I’ve spent about 2 days on dealing with the ADC (Analog to Digital Converter) of the Arduino platform. Turns out there’s more problems to overcome when dealing with taking analog signal from a sensor and converting it to meaningful digital reading.
The issue is that even with a constant input voltage the ADC reading sampling error rate is larger than I’d like (~1.2%) with some random spikes in excess of 3%.
I built a test rig to troubleshoot the issue. Using constant input voltage and a potentiometer to simulate the analog input from a sensor.
Goal:
– At least 4 sample updates per second. (Smooth gauge updates)
– Stable voltage regardless of outside interference.
Things I found:
– Arduino Atmel chip uses a single ADC and multiplexes the input pins.
– Switching read pin on the ADC causes noise in the system. Solution to read the first value and totally discard it. Then wait at least 10ms before sampling actual data.
– Sample accuracy is inversely proportional to the delay between samples. Shorter delay between reads, less drift. But over a static period of time, the error rate is the same.
– ADC is quite sensitive to electronic noise. Adjacent pins seem to affect reading.
Things I tried:
– Average voltage over multiple samples (currently 11 samples per read)
– Add delay between readings (1 to 12 ms) (currently 2ms)
– Discard top and bottom values and average the rest (currently discard 25%)
– Add a delta value, discard new value if difference is less than delta (currently 0.011V)
– vary delay on multiplex debounce (currently 10ms)
Current sampling rate: 3.7 samples / second.
Final Arduino Code:
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 | const int LED_RED = 9; const int LED_GREEN = 8; const int LED_BLUE = 7; const int LED_ACT = 11; const int INPUT_REF = 22; const int INPUT_RATE = 20; int refPins[4] = {19, 17, 18, 16}; int sensePins[4] = {15, 13, 14, 12}; double lastRefVolts[4] = { 0, 0, 0, 0 }; double lastSenseVolts[4] = { 0, 0, 0, 0 }; const float LOW_VOLTAGE = 4.5; //alert voltage for 5V bus const int MAX_SAMPLES = 50; //max sample on trim pot const float DISCARD_PCT = 0.25; //percent of samples to discard (top and bottom) const float MAX_DELTA = 0.011; //ignore changes less than this const int SAMPLE_DELAY = 2; //delay MS between sample reads const int INITIAL_DELAY = 10; //delay MS on pin change void setup() { //set pin IO modes pinMode(LED_RED, OUTPUT); pinMode(LED_GREEN, OUTPUT); pinMode(LED_BLUE, OUTPUT); pinMode(LED_ACT, OUTPUT); for (int pin = 0; pin < 4; pin++) { pinMode(refPins[pin], INPUT); pinMode(sensePins[pin], INPUT); } pinMode(INPUT_REF, INPUT); pinMode(INPUT_RATE, INPUT); //cycle leds digitalWrite(LED_RED, HIGH); digitalWrite(LED_BLUE, HIGH); digitalWrite(LED_GREEN, HIGH); setLED(LOW, HIGH, HIGH); delay(200); setLED(HIGH, LOW, HIGH); delay(200); setLED(HIGH, HIGH, LOW); delay(200); setLED(LOW, LOW, LOW); delay(200); setLED(HIGH, HIGH, HIGH); //emulated serial, speed ignored Serial.begin(38400); } void loop() { //read serial to clear buffer if (Serial.available() > 0) { Serial.flush(); } //get number of samples to read int readCount = getReadCount(); //write +5V bus voltage writeRefVoltage(readCount); //write all input sensors for (int pin = 0; pin < 4; pin++) { writeSerialVoltage(pin, readCount); } } void writeSerialVoltage(int pin, int readCount) { digitalWrite(LED_BLUE, HIGH); //blue off - start sending digitalWrite(LED_ACT, LOW); //internal off //ref volt, seems more volatile float refVolt = getVoltage(refPins[pin], readCount); float newRefVolt = processRefVoltage(pin, refVolt); //get sensor voltage float senseVolt = getVoltage(sensePins[pin], readCount); float newSenseVolt = processSenseVoltage(pin, senseVolt); Serial.print("IN"); //write identifier Serial.print(pin); Serial.print("\t"); Serial.print(newRefVolt, 4); Serial.print("\t"); Serial.print(newSenseVolt, 4); Serial.print("\r"); Serial.println(""); digitalWrite(LED_ACT, HIGH); //internal on digitalWrite(LED_BLUE, LOW); //blue on - sending done } //do a delta comparison on Ref voltage float processRefVoltage(int Pin, float refVolt) { float lastRefVolt = lastRefVolts[Pin]; if (abs(refVolt - lastRefVolt) < MAX_DELTA) refVolt = lastRefVolts[Pin]; else lastRefVolts[Pin] = refVolt; return refVolt; } //do delta comparison on sensor voltage float processSenseVoltage(int Pin, float senseVolt) { float lastSenseVolt = lastSenseVolts[Pin]; if (abs(senseVolt - lastSenseVolt) < MAX_DELTA) senseVolt = lastSenseVolts[Pin]; else lastSenseVolts[Pin] = senseVolt; return senseVolt; } //send bus voltage to host void writeRefVoltage(int readCount) { float refVoltage = getVoltage(INPUT_REF, readCount); if (refVoltage < LOW_VOLTAGE) { digitalWrite(LED_RED, LOW); digitalWrite(LED_GREEN, HIGH); } else { digitalWrite(LED_RED, HIGH); digitalWrite(LED_GREEN, LOW); } Serial.print("REF\t"); Serial.print(refVoltage, 4); Serial.println(""); } //read value from ADC (0-1023) and convert to voltage (0-5) float getVoltage(int PIN, int samples) { //allow ADC to stablize analogRead(PIN); //ignore value delay(INITIAL_DELAY); //wait for debounce float sampleList[samples]; //read samples for (int i = 0; i < samples; i++) { float voltage = (float)analogRead(PIN) * (5.0 / 1024.0); //round to 2 decimals sampleList[i] = (ceil(voltage * 100.0)) / 100.0; delay(SAMPLE_DELAY); } //sort array (shitty bubble sort, cause i'm lazy) float swapper; for (int o = samples-1; o > 0; o--) { for (int i = 1; i <= o; i++) { if (sampleList[i-1] > sampleList[i]) { swapper = sampleList[i-1]; sampleList [i-1] = sampleList[i]; sampleList[i] = swapper; } } } //discard % of top and bottom values, average the rest int avgStart = max(samples * DISCARD_PCT, 1); //array start int avgEnd = min(samples * (1.0 - DISCARD_PCT), samples); //array end int avgSamples = 0; float ret = 0; //average out the values for (int cntr = avgStart; cntr < avgEnd; cntr++) { avgSamples++; ret += sampleList[cntr]; } return ret / (float)avgSamples; } //read trim pot, get average samples int getReadCount() { analogRead(INPUT_RATE); int readCount = analogRead(INPUT_RATE); return map(readCount, 0, 1023, 4, MAX_SAMPLES); } //set RGB led values void setLED(int RED, int GREEN, int BLUE) { digitalWrite(LED_RED, RED); digitalWrite(LED_GREEN, GREEN); digitalWrite(LED_BLUE, BLUE); } |
Binary sketch size: 7,196 bytes (of a 32,256 byte maximum)
Estimated memory use: 103 bytes (of a 2,560 byte maximum)
This is probably as close as I can get to get an accurate reading that doesn’t jump around too much. Filters most noise while giving a decent sample rate. Currently reading all 4 inputs. Technically could reduce to 3 inputs since 4th won’t be used for a while. Will see how well Gauge Pod software deals with current feed rate.