| title | layout | meta-description | share | author | about | cats | simple-description | date |
|---|---|---|---|---|---|---|---|---|
Real-time Graphing of Microbit Data |
text-width-sidebar |
Show a real-time graph of data streamed from the microbit over Bluetooth. |
true |
jez |
Stream Data from the Microbit. |
datalogging |
Graphing in Real-time |
2016-12-23 10:20:00 UTC |
DRAFT NOTES:
Everything that follows is wrong.
ARM have a very cool video on YouTube about juggling and sensor reading. They wirelessly graph the readings from accelerometer readings on three microbits.
This example shows you how to do this with one microbit; I'm not as clever as ARM! You will need two microbits:
- TX microbit: sends the accelerometer data.
- RX microbit: receives the data and streams it to our computer over a serial connection.
The data is then charted on a computer attached to the RX microbit.
In this example, a TX microbit running PXT streams accelerometer readings to a RX microbit running Python. The RX microbit then sends data to the computer over serial.
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By default, the accelerometer has a sensitivity of 1g in addition to earth's gravity. If we're measuring the acceleration of a ball being thrown, we will need to measure a wider range.
This can be done with the sensitivity block:
Set it to 4 or 8g. Microsoft README about the sensitivity
The accelerometer on the microbit returns a value in milli-g across three axes: x,y & z. It can also calculate the total acceleration across all the axes.
Set it to return 'strength'. This is the total acceleration across each axis. PXT README about the acceration
We use the 'RADIO SEND' block to send the value wirelessly. The final program looks like this:
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We will use Python for the RX microbit and connect it to our computer with a USB cable. It will take the readings from the TX microbit and output the readings to the computer over a serial connection. This streaming data will be used to chart the readings.
The radio module must be imported and the radio switched on:
{% highlight python %}
import radio
from microbit import *
radio.on()
{% endhighlight %}
The program used on the computer is called SerialPortPlotter. It will take the data from the serial port and chart it.
The returned data must be in a particular format for the program to understand. For this program, it must be:
$variable1 variable2 variable3;
A maximum of three integers are separated from the other with a space. The frame of data begins with a $ and ends with ;.
We are charting only one variable so the output from the microbit is:
$variable1$
{% highlight python %} from microbit import *
import radio
radio.on()
while True: incoming_data = radio.receive() if incoming_data: print("$%s;" % incoming_data) # format: $incoming_data; sleep(10) {% endhighlight %}
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Only for use if using Python on the TX microbit as well.
In this example, a TX microbit running Python streams accelerometer readings to a RX microbit also running Python. The RX microbit then sends data to the computer over serial.
There are two issues to overcome if using Python: setting the sensitivity and returning total strength.
The sensitivity of the accelerometer cannot be set directly in Python. An i2c command must be sent to the accelerometer to change its sensitivity. This is done with:
{% highlight python %} i2c.write(0x1d, b'\x2a\x00') # disable the device
i2c.write(0x1d, b'\x0e\x02') # set range to 8g
i2c.write(0x1d, b'\x2a\x21') # enable device, sample rate of 50Hz {% endhighlight %}
The total acceleration (as plotted in the video) needs to be calculated. This is done with:
{% highlight python %} from microbit import *
from math import sqrt
i2c.write(0x1d, b'\x2a\x00') # disable the device
i2c.write(0x1d, b'\x0e\x00') # set range to 2g
i2c.write(0x1d, b'\x2a\x21') # enable with sample rate of 50Hz
while True: x = accelerometer.get_x() y = accelerometer.get_y() z = accelerometer.get_z()
total = sqrt(x**2 + y**2 + z**2)
print(int(total))
sleep(100)
{% endhighlight %}
This is the root of the sum of squares, or Pythagoras in 3d.
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Draft Notes:
I cannot do this with bluetooth in PXT.
I can do it in C++ with:
{
"microbit-dal": {
"bluetooth": {
"pairing_mode": 1,
},
}
}
I either need to turn off pairing in PXT
or work out how to pair with the Bluegiga BLED112 dongle.
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The output from the TX microbit is received by the RX microbit and read by the computer over a serial connection.
A charting program is used to plot the data.
Download the SerialPortPlotter program from the ARM website. Extract and run the SerialPortPlotter.exe program.
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In the top left you must select options for the port you wish to connect to. These are:
- PORT: Whichever COM corresponds to your microbit
- BAUD: 115200 is the default baud rate of the microbit in Python
- DATA: 8 bits
- PARITY: none
- STOP: 1 bits
Click 'connect' to chart data.
The frequency at which the chart is updated can be changed by introducing a sleep command on the microbit.
- AXES: The number of variables sent as data. Maximum of 3
- POINTS: Number of data points plotted
- YSTEP: Y-axis steps
- MIN: Minimum value charted
- MAX: Maximum value charted
DRAFT NOTES:
Ideally the plotting program should be in Python.
I cannot get it working with matplotlib.
I think I need to have a buffer for it to update smoothly.