Quadrature encoder interface library.

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API Documentation at this revision

Comitter:
aberk
Date:
Thu Sep 02 16:48:55 2010 +0000
Commit message:
Added X2 encoding, which the library now uses by default. This reduces the number of interrupts generated.

Changed in this revision

QEI.cpp Show annotated file Show diff for this revision Revisions of this file
QEI.h Show annotated file Show diff for this revision Revisions of this file
diff -r 000000000000 -r 5c2ad81551aa QEI.cpp
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/QEI.cpp	Thu Sep 02 16:48:55 2010 +0000
@@ -0,0 +1,289 @@
+/**
+ * @author Aaron Berk
+ *
+ * @section LICENSE
+ *
+ * Copyright (c) 2010 ARM Limited
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ *
+ * @section DESCRIPTION
+ *
+ * Quadrature Encoder Interface.
+ *
+ * A quadrature encoder consists of two code tracks on a disc which are 90
+ * degrees out of phase. It can be used to determine how far a wheel has
+ * rotated, relative to a known starting position.
+ *
+ * Only one code track changes at a time leading to a more robust system than
+ * a single track, because any jitter around any edge won't cause a state
+ * change as the other track will remain constant.
+ *
+ * Encoders can be a homebrew affair, consisting of infrared emitters/receivers
+ * and paper code tracks consisting of alternating black and white sections;
+ * alternatively, complete disk and PCB emitter/receiver encoder systems can
+ * be bought, but the interface, regardless of implementation is the same.
+ *
+ *               +-----+     +-----+     +-----+
+ * Channel A     |  ^  |     |     |     |     |
+ *            ---+  ^  +-----+     +-----+     +-----
+ *               ^  ^
+ *               ^  +-----+     +-----+     +-----+
+ * Channel B     ^  |     |     |     |     |     |
+ *            ------+     +-----+     +-----+     +-----
+ *               ^  ^
+ *               ^  ^
+ *               90deg
+ *
+ * The interface uses X2 encoding by default which calculates the pulse count
+ * based on reading the current state after each rising and falling edge of
+ * channel A.
+ *
+ *               +-----+     +-----+     +-----+
+ * Channel A     |     |     |     |     |     |
+ *            ---+     +-----+     +-----+     +-----
+ *               ^     ^     ^     ^     ^
+ *               ^  +-----+  ^  +-----+  ^  +-----+
+ * Channel B     ^  |  ^  |  ^  |  ^  |  ^  |     |
+ *            ------+  ^  +-----+  ^  +-----+     +--
+ *               ^     ^     ^     ^     ^
+ *               ^     ^     ^     ^     ^
+ * Pulse count 0 1     2     3     4     5  ...
+ *
+ * This interface can also use X4 encoding which calculates the pulse count
+ * based on reading the current state after each rising and falling edge of
+ * either channel.
+ *
+ *               +-----+     +-----+     +-----+
+ * Channel A     |     |     |     |     |     |
+ *            ---+     +-----+     +-----+     +-----
+ *               ^     ^     ^     ^     ^
+ *               ^  +-----+  ^  +-----+  ^  +-----+
+ * Channel B     ^  |  ^  |  ^  |  ^  |  ^  |     |
+ *            ------+  ^  +-----+  ^  +-----+     +--
+ *               ^  ^  ^  ^  ^  ^  ^  ^  ^  ^
+ *               ^  ^  ^  ^  ^  ^  ^  ^  ^  ^
+ * Pulse count 0 1  2  3  4  5  6  7  8  9  ...
+ *
+ * It defaults
+ *
+ * An optional index channel can be used which determines when a full
+ * revolution has occured.
+ *
+ * If a 4 pules per revolution encoder was used, with X4 encoding,
+ * the following would be observed.
+ *
+ *               +-----+     +-----+     +-----+
+ * Channel A     |     |     |     |     |     |
+ *            ---+     +-----+     +-----+     +-----
+ *               ^     ^     ^     ^     ^
+ *               ^  +-----+  ^  +-----+  ^  +-----+
+ * Channel B     ^  |  ^  |  ^  |  ^  |  ^  |     |
+ *            ------+  ^  +-----+  ^  +-----+     +--
+ *               ^  ^  ^  ^  ^  ^  ^  ^  ^  ^
+ *               ^  ^  ^  ^  ^  ^  ^  ^  ^  ^
+ *               ^  ^  ^  +--+  ^  ^  +--+  ^
+ *               ^  ^  ^  |  |  ^  ^  |  |  ^
+ * Index      ------------+  +--------+  +-----------
+ *               ^  ^  ^  ^  ^  ^  ^  ^  ^  ^
+ * Pulse count 0 1  2  3  4  5  6  7  8  9  ...
+ * Rev.  count 0          1           2
+ *
+ * Rotational position in degrees can be calculated by:
+ *
+ * (pulse count / X * N) * 360
+ *
+ * Where X is the encoding type [e.g. X4 encoding => X=4], and N is the number
+ * of pulses per revolution.
+ *
+ * Linear position can be calculated by:
+ *
+ * (pulse count / X * N) * (1 / PPI)
+ *
+ * Where X is encoding type [e.g. X4 encoding => X=44], N is the number of
+ * pulses per revolution, and PPI is pulses per inch, or the equivalent for
+ * any other unit of displacement. PPI can be calculated by taking the
+ * circumference of the wheel or encoder disk and dividing it by the number
+ * of pulses per revolution.
+ */
+
+/**
+ * Includes
+ */
+#include "QEI.h"
+
+QEI::QEI(PinName channelA,
+         PinName channelB,
+         PinName index,
+         int pulsesPerRev,
+         Encoding encoding) : channelA_(channelA), channelB_(channelB),
+        index_(index) {
+
+    pulses_       = 0;
+    revolutions_  = 0;
+    pulsesPerRev_ = pulsesPerRev;
+    encoding_     = encoding;
+
+    //Workout what the current state is.
+    int chanA = channelA_.read();
+    int chanB = channelB_.read();
+
+    //2-bit state.
+    currState_ = (chanA << 1) | (chanB);
+    prevState_ = currState_;
+
+    //X2 encoding uses interrupts on only channel A.
+    //X4 encoding uses interrupts on      channel A,
+    //and on channel B.
+    channelA_.rise(this, &QEI::encode);
+    channelA_.fall(this, &QEI::encode);
+
+    //If we're using X4 encoding, then attach interrupts to channel B too.
+    if (encoding == X4_ENCODING) {
+        channelB_.rise(this, &QEI::encode);
+        channelB_.fall(this, &QEI::encode);
+    }
+    //Index is optional.
+    if (index !=  NC) {
+        index_.rise(this, &QEI::index);
+    }
+
+}
+
+void QEI::reset(void) {
+
+    pulses_      = 0;
+    revolutions_ = 0;
+
+}
+
+int QEI::getCurrentState(void) {
+
+    return currState_;
+
+}
+
+int QEI::getPulses(void) {
+
+    return pulses_;
+
+}
+
+int QEI::getRevolutions(void) {
+
+    return revolutions_;
+
+}
+
+// +-------------+
+// | X2 Encoding |
+// +-------------+
+//
+// When observing states two patterns will appear:
+//
+// Counter clockwise rotation:
+//
+// 10 -> 01 -> 10 -> 01 -> ...
+//
+// Clockwise rotation:
+//
+// 11 -> 00 -> 11 -> 00 -> ...
+//
+// We consider counter clockwise rotation to be "forward" and
+// counter clockwise to be "backward". Therefore pulse count will increase
+// during counter clockwise rotation and decrease during clockwise rotation.
+//
+// +-------------+
+// | X4 Encoding |
+// +-------------+
+//
+// There are four possible states for a quadrature encoder which correspond to
+// 2-bit gray code.
+//
+// A state change is only valid if of only one bit has changed.
+// A state change is invalid if both bits have changed.
+//
+// Clockwise Rotation ->
+//
+//    00 01 11 10 00
+//
+// <- Counter Clockwise Rotation
+//
+// If we observe any valid state changes going from left to right, we have
+// moved one pulse clockwise [we will consider this "backward" or "negative"].
+//
+// If we observe any valid state changes going from right to left we have
+// moved one pulse counter clockwise [we will consider this "forward" or
+// "positive"].
+//
+// We might enter an invalid state for a number of reasons which are hard to
+// predict - if this is the case, it is generally safe to ignore it, update
+// the state and carry on, with the error correcting itself shortly after.
+void QEI::encode(void) {
+
+    int change = 0;
+    int chanA  = channelA_.read();
+    int chanB  = channelB_.read();
+
+    //2-bit state.
+    currState_ = (chanA << 1) | (chanB);
+
+    if (encoding_ == X2_ENCODING) {
+
+        //11->00->11->00 is counter clockwise rotation or "forward".
+        if ((prevState_ == 0x3 && currState_ == 0x0) ||
+                (prevState_ == 0x0 && currState_ == 0x3)) {
+
+            pulses_++;
+
+        }
+        //10->01->10->01 is clockwise rotation or "backward".
+        else if ((prevState_ == 0x2 && currState_ == 0x1) ||
+                 (prevState_ == 0x1 && currState_ == 0x2)) {
+
+            pulses_--;
+
+        }
+
+    } else if (encoding_ == X4_ENCODING) {
+
+        //Entered a new valid state.
+        if (((currState_ ^ prevState_) != INVALID) && (currState_ != prevState_)) {
+            //2 bit state. Right hand bit of prev XOR left hand bit of current
+            //gives 0 if clockwise rotation and 1 if counter clockwise rotation.
+            change = (prevState_ & PREV_MASK) ^ ((currState_ & CURR_MASK) >> 1);
+
+            if (change == 0) {
+                change = -1;
+            }
+
+            pulses_ -= change;
+        }
+
+    }
+
+    prevState_ = currState_;
+
+}
+
+void QEI::index(void) {
+
+    revolutions_++;
+
+}
diff -r 000000000000 -r 5c2ad81551aa QEI.h
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/QEI.h	Thu Sep 02 16:48:55 2010 +0000
@@ -0,0 +1,244 @@
+/**
+ * @author Aaron Berk
+ *
+ * @section LICENSE
+ *
+ * Copyright (c) 2010 ARM Limited
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ *
+ * @section DESCRIPTION
+ *
+ * Quadrature Encoder Interface.
+ *
+ * A quadrature encoder consists of two code tracks on a disc which are 90
+ * degrees out of phase. It can be used to determine how far a wheel has
+ * rotated, relative to a known starting position.
+ *
+ * Only one code track changes at a time leading to a more robust system than
+ * a single track, because any jitter around any edge won't cause a state
+ * change as the other track will remain constant.
+ *
+ * Encoders can be a homebrew affair, consisting of infrared emitters/receivers
+ * and paper code tracks consisting of alternating black and white sections;
+ * alternatively, complete disk and PCB emitter/receiver encoder systems can
+ * be bought, but the interface, regardless of implementation is the same.
+ *
+ *               +-----+     +-----+     +-----+
+ * Channel A     |  ^  |     |     |     |     |
+ *            ---+  ^  +-----+     +-----+     +-----
+ *               ^  ^
+ *               ^  +-----+     +-----+     +-----+
+ * Channel B     ^  |     |     |     |     |     |
+ *            ------+     +-----+     +-----+     +-----
+ *               ^  ^
+ *               ^  ^
+ *               90deg
+ *
+ * The interface uses X2 encoding by default which calculates the pulse count
+ * based on reading the current state after each rising and falling edge of
+ * channel A.
+ *
+ *               +-----+     +-----+     +-----+
+ * Channel A     |     |     |     |     |     |
+ *            ---+     +-----+     +-----+     +-----
+ *               ^     ^     ^     ^     ^
+ *               ^  +-----+  ^  +-----+  ^  +-----+
+ * Channel B     ^  |  ^  |  ^  |  ^  |  ^  |     |
+ *            ------+  ^  +-----+  ^  +-----+     +--
+ *               ^     ^     ^     ^     ^
+ *               ^     ^     ^     ^     ^
+ * Pulse count 0 1     2     3     4     5  ...
+ *
+ * This interface can also use X4 encoding which calculates the pulse count
+ * based on reading the current state after each rising and falling edge of
+ * either channel.
+ *
+ *               +-----+     +-----+     +-----+
+ * Channel A     |     |     |     |     |     |
+ *            ---+     +-----+     +-----+     +-----
+ *               ^     ^     ^     ^     ^
+ *               ^  +-----+  ^  +-----+  ^  +-----+
+ * Channel B     ^  |  ^  |  ^  |  ^  |  ^  |     |
+ *            ------+  ^  +-----+  ^  +-----+     +--
+ *               ^  ^  ^  ^  ^  ^  ^  ^  ^  ^
+ *               ^  ^  ^  ^  ^  ^  ^  ^  ^  ^
+ * Pulse count 0 1  2  3  4  5  6  7  8  9  ...
+ *
+ * It defaults
+ *
+ * An optional index channel can be used which determines when a full
+ * revolution has occured.
+ *
+ * If a 4 pules per revolution encoder was used, with X4 encoding,
+ * the following would be observed.
+ *
+ *               +-----+     +-----+     +-----+
+ * Channel A     |     |     |     |     |     |
+ *            ---+     +-----+     +-----+     +-----
+ *               ^     ^     ^     ^     ^
+ *               ^  +-----+  ^  +-----+  ^  +-----+
+ * Channel B     ^  |  ^  |  ^  |  ^  |  ^  |     |
+ *            ------+  ^  +-----+  ^  +-----+     +--
+ *               ^  ^  ^  ^  ^  ^  ^  ^  ^  ^
+ *               ^  ^  ^  ^  ^  ^  ^  ^  ^  ^
+ *               ^  ^  ^  +--+  ^  ^  +--+  ^
+ *               ^  ^  ^  |  |  ^  ^  |  |  ^
+ * Index      ------------+  +--------+  +-----------
+ *               ^  ^  ^  ^  ^  ^  ^  ^  ^  ^
+ * Pulse count 0 1  2  3  4  5  6  7  8  9  ...
+ * Rev.  count 0          1           2
+ *
+ * Rotational position in degrees can be calculated by:
+ *
+ * (pulse count / X * N) * 360
+ *
+ * Where X is the encoding type [e.g. X4 encoding => X=4], and N is the number
+ * of pulses per revolution.
+ *
+ * Linear position can be calculated by:
+ *
+ * (pulse count / X * N) * (1 / PPI)
+ *
+ * Where X is encoding type [e.g. X4 encoding => X=44], N is the number of
+ * pulses per revolution, and PPI is pulses per inch, or the equivalent for
+ * any other unit of displacement. PPI can be calculated by taking the
+ * circumference of the wheel or encoder disk and dividing it by the number
+ * of pulses per revolution.
+ */
+
+#ifndef QEI_H
+#define QEI_H
+
+/**
+ * Includes
+ */
+#include "mbed.h"
+
+/**
+ * Defines
+ */
+#define PREV_MASK 0x1 //Mask for the previous state in determining direction
+//of rotation.
+#define CURR_MASK 0x2 //Mask for the current state in determining direction
+//of rotation.
+#define INVALID   0x3 //XORing two states where both bits have changed.
+
+/**
+ * Quadrature Encoder Interface.
+ */
+class QEI {
+
+public:
+
+    typedef enum Encoding {
+
+        X2_ENCODING,
+        X4_ENCODING
+
+    } Encoding;
+
+    /**
+     * Constructor.
+     *
+     * Reads the current values on channel A and channel B to determine the
+     * initial state.
+     *
+     * Attaches the encode function to the rise/fall interrupt edges of
+     * channels A and B to perform X4 encoding.
+     *
+     * Attaches the index function to the rise interrupt edge of channel index
+     * (if it is used) to count revolutions.
+     *
+     * @param channelA mbed pin for channel A input.
+     * @param channelB mbed pin for channel B input.
+     * @param index    mbed pin for optional index channel input,
+     *                 (pass NC if not needed).
+     * @param pulsesPerRev Number of pulses in one revolution.
+     * @param encoding The encoding to use. Uses X2 encoding by default. X2
+     *                 encoding uses interrupts on the rising and falling edges
+     *                 of only channel A where as X4 uses them on both
+     *                 channels.
+     */
+    QEI(PinName channelA, PinName channelB, PinName index, int pulsesPerRev, Encoding encoding = X2_ENCODING);
+
+    /**
+     * Reset the encoder.
+     *
+     * Sets the pulses and revolutions count to zero.
+     */
+    void reset(void);
+
+    /**
+     * Read the state of the encoder.
+     *
+     * @return The current state of the encoder as a 2-bit number, where:
+     *         bit 1 = The reading from channel B
+     *         bit 2 = The reading from channel A
+     */
+    int getCurrentState(void);
+
+    /**
+     * Read the number of pulses recorded by the encoder.
+     *
+     * @return Number of pulses which have occured.
+     */
+    int getPulses(void);
+
+    /**
+     * Read the number of revolutions recorded by the encoder on the index channel.
+     *
+     * @return Number of revolutions which have occured on the index channel.
+     */
+    int getRevolutions(void);
+
+private:
+
+    /**
+     * Update the pulse count.
+     *
+     * Called on every rising/falling edge of channels A/B.
+     *
+     * Reads the state of the channels and determines whether a pulse forward
+     * or backward has occured, updating the count appropriately.
+     */
+    void encode(void);
+
+    /**
+     * Called on every rising edge of channel index to update revolution
+     * count by one.
+     */
+    void index(void);
+
+    Encoding encoding_;
+
+    InterruptIn channelA_;
+    InterruptIn channelB_;
+    InterruptIn index_;
+
+    int          pulsesPerRev_;
+    int          prevState_;
+    int          currState_;
+
+    volatile int pulses_;
+    volatile int revolutions_;
+
+};
+
+#endif /* QEI_H */