IEEE Spectrum February, 2013 - 22

A keyboard command switches the spacecraft
between rotation and translation modes, which
simulates the action of the small thrusters that
real spacecraft use for both rotation and translation, but with different firing patterns. Another
vital keyboard command stops all spacecraft
rotation to prevent the spacecraft from drifting
into a confusing multiaxis spin.
With this in mind, I decided on two buttons-
one to switch thruster modes and the other to
kill any spin-along with small left- and righthand joysticks.
In rotation mode, the right joystick acts like a
traditional airplane joystick-pushing forward
pitches the nose down, pushing left rolls the
spacecraft to the left, and so on. Meanwhile,
moving the other joystick left and right makes
the spacecraft yaw back and forth. In translation mode, pushing forward, back, left, and
right on the right-hand joystick will accelerate
the spacecraft in the matching direction. Moving the left joystick forward and back accelerates the spacecraft up and down.
The joysticks, made by Parallax, cost $5 apiece.
These spring-loaded two-axis joysticks are
essentially two potentiometers set at right angles
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NORTh AMERICAN

to each other. Electrical resistance is proportional
to the position of the joystick along each axis.
I also added a switch that turns the Leonardo's
ability to send keystrokes on and off (as indicated
by an LED on the side of the controller). This switch
means I can leave the USB controller plugged in
when I'm not using Orbiter, without having to worry about, say, knocking against the device and inserting random characters into an e-mail.
Wiring up the components to the Leonardo was
easy by design. The Leonardo, like all Arduinos,
provides a 5-volt reference voltage and a number
of built-in analog-to-digital converters for making exactly the kind of resistance measurements
I needed to use the joysticks. The digital input pins
can be configured to use internal pull-up resistors
to prevent spurious readings, so I could connect
the switch and buttons directly to the Leonardo.
Digital output pins provide enough current to
power my indicator LED, so the only additional
component required in the whole system was a
current-limiting resistor in series with the LED.
Programming the Leonardo was only slightly
more difficult than wiring it up-about 100 lines
of code to map the inputs from the joysticks
and buttons to outgoing keystrokes, written

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stephen Cass (3)

E-Z USB: Arduino microcontrollers have supporting input/
output circuitry built in, so wiring up joysticks was trivial to do in
my test setup [top left]. To mimic signals from a USB keyboard,
I used an Arduino Leonardo. Wooden supports held the joysticks
at the right height [left]. After a minor tweak to the simulator's
configuration, I was in command of my virtual spacecraft [above].

in a version of C developed for
the Arduino.
The trickiest part was constructing the controller's enclosure. As
I live in a third-floor apartment,
I don't have space for a workshop
(or even a dedicated workbench). My
equipment is restricted to the small
hand tools I occasionally use to build
scale models. So a metal enclosure,
for example, wasn't really an option.
I settled on basswood. It's commonly used to make architectural
models, so beams and strips are
available from art supply stores in
precut shapes and sizes. Some sawing and gluing of about $5 worth
of wood produced an enclosure.
The joysticks were held in place by
grooves cut into supporting beams.
While this arrangement probably
isn't robust enough to withstand
the kind of button-mashing frenzy
a first-person-shooter video game
would elicit, it's just fine for the more
subtly operated Orbiter.
A micro USB cable connects the controller to
the computer running Orbiter: The computer's
USB port provides all the power the controller requires, but I had to install a driver to make
Microsoft Windows recognize the Leonardo.
When you operate the controller, the existing
keyboard and mouse remain active, so Orbiter's
full set of commands is available.
I did have to tweak Orbiter's configuration
to use the controller. Normally, Orbiter assigns
the rotation and translation commands to the
numeric keypad found on full-size keyboards.
However, the Leonardo does not support the
ability to distinguish between, say, the 8 on the
keypad and the 8 in the row above the letters.
Fortunately, Orbiter stores its key assignments
in an easily edited text file, so a few moments'
work mapped the relevant commands onto the
keystrokes I had programmed into the Leonardo.
Once that was done, I launched Orbiter and
was instantly able to control my virtual spacecraft with my new controller. A quick flight
around the ISS and a perfect docking confirmed
the new ease in spacecraft handling-and the
new ease in building custom USB hardware-
provided by the Leonardo. -stephen CAss

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Table of Contents for the Digital Edition of IEEE Spectrum February, 2013