Glossary of Terms
Ndrive HP 10/20/30 Manual
A-2
www.aerotech.com
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.
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
Non-uniform angular/linear velocity. Cogging appears as a
jerkiness, especially at low speeds, and is due to magnetic poles
attracting 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 motor’s back EMF.
The result is smoother control and less ripple. Aerotech’s BA
series self-commutate using this method.
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 motor’s 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. All Aerotech controllers, as
well as the BAS series amplifiers, commutate using this method.
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