Fric by Chris Burns

Fric is a small, STAMP powered, autonomous robot.  It's 
inspiration comes from EPFL's ALICE robot and DIDEL's
SWIBOT.  These are tiny robots built using SWATCH stepper 
motors.  These motors draw a fracion of the power
that DC motors draw (15 to 20mA compared 75 mA for a pager 
motor) and can be run directly from the STAMP module
without intervening circuitry.  This means you can make a 
robot with very few components, basically, a CPU, the
motors and a power supply.

Fric's specs:

size:  7 cm long by 9 cm wide by 2 cm tall.
Electronics:  Stamp BS2 and a 74hc164 shift register
motor:   2 SWATCH bipolar stepper motors
power suply: 9v battery
sensors: IR detector and 2 emmitters for obstacle detection
cost:  $20 for the SWATCH motors, $49 for the STAMP, $5 for the rest.

I used a shift register to actually run the motors.  
Originally I ran it directly from the STAMP, but that uses
6 pins.  The hc164 only uses two pins.  I had to experiment 
with different famillies of chips before I hit on
the "hc" familly, the others couldn't sink or source enough 
current to drive the motors effectively.  

There are two problems with using these SWATCH motors. 
Unlike a DC motor, the CPU has to be an active
participant in driving it.  With a DC motor and an H-bridge 
the CPU sets the control pins and goes it's merry
way.  With stepper motors the CPU has to constantly change 
the state of the control pins, AND it has to do that
rather quickly.  The stepper motors won't move otherwise.  
This means that the CPU spends most of it's time
moving the motors.

The other problem is that these motors deliver very little 
torque.  That is the trade off for using very little
current.  Fric's environment is my linoleum floor or table 
top.  It can't handle rough surfaces.  Part of the
problem is the gear ratio of 1:180.  I found that by adding 
another gear train the torque can be improved
significantly.  The price is, however, speed.  The fastest 
you can move the steppers is 600 degrees a second,
100 RPM.  So, adding gears slows it down to a snail's pace.

Future plans are to complete Fric's brother, Frac (he is 
awaiting a brain), finding a lighter power supply (the
battery amounts to half of Fric's weight) and trying 
smaller CPU (I figure I can make a robot about a quarter
of the size of Fric).


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