| 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. |
