QEI.cpp

00001 /**
00002  * @author Aaron Berk
00003  *
00004  * @section LICENSE
00005  *
00006  * Copyright (c) 2010 ARM Limited
00007  *
00008  * Permission is hereby granted, free of charge, to any person obtaining a copy
00009  * of this software and associated documentation files (the "Software"), to deal
00010  * in the Software without restriction, including without limitation the rights
00011  * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
00012  * copies of the Software, and to permit persons to whom the Software is
00013  * furnished to do so, subject to the following conditions:
00014  *
00015  * The above copyright notice and this permission notice shall be included in
00016  * all copies or substantial portions of the Software.
00017  *
00018  * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
00019  * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
00020  * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
00021  * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
00022  * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
00023  * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
00024  * THE SOFTWARE.
00025  *
00026  * @section DESCRIPTION
00027  *
00028  * Quadrature Encoder Interface.
00029  *
00030  * A quadrature encoder consists of two code tracks on a disc which are 90
00031  * degrees out of phase. It can be used to determine how far a wheel has
00032  * rotated, relative to a known starting position.
00033  *
00034  * Only one code track changes at a time leading to a more robust system than
00035  * a single track, because any jitter around any edge won't cause a state
00036  * change as the other track will remain constant.
00037  *
00038  * Encoders can be a homebrew affair, consisting of infrared emitters/receivers
00039  * and paper code tracks consisting of alternating black and white sections;
00040  * alternatively, complete disk and PCB emitter/receiver encoder systems can
00041  * be bought, but the interface, regardless of implementation is the same.
00042  *
00043  *               +-----+     +-----+     +-----+
00044  * Channel A     |  ^  |     |     |     |     |
00045  *            ---+  ^  +-----+     +-----+     +-----
00046  *               ^  ^
00047  *               ^  +-----+     +-----+     +-----+
00048  * Channel B     ^  |     |     |     |     |     |
00049  *            ------+     +-----+     +-----+     +-----
00050  *               ^  ^
00051  *               ^  ^
00052  *               90deg
00053  *
00054  * The interface uses X2 encoding by default which calculates the pulse count
00055  * based on reading the current state after each rising and falling edge of
00056  * channel A.
00057  *
00058  *               +-----+     +-----+     +-----+
00059  * Channel A     |     |     |     |     |     |
00060  *            ---+     +-----+     +-----+     +-----
00061  *               ^     ^     ^     ^     ^
00062  *               ^  +-----+  ^  +-----+  ^  +-----+
00063  * Channel B     ^  |  ^  |  ^  |  ^  |  ^  |     |
00064  *            ------+  ^  +-----+  ^  +-----+     +--
00065  *               ^     ^     ^     ^     ^
00066  *               ^     ^     ^     ^     ^
00067  * Pulse count 0 1     2     3     4     5  ...
00068  *
00069  * This interface can also use X4 encoding which calculates the pulse count
00070  * based on reading the current state after each rising and falling edge of
00071  * either channel.
00072  *
00073  *               +-----+     +-----+     +-----+
00074  * Channel A     |     |     |     |     |     |
00075  *            ---+     +-----+     +-----+     +-----
00076  *               ^     ^     ^     ^     ^
00077  *               ^  +-----+  ^  +-----+  ^  +-----+
00078  * Channel B     ^  |  ^  |  ^  |  ^  |  ^  |     |
00079  *            ------+  ^  +-----+  ^  +-----+     +--
00080  *               ^  ^  ^  ^  ^  ^  ^  ^  ^  ^
00081  *               ^  ^  ^  ^  ^  ^  ^  ^  ^  ^
00082  * Pulse count 0 1  2  3  4  5  6  7  8  9  ...
00083  *
00084  * It defaults
00085  *
00086  * An optional index channel can be used which determines when a full
00087  * revolution has occured.
00088  *
00089  * If a 4 pules per revolution encoder was used, with X4 encoding,
00090  * the following would be observed.
00091  *
00092  *               +-----+     +-----+     +-----+
00093  * Channel A     |     |     |     |     |     |
00094  *            ---+     +-----+     +-----+     +-----
00095  *               ^     ^     ^     ^     ^
00096  *               ^  +-----+  ^  +-----+  ^  +-----+
00097  * Channel B     ^  |  ^  |  ^  |  ^  |  ^  |     |
00098  *            ------+  ^  +-----+  ^  +-----+     +--
00099  *               ^  ^  ^  ^  ^  ^  ^  ^  ^  ^
00100  *               ^  ^  ^  ^  ^  ^  ^  ^  ^  ^
00101  *               ^  ^  ^  +--+  ^  ^  +--+  ^
00102  *               ^  ^  ^  |  |  ^  ^  |  |  ^
00103  * Index      ------------+  +--------+  +-----------
00104  *               ^  ^  ^  ^  ^  ^  ^  ^  ^  ^
00105  * Pulse count 0 1  2  3  4  5  6  7  8  9  ...
00106  * Rev.  count 0          1           2
00107  *
00108  * Rotational position in degrees can be calculated by:
00109  *
00110  * (pulse count / X * N) * 360
00111  *
00112  * Where X is the encoding type [e.g. X4 encoding => X=4], and N is the number
00113  * of pulses per revolution.
00114  *
00115  * Linear position can be calculated by:
00116  *
00117  * (pulse count / X * N) * (1 / PPI)
00118  *
00119  * Where X is encoding type [e.g. X4 encoding => X=44], N is the number of
00120  * pulses per revolution, and PPI is pulses per inch, or the equivalent for
00121  * any other unit of displacement. PPI can be calculated by taking the
00122  * circumference of the wheel or encoder disk and dividing it by the number
00123  * of pulses per revolution.
00124  */
00125 
00126 /**
00127  * Includes
00128  */
00129 #include "QEI.h"
00130 #include "mbed.h"
00131 
00132 QEI::QEI(PinName channelA,
00133          PinName channelB,
00134          PinName index,
00135          int pulsesPerRev,
00136          Encoding encoding) : channelA_(channelA), channelB_(channelB),
00137         index_(index) {
00138 
00139     pulses_       = 0;
00140     revolutions_  = 0;
00141     pulsesPerRev_ = pulsesPerRev;
00142     encoding_     = encoding;
00143 
00144     //Workout what the current state is.
00145     int chanA = channelA_.read();
00146     int chanB = channelB_.read();
00147 
00148     //2-bit state.
00149     currState_ = (chanA << 1) | (chanB);
00150     prevState_ = currState_;
00151 
00152     //X2 encoding uses interrupts on only channel A.
00153     //X4 encoding uses interrupts on      channel A,
00154     //and on channel B.
00155     channelA_.rise(callback(this, &QEI::encode));
00156     channelA_.fall(callback(this, &QEI::encode));
00157 
00158     //If we're using X4 encoding, then attach interrupts to channel B too.
00159     if (encoding == X4_ENCODING) {
00160         channelB_.rise(callback(this, &QEI::encode));
00161         channelB_.fall(callback(this, &QEI::encode));
00162     }
00163     //Index is optional.
00164     if (index !=  NC) {
00165         index_.rise(callback(this, &QEI::index));
00166     }
00167 
00168 }
00169 
00170 void QEI::reset(void) {
00171 
00172     pulses_      = 0;
00173     revolutions_ = 0;
00174 
00175 }
00176 
00177 int QEI::getCurrentState(void) {
00178 
00179     return currState_;
00180 
00181 }
00182 
00183 int QEI::getPulses(void) {
00184 
00185     return pulses_;
00186 
00187 }
00188 
00189 int QEI::getRevolutions(void) {
00190 
00191     return revolutions_;
00192 
00193 }
00194 
00195 // +-------------+
00196 // | X2 Encoding |
00197 // +-------------+
00198 //
00199 // When observing states two patterns will appear:
00200 //
00201 // Counter clockwise rotation:
00202 //
00203 // 10 -> 01 -> 10 -> 01 -> ...
00204 //
00205 // Clockwise rotation:
00206 //
00207 // 11 -> 00 -> 11 -> 00 -> ...
00208 //
00209 // We consider counter clockwise rotation to be "forward" and
00210 // counter clockwise to be "backward". Therefore pulse count will increase
00211 // during counter clockwise rotation and decrease during clockwise rotation.
00212 //
00213 // +-------------+
00214 // | X4 Encoding |
00215 // +-------------+
00216 //
00217 // There are four possible states for a quadrature encoder which correspond to
00218 // 2-bit gray code.
00219 //
00220 // A state change is only valid if of only one bit has changed.
00221 // A state change is invalid if both bits have changed.
00222 //
00223 // Clockwise Rotation ->
00224 //
00225 //    00 01 11 10 00
00226 //
00227 // <- Counter Clockwise Rotation
00228 //
00229 // If we observe any valid state changes going from left to right, we have
00230 // moved one pulse clockwise [we will consider this "backward" or "negative"].
00231 //
00232 // If we observe any valid state changes going from right to left we have
00233 // moved one pulse counter clockwise [we will consider this "forward" or
00234 // "positive"].
00235 //
00236 // We might enter an invalid state for a number of reasons which are hard to
00237 // predict - if this is the case, it is generally safe to ignore it, update
00238 // the state and carry on, with the error correcting itself shortly after.
00239 void QEI::encode(void) {
00240 
00241     int change = 0;
00242     int chanA  = channelA_.read();
00243     int chanB  = channelB_.read();
00244 
00245     //2-bit state.
00246     currState_ = (chanA << 1) | (chanB);
00247 
00248     if (encoding_ == X2_ENCODING) {
00249 
00250         //11->00->11->00 is counter clockwise rotation or "forward".
00251         if ((prevState_ == 0x3 && currState_ == 0x0) ||
00252                 (prevState_ == 0x0 && currState_ == 0x3)) {
00253 
00254             pulses_++;
00255 
00256         }
00257         //10->01->10->01 is clockwise rotation or "backward".
00258         else if ((prevState_ == 0x2 && currState_ == 0x1) ||
00259                  (prevState_ == 0x1 && currState_ == 0x2)) {
00260 
00261             pulses_--;
00262 
00263         }
00264 
00265     } else if (encoding_ == X4_ENCODING) {
00266 
00267         //Entered a new valid state.
00268         if (((currState_ ^ prevState_) != INVALID) && (currState_ != prevState_)) {
00269             //2 bit state. Right hand bit of prev XOR left hand bit of current
00270             //gives 0 if clockwise rotation and 1 if counter clockwise rotation.
00271             change = (prevState_ & PREV_MASK) ^ ((currState_ & CURR_MASK) >> 1);
00272 
00273             if (change == 0) {
00274                 change = -1;
00275             }
00276 
00277             pulses_ -= change;
00278         }
00279 
00280     }
00281 
00282     prevState_ = currState_;
00283 
00284 }
00285 
00286 void QEI::index(void) {
00287 
00288     revolutions_++;
00289 
00290 }