Working With Electronics: Controlling the Servo Over the Web

Python Scripts

Using the script that I had created in Python which allows the servo to turn to both extremes (left and right) and the neutral position, I thought about possible ways of the user selecting the direction that they want the rover to travel in and how I would create a Python script to do this.

  1. Allow the user to specify the exact angle that they wish the wheels to turn to (from 180 degrees as full right, to -180 degrees full left). This is the most preferable option as it gives the user maximum control of the vehicle and is probably closest to how Martian Rovers actually operate since the drivers would need a high level of precision to navigate tricky terrain. I therefore calculated the following formula(a/170)+1.5/20.00*100.00Where a is the angle that you wish to turn to. However, when using this in Python it did not seem to want to calculate it correctly. This would also require me to have Python edit the script based upon what the user inputted on the webpage.
  2. Have 3 options for wheel turn angle – Left, Centre and Right. This would allow me to use the values that I already know, and I could very easily create a script where only 1 value would need to be changed in order for this to work. I could do this with one script that takes feedback from the web, or I could do it with multiple scripts that activate when necessary.

Setting Up The Web Server
Using this Instructables tutorial, I managed to install Apache and php5 as well as clients to allow me to upload files to the Pi remotely. Once I figured out

the www folder

the /www/ folder

that the Raspberry Pi’s web server path /home/pi/www/ worked the same as any other web directory it was easy for me to create folders and pages within it that could easily be referenced through the HTML. I decided that all my images would go in /images and all my python scripts in /python.

Creating a Basic Control Interface

I discovered that in order to run python scripts from the web you need to call .php pages to open which have “<?php exec(‘sudo python /full/directory/here/scriptname.py’);?>” in their body. Therefore I figured out that I could link from the index.php page to a similar or identical page except that it calls the Python script on opening. This would cause the script to run and the servo to move.

I therefore created some basic images that said left, centre and right on them and put in some basic text that told the user what direction the wheels would now be facing and created two similar .php files – one with and one without the ‘exec’ function.

web<!DOCTYPE html>
<html>
<head>
</head>
<body>
<?php exec(‘sudo python /home/pi/www/python/servoleft.py’);?>
<a href= left.php><img src=”images/left.png” alt=”Turn Left”></a>
<a href= centre.php><img src=”images/centre.png” alt=”Centre Wheels”></a>
<a href= right.php><img src=”images/right.png” alt=”Turn Right”></a>
</br>
Current Wheel Direction: Left
</body>
</html>

Once I was sure that this worked, I then duplicated and edited the Python scripts appropriately, putting them in the /python folder and created the missing .php files using the above template, before ensuring everything linked up correctly.

This allowed me to remotely change the direction of the servos.

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