Installation manual

Appendix B 08/04 Danaher Motion Kollmorgen
96 M-SS-001-01 Rev. T SERVOSTAR S Installation
The 90º electrical phase shift between the two channels is referred
to as quadrature-encoded. The encoder output appears as a
frequency, but the pulse rate is dependent on the motor's rotational
velocity, not time. Since the two channels are phase-shifted by 90º,
there are actually four states available per electrical cycle of these
signals. The SERVO
STAR is able to receive four counts for
position feedback for one line of motion of the encoder. The actual
decode of the four position counts-per-line of the encoder is called
quadrature decode.
Additionally, since the encoder signals A and B are phase shifted
by 90º, it is easy to design electronics that recognize whether A
came before B or B came before A, thus supplying directional
information. Encoders are often provided with an additional channel
called a Marker channel, Zero Pulse, or an Index channel - different
names for the same function. This channel outputs one pulse per
revolution and is typically an extremely narrow pulse equating to
roughly ¼ of the width of an A or B channel pulse but can be wider.
This is a reference position marker used for homing (absolute
position reference) and commutation alignment.
One challenge when using encoders is that they are incremental
rather than absolute devices. When using an absolute device (such
as a resolver) to determine the motor shaft position within the
rotation, the transmitted code is unambiguous. In other words, at
power-up, the system knows the position of the motor shaft.
Incremental encoders only detects how far the motor shaft has
moved from its original position. This presents a problem with
three-phase brushless motors in terms of commutation alignment.
Generally, it is extremely important to establish the appropriate
commutation angle within the controller. (Commutation refers to the
alignment of the electromagnetic field armature winding to the
permanent magnet fields to create optimal torque.)
For this reason, encoders or motors are often provided with
additional channels sometimes called commutation tracks or Hall
emulation tracks which provide 1-part-in-6 absolute position
information as shown in Commutation tracks.