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3. ACCELERATION SENSOR

 

 

YOU LEARN HERE...

 

how you can use the acceleration sensor to detect changes in position and movements of the micro:bit.

 

 

SENSOR VALUES

 

The micro:bit's acceleration sensor measures both the constant gravitational acceleration of around 10 m/s2, which points vertically downwards, and accelerations caused by movements.

The acceleration sensor is clearly visible on the board. Similar sensors are also built into most smartphones.

The sensor can measure acceleration (including gravitational acceleration) in the x, y and z directions. The forward inclination (pitch) and sideways inclination (roll) can be calculated from these components.

 

It is similar to the attitude display of aeroplanes. If you tilt the board forwards or backwards, the pitch angle changes; if you tilt it sideways, the roll angle changes.

In the programme, you use the accelerometer object and call accelerometer.get_x(), accelerometer.get_y() or accelerometer.get_z(), which return values in the range from approximately -2000 to 2000, corresponding to accelerations of -20 m/s2 and 20 m/s2. With acceloremeter.get_values() you get back all three values in one tuple.

 

 

EXAMPLES
 
Query sensor values
Start the programme below, tilt the micro:bit bit from the horizontal position to the left or down to the right and observe the sensor values that are written out in the terminal window.

In the programme, query the acceleration in the x-direction every 100 milliseconds in an endless loop and write it out in the terminal window using print(acc). In addition, you show a left arrow on the display for a positive value and a right arrow for a negative value.
  

Program: 

from microbit import *

while True:
    acc = accelerometer.get_x()
    print (acc)
    if acc > 0:
        display.show(Image.ARROW_E)
    else:
        display.show(Image.ARROW_W)
    sleep(100)
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Spirit level vial

 
The micro:bit should function like a spirit level. You use the fact that the x and y components of the accelerations are approximately 0 when the board is in a horizontal position.
 

 

Only one pixel is displayed, which shifts to the right, left, up or down depending on the tilt. The aim is to align the level so that the centre LED lights up.

In the programme, you use four conditions to determine the x and y values of the pixel and then delete the pixel before lighting the new one.

Program: 

from microbit import *

x = 1
y = 1

while True:
    accX = accelerometer.get_x()
    accY = accelerometer.get_y() 
    if accX > 100 and x < 4:
        x += 1
    elif accX < -100 and x > 0:
        x -= 1
    elif accY > 100 and y < 4:
        y += 1
    elif accY < -100 and y > 0:
        y -= 1
    display.clear()
    display.set_pixel(x, y, 9)
    sleep(100)
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REMEMBER YOUR...
 

You can use the acceleration sensor to record the movements and position of the micro:bit. The sensor values are usually scanned regularly every 10 - 100 milliseconds in a measuring loop. This is also known as polling the sensor values.

 

 

TO SOLVE BY YOURSELF

 

 
1.
Add to the first example so that the rotation of the micro:bit in the y-direction is also measured and displayed with the appropriate arrows Image.ARROW_N or Image.ARROW_S.

2.

By tilting the micro:bit, all pixels at the edge should be switched on individually.

 

  

 

 
 
     

 

 

   
3-2
Note:

You can calculate the pitch and roll angles from the three acceleration components. Start the following programme and tilt the board sideways or forwards and backwards. The rotation angles are displayed in degrees in the terminal window.

from microbit import *
from math import *

def getPitch():
    a = accelerometer.get_values()
    pitch = atan2(a[1], -a[2])
    return int(degrees(pitch))

def getRoll():
    a = accelerometer.get_values()
    anorm = sqrt(a[0] * a[0] + a[1] * a[1] + a[2] * a[2])
    roll = asin(a[0] / anorm)
    return int(degrees(roll))
   
while True:
    pitch = getPitch()
    roll = getRoll()
    print("p: " + str(pitch))
    print("r: " + str(roll))
    print()
    sleep(100)