7.1 KiB
SRF05
Arduino library for the SRF05 distance sensor and compatibles.
Description
The library allows to adjust to the speed of sound. Reasons to use a different value are temperature, humidity, type of gas, composition etc.
Default value for the speed of sound is set to 343 m/s. (~20°C)
Several correction formulas for the speed of sound are available on the internet.
// temperature in °C
v = 331.3 + 0.606 * temperature (m/s)
v = 331.4 + 0.606 * temperature + 0.0124 * rel_humidity (m/s)
v = 20.05 * sqrt(273.16 + temperature) (m/s)
In fact humidity has an effect which increases with temperature so the formula is more complex See - https://forum.arduino.cc/t/ultrasonic-sensor-to-determine-water-level/64890/12
| temp | 10% | 20% | 30% | 40% | 50% | 60% | 70% | 80% | 90% | notes |
|---|---|---|---|---|---|---|---|---|---|---|
| -30 | 313.5 | 329.5 | 312.9 | 329.6 | 313.0 | 329.6 | 313.1 | 329.7 | 329.7 | extrapolated indication |
| -25 | 316.5 | 332.6 | 316.0 | 332.7 | 316.1 | 332.7 | 316.2 | 332.8 | 332.9 | extrapolated indication |
| -20 | 319.5 | 335.6 | 319.1 | 335.7 | 319.2 | 335.9 | 319.3 | 336.0 | 336.0 | extrapolated indication |
| -15 | 322.5 | 330.5 | 322.2 | 330.6 | 322.3 | 330.6 | 322.4 | 330.7 | 330.7 | extrapolated indication |
| -10 | 325.5 | 333.6 | 325.3 | 333.7 | 325.4 | 333.7 | 325.5 | 333.8 | 333.9 | extrapolated indication |
| -5 | 328.5 | 336.6 | 328.4 | 336.7 | 328.5 | 336.9 | 328.6 | 337.0 | 337.0 | extrapolated indication |
| 0 | 331.5 | 331.5 | 331.5 | 331.6 | 331.6 | 331.6 | 331.7 | 331.7 | 331.7 | |
| 5 | 334.5 | 334.6 | 334.6 | 334.7 | 334.7 | 334.7 | 334.8 | 334.8 | 334.9 | |
| 10 | 337.5 | 337.6 | 337.7 | 337.7 | 337.8 | 337.9 | 337.9 | 338.0 | 338.0 | |
| 15 | 340.5 | 340.6 | 340.7 | 340.8 | 340.9 | 341.0 | 341.1 | 341.2 | 341.2 | |
| 20 | 343.5 | 343.6 | 343.7 | 343.9 | 344.0 | 344.1 | 344.2 | 344.4 | 344.5 | |
| 25 | 346.4 | 346.6 | 346.8 | 347.0 | 347.1 | 347.3 | 347.5 | 347.6 | 347.8 | |
| 30 | 349.4 | 349.6 | 349.9 | 350.1 | 350.3 | 350.5 | 350.8 | 351.0 | 351.2 | |
| 35 | 352.4 | 352.6 | 353.0 | 353.2 | 353.5 | 353.7 | 354.1 | 354.4 | 354.6 | extrapolated indication |
| 40 | 355.4 | 355.6 | 356.1 | 356.3 | 356.7 | 356.9 | 357.4 | 357.8 | 358.0 | extrapolated indication |
| 45 | 358.4 | 358.6 | 359.2 | 359.4 | 359.9 | 360.1 | 360.7 | 361.2 | 361.4 | extrapolated indication |
| 50 | 361.4 | 361.6 | 362.3 | 362.5 | 363.1 | 363.3 | 364.0 | 364.6 | 364.8 | extrapolated indication |
| 55 | 364.4 | 364.6 | 365.4 | 365.6 | 366.3 | 366.5 | 367.3 | 368.0 | 368.2 | extrapolated indication |
| 60 | 367.4 | 367.6 | 368.5 | 368.7 | 369.5 | 369.7 | 370.6 | 371.4 | 371.6 | extrapolated indication |
(table based upon https://www.engineeringtoolbox.com/air-speed-sound-d_603.html)
The library has several ways to improve the quality of the measurements. By taking the average or the median of multiple readings.
The library allows to set a correction factor to compensate for the timing of the pulseIn() function. This has in the end the same effect as changing the speed of sound however it is technically more correct to split the two.
Interface
- SRF05(const uint8_t trigger, const uint8_t echo, const uint8_t out = 0) constructor to set the trigger and echo pin. It is not clear yet what the purpose of the OUT pin is.
Configuration
- void setSpeedOfSound(float sos) adjust the speed of sound.
- float getSpeedOfSound() return set value.
- void setCorrectionFactor(float cf = 1) adjust the timing by a few percentage e.g. to adjust clocks. Should not be used to correct the speed of sound :)
- float getCorrectionFactor() returns the current correction factor.
Normally a single read should be OK.
- void setModeSingle() read single time.
- void setModeAverage(uint8_t count) read count times and take the average.
- void setModeMedian(uint8_t count) read count times and take the median. count = 3..15
- void setModeRunningAverage(float alpha) use a running average algorithm with a weight alpha. (value depends on your application).
- uint8_t getOperationalMode() returns the operational mode 0..3.
| operational mode | value | Notes |
|---|---|---|
| SRF_MODE_SINGLE | 0 | |
| SRF_MODE_AVERAGE | 1 | |
| SRF_MODE_MEDIAN | 2 | |
| SRF_MODE_RUN_AVERAGE | 3 | |
| other | error |
Get distance
- uint32_t getTime() returns distance in microseconds.
- uint32_t getMillimeter() returns distance in millimetre.
- float getCentimeter() returns distance in centimetre.
- float getMeter() returns distance in meter.
- float getInch() returns distance in inches. (1 inch = 2.54 cm).
- float getFeet() returns distance in feet. (1 feet = 12 inch).
Experimental - calibration
Put the sensor at e.g. exactly 1.00 meter from a wall, and based upon the timing it will give an estimate for the speed of sound. 0.1.2 version seems accurate within 5 %.
- float determineSpeedOfSound(uint16_t distance)
Need to compensate for temperature and humidity.
Performance
Assumes default speed of sound of 340 m/sec. Distance is in meters and equals total distance (forth and back).
| Distance | time (us) |
|---|---|
| 1 | 2.94 |
| 2 | 5.88 |
| 5 | 14.71 |
| 10 | 29.41 |
| 20 | 58.82 |
| 50 | 147.06 |
| 100 | 294.12 |
| 200 | 588.24 |
| 300 | 882.35 |
| 400 | 1176.47 |
Operational
See examples.
Future
must
- investigate purpose of the OUT pin.
should
- add examples
- DHT22 and the formula for SOS
- add
get- setTriggerLength() - add
float calcSOS(float temp, float humidity = 0); - fix magic conversion numbers.
could
- add
float lastValue()?- not all paths update this variable.
- add
float delta()difference with last value.- how to handle different units? or only time?
- set default SOS to an SOS from the table instead of 340.
wont
- print feet as 3'2" or 3-7/8 feet (is that needed in this lib)
- in printHelpers ?