…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: