
A linear or angular position feedback device using light
fringes to develop position information.
Opto-isolated
System or circuit that transmits signal with no direct
electrical connections, using photoelectric coupling between
elements.
Orthogonality The condition of a surface or axis which is
perpendicular (offset 90 degrees) to a second surface or
axis. Orthogonality specification refers to the error from
90 degrees from which two surfaces of axes are aligned.
Overshoot
In a servo system, referred to the amount of velocity
and/or position overrun from the input command.
Overshoot is a result of many factors including mechanical
structure, tuning gains, servo controller capability, and
inertial mismatch.
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P
PID A group of
gain terms in classical control theory (Proportional
Integral Derivative) used in compensation of a closed-loop
system. The terms are optimally adjusted to have the output
response equal the input command.
Pitch (of travel)
Angular motion of a carriage around an axis perpendicular
to the motion direction and perpendicular to the yaw axis.
Pitch error
Positioning error
resulting from a pitching motion.
PLC
Programmable Logic Controller. A programmable device that
utilizes ladder logic to control a number of input and
output discrete devices.
PWM
Pulse Width Modulation. Switch-mode technique used in
amplifiers and drivers to control motor current. The output
voltage is constant and switched at the bus value (160 VDC
with a 115 VAC input line).
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Q
Quadrature
Refers to the property of position transducers that allows
them to detect direction of motion using the phase
relationship of two signal channels. A 1000-line encoder
will yield 4000 counts via quadrature.
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R
Radial runout
Positioning error of the rotary stage in the horizontal
direction when the tabletop is oriented in the horizontal
plane. Radial runout is defined as the total indicator
reading on a spherical ball positioned 50mm above the
tabletop and centered on the axis of rotation.
Ramp time
Time it takes to
accelerate from one velocity to another.
Range
The maximum allowable travel of a positioning stage.
RDC
Resolver to Digital Converter. Electronic component that
converts the analog signals from a resolver (transmitter
type) into a digital word representing angular position.
Repeatability
The maximum deviation from the mean (each side) when
repeatedly approaching a position.
Unidirectional repeatability refers to the value established
by moving toward a position in the same direction.
Bidirectional repeatability refers to the value established
by moving toward a position in the same or opposite
direction.
Resolution
The smallest change in distance that a device can measure.
Resonance
- The tendency of a system to oscillate with high amplitude
when excited by energy at a certain frequency. This
frequency is known as the system's natural frequency of
vibration, resonant frequency, or eigenfrequency.
Retroreflector
An optical element with the property that an input light
beam is reflected and returns along the same angle as the
input beam. Used with laser interferometers.
Roll (of travel)
Angular motion of a carriage around an axis parallel to
the motion direction and perpendicular to the yaw axis.
Roll error
Positioning error resulting from a roll motion.
Rotor
The rotating part of a magnetic structure. In a motor, the
rotor is connected to the motor shaft.
RS-232C
Industry standard for sending signals utilizing a
single-ended driver/receiver circuit. As such, the maximum
distance is limited based on the baud rate setting but is
typically 50-100 feet. This standard defines pin
assignments, handshaking, and signal levels for receiving
and sending devices.
RS-274
Industry standard programming language. Also referred to
as G-code machine programming. A command set specific for
the machine tool industry that defines geometric moves.
RS-422
Industry communication standard for sending signals over
distances up to 4000 feet. Standard line driver encoder
interfaces utilize RS-422 because of the noise immunity.
Runout
The deviation from the desired form of a surface during
full rotation (360 degrees) about an axis. Runout is
measured as Total Iindicated Reading (TIR). For a rotary
stage, axis runout refers to the deviation of the axis of
rotation from the theoretical axis of rotation.
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S
Servo system
Refers to a closed loop control system where a command is
issued for a change in position and the change is then
verified via a feedback system.
Settling time
Time required for a motion system to cease motion once the
command for motion has ended.
Shaft radial load
Maximum radial load that can be applied to the end of the
motor shaft at maximum motor speed.
Shaft runout
Deviation from straight line travel.
Slotless
Describes the type of laminations used in a motor that
eliminates cogging torque due to magnetic attraction of the
rotor to the stator slots.
Stator
Non-rotating part of a magnetic structure. In a motor, the
stator usually contains the mounting surface, bearings, and
non-rotating windings.
Stiction
Friction encountered when accelerating an object from a
stationary position. Static friction is always greater than
moving friction, and limits the smallest possible increment
of movement.
Straightness of travel
Measure of the side-to-side deviation of a stage as it
travels in a horizontal plane.
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T
Torque Rotary
equivalent to force. Equal to the product of the force
perpendicular to the radius of motion and distance from the
center of rotation to the point where the force is applied.
Torque, continuous
Torque needed to drive a load over a continuous time.
Torque, peak
Maximum amount of torque a motor can deliver when the
highest allowable peak currents are applied.
Torque, rms
Root Mean Square (rms) is a mathematical method to
determine a steadfast or average torque for a motor.
Torque, stall
The maximum torque without burning out the motor.
Total Indicated Reading (TIR)
The full indicator reading observed when a dial indicator
is in contact with the part surface during one full
revolution of the part about its axis of rotation.
Tuning
In a servo system, the process of optimizing loop gains
(usually PID terms) to achieve the desired response from a
stage or mechanism from an input command.
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U
Unidirectional repeatability
See
Repeatability
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V
Velocity command
Motor driver or amplifier configuration where the input
signal is commanding motor velocity. Motors with analog
tachometers are normally driven by this driver
configuration.
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W
Wobble An
irregular, non-repeatable rocking or staggering motion of
the table top of a rotary stage. Wobble is defined as an
angular error between the actual axis of rotation and the
theoretical axis of rotation.
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Y
Yaw (of travel)
Rotation about the vertical axis, perpendicular to the axis
of travel. Angular movement (error) that affects
straightness and positioning accuracy.
Yaw error
Positioning error resulting from a yaw motion.
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