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comparison: lib/QEI/QEI.cpp

lib/QEI/QEI.cpp

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1 /**
2 * @author Aaron Berk
3 *
4 * @section LICENSE
5 *
6 * Copyright (c) 2010 ARM Limited
7 *
8 * Permission is hereby granted, free of charge, to any person obtaining a copy
9 * of this software and associated documentation files (the "Software"), to deal
10 * in the Software without restriction, including without limitation the rights
11 * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
12 * copies of the Software, and to permit persons to whom the Software is
13 * furnished to do so, subject to the following conditions:
14 *
15 * The above copyright notice and this permission notice shall be included in
16 * all copies or substantial portions of the Software.
17 *
18 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
19 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
20 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
21 * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
22 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
23 * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
24 * THE SOFTWARE.
25 *
26 * @section DESCRIPTION
27 *
28 * Quadrature Encoder Interface.
29 *
30 * A quadrature encoder consists of two code tracks on a disc which are 90
31 * degrees out of phase. It can be used to determine how far a wheel has
32 * rotated, relative to a known starting position.
33 *
34 * Only one code track changes at a time leading to a more robust system than
35 * a single track, because any jitter around any edge won't cause a state
36 * change as the other track will remain constant.
37 *
38 * Encoders can be a homebrew affair, consisting of infrared emitters/receivers
39 * and paper code tracks consisting of alternating black and white sections;
40 * alternatively, complete disk and PCB emitter/receiver encoder systems can
41 * be bought, but the interface, regardless of implementation is the same.
42 *
43 * +-----+ +-----+ +-----+
44 * Channel A | ^ | | | | |
45 * ---+ ^ +-----+ +-----+ +-----
46 * ^ ^
47 * ^ +-----+ +-----+ +-----+
48 * Channel B ^ | | | | | |
49 * ------+ +-----+ +-----+ +-----
50 * ^ ^
51 * ^ ^
52 * 90deg
53 *
54 * The interface uses X2 encoding by default which calculates the pulse count
55 * based on reading the current state after each rising and falling edge of
56 * channel A.
57 *
58 * +-----+ +-----+ +-----+
59 * Channel A | | | | | |
60 * ---+ +-----+ +-----+ +-----
61 * ^ ^ ^ ^ ^
62 * ^ +-----+ ^ +-----+ ^ +-----+
63 * Channel B ^ | ^ | ^ | ^ | ^ | |
64 * ------+ ^ +-----+ ^ +-----+ +--
65 * ^ ^ ^ ^ ^
66 * ^ ^ ^ ^ ^
67 * Pulse count 0 1 2 3 4 5 ...
68 *
69 * This interface can also use X4 encoding which calculates the pulse count
70 * based on reading the current state after each rising and falling edge of
71 * either channel.
72 *
73 * +-----+ +-----+ +-----+
74 * Channel A | | | | | |
75 * ---+ +-----+ +-----+ +-----
76 * ^ ^ ^ ^ ^
77 * ^ +-----+ ^ +-----+ ^ +-----+
78 * Channel B ^ | ^ | ^ | ^ | ^ | |
79 * ------+ ^ +-----+ ^ +-----+ +--
80 * ^ ^ ^ ^ ^ ^ ^ ^ ^ ^
81 * ^ ^ ^ ^ ^ ^ ^ ^ ^ ^
82 * Pulse count 0 1 2 3 4 5 6 7 8 9 ...
83 *
84 * It defaults
85 *
86 * An optional index channel can be used which determines when a full
87 * revolution has occured.
88 *
89 * If a 4 pules per revolution encoder was used, with X4 encoding,
90 * the following would be observed.
91 *
92 * +-----+ +-----+ +-----+
93 * Channel A | | | | | |
94 * ---+ +-----+ +-----+ +-----
95 * ^ ^ ^ ^ ^
96 * ^ +-----+ ^ +-----+ ^ +-----+
97 * Channel B ^ | ^ | ^ | ^ | ^ | |
98 * ------+ ^ +-----+ ^ +-----+ +--
99 * ^ ^ ^ ^ ^ ^ ^ ^ ^ ^
100 * ^ ^ ^ ^ ^ ^ ^ ^ ^ ^
101 * ^ ^ ^ +--+ ^ ^ +--+ ^
102 * ^ ^ ^ | | ^ ^ | | ^
103 * Index ------------+ +--------+ +-----------
104 * ^ ^ ^ ^ ^ ^ ^ ^ ^ ^
105 * Pulse count 0 1 2 3 4 5 6 7 8 9 ...
106 * Rev. count 0 1 2
107 *
108 * Rotational position in degrees can be calculated by:
109 *
110 * (pulse count / X * N) * 360
111 *
112 * Where X is the encoding type [e.g. X4 encoding => X=4], and N is the number
113 * of pulses per revolution.
114 *
115 * Linear position can be calculated by:
116 *
117 * (pulse count / X * N) * (1 / PPI)
118 *
119 * Where X is encoding type [e.g. X4 encoding => X=44], N is the number of
120 * pulses per revolution, and PPI is pulses per inch, or the equivalent for
121 * any other unit of displacement. PPI can be calculated by taking the
122 * circumference of the wheel or encoder disk and dividing it by the number
123 * of pulses per revolution.
124 */
125
126 /**
127 * Includes
128 */
129 #include "QEI.h"
130
131 QEI::QEI(PinName channelA,
132 PinName channelB,
133 PinName index,
134 int pulsesPerRev,
135 Encoding encoding,
136 const event_callback_t& ev_callback) :
137 channelA_(channelA),
138 channelB_(channelB),
139 index_(index),
140 _callback(ev_callback) {
141
142 pulses_ = 0;
143 revolutions_ = 0;
144 pulsesPerRev_ = pulsesPerRev;
145 encoding_ = encoding;
146
147 //Workout what the current state is.
148 int chanA = channelA_.read();
149 int chanB = channelB_.read();
150
151 //2-bit state.
152 currState_ = (chanA << 1) | (chanB);
153 prevState_ = currState_;
154
155 //X2 encoding uses interrupts on only channel A.
156 //X4 encoding uses interrupts on channel A,
157 //and on channel B.
158 channelA_.rise(callback(this, &QEI::encode));
159 channelA_.fall(callback(this, &QEI::encode));
160
161 //If we're using X4 encoding, then attach interrupts to channel B too.
162 if (encoding == X4_ENCODING) {
163 channelB_.rise(callback(this, &QEI::encode));
164 channelB_.fall(callback(this, &QEI::encode));
165 }
166 //Index is optional.
167 if (index_ != NC) {
168 index_.rise(callback(this, &QEI::index));
169 }
170
171 }
172
173 void QEI::reset(void) {
174
175 pulses_ = 0;
176 revolutions_ = 0;
177
178 }
179
180 int QEI::getCurrentState(void) {
181
182 return currState_;
183
184 }
185
186 int QEI::getPulses(void) {
187
188 return pulses_;
189
190 }
191
192 int QEI::getRevolutions(void) {
193
194 return revolutions_;
195
196 }
197
198 // +-------------+
199 // | X2 Encoding |
200 // +-------------+
201 //
202 // When observing states two patterns will appear:
203 //
204 // Counter clockwise rotation:
205 //
206 // 10 -> 01 -> 10 -> 01 -> ...
207 //
208 // Clockwise rotation:
209 //
210 // 11 -> 00 -> 11 -> 00 -> ...
211 //
212 // We consider counter clockwise rotation to be "forward" and
213 // counter clockwise to be "backward". Therefore pulse count will increase
214 // during counter clockwise rotation and decrease during clockwise rotation.
215 //
216 // +-------------+
217 // | X4 Encoding |
218 // +-------------+
219 //
220 // There are four possible states for a quadrature encoder which correspond to
221 // 2-bit gray code.
222 //
223 // A state change is only valid if of only one bit has changed.
224 // A state change is invalid if both bits have changed.
225 //
226 // Clockwise Rotation ->
227 //
228 // 00 01 11 10 00
229 //
230 // <- Counter Clockwise Rotation
231 //
232 // If we observe any valid state changes going from left to right, we have
233 // moved one pulse clockwise [we will consider this "backward" or "negative"].
234 //
235 // If we observe any valid state changes going from right to left we have
236 // moved one pulse counter clockwise [we will consider this "forward" or
237 // "positive"].
238 //
239 // We might enter an invalid state for a number of reasons which are hard to
240 // predict - if this is the case, it is generally safe to ignore it, update
241 // the state and carry on, with the error correcting itself shortly after.
242 void QEI::encode(void) {
243
244 int change = 0;
245 int chanA = channelA_.read();
246 int chanB = channelB_.read();
247
248 //2-bit state.
249 currState_ = (chanA << 1) | (chanB);
250
251 if (encoding_ == X2_ENCODING) {
252
253 //11->00->11->00 is counter clockwise rotation or "forward".
254 if ((prevState_ == 0x3 && currState_ == 0x0) ||
255 (prevState_ == 0x0 && currState_ == 0x3)) {
256 change = 1;
257 pulses_++;
258
259 }
260 //10->01->10->01 is clockwise rotation or "backward".
261 else if ((prevState_ == 0x2 && currState_ == 0x1) ||
262 (prevState_ == 0x1 && currState_ == 0x2)) {
263 change = -1;
264 pulses_--;
265
266 }
267
268 } else if (encoding_ == X4_ENCODING) {
269
270 //Entered a new valid state.
271 if (((currState_ ^ prevState_) != INVALID) && (currState_ != prevState_)) {
272 //2 bit state. Right hand bit of prev XOR left hand bit of current
273 //gives 0 if clockwise rotation and 1 if counter clockwise rotation.
274 change = (prevState_ & PREV_MASK) ^ ((currState_ & CURR_MASK) >> 1);
275
276 if (change == 0) {
277 change = -1;
278 }
279
280 pulses_ -= change;
281 }
282
283 }
284
285 if (_callback && (change != 0))
286 _callback.call(change==1?1:0);
287
288 prevState_ = currState_;
289
290 }
291
292 void QEI::index(void) {
293
294 revolutions_++;
295
296 }

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