that I did during the design phase was to choose the wheel
motors. Picking out wheel motors tends to be one of those
things that you can spend an inordinate amount of time
doing. I think it’s partly due to the sticker shock. You buy
your microcontroller (in this case, a Parallax Propeller demo
board for ~$20), but then the motors, wheels, and motor
controllers end up costing around $300-$400 for
everything. That’s a lot of money — especially if your last
robot used continuous rotation servos that cost $25 total
and included the wheels. You start looking around, finding
surplus windshield motors on eBay, but then you realize
that you need to find the right size tire. So, you look
around some more, possibly looking at lawnmower wheels
at some home improvement store. However, you realize
that windshield motors don’t naturally afford themselves to
being attached to lawnmower wheels (weird, huh?), so
you’re going to need to create a coupling for between the
wheel and motor. Don’t forget the last step — you still need
to create a mount for attaching the motor to the robot.
Wow, so that’s a lot of work just to put wheels on your
medium size robot. It’s not too difficult, but it does take
time. It helps to have access to a CNC machine or a 3D
printer. It’s up to you, and you may end up saving some
money. At the very least, everyone should look around and
price out the components; it will make you appreciate the
work that goes into producing a high quality wheel and
motor set like the ones Parallax sells.
Parallax now sells two different motor and wheel sets
that include integrated quadrature encoders (0.14” linear
travel resolution). The only difference is that one set has its
rims and motor mounting block milled from a 6061
aluminum billet ( 60 lb payload), and the other is injected
molded plastic ( 20 lb payload). I used the aluminum version
because I wanted the option of using the motors on an
even larger robot in the future, and I really liked the way
they looked (Figure 3). One last thing about this wheel set
is that it uses pneumatic tires that help give the robot a
smooth ride and better traction in the grass.
Motor Controllers and
Now that you’ve selected the wheel motors, you need
to have a way to control them. I decided to use the HB- 25
motor controller for a couple of reasons. The first thing that
you should always check is the amount of current (in amps)
that your motor may require (continuous and surge value).
Looking at the datasheet, I saw that the motors may draw
a bit more than eight amps depending on the load and
terrain. The HB-25s are rated for 25 amps continuous duty,
so they won’t have a problem with these motors.
The second reason that I chose these motors is that
you’re able to control them the same way you control a
servo using pulse width modulation (PWM).
Lastly, remember the quadrature encoders? It’s possible
to connect the quadrature encoder from one wheel to its
HB- 25 motor controller for closed-loop control that’s useful
if you want your robot to drive in a straight line or go at a
constant speed while traveling up or down hill. The
quadrature encoder tells the HB- 25 how fast the wheel is
turning; the HB- 25 cheeks to see if it’s going at the
correct speed based on the PWM that you gave it; and
automatically adjusts the amount of power it gives the
48 SERVO 04.2014
Figure 5. Robot base with wheels, motors, motor controllers,
and the power distribution wire harness installed.
Figure 3. Parallax motor and wheel set with rims and motor
mounting block milled from 6061 aluminum billet.
Figure 4. Power distribution wire harness. From left to right,
on/off switch, HB-25s, and battery connectors.