Hopefully, you were able to reproduce that walker and proved yourself to be a true hacker and roboticist. I left you with the challenge of controlling the walker with your IR remote. How did you make out? This fellow in the
PICAXE forum did just fine: www.youtube.com/watch
?v=_UnITl-VYx0. He links to his post and code in the
description. Another builder on a new robot forum made a
four-legged bot “inspired” by my walker at
www.robotrebels.org/index.php?topic=184.0. This is a
nice variation of my passive leg design, and shows how
roboticists will take something, modify it, and improve it.
Robotics depends on linkages and gears just as much
as electronics and programming, and there’s a lot of work
remaining in all those fields. For me, it comes down to
controlling motion in order to get where you want to go.
This month, we’re going to learn enough to make a simple
self-driving robot car. We can’t let DARPA and Google have
all the fun!
We’ll be using DC motors which are most often used
with several gears to reduce their RPM and increase torque.
Gearmotors are handy modules containing a DC motor,
gears, and an output shaft. They come in a variety of
shapes and sizes, as well as a wide range of gear
reductions and power outputs.
I prefer slow torquey ones with gear ratios of 1:100 or
more for fine motor control.
The traditional differentially-steered robot chassis in
Figure 1 is inexpensive and widely available. It’s not my
favorite combination since it uses 1: 48 gearmotors with
fairly large wheels. It’s also under-geared: too much speed
and not enough torque for accurate control.
Yes, Virginia, there are motor speed controllers, but
they can’t fully make up for poor gearmotor choice. When
in doubt, select a higher gear ratio. Robots generally
benefit from more torque and less speed where accuracy
and repeatability are concerned.
The output pins on a PICAXE (and other
microcontrollers) can’t supply enough current to drive a
motor directly. They must use an intermediate component
to supply higher current. Relays are the simplest way to
switch motors and other things on and off. Figure 2 shows
inexpensive one- and four-relay modules which are easily
controlled by our PICAXE 08M2. A relay is an
electromechanical switch, providing full isolation between
the PICAXE and anything switched.
Our 4.5V PICAXE can just as easily switch a 24V motor
and battery as a 120 VAC table lamp (please use common
sense and caution if you do). The module’s onboard
circuitry allows direct connection to low power PICAXE
output pins, and simple HIGH, LOW, and TOGGLE
commands are used to switch the relays on and off.
Insider tip: Programming pin C.0 can be used as an
extra output (but not input) pin. So, C.0 can control an
LED, relay, or servo much like GPIO pins C.1, C. 2, and C. 4,
for a total of FOUR outputs. Just note that anything
connected to C.0 may flicker, buzz, twitch, or trigger during
programming or while the programming cable is attached.
See the amazing PICAXE control four relays at
SERVO 12.2015 45
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