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DRIVING STEPPER MOTORS WITH THE L293D
Stepper motors are great to use in robotics. By energizing the coils in the motor
in a particular sequence, a motor takes 48 or more small, precise steps
to make one full revolution. If you are using two of these motors to drive your
robot’s wheels, you can get good control over how far it travels by making the
motors travel x many steps forwards or backwards. That’s assuming you can
find stepper motor drivers, of course. I’ve always had problems finding
good servo drivers. Once I was even reduced to using discreet components
(say it isn’t so!) to drive my steppers.
Then I got my hands on the L293D motor driver chip (See motors part 1)
and life got a lot easier. The L293D contains two H-bridges for driving small DC
- Now the home-viewing audience might say: "Rob, it’s easy to drive DC
- What I need to do is drive a stepper motor." No problem. After all,
what is a DC motor but a coil and the L293D drives two of them backwards
or forwards. That’s what stepper are - two (or more) coils being
driven in a sequence, backwards and forwards. So one L293D can, in
theory, drive one bi-polar 2 phase stepper, if you supply the sequence.
I found some close-out two-phase bi-polar steppers (part#117954) in the
Jameco Catalog. For six bucks plus shipping I get a 7.5 degree stepper
with 48 steps per revolution. The stepper motor runs at about 5
volts and pulls 800 milli-amps of current. That’s a lot of current so
I can expect my chips to heat up. The way I fix the heat problem
is by gluing a heat sink to the top of the chip. Any piece of metal
three or four times the size of the chip will do. If things get real
hot, I use a small fan to move air over the top of the heat sink.
STEPPER CONTROLLER SCHEMATIC
I realize that Seattle has developed it’s own mutant breed of
controller circuits, I think it’s based on the 68HC12, but I have
successfully resisted all attempts to be lured to the dark side
of the force and will continue to use the BS-2 (Basic Stamp II). Here
is a real quick description of the L293D inputs:
1, 9 Enable pins. Hook them together and you can either keep them high
and run the motor all the time, or you can control them
with your own controller.
2,7,10, 15 Control the two coils. Here is how you pulse them
for a single cycle:
