Download Encoder Signal Processing

Beyond Bits
Motor Control Edition
Encoder Signal Processing
Devices for industrial applications
Introduction
Quadrature encoders are widely used
in industrial motor control applications
as precise rotor position sensors
mounted directly on the motor shaft.
This article will provide information on
the principle of the encoder operation
and the quadrature decoder MCU
module used for decoding of the
position information.
Quadrature Incremental
Encoder
processed by the on-chip hardware
of the controlling MCU.
The quadrature incremental encoder
is a position feedback device that
provides incremental counts. The
incremental encoder provides relative
information, where the feedback
signal is always referenced to a start
position. The signals are directly
The quadrature encoder typyically
has three output signals. The “phase
A” and “phase B” signals consist
of a series of pulses, the phase
shifted by 90° (therefore the term
“quadrature” is used). The third signal,
called “index,” provides the absolute
position information, in motion
Figure 1: Quadrature Encoder Output Signals and Position Encoding Modes
Phase A
Phase B
Counter
Increment/
Decrement
+1 +1 +1 +1 +1 +1 +1
+1
-1 -1 -1 -1 -1 -1
-1
+1 +1 +1 +1 +1 +1 +1
+1
-1 -1 -1 -1 -1 -1
-1
+1 +1 +1 +1 +1 +1 +1
Counter
Value
Time
Phase A
Phase B
Counter
Increment/
Decrement
+1 +1 +1
+1 +1
+1 +1 +1
-1
-1
-1
-1
-1
Counter
Value
Time
Beyond Bits
control used to check the pulse
counting consistency. That means,
after each revolution, the value of
the counted pulses is captured and
compared against the defined value.
If a difference is detected, the control
algorithm then must perform the
position offset compensation.
Quadrature Decoder
The quadrature decoder is the MCU
peripheral module used for hardware
decoding of the encoder signals
and is a standard component of
Freescale motor control dedicated
chips. Though there are differences in
the implementation of the quadrature
decoders in particular MCU families, the
principle is the same: it counts rising
and falling edges of both phase signals.
The internal logic of the quadrature
decoder evaluates the direction of the
rotation by increasing or decreasing
the counter value. There are also other
arrangements of the output encoder
signals. For example, the phase A
signal is a chain of the pulses while
the polarity of phase B determines the
counting direction, as shown in the
lower time diagram in figure 1.
Implementation of the
Quadrature Decoder
on DSCs
The special purpose enhanced
quadrature encoder/decoder module
is part of the newly introduced
MC56F84xx family of DSCs. It
provides interfacing capability to
position/speed sensors used in
industrial motor control applications
and has four input signals: phase
A, phase B, index and home. This
module is used to decode shaft
position, revolution count and speed.
It also supports different sources of
interrupt events.
Implementation of the
Quadrature Decoder
on Kinetis MCUs
The quadrature decoder is one of
several features of the FlexTimer
module that is part of the Kinetis family
of MCUs. It is not as rich in features
as the previously mentioned module
and only has two input signals, phase
A and phase B, so only the position
can be evaluated directly. If required,
processing of the index signal has to
be made by another on-chip timer.
Motor Control Edition
More information on the initialization
and configuration of the quadrature
decoders can be found in the
following application notes, design
reference manuals and device
reference manuals:
•A
N4381 (Configuring the
FlexTimer for Position and Speed
Measurement with an Encoder)
•D
RM128 (PMSM Vector Control
with Quadrature Encoder on Kinetis)
RM102 (PMSM Vector Control
•D
with Single-Shunt Current-Sensing
Using MC56F8013/23)
•M
C56F844XXRM,
K40P100M72SF1RM
These documents are available at
freescale.com/motorcontrol.
Beyond Bits
Motor Control Edition
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