Installation manual

Danaher Motion Kollmorgen Appendix C
SERVOSTAR
®
S and SERVOSTAR
®
CD Series 108
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 SERVOSTAR 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 the following
figure.