Important

This project was part of my coursework. Since the course has ended, it will no longer be updated.

SR04_Driver

A MicroPython SR04 Ultrasonic Distance Sensor driver.

SR04 not like other similar sensors, SR04 not use any bus protocols. SR04 use pulse time to return result. A typical SR04 Sensor like:

How to use

Create an SR04 object. Then use SR04.measure_centimeter() to get data.

How SR04 Measuring

Before measuring, SR04 need to initialize. To initialize properly, SR04 must connect ground first and wait 1 second after connecting the power. After initializing, give Trig pin 10μs HIGH pulse to trigger measurement. Then Echo pin will set HIGH when received reflected ultrasonic. Measuring Echo pin's HIGH pulse time to get ultrasonic reflected back time.

To get distance $d$, need to calculate with speed of sound $C$ and Echo pin's pulse time $t_{echo}$, like:

$$ d = C \frac{t_{echo}}{2} $$

For example, to get centimeter distance:

$$ d = \frac{1}{2} \times 0.034 cm/μs \times t_{echo} μs $$

$$ d = 0.017 \times t_{echo}(cm) $$

SR04's accuracy is 3mm and max measuring distance is 4 meter, so keeping one decimal place is necessary:

$$ d = \mathrm{round}(0.017 \times t_{echo}, 1) $$

Compensation Error

For the speed of sound $C$, it's about $340 m/s$ when air is 15℃ and at 1atm. Normally, $340 m/s$'s accuracy is enough. But for extreme conditions, speed of sound $C$ need compensation. $T$ is temperature in ℃:

$$ c_{fixed} = C \times \sqrt{1 + \frac{T}{273}} \approx 331.4 + 0.607T(m/s) $$

Here is relationship between speed of sound and common temperature:

Temperature(℃)	Speed of sound(m/s)
-30	313
-20	319
-10	325
0	332
10	338
15	340
20	344
30	349
100	386

To apply compensation, need to give speed of sound param when construct an object:

from library.SR04_driver.sr04_driver import SR04
from machine import Pin
driver = SR04(pin_trig=Pin(14),
              pin_echo=Pin(2),
              speed_of_sound=344)