The Electronic Speed Controller (ESC) can be controlled in the same way as the servo using the Raspberry Pi’s GPIO pins. The biggest difference is that the pulse widths to control it are different.
To find out what pulses made it do what I decided that it would be easier to set the duty cycle to 10o and then to change the pulse widths based on this. I discovered that then at a pulse width of 15 the ESC made the motor go forwards, and after much trial and error, that a pulse width of 13 made it go backwards.
Control via PHP
PHP runs python scripts on page load, so I needed to have the control buttons link to a page with the python script called on it. I could do this in a similar way to how I did with the servo control and have several different pages that look the same but with different scripts called when they load, however this would leave me with over 10 different pages, and this seems like a lot.
window.setTimeout(‘history.back();’, 1000); // waits a specified number of seconds before going back
This simple bit of script went into the <head> of my document. By changing the value ‘1000’ to another number you can change the time period that the page waits before going back. I then decided to add a simple message to the page that told the user that their previous command was currently in progress. I decided a .gif would make the page more interesting.
After many unsuccessful attempts at working the stepper motor I came across other projects using the Raspberry Pi that connected it to the servo and speed controller of a radio controlled car. Although in total more expensive and less customisable, it would save me plenty of time and effort to create the rover itself this way.
The biggest drawback would be the battery life. The rechargeable battery pack for the Pi lasts about 6 hours but the battery for the car is supposed to last only 15 minutes. I guessed it would last longer however because I would only be using it in short bursts of less than a second at a time, and the ESC would be drawing power from the Pi rather than that battery.
I chose a 1/18th scale model because it is smaller and therefore will have more room to move around.
After purchase the first thing I did was connect the servo in the same way as the one I was planning on using to move the camera with. This worked fine, however, the wheels obviously do not turn 180 degrees in each direction so I had to adjust the pwm time so that it would turn the wheels up to a maximum of 45 degrees in each direction. I also upgraded to a metal geared servo to stop the gears from shredding so easily if it was trying to turn too far in one direction or the other.
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.
- 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.
- 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
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.
<?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>
Current Wheel Direction: Left
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.