Stepper Motor Glossary
Absolute Positioning Refers to a motion control system employing position feedback devices (absolute encoders) to maintain a given mechanical location.
Absolute Programming A positioning coordinate reference wherein all positions are specified relative to some reference, or "home," position. This is different from incremental programming, where distances are specified relative to the current position.
Acceleration Acceleration is the time rate of change of velocity with respect to a fixed reference frame. Angular acceleration is the time rate of change of angular displacement with respect to a fixed rotational reference axis. The commanded step rate is started at a base velocity and accelerated to the slew velocity at a defined and controlled rate or rate of changes.
Acceleration (Linear) Linear acceleration is the most commonly utilized form of accelerating the commanded pulse rate, and is expressed mathematically as:
a = dv/dt (constant)
For rotating bodies, the angular acceleration is the ratio of torque to inertia, and is expressed mathematically as:
a = dw/dt = Torque/Jsystem (constant)
Acceleration (Nonlinear) Nonlinear acceleration is a constantly changing acceleration of the commanded step rate and can be customized to reflect an "S-Curve" acceleration or any other required shape to provide control of the stepper motor system. The Optimal Nonlinear acceleration technique utilized in some controller designs, allow for the high acceleration rates at low commanded pulse rates where stepper motors exhibit high torque capabilities, and a reduced acceleration rate as the slew speed commanded pulse rate is achieved. Optimal nonlinear ramping techniques allow for greater torque utilization and a "faster" point-to-point positioning than for linear acceleration techniques.
Accuracy A measure of the difference between expected position and actual position of a motor or mechanical system. stepper motor accuracy is usually specified an angle representing the maximum deviation from expected position.
Ambient Temperature The temperature of the cooling medium, usually air, immediately surrounding the motor or another device.
ASCII (American Standard Code for Information Interchange) This code assigns a number to each numeral and letter of the alphabet. In this manner, alphanumeric information can be transmitted between machines as a series of binary numbers.
Automation The implementation of processes by automatic means. The theory, art, or technique of making a process more automatic. The investigation, design, development and application of methods of rendering processes automatic, self-moving or self-controlling.
Axial Play (End Play) The shaft displacement axially, due to a reversal of an axial force.
Back (End of Motor) This is considered the output end, the end which drives the load.
Bandwidth The frequency range in which the magnitude of the system gain expressed in dB is greater than -3dB.
Baud A unit of signaling speed equal to the number of code elements per second.
BCD (Binary Coded Decimal) An encoding technique used to describe the numbers 0 through 9 with four digital (on or off) signal lines. Popular in machine tool equipment, BCD interfaces are now giving way to interfaces requiring fewer wires, such as RS232C.
Back EMF (Back Electro-Motive Force) A reversed bias generated by rotation of the magnetic field (rotor of a hybrid motor), across a stator's windings.
Base Speed Response range of a motor to commanded pulses over which the unloaded motor can accelerate to command pulse rate from standstill, decelerate from command pulse rate to standstill, and reverse direction (on command) without loss of synchronism.
Bifilar Winding Refers to the winding configuration of a stepper motor where each stator pole has a pair of windings, (4 electrical phases), the motor will have either 4, 6 or 8 lead wires depending on termination. This winding configuration can be driven from a unipolar or bipolar driver design.
Bipolar Drive Refers to specific type driver that is connected to a stepper motor configured for a 2 phase operation. The 4 electrical cycles required for operation are generated when the direction of current is reversed in each motor phase. A bipolar driver can be utilized with a 4, 6 or 8 lead motor.
Bit An abbreviation of binary digit. A single character in a binary number. A single pulse in a group of pulses. A unit of information capacity of a storage device.
Block Diagram A simplified schematic representing components and signal flow through a system.
Bode Plot A graph of system gain and phase versus input frequency, which graphically illustrates the steady state characteristics of the system.
Break Frequency Frequency(ies) at which the gain changes slope on a Bode Plot. Break Frequencies correspond to the poles and zeros of the system.
Buffer A storage device used to compensate for a difference in rate of flow of data, or time of occurrence of events, when transmitting data from one device to another.
Bus A circuit over which data or power is transmitted. Often one which acts as a common connection among a number of locations. Synonymous with trunk. A communications path between two switching points.
Byte A group of eight bits treated as a whole, with 256 possible combinations of one's and zero's, each combination representing a unique piece of information.
Clock A pulse generator, which controls the timing of switching circuits that control the speed of the stepper motor.
Closed Loop System A stepper motor system can be operated in a closed loop application where the output is measured and compared to the input. The output is then adjusted to reach the desired input condition. In motion control, this term is used to describe a system wherein a velocity or position sensor is used to generate signals for comparison to desired parameters. For cases where loads are not predictable, the closed loop feedback from an external encoder to the controller may be used for stall detection, position verification or position maintenance.
Command An electronic pulse, signal, or set of signals to start, stop, or continue some operation.
Compliant Coupling Complying, yielding. Limited motion of one shaft without motion of coupled shaft. Does not permit permanent displacement of one shaft with respect to the other.
Constant Current Drive Device or control for adjusting the voltage to force and maintain design current in the winding when switching from one winding to another.
Controller (stepper motor) A regulating mechanism, essentially a DC power supply, plus power switching and circuits for controlling the proper step sequence.
Counter A device capable of changing states in a specified sequence upon receiving appropriate input signals. The output of the counter indicates the number of pulses that have been applied.
Critical Damping A system is critically damped when the response to an incremental change in desired velocity or position is achieved in a minimum possible time with little or no overshoot.
Crossover Frequency The frequency at which the gain intercepts the 0dB point on a Bode plot. Used in reference to the open-loop gain plot.
Cycle (Incremental Motion) One of a sequence or series of identical events. Includes starting, moving and stopping of the mechanism.
Daisychain The term daisychain is used to describe the linking of several RS-232/422/485 devices in sequence, such that a single data stream flows through one device and on to the next. The devices are usually distinguished by device addresses which serve to indicate the desired destination for the data in the stream.
Damper A device that attaches to the stepper motor shaft to absorb energy. It is useful in damping step oscillations and preventing resonances.
Damping An indication of the rate of decay of a signal to its' steady state value. Related to settling time. Suppression of oscillations at the end of motion or during motion.
Damping Ratio Ratio of actual damping to critical damping. Less than one is an under-damped system and greater than one is an over-damped system.
Dead Range or Dead Band The Dead Band window is the range of input signals for which there is no system response.
The angle between clockwise and counterclockwise limits to which the rotor of an energized stepper motor can stop due to internal and external friction.
Deadbeat (Response) Critically damped. Moving from one step position to another without overshoot or oscillation.
Decibel A logarithmic measurement of gain. If G is a systems gain (ratio of output to input) then 20LogG = gain in decibels (dB).
Decimal, Binary Coded A decimal notation in which each individual decimal digit is represented by a pattern of ones and zeros; e.g. in the 8-4-2-1 coded decimal notation the number twelve is represented as 0001 0010 for one and 2 respectively; whereas, in pure or straight binary notation, 12 is represented by 1100.
Deceleration See Acceleration.
Detent Position This position is the static angular position in which the shaft of an unloaded and unenergized stepper motor comes to rest.
Detent Torque Sometimes noted as "Cogging Torque," is the periodic torque ripple resulting from the tendency of the magnetic rotor poles and stator poles to align themselves to positions of minimal reluctance. The measurement is taken with all phases de-energized.
Digital Means to operate in the manner of a switch, meaning in one of two states, either "on" or "off." Could also be two distinct states or levels.
Diode A device used to permit current flow in one direction in a circuit and to inhibit current flow in the other.
Direction of Rotation The direction the shaft rotates when viewed from the mounting shaft end. The "standard" (positive) direction is defined as counterclockwise.
Driver (stepper motor) Often referred to as a translator. Drives a stepper motor based on pulses from a clock source, pulse generator, or computer. Translates the train of pulses and applies power to the appropriate stepper motor windings.
Duty Cycle For a repetitive cycle, the ratio of on time to total cycle time.
Duty Cycle = On Time
On Time + Off Time
Dynamic Energy in motion, effective action; active, such as in dynamic torque, which indicates the torque while the stepper motor is producing motion.
Dynamic Torque The torque developed by a motor at low stepping rates.
Efficiency The ratio of power output to power input, expressed in like units; watts, for example.
Electronic Damping A means of suppressing oscillation of the stepper motor output by switching the motor winding in sequence such that the motor and load have come to rest when the final step position has been reached.
Encoder An encoder is an electromechanical device which translates mechanical motion into electronic signals utilized by the system controller for monitoring position or velocity, (examples: position maintenance, stall detect and home on encoder Z channel). Sometimes called a pulse generator. It consists of a disc, vane or reflector attached to a stepper motor shaft to provide digital pulses to the system controller and or counters.
End Play (Axial Play) The axial shaft displacement, due to reversal of an axial force.
Excitation Current or voltage applied to the stepper motor in order to provide motive power or to hold the rotor in a particular place.
Feedback (Loops, Systems/Transducers) In a closed-loop system, a device that detects the condition being controlled and relates the information back (feedback) to the controlling device or system for comparison with the input values.
Friction (Drag or Coulomb) Friction is defined as the resistance to motion caused by surfaces rubbing together. Friction can be a constant with varying speed (Coulomb) or proportional to speed (Viscous). Limits top speed of stepper motor, slows down acceleration, increases positional error, but enables the motor to stop in less time with minimal oscillations.
Friction Torque In a stepper motor, the bearing friction, usually called coulomb or drag friction, is a representative friction torque component. In a permanent magnet stepper motor, a cog friction torque is also present and is caused by the magnetic drag between the permanent magnet in the rotor assembly and the stator lamination teeth. A viscous friction torque is also possible and is a function of drag torque, proportional to changing rotor speeds.
Friction (Viscous) A resistance to motion, proportional to velocity.
Gain The ratio of system output signal to system input signal.
Gate A circuit whose output signal is dependent on some function of its input signals.
Holding Torque The maximum torque that can be externally applied to the stepper motor shaft without causing continuous rotation when one or more phases of the motor are energized.
Home A reference position in a motion control system, usually
derived from a mechanical datum. Often designated as the "zero" point.
Hysteresis The difference in response of a system to an increasing or decreasing input signal.
IEEE-488 A digital data communications standard popular in instrumentation electronics. This parallel interface is also known as GPIB, or Generic Purpose Interface Bus.
Incremental Motion A motion control term that is used to describe a device that produces on step of motion for each step command (usually a pulse) received. Motion made up of starts, moves, and stops. Motion caused by pulses. A small envelope or program of steps.
Incremental Programming A coordinate system where positions or distances are specified relative to the current position.
Inductance (Mutual) Mutual inductance is the property that exists between two current-carrying conductors or coils when magnetic lines of force from one conductor or coil are linked with those of the other.
Inductance (Self) The self-inductance of a coil is the constant by which the time rate of change of the current in the coil must be multiplied to give the self-induced counter EMF.
Inertia A measure of an object's resistance to a change in velocity. The larger an object's inertia, the greater the torque required to accelerate or decelerate it. Inertia is a function of an object's mass and shape. For the most efficient operation, the system coupling ratio should be selected so that the reflected inertia of the load is equal to or no greater than 10 times the rotor inertia of the stepper motor.
Inertia (Reflected) Inertia as seen by the stepper motor when driving through a speed change, reducer or gear train.
Inertia Match (See Inertia)
Input-Output The equipment used to communicate with a computer. Also, the data involved in the communication. Synonymous with (I/O).
LCD Digital instruments employ LCD (Liquid Crystal Display) readouts because they utilize minuscule amounts of power, thereby making them excellent for battery-operated instruments. LCDs are best in high ambient light levels, as they do not wash out but instead gain greater contrast in bright light.
Lead(1) A wire or terminal of the stepper motor internally connected to the motor phase windings and externally connected to the driver output(s) terminals.
Lead(2) The axial distance a nut on a leadscrew travels during one revolution of the lead screw, e.g. in./rev. The inverse of pitch.
Lead Compensation Algorithm A mathematical equation implemented by a computer to decrease the delay between the input and output of a system.
LED Light pipe LED (Light Emitting Diode) displays provide a bright, clear numeric presentation of readings in digital instruments. They generally are best for indoor environments, and can be viewed from a greater distance in normal lighting conditions. Most LED displays are red, but are also available in yellow and green.
Limits A properly designed stepper motor system has sensors called limits that alert the control electronics that a physical end-of-travel is being approached and that the motion is not allowed in a specific direction.
Linear Motion in a straight line.
Load Any external static or dynamic resistance to motion that is applied to the motor. The characteristics of the load can be defined as: Coulomb Friction, Viscous Friction, Inertial, etc.
Load Angle
Static Load Angle Static Load Angle is the angle through which the rotor is displaced from it's energized stable equilibrium position by a given applied torque at a specified current.
Dynamic Load Angle The Dynamic Load Angle is the angle between the loaded and unloaded position (theoretical zero) of the rotor at a given instant under otherwise identical conditions at a specified command pulse rate, mode of winding excitation and phase current.
Logic Ground The logic ground is the reference "zero" voltage to which a group of control signals in a particular system are referenced.
Loop, Closed (See Closed-Loop)
Matched Load (See Inertia)
Maximum Running Torque The maximum torque load that the motor can drive without missing a step. This typically occurs when the windings are sequentially energized at approximately 5 pulses-per-second.
Maximum Slew Rate The maximum slew rate is the maximum velocity at which the unloaded stepper motor can remain synchronous with the command pulses under the specified drive conditions. This velocity is usually defined in the Full Step Mode of 1.8° steps or as shaft speed in revolutions per second.
Maximum Safe Operating Temperature The maximum temp-erature the stepper motor, either continuously or intermittently rated, may safely be allowed to achieve (measured by the change of winding resistance method). They may bear little or no relation to the class on insulation needed in the construction of the motor, but may be dictated by considerations such as bearing lubricant, etc.
Maximum Start-Stop Rate The maximum switching rate at which an unloaded stepper motor can start and run without losing sychronism (missing steps) or stop without taking more steps than pulses.
Microsecond One millionth of a second.
Microstepping Microstepping refers to a control technique that proportions the current in a stepper motor's windings to provide additional intermediate positions between poles. The advantages of microstepping is the smooth rotation with a reduction of system resonances over a wide speed range and semi-high positional resolution.
Millisecond One thousandth of a second.
Mini-Stepping (See Microstepping)
Mode A particular sequence of excitation defined by the drive circuit, which, when applied to a stepper motor, will energize certain windings or phases.
Multi-Level Drive (See Bilevel Drive)
Nanosecond One billionth of a second.
Noncumulative Error An error that does not repeat or accumulate for multiple steps or increments.
Open Collector A term used to describe a signal output that is performed with a transistor. An open collector output acts like a switch closure with one end of the switch at ground potential and the other end of the switch accessible. Also called Open Drain.
Open Loop System An open loop stepper motor system refers to a system where no external sensors are used to provide position or velocity feedback signals, such as encoder feedback of position. When an application is selected that consists of loads without discontinuity and the proper motor and drive is utilized for positional accuracy, the motor will remain in synchronism with the command pulse rate and the expected results will occur.
Opto-Isolated A method of sending a signal from one piece of equipment to another without the usual requirement of common ground potentials. The signal is transmitted optically with a light source (usually a Light Emitting Diode) and a light sensor (usually a photosensitive transistor). These optical components provide electrical isolation.
Overshoot (Permanent) The amount (in degrees) that the shaft of a stepper motor remains beyond the commanded position.
Overshoot (Transient) The Overshoot (transient) is the peak angular distance the shaft of the stepper motor rotates beyond the actual final position under the specified drive and load conditions.
Parallel Refers to a data communication format wherein many signal lines are used to communicate more than one piece of data at the same time.
Permanent Magnet stepper motor A stepper motor having permanent-magnet poles.
Permeance Conducting power of a magnetic circuit for magnetic flux.
Phase Angle The angle at which the steady state input signal to a system leads the output signal.
Phase Angle Rotor-Stator The angle of lag of the rotor to the axis of the stator magnetic field under load. The angle of lag between rotor and stator teeth under load.
Phase Margin The difference between 180 degrees and the phase angle of a system at its crossover frequency.
Phase (stepper motor) A motor phase is a set of electrically excited stator poles, consisting of one or more pairs of oppositely polarized poles. stepper motor manufacturers provide 4 lead motors with bifilar ratings and 6 or 8 lead motors with unifilar ratings. (See the section on "Speed/Torque Relation- ships" for benefits on driving a motor with a unifilar or bifilar winding configuration).
Positional Error Position error (sometimes designated "Absolute Accuracy") is the deviation from the theoretically correct angular position of any step position in a complete revolution. The zero position used in determining the theoretically correct angular position shall be the midpoint between the two extremes of position error. It is expressed as either percentage of the nominal full step or as an angular measure and is noncumulative. It is measured under rated motor conditions.
Pitch See Lead
Pole That part of the magnetic circuit where a magnetic pole is generated either by a permanent magnet or by windings. A frequency at which the transfer function of a system goes to infinity.
Power (Motor Heat Dissipation) The heat generated by a stepper motor during standstill operation or while responding to a command pulse rate is expressed by the mathematical equation:
P (watts) = I2 R for single phase operation
P (watts) = (I2 R)2 for dual phase or microstep operation
Where the square of the drive output current (I) times the motor resistance (R) is the dissipated heat generated in the motor.
Power (RMS) The "Root Mean Square" power of a stepper motor is the effective value of time varying power consumption of the stepper motor.
Pull-In Step Rate The pull-in step rate or error-free start speed is the maximum command pulse rate (constant) at which the energized stepper motor can accelerate an applied load from standstill to command pulse rate, and run synchronously without missing any steps.
Pull-In Torque The pull-in torque is the maximum positive coulomb friction torque which an energized stepper motor will accelerate to command pulse rate and run synchronously without missing any steps.
Pull-Out Step Rate The pull-out step rate is the maximum command pulse rate (constant) at which the energized stepper motor can run in synchronism.
Pull-Out Torque Pull-out torque is the maximum positive coulomb friction torque which can be applied to the rotating
shaft of a stepper motor (already running at Slew Speed) at a given pulse rate without missing any steps.
Pulse Width Modulation (PWM) Refers to a technique of con-trolling the average current in a motor winding by varying the duty cycle of an applied voltage.
Ramping See Acceleration.
Resonance Since a stepper motor system is a discrete incremental positioning system, it is subject to the effects of resonance, where the system is operated at this given frequency, it may begin to oscillate. Primary resonance frequency occurs at about one revolution per second. This oscillating will cause a loss of effective torque and may result in loss of synchronism. When an application is being considered, the design should allow for working outside the primary resonance frequency or by utilizing half-step or microstep techniques to reduce or shift the resonance frequency. The resonance frequency may also be shifted by changing the system friction or inertia.
Ringing Refers to the oscillation resulting in a system following a sudden change in velocity or position state.
Settling Time Refers to the total time from the application of the last pulse signal until the amplitude of the oscillatory motion of the rotor has diminished to a specified level under certain conditions.
Slew Refers to the position of a move profile where the motor is operating at a constant velocity.
Static Torque This is the peak torque that can be applied to the shaft of an energized motor at standstill, also called "holding torque". The mode of winding excitation and applied current shall be specified.
stepper motor A stepper motor is a polyphase synchronous inductor motor, the rotor of which rotates in discrete angular increments when the stator windings thereof are energized in a programmed manner either by appropriately timed DC states or by polyphase AC states. Rotation occurs because of the magnetic interaction between the rotor poles and the poles of the sequentially energized stator phases.
Variable Reluctance (VR) A variable reluctance stepper motor utilizes a rotor which has pole salients (soft iron) without magnetic bias in the de-energized state.
Permanent Magnetic (PM) A permanent magnet stepper motor utilizes a rotor which has magnetized poles.
Hybrid (HY) A hybrid stepper motor utilizes a permanent magnet to polarize soft iron pole pieces.
Stiffness (Sometimes called "Torque Gradient") is the derivative (slope) of the torque-verse-angle curve. The curve is the sum of the stiffness due to holding torque and detent torque.
Synchronism Synchronism exists when the motor's output is correctly corresponding to the system's input signals. Load torques exceeding the motor's capabilities will cause loss of synchronism. This condition will not damage the stepper motor.
Thermal Resistance Thermal resistance is the opposition to the flow of heat in the materials of which the motor is constructed. It is expressed as degrees Celsius per watt. All measurements are taken after steady state conditions have been achieved and without heatsinking in still air.
Thermal Resistance (Winding to Frame) This is the measured difference in temperature between the winding and the specified point on the surface of the motor divided by the total electrical Power input to the motor.
Thermal Resistance (Frame to Air) This is the same as
Thermal Resistance (Winding to Frame), except that the temperature difference is
the temperature at a specified point on the surface of the motor and the ambient
air surrounding the motor.
Thermal Time Constant This is the time required for the winding temperature of a motor to reach 63% of its' steady state temperature rise with constant power applied to the motor. It is measured by allowing the motor to reach steady state temperature and then disconnecting the electrical power input. The winding temperature is recorded as a function of time; zero time being the time at which the power source is disconnected. The time required to drop 37% of the steady-state temperature rise is the thermal time constant. Usually expressed in seconds, conditions will be specified.
Torque Gradient See Stiffness.
Translator Logic Translator logic (Driver Logic) converts the signal channel pulse train-into multichannel states to be applied to the power amplifier (Driver) which energizes the motor.
Unifilar Winding The term Unifilar winding refers to the winding configuration of a stepper motor where each stator pole has one set of windings, (4 electrical phases), the motor will have only 4 lead wires. This winding configuration can only be driven from a bipolar driver design.
Unipolar Drive The term Unipolar refers to specific type driver that is connected to a stepper motor configured for 4 phase operation. A unipolar driver can only be utilized with a bifilar wound motor, 6 or 8 leads.
Winding Inductance The winding inductance of a stepper motor winding varies both with rotor position and with excitation current. Measurements can also be effected by the rate of change of current; thus, when a figure for inductance is given, the conditions under which the measurements were taken must be quoted.
Winding Inductance, Incremental Unenergized An inductance bridge having a test frequency of 1KHz 1 volt RMS open circuit voltage is used to make this measurement. The inductance is measured with the rotor locked in the "aligned" or "unaligned" position, with no DC current applied to any of the windings, the conditions will be stated.
Winding Resistance Winding resistance is the lead-to-lead (terminal-to-terminal) ohmic resistance measured with the windings at 25°C.
Unipolar Drive Excitation applied such that torque generating current in each winding occurs in one direction only. The polarity of voltage to each winding is always the same.
Viscous Damping A damping that provides a retarding torque during motion. At zero velocity there is no retarding torque. The higher the velocity, the higher the retarding torque.
Zero A frequency at which the transfer function of a system goes to zero.
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