Absolute move A positioning move referencing a fixed origin position. Contrast with incremental move.
Acceleration The change in motor speed as a function of time. Acceleration (also referred to as ramping) is used in stepper motor control to achieve higher speeds than otherwise possible.
Accuracy A measure of the difference between expected position and actual position of a mechanical system. As it relates to linear travel, accuracy is usually specified as linear inches per inch.
Back EMF The voltage generated when the shaft of a permanent magnet motor is rotated. This voltage is proportional to rotational speed.
Backlash Free play in an axis assembly along the path of travel. The rotational shaft movement that results in no linear movement of the linear way.
Bipolar chopper driver A type of stepper motor driver which uses a switch mode technique to control motor current and polarity. Bipolar indicates the driver can apply current to the motor windings in either direction. This provides better efficiency than unipolar drivers.
Brushless DC motor A D.C. motor with a permanent magnet rotor and coils in the stator. The stator coil currents are sequenced by an external brushless D.C. motor controller. The main advantage to this type of motor is the elimination of EMI caused by the arcing brushes and improved motor life.
Closed loop positioning A mechanical system in which the output is measured and compared to the input. The output is then adjusted to reach the desired condition.
Detent torque Also referred to as the residual torque; it is the torque present in an unenergized stepper motor caused by its magnetic rotor. Due to the detent torque, stepper motors tend to hold their position even when unenergized.
Driver Electronic device which accepts step and direction signals and provides the high power currents and voltages to drive a stepper motor.
Efficiency The ratio of power output to power input.
Encoder An electro-mechanical device for translating the incremental angular motion of a rotating shaft into a corresponding series of digital signals. For example, a 400-line encoder generates 400 pulses for every shaft revolution. Encoders may consist of a metal wheel with notched stripes, which pass through a photodetector to produce the electronic signals.
Feedback Used in completing a closed loop system. A signal transmitted from the output of a system back to the input where it is used to adjust for any errors between desired and actual output position. (see encoder and closed loop)
Flatness The deviation from the ideal straight line travel in a vertical plane. Also referred to as the vertical run-out.
Friction A resistance to mechanical motion caused by two surfaces rubbing against each other. The three types of friction are: 1) Static; 2) Dynamic (constant with varying speed); and 3) Viscous (increases with increased speed).
Half stepping A method of driving stepper motors that improves the performance, smoothness, and resolution of the system.
Holding torque Specifies the maximum external torque that can be applied to a stopped, energized motor without causing the rotor to rotate continuously.
Home switch A limit sensor that is used to establish an initial reference point. One is used per axis.
Hybrid stepper motor A motor designed to move in discrete increments or steps. The motor is brushless and has a permanent magnet rotor and a wound stator. Motion is generated by sequencing the current to the windings.
Incremental move A positioning move where the current location is assumed to be zero.
Idle current reduction A feature of the driver that reduces the current to the motor when inactive for a set period of time. This feature reduces motor heating and saves power.
Indexer Electronic device or software which converts motion commands from a host computer, PLC, or control panel switches into a set of step and direction signals for use by the stepper motor driver. Indexers can be broadly divided into two classes. A preset indexer typically accepts distance, speed and ramp time inputs only. The more sophisticated programmable indexer is capable of complex and coordinated motion control.
Inertia A measure of an object’s resistance to a change in velocity. The force needed to accelerate or decelerate an object is directly proportional to its mass. The torque required to rotationally accelerate or decelerate a cylindrical object is directly proportional to its mass and radius.
Inertial match When the reflected system inertia at the shaft is equal to the rotor inertia of the motor. This provides a maximum transfer of power and thus efficiency.
Leadscrew pitch The number of turns a leadscrew must make to cause a linear travel of 1 inch. Common lead screw pitches are 5, 10 and 20 turns per inch.
Limit switch A mechanical, hall effect, or optical sensor used to detect end of travel on each axis. This can also be used for home referencing.
Microstepping Some stepper motor drivers employ microstepping circuitry to increase a stepper motor’s resolution and rotational smoothness by applying intermediate amounts of winding currents.
Open loop positioning A motion control system that does not use external sensors (i.e. encoders) to provide position or speed feedback signals. Most commonly used in stepper motor systems.
Pull-out torque The maximum torque that can be applied to a stepper motor running at constant speed without causing a loss of synchronism.
Pulse rate The speed of the ON-OFF signals applied to a stepper motor driver. The pulse rate, divided by the logical steps per revolution of the motor/driver combination, is the rotational speed in revolutions per second (RPS).
PWM Pulse width modulation (PWM) is an efficient electronic control technique used in stepper motor drivers to set average winding current. PWM is commonly used in high power amplifiers and power supplies.
Ramping Acceleration or deceleration of a stepping motor performed to increase the speed beyond the start-stop point of the motor. The ramp speed is dependent on the load, screw pitch, motor and drive voltage.
Repeatability A measure of the ability to repeatedly perform an identical move (or a sequence of moves).
Resolution The number of steps required for a motor’s shaft to rotate one full revolution. 200 steps/rev is the resolution of a commonly used 1.8o per step motor.
Resonance Loss of synchronism caused by mechanical limitations. Usually occurs if the natural frequency of the rotor coincides with input frequency of the pulses. Possible to overcome by use of magnetic dampener.
RS-232C A popular serial data communications protocol. This standard specifies signal levels, data formats, maximum transmission distance, etc. commonly used in micro computers and stand-alone motion, CNC, and robotic controllers.
Slew speed The constant speed portion of a move after acceleration is completed and before deceleration begins.
Start/stop speed The highest step rate that can be immediately applied to a stepper motor without loosing synchronism.
Step angle The angular increment of the motor shaft caused by a single step pulse.
Synchronism A motor is in synchronism if its rotational behavior corresponds directly to the input step pulse rate. Loss of synchronism occurs if the motor fails to rotate at the commanded speed, this is frequently the result of excessive load or very high commanded speed.
Torque A measurement of rotational force which is usually expressed in oz-in or lb-ft.
Torque-to-inertia ratio The motor’s holding torque divided by its rotor inertia. A high torque-to-inertia ratio indicates a high speed capable motor.
Unipolar driver A stepper motor driver in which the current to the motor coils always flows in the same direction. Very common due to ease of design and low cost but produces less torque because it does not fully utilize the windings of the motor.