EN articles update

8 new English articles

docs/en/sonar.md

@@ -0,0 +1,133 @@
+# Working with the ultrasonic distance gage
+
+Ultrasonic distance gage (*"sonar"*) is a distance gage 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.
+
+## Distance gage HC-SR04
+
+<img src="../assets/hc-sr04.jpg" alt="hc-sr04" width=200>
+
+## Installation
+
+The distance gage 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:
+
+<img src="../assets/raspberry-hc-sr04.png" alt="Connecting HC-SR04" height=600>
+
+> **Hint** Instead of a 2.2 kΩ resistor, you can use two 1 kΩ resistors connected in series.
+
+<!-- -->
+
+> **Hint** 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 gage HC-SR04 library for working with <abbr title="General-Purpose Input/Output">GPIO</abbr> is used – [`pigpio`](http://abyz.me.uk/rpi/pigpio/index.html). This library is pre-installed in the [Clever image](microsd_images.md), starting with version **v0.14**. For older versions of the image, use [an installation guide](http://abyz.me.uk/rpi/pigpio/download.html).
+
+To work with `pigpio`, start appropriate daemon:
+
+```(bash)
+sudo systemctl start pigpiod.service
+```
+
+You can also enable `pigpiod` auto launch on system startup:
+
+```(bash)
+sudo systemctl enable pigpiod.service
+```
+
+Thus, it becomes possible to interact with the `pigpiod` daemon from Python:
+
+```python
+import pigpio
+pi = pigpio.pi()
+```
+
+> **Hint** See detailed description of Python API in [`pigpio` documentation](http://abyz.me.uk/rpi/pigpio/python.html).
+
+An example of the code for reading data from HC-SR04:
+
+```python
+import time
+import threading
+import pigpio
+
+TRIG = 23 is the No. of the pin connected to the Trig contact of the distance gage
+TRIG = 24 is the No. of the pin connected to the Echo contact of the distance gage
+
+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():
+    done.clear()
+    pi.gpio_trigger(TRIG, 50, 1)
+    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 [emission](https://ru.wikipedia.org/wiki/Outbreak_%28statistics%29) [Kalman filter] (https://ru.wikipedia.org/wiki/Фильтр_Калмана) or a simple [median filter](https://ru.wikipedia.org/wiki/Median_filter) can be used. An example of median filtering implementation:
+
+```python
+import collections
+import numpy
+
+# ...
+
+history = collections.deque(maxlen=10)  # 10 - количество сэмплов для усреднения
+
+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:
+
+<img src="../assets/sonar-filtered.png">
+
+The source code of the ROS-node used for building the chart can be found [on Gist](https://gist.github.com/okalachev/feb2d7235f5c9636802c3cda43add253).
+
+## Distance gage RCW-0001
+
+<img src="../assets/rcw-0001.jpg" width=200>
+
+Ultrasonic distance gage RCW-0001 is compatible with distance gage 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](simple_offboard.md), which makes the copter fly forward until the connected ultrasonic distance gage detects an obstacle:
+
+```python
+set_velocity(x=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)
+```