Gabriel das Neves
Quadrature encoder interface library.
Diff: QEI.cpp
- Revision:
- 0:5c2ad81551aa
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
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+/**
+ * @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_++;
+
+}