Last time around, on day 23 of the build diary, we connected up an I2C rangefinder and compass module to the Raspberry Pi, and checked their functionality using stand-alone applications. Today, we’re going to integrate them into the main code that runs the tank,
Step 1. To the C!
Each of the sensors was provided with a few lines of C code which could be used to read data from them and display it on the screen.
It proved very easy to combine these into a single routine – we don’t need to open and close the I2C port once for the compass and again for the rangefinder – and add it to
rt_http. The new sensor sampling routine runs in its own thread, and every second or so uses a mutex lock to dump the sensor readings to global variables. (Hardly elegant, I know.)
It also dumps the readings to
stdout for debug purposes, which you can see in action below.
rt_http Console Output
You can download the latest rt_http code from GitHub.
The intention was to add more functionality to the embedded HTTP server by providing a URL (e.g.
http://raspberrytank:3000/?get) that could be used to return this data. I tried this and it worked fine – you can still see the code in
rt_http.c, although it is commented out.
<div/> on the page. That would work in this case, except that the server that provides the Web UI pages runs at
http://raspberrytank:80/, while the Mongoose server embedded in
rt_http runs at
As a work-around, I decided to put the sensor data somewhere that
lighttpd – the server behind the main web interface – could find it. Unfortunately I achieved this by getting the sensor polling function to write the sensor data to
/var/www/sensordata.txt as well as
There are a few problems with this: it assumes that the web UI will be served from
/var/www, there is a potential conflict if the web server tries to read the file while the control code is writing to it, and it’s just plain ugly. But it works. I will make this more elegant later!
Step 3. Pulling in the Sensor Data
As I mentioned just now, the Web UI now uses jQuery to pull the sensor data into the web pages it serves. This is done with a
$.get() request using the jQuery library, which fires once a second.
Step 4. Forking the Web UI
The Web UI previously existed as a portrait-orientated interface that assumed the user was using a touch device such as a tablet or phone. As we discovered to our cost on day 17, the “press and hold” behaviour we used for the control buttons wasn’t appropriate for these devices, so we had to drop back to using a single press to start an action and a single press on the STOP button to stop it.
During testing I have mostly been using a laptop. Not only is “press and hold” possibly on a laptop, it’s much more user-friendly. A landscape orientation would also be much more usable on a laptop screen.
For this reason, the Web UI now comes in two flavours – laptop (non-touch) and phone/tablet (touch). Visiting
http://raspberrytank/ will give you a menu to choose which UI you prefer.
An example of the new laptop orientation, including the sensor data, is shown below.
Laptop Web UI with Sensor Data
As always, the Web UI code is available on GitHub.
All my code is released under the free, open-source BSD licence. (But we do use libraries such as Mongoose and jQuery that have their own licence terms.)