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DRIVING STEPPER MOTORS WITH THE L293D
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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
motors. Now the home-viewing audience might say: "Rob, it’s easy to drive DC
motors. 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:
STEPPER TABLE
COIL A1
COIL B1
COIL A2
COIL B2
STEP 1
ON
ON
OFF
OFF
STEP2
OFF
ON
ON
OFF
STEP3
OFF
OFF
ON
ON
STEP4
ON
OFF
OFF
ON
3,6,11,14 Here is where you plug in the two coils. You want to ohm them out
and make sure you get one coil hooked up to 3,6 and another
one hooked up to 11,14.
4,5,12,13 Gets hooked to ground.
8 Motor voltage, usually about 6 volts.
16 +5 volts. It’s a good idea to keep this power supply separate
from your motor power.
And here is the schematic to hook the BS-2 and the L293D together. Hey, I’ve even
included the code!
The software to program the Basic Stamp II can be found in stepprog.bs2
Notice that this single chip can handle a two coil stepper otherwise known
as a two-phase stepper motor. If you want to do more than that, you need more
chips. The cool thing is that if you have multiple steppers and you want to drive
them one at a time (say on a PC board driller), it’s easy to have multiple L293Ds
driven on the same line.
BONUS! TROUBLE-SHOOTING SCENARIOS:
Let’s say you get this thing all hooked up to your microcontroller
and it doesn’t work. Here are a couple of symptoms and what they
point to as the cause. Since I’ve had the misfortune of experiencing
each one of these problems personally, I thought I might be able
to save you a little time and pain.
Symptom: The stepper motor shaft turns easily with fingers.
Cause/solution: No power to the circuitry, no power to the motor, or enable pin is low.
Symptom: Everything on, but motor is locked in one position.
Cause/solution: The L293D got the first position and that’s all. Check software, check connections, and check that the hard-wired ports match the software.
You might want to pull out the logic probe.
Symptom: Everything on, stepper is shifting in position, but the motor won’t turn.
Cause/solution: The correct line isn’t getting pulsed. Either the
