Wednesday, March 15, 2017

Controlling the Raspberry Pi via a web browser

Web Controlled Robot

Now that we can stream video to a web page it would be nice to be able to remotely control our robot. To do this we will us the Raspberry Pi to run a web server that serves the page used to control the robot. Once we have this up and running you will be able to drive your robot around using a browser on your laptop via WiFi on your LAN.

As shown in the previous post, you can use the python command print(server) to see what URL you need to point your browser at to see the video and control your robot. The way the controls work is as follows:
  1. Typing the address of your Pi served page (e.g. into your browser will send a web request to the python program running the server, in our case
  2. RS_Server responds with the contents of index.html. Your browser renders this HTML and it appears in your browser.
  3. The broadcasting of video data is handled by the broadcast thread object in RS_Sever. The BroadcastThread class implements a background thread which continually reads encoded MPEG1 data from the background FFmpeg process started by the BroadcastOutput class and broadcasts it to all connected websockets. More detail on this can be found at pistreaming if you are interested. Basically the camera is continually taking photos, converting them to MPEG's and sending them at the frame rate to a canvas in your browser.
  4. You will see below that we have modified the index.html file to display a number of buttons to control our robot. Pressing one of these buttons will send a GET request to the server running on your Pi with a parameter of "command" and the value of the button pressed. We then handle the request by passing on the appropriate command to our MotorControl class. To do this we will need to bring together RS_Server and RS_MotorControl in our new RS_Robot class.

Modifying index.html

The index.html file provided by pistreaming just creates a canvas in which to display our streaming video. To this we will add a table with 9 command control buttons for our robot. You could get away with only 5 (Forward, Back, Left, Right and Stop) but looking ahead we know we will also need 4 more (speed increase, speed decrease, auto and manual). Auto and Manual will toggle between autonomous control and remote control (i.e. via the browser). Associated with each button is a JavaScript script that will send the appropriate command when the button is clicked.

In addition to controlling your robot via the on screen buttons you can use the keyboard. We have mapped the following functionality:

Up Arrow      = Forward
Down Arrow = Back
Left Arrow    = Left
Right Arrow  = Right
Space             = Stop
-                     = Decrease Speed
+                    = Increase Speed
m                   = Manual
a                    = Autonomous

You can modify the index.html to map whatever keybindings you want. Be aware that the keycode returned by different browsers isn't always consistent. You can use the JavaScript Event KeyCode Test Page to find out what key code your browser returns for different keys.

The manual and auto modes don't do anything at this stage. 

The modified index.html file is shown below.

<!DOCTYPE html>
    <meta name="viewport" content="width=${WIDTH}, initial-scale=1"/>
    <title>Alexa M</title>
    <script src="" type="text/javascript" charset="utf-8"></script>

        .controls {
            width: 150px;
            font-size: 22pt;
            text-align: center;
            padding: 15px;
            background-color: green;
            color: white;

    <style type="text/css">
            body {
                background: ${BGCOLOR};
                text-align: center;
                margin-top: 2%;
            #videoCanvas {
                // Always stretch the canvas to 640x480, regardless of its internal size.
                width: ${WIDTH}px;
                height: ${HEIGHT}px;

    function sendCommand(command)
        $.get('/', {command: command});
    function keyPress(event)
        keyCode = event.keyCode;
        switch (keyCode) {
            case 38:                // up arrow
            case 37:                // left arrow
            case 32:                // space
            case 39:                // right arrow
            case 40:                // down arrow
            case 109:               // - = decrease speed
            case 189:
            case 107:
            case 187:
                sendCommand('+');   // + = increase speed
            case 77: 
                sendCommand('m');   // m = manual (remote control)
            case 65:
                sendCommand('a');   // a = autonomous
            default: return;        // allow other keys to be handled
        // prevent default action (eg. page moving up/down with arrow keys)


    <h1><FONT color=white>Alexa M</h1>

    <!-- The Canvas size specified here is the "initial" internal resolution. jsmpeg will
        change this internal resolution to whatever the source provides. The size the
        canvas is displayed on the website is dictated by the CSS style.
    <canvas id="videoCanvas" width="${WIDTH}" height="${HEIGHT}">
            Please use a browser that supports the Canvas Element, like
            <a href="">Chrome</a>,
            <a href="">Firefox</a>,
            <a href="">Safari</a> or Internet Explorer 10
    <script type="text/javascript" src="jsmpg.js"></script>
    <script type="text/javascript">
        // Show loading notice
        var canvas = document.getElementById('videoCanvas');
        var ctx = canvas.getContext('2d');
        ctx.fillStyle = '${COLOR}';
        ctx.fillText('Loading...', canvas.width/2-30, canvas.height/3);
        // Setup the WebSocket connection and start the player
        var client = new WebSocket('ws://${ADDRESS}/');
        var player = new jsmpeg(client, {canvas:canvas});

    <table align="center">
    <tr><td  class="controls" onClick="sendCommand('-');">-</td>
        <td  class="controls" onClick="sendCommand('f');">Forward</td>
        <td  class="controls" onClick="sendCommand('+');">+</td>
    <tr><td  class="controls" onClick="sendCommand('l');">Left</td>
        <td  class="controls" onClick="sendCommand('s');">Stop</td>
        <td  class="controls" onClick="sendCommand('r');">Right</td>
    <tr><td  class="controls" onClick="sendCommand('m');">Manual</td>
        <td  class="controls" onClick="sendCommand('b');">Back</td>
        <td  class="controls" onClick="sendCommand('a');">Auto</td>


Python Robot Class

As Alexa M continues to evolve, so too will this robot class. For now we can keep things pretty simple. In addition to creating a robot class we have updated the motor control, servo and server classes. Rather than reproduce all the code, we will provide links to our Gist Repository where you can download the latest versions. For completeness, I will also provide links to the HTML and JavaScript library that you will need. All these files need to be in the same directory.

  1. version 1.0 - Run this script on your Pi to create a telepresence rover.
  2. version 1.1 - Updated to include command parsing.
  3. version 1.1 - New motor control methods.
  4. version version 1.2 - License added.
  5. index.html version 1.0 - The file shown in the previous section.
  6. jsmpg.js - Dominic Szablewski's Javascript-based MPEG1 decoder.
That completes the remote control and video streaming portion of the design. We hope you have as much fun driving around your robot as we do. Next up we will look at battery monitoring and autonomous control of the robot.

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