This is going to be a bit of a major build, so let’s break it down before we get started. The previous article explained the overall concept and delved into detail on a few critical concepts. Now, we’re going
to go into detail while we walk through the build
starting with the robot, the controller, and then
the program for each.
Nuntius — A Road Map
Prior to starting any build, it’s important to have a clear
picture of what you want to achieve. I’m not saying that
you need to first create a CAD model or even a drawing,
but you should know what you want it to do. For Nuntius,
I wanted to build a telepresence robot with a manipulator
that was intuitive to control and simple to set up. Just turn
on the robot, plug the controller into the TV, and go. So
with that, let’s begin by looking at building the robot.
Before I began designing the robot, I mentally made a
requirements list (road map) of what it needed to be able
to do (features) and uses for it. The key physical feature of
Nuntius is that it is a large robotic arm (manipulator) on a
mobile platform that can drive through rough terrain.
Therefore, the size of the manipulator determined how big
the robot body needed to be, which then dictated the size
of the motors. The motors then determined the kind of
motor controllers required, and suddenly the requirements
of the major systems are mapped out. We’re now ready to
start designing the body and look of the robot.
What's in a Shape?
Designing the shape and layout of your robot is very
important; you should spend some time just thinking about
it. I start by considering the forces that are going to be
acting on the robot and the environment that it’s going to
be operating in. I then try and craft it such that it’s also
aesthetically pleasing. There’s no reason why you can’t
make it work well and look good. Your robot wants to be
as popular as you are. I started with the robot’s arm. First
of all, it was going to be lifting and pulling things while
swinging around, so it needed to have a firm platform and
be close to the ground to prevent tipping. Second, it was
going to be driving through the grass and tight spaces.
These requirements led me to choose a differential drive
system on a teardrop platform with a single coaster wheel
in the rear (Figure 1). The robot was narrow enough to
drive through small gaps, but wide enough to be stable and
fit the control board between the wheel motors.
I decided that I wanted the manipulator as far forward
as possible, so that it would have maximum reach and
accessibility. Having the manipulator in front of the wheels
could have caused the robot to tip forward under some
circumstances, so I placed an oversized battery inside the
center of gravity triangle formed by the two drive wheels
and the coaster wheel in the tail (Figure 2). The oversized
battery added stability and aesthetics. (The size and shape
of the battery fills out Nuntius and creates the impression
that it has a body.)
Sketching out the body of the robot, I started by
placing the base of the manipulator and other major
components on a sheet of birch plywood from the local
home improvement store. I used birch plywood because it’s
strong, looks nice, and it’s readily available compared to
plastic sheets. I then cut it out with a hand jig saw and
rounded the corners with a hand-held belt sander.
Motors and Wheels
I briefly mentioned before that one of the first things
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Figure 1. Tear shaped wooden body of the robot. Notice the
partial motor cutouts allowing the robot to have a lower center
of gravity. The strange plastic curves on the robot platform are
from a drawing tool called a "French Curve" and are useful for
drawing more graceful curves. You can see how the curves for
the platform were taken from the two French curves.
Figure 2. The robot with the motors and wheels attached.
The center of gravity triangle extends between the drive
wheels and the coaster wheel in the back.