…code, cg, electronics, 3d printing…

More photos at the bottom. Click to enlarge.

Armed with the knowledge of my previous “Ponar” post, I successfully completed construction and programming of my “Arduino-based 4wd robot with servo-actuated Ping))) sensor”.   It was an enjoyable process with a lot of trial and error since: I’m still learning a lot about how the Arduino works, how hardware hooked up to it works, and electronics work in general.  To get things started, here is a video of it driving around my kitchen:

Here’s the parts list:

• Arduino Duemilanove (I’m sure any modern Arduino board will work though)
• Adafruit Motorshield
• Paralax Ping))) sensor
• Paralax (Fubata) “standard servo” and mounting hardware
• DFRobot 4wd mobile platform
• Extra 4xC battery holder (6v).
• Extra toggle switch (the robot kit comes with one, I needed two).

Here’s the Arduino program:

• robot4wd_06.pde

How it works:

• I wanted a simple robot to drive around avoiding obstacles using the ping sensor.  To do that, I sketched out (on paper) the general program, that went like this:
  • Ping to see if there is any obstacles in front.
  • If not, drive forward, while pinging.
  • If there is an obstacle, stop, and take a distance reading to the left and right of the robot.
  • Turn for a fixed period of time in the further direction.
  • Ping again:  If still blocked, keep turning that direction.  If not blocked, go back to drive mode.
• There are basically three modes the robot can live in:  Drive forward (mode 1), stop and scan (mode 2), and turn (mode 3).  When driving, it can do three things:  Drive straight, turn left, turn right.
• The hardware is configured like so:
  • The Adafruit Motorshield is plugged into the top of the Arduino.
  • The Arduino is powered off of 5xAA batteries (7.5v) in the belly of the robot.  These are wired to a switch sticking out the back of the robot.
  • The Motorshiled is powered off a separate 4xC battery pack (6v) on top of the robot.  These are wired to another switch on the back of the robot.
  • The 4 DC wheel motors & servo are powered off the Motorshield.
  • The Ping))) sensor draws power form the Arduino.

Things Learned:

• Next time, use 2-wheeled robot instead:  easier to steer around the house.  The “tank steering” method of this robot seems like it uses a lot of power and doesn’t work very good.
• I read it in a couple places, and it turns out it’s probably needed:  Solder a 1uf capacitor between the leads on the DC motors to help “smooth out” their operation.  Otherwise it seems little “hiccups” can happen while driving.
• Voltage, voltage, voltage:
  • I was having all sorts of problems getting the motors to work properly:  They’d start\stop “hiccuping” a lot.  Per the above note I added in extra capacitors on the motors themselves, but it didn’t solve all the problems:
  • Originally I had the motors hooked up to the 5xAA battery pack (7.5v) that came with the robot chassis.  The motors per the specs are rated at max for 6v.  But… I didn’t know this.  I thought it needed moar power.  So I went out and got a 6xC (9v) pack and powered the motors through that.  The stuttering got even worse (although, it went pretty fast when it behaved) and even did something really stupid:  Put one of the batteries in backwards… which caused it to melt-down and leak acid everywhere.  Sigh…
  • Finally (after I knew the correct voltage for the motors and servo) I went out and got a 4xC pack (6v), and that, combined with the extra capacitors, finally gave it a smooth ride.
• I had to solder in extra connections to the analog pins on the motorshield to allow fo the Ping))) sensor to passthrough it to the Arduino (acting as a digital pin).  However, the motorshield has the pins in the order of “signal\-\+”, while the cables from the Ping))) (and servo) are “signal\+\-“.  Not sure why the motorshield would break ordering convention, but I had to splice and re-wire the Ping))) cable to match the board.
• The motorshield docs say that servo2 is pin9, but it’s actually pin10.  That took me a while to figure out… :-S
• I ran into a lot of problems with the programming of the robot itself:
  • The behavior is to run, ping, scan, repeat.  But during “scan” it wouldn’t drive the servo full left\right:  Sometimes it would only  got left, never make it right.
  • To solve this, I put a “timer” in the code, that would only execute the main loop if a certain amount of time in ms had passed (30ms to be exact, which makes it run around 30fps in game terms).  This seemed to make it behave exactly how I wanted.
  • I thought I needed to tell the motors to run on every loop:  It turns out they’re a state machine:  once you tell them to run, they keep running until you tell them otherwise.  Knowing this helped me clean up the code.
• There doesn’t seem to be any official documentation I could find to the motorshield library, I had to crack open this header file to deduce what it could do:  AFMotor.h

Picture time!  Click to enlarge:

And a final shot of the wiring on the board per request:

Based on my PyLolGraph app , PyLolDraw extends it by creating a simple ‘paint’ application for the Arduino/ lolshield.

See its page for source code and examples.

(any flickering you see is due to the video capture, and not the application)

Or pylolgraph for short.  See latest info for this subject over on it’s official page.

My son and I soldered together a lolshield for the Arduino Duemilanove we picked up at the 2010 Maker Faire:

(Video Note: The flickering you see is based on the camera capture, it doesn’t actually do that…)

After making it… what to do with it?  I thought it would be fun to make a Python \ Arduino library that would allow you to send Pygame graphics to the lolshield, and that’s what I implemented.

There are two main parts to the library:

• The Python module pylolgraph.py for sending graphics to the Arduino\lolshield.
  • It requires Pygame, and pyserial.  Was made with Python 2.6.5, and Pygame 1.9.1
• The Arduino sketch pylolgraph.pde for receiving serial data, and displaying it.
  • It requires  the lolshield library.

And, I’ve made an example Pygame application illustrating usage:

• example01.py

You can also download all three in a zip here pylolgraph.zip

How to use:

• My system is WinXP.  Not tested on anything else.
• Download pylolgraph.pde, and upload it to your Arduino.  Make sure the BAUD variable matches that in your Python module (below).
• Download pylolgraph.py, put it in your Python path.
• Download example01.py:  Set it’s BAUD variable to match that in pylolgraph.pde, and set its PORT variable to be the port your Arduino is on (will vary depending on your OS).
• Start pylolgraph.pde on the Arduino.  Run example01.py from the command-line or double-click the icon: You should see a small Pygame window pop up:  When you click the LMB, a white donut will appear in the Pygame window.  The same donut should then appear on the lolshield.

Inspect the documentation in pylolgraph.py for more details on implementation.  Here is example pseudo-code for writing your own Pygame application with the code:

import pygame
import pylolgraph

# Setup some constants:
# Create a resolution that corresponds to the lolshield.
# See docs on the pylolgraph.LOLShield.setSurface() method
# for acceptable resolutions.
WIDTH, HEIGHT = (448, 288)
# Baud rate must be the same as setup in pylolgraph.pde
BAUD = 28800
# Port the Arduino is on, on my WinXP box.
PORT = 'COM4'

# Start Pygame application:
# Make surface that will have its data sent to lolshield:
screenSurf = pygame.display.set_mode((WIDTH, HEIGHT))

# Setup lolshield object:
lolshield = pylolgraph.LolShield(port=PORT, baudrate=BAUD)
# Set the surface that will have its pixel data sent to the Arduino\lolshield:
lolshield.setSurface(screenSurf)

# inside main loop:
    # Draw stuff to screenSurf.  Then:
    lolshield.sendData()