[Yaaarc] Joshua's Barbie Jeepbot project

[Keith Mc. (Route-To YAR)]

In another thread (now moved to this one... :-), 
"Joshua Bauer" <flamingtennisball at gmail.com> wrote:
> hello I'm Joshua Bauer, I'm 15 and I have been in some or Dr. George's
> robotic classes. [...] [My friend and I have a Barbie car and want to 
> make a large R/C car out of it]

> [Questions]
> 1. getting the remote controlled car to control the Barbie car

Use a Y3MD controller board.  See us at the meeting about obtaining one.

If you wish to use it with an R/C receiver instead of a micro board,
there's already a hack where you take the guts out of a dirt cheap R/C
servo and drive the Y3MD with it.  This makes the Y3MD into a high
current servo controller.  We can describe it to you in more detail 
at the next meeting.

> 2. getting the Barbie car to turn [...]

Two methods are easy:
A) Add a motor to steer it, as you suggest.  But this limits you
   to car style turns.

B) *IF* your jeep uses two drive motors, you can go "differential drive".
   First, convert the two front wheels to large swivel castors, available
   at any hardware store.  (Use ball bearing swivels for best performance.)
   Now split the wiring between the two wheel units, and control each
   one independently with separate Y3MD channels, "tank style".  This
   is best controlled with a two joystick R/C transmitter, vs a "trigger 
   and knob" one.

> I bought a drill for a few bucks at a yard sale it is 9.6 volts and is
> enough to turn the barbie cars wheels. i just need to figure out how 
> to hook it up to go both directions.

The drill is a Permanent Magnet (PM) motor, which is inherently 
reversable. Simply reverse the polarity of the voltage applied to it.  
This can be done several ways:

Simplest - Again, just add a Y3MD control channel to it, plus the
servo hack as described above. (THIS one can be easily controlled
with a trigger and knob controller.)

Now you'll have to be careful about overvoltage, if you are
using 12VDC.  The simplest "cheap and dirty" safe way is to just add 
an appropriate value power resistor in series with the motor after
the H-bridge, to drop off 1/5 of the voltage.  You'll need to know
the current the motor draws at a low voltage, and then use Ohms Law 
(E=I*R) to calculate that value.  We can show you how to do that at 
the next meeting.  Be sure to bring in your drill motor for testing,
so we can determine its current.

You mecanically have the motor drive a lead screw and nut system.
Be sure to add limit switches at each end to prevent overtravel
of your lead screw. (We can describe how to hook these switches
up electrically to allow you to back off of the end of travel
state, when you get that far in the build.)

Y3MD -

The basic Y3MD board allows up to three channels to be populated,
so you'll have plenty of channels for this application.

Both a "steering motor version with a single drive motor", or
a tank steer version (castors and two back drive motors) would
use only two channels.  

A steerable version with TWO drive motors should use three channels,
one for each motor, to prevent overloading the drive channel with the
pair of back motors. Simply common the *data* to the two servo modules
for the back two wheels, with a servo Y-cord from the receiver.

I hope this helps!  

- Keith Mc.