Eder Torres
Maria Sosa
3/30/14
Mercury Robotics: Robot Specifications
Physical appearance: Robot’s chassis is made out of aluminum, main body measures 8” X 9” X
2.3”. Max dimensions (edge to edge of the wheels, floor to highest point): 10.5” X 11” X 8”
Net Weight: 3 pounds
Number of motors: 4
Motors’ voltage: 12 V
Motors’ Gear Ratio: 1:29
Wheels’ diameter: 2.75”
Number of batteries: 2
Number of cells per battery: 4 and 3
Cells’ chemistry: Po-Li
Cells’ capacity: 4000 mAh
Cells’ max discharge rate: 10C
Motor controller type: 2 quad half-bridges (l293), Sign-Magnitude signal received through a
NAND gates digital circuit. Sign-Magnitude signal is generated by a PIC18F4550 microcontroller. There is
circuit isolation through opto-isolators between the NAND gates circuit and the half-bridges (iso721).
Wi-Fi receiver/transmitter: LinkSys E2500 with dd-wrt as the OS, it is enabled as bridge-repeater.
Data Controller: Beaglebone with Ubuntu 13.10 as the OS. It collects video signal and sends it
out as rtp through GSTREAMER. It transmits and receives information through serial port 1 with
minicom. The PIC18F4550 sends and receive data through its serial port.
Robotic arm: Aluminum with clutch.
Sensors: Ultrasonic Sensors, c920 Logitech camera.
The design was chosen to be able to add functions such off-road cruise, autonomy, races, etc.
The number of motors will help with this goal. Also, each motor contains an encoder to later be
interfaced and efficiency to be increased. The number of motors was also chosen to have a high speed
capability without sacrificing torque. The aluminum chassis was chosen due to it being light and
relatively resistant. The four wheels also allow the robot to rotate around its own center as axis, giving
more control on tight places.
We chose Li-Po batteries due to its high density of charge and their capability to supply high
currents without a sweat. The necessity of that quantity of current comes mainly from the Bridgerepeater and the motors. The decision for two different batteries was to avoid having common ground
between the motors and the controllers.
The reason we used an 18F4550 microcontroller was due to its instruction set being optimized
for C compilers, it also features a Phase Lock Loop which allows for the system clock frequency to go
from 20 MHz to 48 MHz.
We also chose the BeagleBone because it makes it possible to use a single node to transport
both command and video in a single node, also because it runs Linux, which we took advantage by
compiling Gstreamer to be able to sink video from a camera. The camera chosen was affected by the
beaglebone, after a long research, we figured out that the c920 has an internal h.264 encoder which
helps any cpu to be free. It’s also a really good compression format allowing sending it through a
network without sacrificing video quality.
We had several difficulties in our decisions, for example, did we want our robot to look like
several ones at competition? Do we want a separate IP camera? Innovation was a big deal in our design,
which only made more challenging since the robot was started in this semester. A hard decision was the
suspension of the close-loop system in the motors, the original plan was to equalize the speed of each
wheel, but it had to be suspended due to an accident with the robotic arm, therefore, making us use an
open-loop system to save time.
There were several components that will benefit the robot’s capabilities. First, we are using a
c920 logitech camera. This camera offers high definition and decodes the video to h.264 which is an
optimal format to transport over the web. This camera will allow the driver to see with no problems.
The beaglebone is the heart of the network communication in our robot. It does nothing else but to
send the video and status of the robot to the driver. It also receives the commands from the driver and
sends them to the robot’s microcontroller. Another extremely important part of our robot is the bridgerepeater, originally, a usb wifi adaptor was used, but the range was relatively low. Although the bridge
consumes more power, its reliability can’t be compared.