BBC Micro:bit some data-logging and control examples

Transcription

1 BBC Micro:bit some data-logging and control examples. Adrian Oldknow 12 th January 2018 A very basic experiment for a simple pendulum is show on the right. I have suspended a BBC micro:bit from a retort stand using its own USB serial cable. The main part of the program, written in the Microsoft MakeCode block editor, is below. It sends sensed data to the PC through the USB cable, using the `Serial blocks in the Advanced menu. It uses 3 functions. `Setup initialises the variables `samples and `gap which control the duration and frequency of the sampling. The variable `pressed is used to check when button A is pressed. The function `WhenA waits until button A is pressed before starting to log the data. `SENDserial writes data to the PC s USB serial port. When all the data have been sent, the micro:bit displays `F. The free Tera Term VT software is used to receive the data from the micro:bit. It detects which Com port is being used, and you need to use Setup to change the baud rate to , the speed with which the micro:bit sends data. The `SENDserial function writes data as strings and puts a, between them.

2 You can copy and paste the data into PC software, such as Excel. But not with great success! Using the `Spreadsheet view in the free GeoGebra software does the trick. Now you can do some analysis in GeoGebra, or copy and paste it back into Excel. I have created columns C and D by dividing the data in each of columns A and B by In order to construct the scattergram of D against C, I have first selected the D column and then, with the Shift key held down, the C column. One of the options in the Spreadsheet view is to perform a `Two Variable Regression Analysis. Selecting this creates a scattergram and offers you a choice of `Regression Model. The screen shot shows an attempt to fit a sine function. Copying and pasting columns C and D into Excel allows you create a scattergram there instead. You can also fit a `trendline to your data, although this does not include a sine function fit. Of course these are not the only applications which can be used with our data set.

3 A very powerful Science tool for datalogging is Vernier s Logger Pro 3 software. This is designed to be used with a wide range of Vernier sensors, but you can also paste in your own data. The Time and Acc data have been pasted into the Data Set to produce the scattergram. Logger Pro has computed its best fit sine function for the data, and added an extra CC column of computed data for comparison. You can also take a video clip of the experiment and import that into Logger Pro. You can play it manually, or you can synchronise it with the data so that tracing a point on the graph moves to the corresponding video frame. The UK distributor, IDS, sells a school site-licence for 350 which covers all students and staff having copies of the software in school and at home. I very much hope that the software will be developed to accept sensed data from micro:bits by either USB serial cable or via BlueTooth Low Energy (BLE) wireless. A science application which will collect data wirelessly using BLE is the very friendly Bitty Data-logger App for Android and Apple mobile devices. Here is a second experiment with a micro:bit and battery strapped to a heavy wooden model elephant suspended from a large spring. You need to download the hex file for the micro:bit here. Once this has been uploaded to the micro:bit, you will be asked to `Draw a circle to calibrate the on-board magnetometers. Once the micro:bit is set up and waiting, you press the `Scan button on App to see which micro:bits are set up with BLE. It has found mine, so when I click on it I can start entering the settings for my experiment. You now need to press `Settings and make some choices about how to collect the data.

4 The first trial just shows accelerometer data from the y- (vertical) sensor. When you press `Start, the datalogging and graphing begin. When you press `Stop, the data is stored in a `comma separated variables file (CSV). You press `Results to see the name of your file, which includes todays date and time. In order to export it to the PC you use `Share, which gives you plenty of options. I usually just the file to myself with the CSV file as the attachment. Then I can open it from the message in my PC s Inbox directly into Excel and chose which columns to use for graphing data. As the elephant only moves up and down, the x-accelerations are zero. As the micro:bit is not quite vertical on Ellie s back, there is a small z-component of acceleration. Now we have the data in Excel we can do some plotting and analysis. The red-blue graph is a scatterplot of the y (blue) and z (red) components of acceleration. The units are those of gravity: g ms -2. The red-green-blue graph shows x-, y- and z- against t. Column E is the values from A divided by 1000 to give times in seconds. Column F is the negative of the magnitude of the acceleration. The blue graph is the data from column F against that from column E so it is the acceleration-time graph for Ellie s swings. I have computed the mean value from column F as The expected value is -1, so a pretty accurate result. The data logger will also capture data from probes attached to the micro:bit s analog pins P1 or P2.

5 This technique uses my expensive Samsung smart phone to collect the micro:bit data wirelessly and to send it my PC via my home wireless broadband. We can do that in a much cheaper and simpler way by using two micro:bits. One micro:bit, call it M1, will be collecting data and sending it using MakeCode s `Radio commands to the second micro:bit M2, which will be attached by a USB cable to a PC. This will use `Radio commands to receive the data, and then `Serial commands to transmit them. Here is the program for M1. There are just three blocks. Òn start turns on the radio and displays `R. The Òn radio block waits for a string to be sent from M2 to trigger the data-logging and to pass values for the variables `max and `gap. The `senddata function uses a counted loop to collect and send the data to M2. `tim is the elapsed time in milliseconds and àcc is the size of the acceleration in milli-g. The program for M2 also has 3 blocks. Button A is pressed to start the data-capture. This sends 3 values to M1 which trigger the data-logging and radio transmission. Each time a value is received from M1, the òn radio block stores it in a variable. When the àcc value is received it calls the `serialline function which sends the 3 variables, count, time and acc, as a serial line.

6 So first open Tera Term and then press button A on M2. After 10 seconds we have collected all the data, ready to copy and paste them into a PC application, such as GeoGebra, where we can do the display and modelling. Instead of using Tera Term to collect the transmitted data, we can send it directly to Excel using a free plug-in from Microsoft called Project Cordoba. You need to request a copy of Cordoba from the Microsoft Garage Project s site. Use this link. You should get a quick response. When you install it there will be an extra `Project Cordoba tab at the end of the toolbar. The first option is to `Connect a Device, which will detect that you have attached a micro:bit. It also tells you which COM port it is using, COM10 in my case.

7 If you click on `Start Data and then on `Stop Data, you will see that three extra tabs have been added at the foot of the sheet. The display shows the `Data In sheet. This has a row of cells which will receive incoming data, and a block beneath them which will build up the received data set. `Data Out has a row of cells which will transmit data to the micro:bit. `Settings allows you set the rate of transmission for the data in ms, the size of the storage area and the number of channels to use for sending and receiving. There is one other important setting to make, which is the baud rate for serial communication, set to 9600 by default. Click on the `Advanced icon to open Cordoba s `Console. Change the 9600 baud setting to the micro:bit s value of We already have the code to send data from micro:bit M1, on the pendulum, wirelessly to micro:bit M2, connected to the PC. We just need to tweak the code for M2 to receive M1 s data and to forward it down the serial cable to Excel s `Data In. We can also use Excel to output the signal to start the experiment s datalogging. First, we ll try a little experiment to use Excel to send data to M2 to signal the start of the experiment. In `Settings I have chosen to have just 3 Data Channels. On the `Data Out page I will use cell A5 (CH1) to control the output from Excel to micro:bit M2.

8 Looking at the `Console display you can see that each output TX is a line consisting of the contents of cells A5, B5 and C5 separated by commas. A new output is sent whenever any cell is changed. Here is the receiving program for the serial output test. When you press Button A, the program will continuously read the lines of transmitted data as a string. The variable CH1 will hold the first character from the received string. Although it was stored as a number in Excel, it is sent as a string for the serial output. The program carries on receiving and displaying incoming data from the `Data Out cell A5 until the value `1 is entered. M2 will be sending 3 readings per sample: the sample count, the calculated time in ms and the `acceleration strength in thousandths of g ms -2. We now have a few decisions to make about the number of samples to collect, the gap between samples and the sensed data to transmit. I have chosen to collect 200 samples at 50 ms intervals. So we can set these up on the Excel `Settings sheet. The number of data rows is stored in the Excel variable `DataRowCountSettings, and the data interval is in `DataDelaySetting. These can be used to copy the values from `Settings into CH2 and CH3 of `DataOut. The new code for M2 is below.

9 In order to receive and decode the comma-separated, serial string sent from Excel, we need to do some text-handling and decoding to extract the data from CH2 and CH3. The line of data is held in the variable `string. The first character is tested until a `1 is received. Then `collectstring function stores the length of the string in `length and calls the `splitstring function which uses commands from the `Advanced and `Text blocks. The variable `left holds the single digit from CH1 as a string. Its numerical value is stored as `trigger. Similarly, `centre holds the three digit string from CH2, and `max holds its numerical value. `right holds the final 2 digit string from CH3, and `gap holds its value. The `senddata function passes the values of trigger, max and gap wirelessly to M1. The other two blocks receive the data wirelessly from M1 and send them by the serial cable to Excel where they are collected in `DataIn. Once there, you can highlight columns C and D, and `Insert a `Scattergram. We have no need to edit the program for M1 which is already set to receive the line of data from M2 carrying the values of trigger, max and gap wirelessly. So with the micro:bit, Excel and Project Cordoba, we have a very simple, powerful and cheap datalogging and modelling system which can be used for telemetry. But actually it is even more powerful than that, because we have spare capacity in the micro:bit to carry out control operations.

10 As a very simple example, we can output a tone to a buzzer attached to micro:bit M1 s P0 pin which varies according to the size of the acceleration. The Kitronik MI-power case for the micro:bit not only provides a 3V power source from a disc battery, but also has an on-board buzzer. We can use the `ring tone command from the `Music menu. The frequency is given as a number of Hertz (Hz), cycles per second. In the little amendment to the M1 program we store the acceleration strength in the `accval variable, which is sent out by the 2 nd `radio send block. We can do a little maths to make sure the frequency is a sensible value say between 0 and 1000 Hz. At the end of the data transfer we can use a `rest command to turn the buzzer off. This is a pretty useless example of control. Maybe you could connect light and temperature sensors to pins P1 and P2 to gather and display data, e.g. in Excel, while also using the micro:bit to control an LED and fan according to the sensed environment? Such decisions could be made locally in the micro:bit program, or remotely by a PC App. Now the principle of 2-way data communication by serial cable or Bluetooth Low Energy radio transmission has been established, the world is indeed your oyster. I am constantly amazed by the creative potential that lurks within such a mighty electronic mouse costing a mere 15.