Quadrature encoder interface library with distance function

Dependents:   QEI_with_distance wheelchaircontrol wheelchaircontrolRos wheelchaircontrol ... more

Fork of QEI by Aaron Berk

Committer:
aberk
Date:
Thu Sep 02 16:48:55 2010 +0000
Revision:
0:5c2ad81551aa
Child:
1:f57640259245
Added X2 encoding, which the library now uses by default. This reduces the number of interrupts generated.

Who changed what in which revision?

UserRevisionLine numberNew contents of line
aberk 0:5c2ad81551aa 1 /**
aberk 0:5c2ad81551aa 2 * @author Aaron Berk
aberk 0:5c2ad81551aa 3 *
aberk 0:5c2ad81551aa 4 * @section LICENSE
aberk 0:5c2ad81551aa 5 *
aberk 0:5c2ad81551aa 6 * Copyright (c) 2010 ARM Limited
aberk 0:5c2ad81551aa 7 *
aberk 0:5c2ad81551aa 8 * Permission is hereby granted, free of charge, to any person obtaining a copy
aberk 0:5c2ad81551aa 9 * of this software and associated documentation files (the "Software"), to deal
aberk 0:5c2ad81551aa 10 * in the Software without restriction, including without limitation the rights
aberk 0:5c2ad81551aa 11 * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
aberk 0:5c2ad81551aa 12 * copies of the Software, and to permit persons to whom the Software is
aberk 0:5c2ad81551aa 13 * furnished to do so, subject to the following conditions:
aberk 0:5c2ad81551aa 14 *
aberk 0:5c2ad81551aa 15 * The above copyright notice and this permission notice shall be included in
aberk 0:5c2ad81551aa 16 * all copies or substantial portions of the Software.
aberk 0:5c2ad81551aa 17 *
aberk 0:5c2ad81551aa 18 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
aberk 0:5c2ad81551aa 19 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
aberk 0:5c2ad81551aa 20 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
aberk 0:5c2ad81551aa 21 * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
aberk 0:5c2ad81551aa 22 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
aberk 0:5c2ad81551aa 23 * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
aberk 0:5c2ad81551aa 24 * THE SOFTWARE.
aberk 0:5c2ad81551aa 25 *
aberk 0:5c2ad81551aa 26 * @section DESCRIPTION
aberk 0:5c2ad81551aa 27 *
aberk 0:5c2ad81551aa 28 * Quadrature Encoder Interface.
aberk 0:5c2ad81551aa 29 *
aberk 0:5c2ad81551aa 30 * A quadrature encoder consists of two code tracks on a disc which are 90
aberk 0:5c2ad81551aa 31 * degrees out of phase. It can be used to determine how far a wheel has
aberk 0:5c2ad81551aa 32 * rotated, relative to a known starting position.
aberk 0:5c2ad81551aa 33 *
aberk 0:5c2ad81551aa 34 * Only one code track changes at a time leading to a more robust system than
aberk 0:5c2ad81551aa 35 * a single track, because any jitter around any edge won't cause a state
aberk 0:5c2ad81551aa 36 * change as the other track will remain constant.
aberk 0:5c2ad81551aa 37 *
aberk 0:5c2ad81551aa 38 * Encoders can be a homebrew affair, consisting of infrared emitters/receivers
aberk 0:5c2ad81551aa 39 * and paper code tracks consisting of alternating black and white sections;
aberk 0:5c2ad81551aa 40 * alternatively, complete disk and PCB emitter/receiver encoder systems can
aberk 0:5c2ad81551aa 41 * be bought, but the interface, regardless of implementation is the same.
aberk 0:5c2ad81551aa 42 *
aberk 0:5c2ad81551aa 43 * +-----+ +-----+ +-----+
aberk 0:5c2ad81551aa 44 * Channel A | ^ | | | | |
aberk 0:5c2ad81551aa 45 * ---+ ^ +-----+ +-----+ +-----
aberk 0:5c2ad81551aa 46 * ^ ^
aberk 0:5c2ad81551aa 47 * ^ +-----+ +-----+ +-----+
aberk 0:5c2ad81551aa 48 * Channel B ^ | | | | | |
aberk 0:5c2ad81551aa 49 * ------+ +-----+ +-----+ +-----
aberk 0:5c2ad81551aa 50 * ^ ^
aberk 0:5c2ad81551aa 51 * ^ ^
aberk 0:5c2ad81551aa 52 * 90deg
aberk 0:5c2ad81551aa 53 *
aberk 0:5c2ad81551aa 54 * The interface uses X2 encoding by default which calculates the pulse count
aberk 0:5c2ad81551aa 55 * based on reading the current state after each rising and falling edge of
aberk 0:5c2ad81551aa 56 * channel A.
aberk 0:5c2ad81551aa 57 *
aberk 0:5c2ad81551aa 58 * +-----+ +-----+ +-----+
aberk 0:5c2ad81551aa 59 * Channel A | | | | | |
aberk 0:5c2ad81551aa 60 * ---+ +-----+ +-----+ +-----
aberk 0:5c2ad81551aa 61 * ^ ^ ^ ^ ^
aberk 0:5c2ad81551aa 62 * ^ +-----+ ^ +-----+ ^ +-----+
aberk 0:5c2ad81551aa 63 * Channel B ^ | ^ | ^ | ^ | ^ | |
aberk 0:5c2ad81551aa 64 * ------+ ^ +-----+ ^ +-----+ +--
aberk 0:5c2ad81551aa 65 * ^ ^ ^ ^ ^
aberk 0:5c2ad81551aa 66 * ^ ^ ^ ^ ^
aberk 0:5c2ad81551aa 67 * Pulse count 0 1 2 3 4 5 ...
aberk 0:5c2ad81551aa 68 *
aberk 0:5c2ad81551aa 69 * This interface can also use X4 encoding which calculates the pulse count
aberk 0:5c2ad81551aa 70 * based on reading the current state after each rising and falling edge of
aberk 0:5c2ad81551aa 71 * either channel.
aberk 0:5c2ad81551aa 72 *
aberk 0:5c2ad81551aa 73 * +-----+ +-----+ +-----+
aberk 0:5c2ad81551aa 74 * Channel A | | | | | |
aberk 0:5c2ad81551aa 75 * ---+ +-----+ +-----+ +-----
aberk 0:5c2ad81551aa 76 * ^ ^ ^ ^ ^
aberk 0:5c2ad81551aa 77 * ^ +-----+ ^ +-----+ ^ +-----+
aberk 0:5c2ad81551aa 78 * Channel B ^ | ^ | ^ | ^ | ^ | |
aberk 0:5c2ad81551aa 79 * ------+ ^ +-----+ ^ +-----+ +--
aberk 0:5c2ad81551aa 80 * ^ ^ ^ ^ ^ ^ ^ ^ ^ ^
aberk 0:5c2ad81551aa 81 * ^ ^ ^ ^ ^ ^ ^ ^ ^ ^
aberk 0:5c2ad81551aa 82 * Pulse count 0 1 2 3 4 5 6 7 8 9 ...
aberk 0:5c2ad81551aa 83 *
aberk 0:5c2ad81551aa 84 * It defaults
aberk 0:5c2ad81551aa 85 *
aberk 0:5c2ad81551aa 86 * An optional index channel can be used which determines when a full
aberk 0:5c2ad81551aa 87 * revolution has occured.
aberk 0:5c2ad81551aa 88 *
aberk 0:5c2ad81551aa 89 * If a 4 pules per revolution encoder was used, with X4 encoding,
aberk 0:5c2ad81551aa 90 * the following would be observed.
aberk 0:5c2ad81551aa 91 *
aberk 0:5c2ad81551aa 92 * +-----+ +-----+ +-----+
aberk 0:5c2ad81551aa 93 * Channel A | | | | | |
aberk 0:5c2ad81551aa 94 * ---+ +-----+ +-----+ +-----
aberk 0:5c2ad81551aa 95 * ^ ^ ^ ^ ^
aberk 0:5c2ad81551aa 96 * ^ +-----+ ^ +-----+ ^ +-----+
aberk 0:5c2ad81551aa 97 * Channel B ^ | ^ | ^ | ^ | ^ | |
aberk 0:5c2ad81551aa 98 * ------+ ^ +-----+ ^ +-----+ +--
aberk 0:5c2ad81551aa 99 * ^ ^ ^ ^ ^ ^ ^ ^ ^ ^
aberk 0:5c2ad81551aa 100 * ^ ^ ^ ^ ^ ^ ^ ^ ^ ^
aberk 0:5c2ad81551aa 101 * ^ ^ ^ +--+ ^ ^ +--+ ^
aberk 0:5c2ad81551aa 102 * ^ ^ ^ | | ^ ^ | | ^
aberk 0:5c2ad81551aa 103 * Index ------------+ +--------+ +-----------
aberk 0:5c2ad81551aa 104 * ^ ^ ^ ^ ^ ^ ^ ^ ^ ^
aberk 0:5c2ad81551aa 105 * Pulse count 0 1 2 3 4 5 6 7 8 9 ...
aberk 0:5c2ad81551aa 106 * Rev. count 0 1 2
aberk 0:5c2ad81551aa 107 *
aberk 0:5c2ad81551aa 108 * Rotational position in degrees can be calculated by:
aberk 0:5c2ad81551aa 109 *
aberk 0:5c2ad81551aa 110 * (pulse count / X * N) * 360
aberk 0:5c2ad81551aa 111 *
aberk 0:5c2ad81551aa 112 * Where X is the encoding type [e.g. X4 encoding => X=4], and N is the number
aberk 0:5c2ad81551aa 113 * of pulses per revolution.
aberk 0:5c2ad81551aa 114 *
aberk 0:5c2ad81551aa 115 * Linear position can be calculated by:
aberk 0:5c2ad81551aa 116 *
aberk 0:5c2ad81551aa 117 * (pulse count / X * N) * (1 / PPI)
aberk 0:5c2ad81551aa 118 *
aberk 0:5c2ad81551aa 119 * Where X is encoding type [e.g. X4 encoding => X=44], N is the number of
aberk 0:5c2ad81551aa 120 * pulses per revolution, and PPI is pulses per inch, or the equivalent for
aberk 0:5c2ad81551aa 121 * any other unit of displacement. PPI can be calculated by taking the
aberk 0:5c2ad81551aa 122 * circumference of the wheel or encoder disk and dividing it by the number
aberk 0:5c2ad81551aa 123 * of pulses per revolution.
aberk 0:5c2ad81551aa 124 */
aberk 0:5c2ad81551aa 125
aberk 0:5c2ad81551aa 126 /**
aberk 0:5c2ad81551aa 127 * Includes
aberk 0:5c2ad81551aa 128 */
aberk 0:5c2ad81551aa 129 #include "QEI.h"
aberk 0:5c2ad81551aa 130
aberk 0:5c2ad81551aa 131 QEI::QEI(PinName channelA,
aberk 0:5c2ad81551aa 132 PinName channelB,
aberk 0:5c2ad81551aa 133 PinName index,
aberk 0:5c2ad81551aa 134 int pulsesPerRev,
aberk 0:5c2ad81551aa 135 Encoding encoding) : channelA_(channelA), channelB_(channelB),
aberk 0:5c2ad81551aa 136 index_(index) {
aberk 0:5c2ad81551aa 137
aberk 0:5c2ad81551aa 138 pulses_ = 0;
aberk 0:5c2ad81551aa 139 revolutions_ = 0;
aberk 0:5c2ad81551aa 140 pulsesPerRev_ = pulsesPerRev;
aberk 0:5c2ad81551aa 141 encoding_ = encoding;
aberk 0:5c2ad81551aa 142
aberk 0:5c2ad81551aa 143 //Workout what the current state is.
aberk 0:5c2ad81551aa 144 int chanA = channelA_.read();
aberk 0:5c2ad81551aa 145 int chanB = channelB_.read();
aberk 0:5c2ad81551aa 146
aberk 0:5c2ad81551aa 147 //2-bit state.
aberk 0:5c2ad81551aa 148 currState_ = (chanA << 1) | (chanB);
aberk 0:5c2ad81551aa 149 prevState_ = currState_;
aberk 0:5c2ad81551aa 150
aberk 0:5c2ad81551aa 151 //X2 encoding uses interrupts on only channel A.
aberk 0:5c2ad81551aa 152 //X4 encoding uses interrupts on channel A,
aberk 0:5c2ad81551aa 153 //and on channel B.
aberk 0:5c2ad81551aa 154 channelA_.rise(this, &QEI::encode);
aberk 0:5c2ad81551aa 155 channelA_.fall(this, &QEI::encode);
aberk 0:5c2ad81551aa 156
aberk 0:5c2ad81551aa 157 //If we're using X4 encoding, then attach interrupts to channel B too.
aberk 0:5c2ad81551aa 158 if (encoding == X4_ENCODING) {
aberk 0:5c2ad81551aa 159 channelB_.rise(this, &QEI::encode);
aberk 0:5c2ad81551aa 160 channelB_.fall(this, &QEI::encode);
aberk 0:5c2ad81551aa 161 }
aberk 0:5c2ad81551aa 162 //Index is optional.
aberk 0:5c2ad81551aa 163 if (index != NC) {
aberk 0:5c2ad81551aa 164 index_.rise(this, &QEI::index);
aberk 0:5c2ad81551aa 165 }
aberk 0:5c2ad81551aa 166
aberk 0:5c2ad81551aa 167 }
aberk 0:5c2ad81551aa 168
aberk 0:5c2ad81551aa 169 void QEI::reset(void) {
aberk 0:5c2ad81551aa 170
aberk 0:5c2ad81551aa 171 pulses_ = 0;
aberk 0:5c2ad81551aa 172 revolutions_ = 0;
aberk 0:5c2ad81551aa 173
aberk 0:5c2ad81551aa 174 }
aberk 0:5c2ad81551aa 175
aberk 0:5c2ad81551aa 176 int QEI::getCurrentState(void) {
aberk 0:5c2ad81551aa 177
aberk 0:5c2ad81551aa 178 return currState_;
aberk 0:5c2ad81551aa 179
aberk 0:5c2ad81551aa 180 }
aberk 0:5c2ad81551aa 181
aberk 0:5c2ad81551aa 182 int QEI::getPulses(void) {
aberk 0:5c2ad81551aa 183
aberk 0:5c2ad81551aa 184 return pulses_;
aberk 0:5c2ad81551aa 185
aberk 0:5c2ad81551aa 186 }
aberk 0:5c2ad81551aa 187
aberk 0:5c2ad81551aa 188 int QEI::getRevolutions(void) {
aberk 0:5c2ad81551aa 189
aberk 0:5c2ad81551aa 190 return revolutions_;
aberk 0:5c2ad81551aa 191
aberk 0:5c2ad81551aa 192 }
aberk 0:5c2ad81551aa 193
aberk 0:5c2ad81551aa 194 // +-------------+
aberk 0:5c2ad81551aa 195 // | X2 Encoding |
aberk 0:5c2ad81551aa 196 // +-------------+
aberk 0:5c2ad81551aa 197 //
aberk 0:5c2ad81551aa 198 // When observing states two patterns will appear:
aberk 0:5c2ad81551aa 199 //
aberk 0:5c2ad81551aa 200 // Counter clockwise rotation:
aberk 0:5c2ad81551aa 201 //
aberk 0:5c2ad81551aa 202 // 10 -> 01 -> 10 -> 01 -> ...
aberk 0:5c2ad81551aa 203 //
aberk 0:5c2ad81551aa 204 // Clockwise rotation:
aberk 0:5c2ad81551aa 205 //
aberk 0:5c2ad81551aa 206 // 11 -> 00 -> 11 -> 00 -> ...
aberk 0:5c2ad81551aa 207 //
aberk 0:5c2ad81551aa 208 // We consider counter clockwise rotation to be "forward" and
aberk 0:5c2ad81551aa 209 // counter clockwise to be "backward". Therefore pulse count will increase
aberk 0:5c2ad81551aa 210 // during counter clockwise rotation and decrease during clockwise rotation.
aberk 0:5c2ad81551aa 211 //
aberk 0:5c2ad81551aa 212 // +-------------+
aberk 0:5c2ad81551aa 213 // | X4 Encoding |
aberk 0:5c2ad81551aa 214 // +-------------+
aberk 0:5c2ad81551aa 215 //
aberk 0:5c2ad81551aa 216 // There are four possible states for a quadrature encoder which correspond to
aberk 0:5c2ad81551aa 217 // 2-bit gray code.
aberk 0:5c2ad81551aa 218 //
aberk 0:5c2ad81551aa 219 // A state change is only valid if of only one bit has changed.
aberk 0:5c2ad81551aa 220 // A state change is invalid if both bits have changed.
aberk 0:5c2ad81551aa 221 //
aberk 0:5c2ad81551aa 222 // Clockwise Rotation ->
aberk 0:5c2ad81551aa 223 //
aberk 0:5c2ad81551aa 224 // 00 01 11 10 00
aberk 0:5c2ad81551aa 225 //
aberk 0:5c2ad81551aa 226 // <- Counter Clockwise Rotation
aberk 0:5c2ad81551aa 227 //
aberk 0:5c2ad81551aa 228 // If we observe any valid state changes going from left to right, we have
aberk 0:5c2ad81551aa 229 // moved one pulse clockwise [we will consider this "backward" or "negative"].
aberk 0:5c2ad81551aa 230 //
aberk 0:5c2ad81551aa 231 // If we observe any valid state changes going from right to left we have
aberk 0:5c2ad81551aa 232 // moved one pulse counter clockwise [we will consider this "forward" or
aberk 0:5c2ad81551aa 233 // "positive"].
aberk 0:5c2ad81551aa 234 //
aberk 0:5c2ad81551aa 235 // We might enter an invalid state for a number of reasons which are hard to
aberk 0:5c2ad81551aa 236 // predict - if this is the case, it is generally safe to ignore it, update
aberk 0:5c2ad81551aa 237 // the state and carry on, with the error correcting itself shortly after.
aberk 0:5c2ad81551aa 238 void QEI::encode(void) {
aberk 0:5c2ad81551aa 239
aberk 0:5c2ad81551aa 240 int change = 0;
aberk 0:5c2ad81551aa 241 int chanA = channelA_.read();
aberk 0:5c2ad81551aa 242 int chanB = channelB_.read();
aberk 0:5c2ad81551aa 243
aberk 0:5c2ad81551aa 244 //2-bit state.
aberk 0:5c2ad81551aa 245 currState_ = (chanA << 1) | (chanB);
aberk 0:5c2ad81551aa 246
aberk 0:5c2ad81551aa 247 if (encoding_ == X2_ENCODING) {
aberk 0:5c2ad81551aa 248
aberk 0:5c2ad81551aa 249 //11->00->11->00 is counter clockwise rotation or "forward".
aberk 0:5c2ad81551aa 250 if ((prevState_ == 0x3 && currState_ == 0x0) ||
aberk 0:5c2ad81551aa 251 (prevState_ == 0x0 && currState_ == 0x3)) {
aberk 0:5c2ad81551aa 252
aberk 0:5c2ad81551aa 253 pulses_++;
aberk 0:5c2ad81551aa 254
aberk 0:5c2ad81551aa 255 }
aberk 0:5c2ad81551aa 256 //10->01->10->01 is clockwise rotation or "backward".
aberk 0:5c2ad81551aa 257 else if ((prevState_ == 0x2 && currState_ == 0x1) ||
aberk 0:5c2ad81551aa 258 (prevState_ == 0x1 && currState_ == 0x2)) {
aberk 0:5c2ad81551aa 259
aberk 0:5c2ad81551aa 260 pulses_--;
aberk 0:5c2ad81551aa 261
aberk 0:5c2ad81551aa 262 }
aberk 0:5c2ad81551aa 263
aberk 0:5c2ad81551aa 264 } else if (encoding_ == X4_ENCODING) {
aberk 0:5c2ad81551aa 265
aberk 0:5c2ad81551aa 266 //Entered a new valid state.
aberk 0:5c2ad81551aa 267 if (((currState_ ^ prevState_) != INVALID) && (currState_ != prevState_)) {
aberk 0:5c2ad81551aa 268 //2 bit state. Right hand bit of prev XOR left hand bit of current
aberk 0:5c2ad81551aa 269 //gives 0 if clockwise rotation and 1 if counter clockwise rotation.
aberk 0:5c2ad81551aa 270 change = (prevState_ & PREV_MASK) ^ ((currState_ & CURR_MASK) >> 1);
aberk 0:5c2ad81551aa 271
aberk 0:5c2ad81551aa 272 if (change == 0) {
aberk 0:5c2ad81551aa 273 change = -1;
aberk 0:5c2ad81551aa 274 }
aberk 0:5c2ad81551aa 275
aberk 0:5c2ad81551aa 276 pulses_ -= change;
aberk 0:5c2ad81551aa 277 }
aberk 0:5c2ad81551aa 278
aberk 0:5c2ad81551aa 279 }
aberk 0:5c2ad81551aa 280
aberk 0:5c2ad81551aa 281 prevState_ = currState_;
aberk 0:5c2ad81551aa 282
aberk 0:5c2ad81551aa 283 }
aberk 0:5c2ad81551aa 284
aberk 0:5c2ad81551aa 285 void QEI::index(void) {
aberk 0:5c2ad81551aa 286
aberk 0:5c2ad81551aa 287 revolutions_++;
aberk 0:5c2ad81551aa 288
aberk 0:5c2ad81551aa 289 }