May
2004, Issue 166
Embedded
Java Controllers
MOTOR
CONTROLLER PROJECT
Now
I’ll describe a simple motor controller that uses the
I/O pins of the JStamp/JStik and controls four servomotors.
Because my application (my robot, Zeppo) uses servos
converted for continuous rotation, I don’t need a lot
of resolution in my motor control.
The
full open-source robotics framework is posted at http://enerd.ws/robots/code/.
The code for the motor controller is taken directly
from this framework. I must warn you that the framework
is constantly evolving, so the code on the site may
be more recent than the code associated with this article.
An
R/C servo motor (because I’m on a first-name basis,
I call it just a "servo") is a standard DC
electric motor with a bit of electronics added. The
electronics are designed to take pulses generated by
an R/C receiver and convert them into a position. Normally,
the pulses control the position of the motor to an angle
of ±90°. However, I use servos that have been modified
for continuous rotation (converted servos).
A
standard servo will be at 0° or no rotation with a pulse
of 1.5-ms duration. Positive rotation is achieved with
a pulse duration of more than 1.5 ms. Negative rotation
is achieved with a pulse duration of less than 1.5 ms.
The typical servo accepts pulses between 1 and 2 ms.
I found that some converted servos will take pulses
between 0.5 and 2.5 ms. For best results, the pulses
should be repeated every 20 ms (see Figure 2).
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Figure
2—This is roughly what the pulses should look
like. My program divides time into 0.10-ms intervals
called "ticks." |
Many
Circuit Cellar authors talk about their hardware first.
I’ve waited until now because the circuit is ridiculously
simple (see Figure 3). I built it on a SimmStick prototyping
board from Dontronics. It is a sea-of-holes board.
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Figure
3—This is about as simple as possible for a schematic.
I just connected all the pins of the first header
to ground, servo power to the pins of the next
header, and the four I/O pins to the third header. |
Because
servos expect male headers in groups of three, I used
three eight-pin male headers. Power goes along one row,
ground the next, and the signals go on the third. I’m
only using four signals at this time, even though I
have eight headers on the board (see Photo 3).
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Photo
3—My latest prototype was built with point-to-point
soldering. It also includes an eight-channel ADC
that is controlled by SPI, but that’s a subject
for another time. |
