Quadrature encoder interface library.
QEI.cpp
- Committer:
- gabrielpdn
- Date:
- 2020-03-11
- Revision:
- 1:ce2f5a0d50b6
- Parent:
- 0:5c2ad81551aa
File content as of revision 1:ce2f5a0d50b6:
/** * @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_++; }