Working with the ultrasonic distance sensor

Ultrasonic distance sensor ("sonar") is a distance sensor based on the principle of measuring the time of a sound wave (about 40 kHz) propagation to the obstacle and back. The sonar can measure the distance up to 1.5 – 3 m with the accuracy of several centimeters.

HC-SR04 distance sensor

Installation

The distance sensor is attached to the body using double-sided tape. For obtaining acceptable results, the use of vibro-insulation is required. A piece of PU foam may be used for vibro-insulation.

Connection

Connect HC-SR04 to Raspberry Pi according to the connection diagram. Use 1.0 and 2.2 kΩ resistors and any free GPIO pins, e.g., 23 and 24:

Instead of a 2.2 kΩ resistor, you can use two 1 kΩ resistors connected in series.

There are several interchangeable pins GND and VCC 5V on Raspberry Pi. Use the [pinout] (https://pinout.xyz) to find them.

Reading the data

To read the data from distance sensor HC-SR04 library for working with GPIO is used – pigpio. This library is pre-installed in the Clover image, starting with version v0.14. For older versions of the image, use an installation guide.

To work with pigpio, start appropriate daemon:

sudo systemctl start pigpiod.service

You can also enable pigpiod auto launch on system startup:

sudo systemctl enable pigpiod.service

Thus, it becomes possible to interact with the pigpiod daemon from Python:

import pigpio
pi = pigpio.pi()

See detailed description of Python API in pigpio documentation.

An example of the code for reading data from HC-SR04:

import time
import threading
import pigpio

TRIG = 23  # pin connected to the Trig pin of the sonar
ECHO = 24  # pin connected to the Echo pin of the sonar

pi = pigpio.pi()
done = threading.Event()

def rise(gpio, level, tick):
    global high
    high = tick

def fall(gpio, level, tick):
    global low
    low = tick - high
    done.set()

def read_distance():
    global low
    done.clear()
    pi.gpio_trigger(TRIG, 50, 1)
    if done.wait(timeout=5):
        return low / 58.0 / 100.0

pi.set_mode(TRIG, pigpio.OUTPUT)
pi.set_mode(ECHO, pigpio.INPUT)
pi.callback(ECHO, pigpio.RISING_EDGE, rise)
pi.callback(ECHO, pigpio.FALLING_EDGE, fall)

while True:
    # Reading the distance:
    print(read_distance())

Filtering the data

To filter (smooth out) the data and delete outliers, Kalman filter or a simple median filter can be used. An example of median filtering implementation:

import collections
import numpy

# ...

history = collections.deque(maxlen=10)  # 10 - number of samples for averaging

def read_distance_filtered():
    history.append(read_distance())
    return numpy.median(history)

while True:
    print(read_distance_filtered())

An example of charts of initial and filtered data:

The source code of the ROS-node used for building the chart can be found on Gist.

RCW-0001 distance sensor

The RCW-0001 distance sensor is compatible with distance sensor HC-SR04. Use the instruction above to connect and work with it.

Flight

An example of a flight program with the use of simple_offboard, which makes the copter fly forward until the connected ultrasonic distance sensor detects an obstacle:

set_velocity(vx=0.5, frame_id='body', auto_arm=True) # flying forward at the velocity of 0.5 mps

while True:
    if read_distance_filtered() < 1:
        # if the obstacle is closer than 1 m, hanging on the spot
        set_position(x=0, y=0, z=0, frame_id='body')
    rospy.sleep(0.1)