YOU LEARN HERE...

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

SENSOR VALUES

The position sensor (acceleration sensor) measures both the constant gravitational acceleration of around 10 m/s², which points vertically downwards, and the accelerations caused by movements.

The position sensor is clearly visible on the board. Similar sensors are also built into most smartphones. The sensor measures the x, y and z components of the acceleration, from which the forward tilt (pitch) and sideways tilt (roll) can be calculated.

It is similar to the attitude display of aeroplanes, which are displayed with an artificial horizon. If you tilt the board forwards or backwards, the pitch angle changes; if you tilt it sideways, the roll angle changes.

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

If the z-axis is pointing vertically downwards, the value of the acceleration due to gravity, i.e. 9.81 m/s²must be measured. The sensor returns approximately 981.

Program: [► WebTigerPython]

from calliope_mini import *

while True:
    ax = accelerometer.get_x()
    ay = accelerometer.get_y()
    az = accelerometer.get_z()
    print(ax, ay, az)
    sleep(1000)

EXAMPLES

Example 1: Query sensor values Start the programme below, tilt the Calliope left and right around the USB port and observe the sensor values that are displayed in the output window with print(acc). The display shows the arrow ARROW_E for a positive value and the arrow ARROW_W for a negative value.

Program: [► WebTigerPython]

from calliope_mini 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)  

Example 2: Spirit level vial   The Calliope should work 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: [► WebTigerPython]

from calliope_mini import *

x = 1
y = 1

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

REMEMBER ...

You can use the acceleration sensor to record the movements and position of the Calliope. The sensor values are regularly queried every 100 milliseconds in an ‘endless’ while true loop. This is also known as polling the sensor values.

TO SOLVE BY YOURSELF