Motion Control Primer
Precision MicroControl Corp.
24
Position Feedback - Quadrature Incremental Encoder
Quadrature Incremental Encoders are the default standard for providing position / velocity feedback for
today's motion control systems. A quadrature incremental encoder is an opto electric feedback device. A
light source and photo sensor pickup are used to detect markings on a glass ‘scale’. The more markings
on the glass scale, the higher the resolution of the encoder. The output of the photo sensor is passed to a
Phase Generator circuit, which is used to generate two wave forms (Phase A and Phase B), which have a
phase difference of 90 degrees. This phase difference is used by the controller
to:
Determine the direction of rotation ( positive or negative) of the encoder/motor
Enhance the resolution of the encoder by a factor of 4.
For noise immunity or applications where the encoder is positioned a significant distance from the motion
controller the encoder can use a differential driver device to output both the generated wave forms ( A+
and B+) and their compliments A- and B-). For Differential or single ended encoders, the controller
provides the user with the the option of enabling Encoder Fault detection, which will indicate an error
upon open circuit, short circuit, low differential voltage signal, and common mode range violation.
Glass scale
Phase
generation
circuitry
Phase A+
Index+
LED
Photo
sensor
Phase A-
Phase B+
Phase B-
Index-
Index mark
A 500 line quadrature incremental encoder will have 2000 quadrature counts per full rotation. The 90
degree phase difference is also used to determine the direction of motion of the axis/encoder. If phase A
comes before phase B, the controller
will indicate positive or clockwise direction. If phase B comes
before phase A, the controller will indicate negative or counter-clockwise direction.
Some quadrature encoders include an additional ‘mark’ on the glass scale, which is used to generate an
index pulse. This signal, which ‘goes active’ once per rotation, is used by the motion controller to
accurately home (re-define the position of an axis) the axis. Please refer to the
Homing Axes
section of
this chapter.
Typically an encoder requires a +5 VDC power supply and ground reference, both of which are available
from the controller .
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