Journal Paper - AUVSI Foundation

Transcription

Robotics Club at UCF:
Classic Boatname
David Pasley
Team Leader
[email protected]
Carolus Andrews
Electrical Team Lead
[email protected]
Ray Brunkow
Electrical
[email protected]
Lucas Pasqualin
Software
lucas.pasqualin@
knights.ucf.edu
Nathan Blake
Mechanical
[email protected]
John Millner
Club President
[email protected]
The Robotics Club at UCF presents their competitive platform for the 8th annual AUVSI
and ONR International RoboBoat competition, ‘<Insert Boatname>’. The Autonomous Surface
Vehicle (ASV) is a mixture of tried and true techniques and designs passed down from previous
members, combined with fresh concepts introduced by members new to the competition.
A sturdy, rounded fiberglass exterior built by our own mechanical team complements a
unique vectored thrust motor configuration which allows for versatile motion through the
water in forward/reverse, rotational, and lateral directions. The electronics are spread out and
easily accessible in its low-profile electrical box, allowing for quick configuration and
modification. Several custom mounts and housings were fabricated for the various hardware
elements necessary to complete this year’s challenges.
The powerful and modular Robot Operating System (ROS) architecture provides the
framework to fuse data from multiple sensors including stereo vision, lidar, GPS, compass and
IMU for robust course navigation and mission completion [1]. The team’s goal for 2015 is to
improve upon an already successful design, in order to give <Insert Boatname> a strong
competitive edge at this year’s Roboboat competition.
Figure 1 <Insert Boatname>’s illuminated interior
Robotics Club at the University of Central Florida
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Introduction
The RoboBoat competition presents the team with several challenges that require
innovative solutions. This year, the team opted to do an overhaul on the club’s existing
platform from 2014, starting with the electrical layout and extending to its software and even
its mechanical design. The team naturally chose to retain features which have proven successful
such as the unique motor configuration and the stable catamaran design, while updating the
vessel’s appearance and inner workings, improving its functionality, and also adding to the
personality of the platform.
From the beginning, the team took a somewhat unconventional but creative approach
to the platform’s redesign which stemmed from a desire by our electrical team to completely
rewire the boat’s dated electrical system. The modularity of the previous electrical system
inadvertently led to stagnant, unused electrical networks with untraceable wire paths;
therefore, the team began by gutting the platform’s electrical system and stripping it down to
the bare essentials. The team then gathered up the necessities and designed a new footprint
for the platform’s electrical system. The design specifications set forth by the footprint led to
the new electrical box’s shallow, rounded design which bares UCF’s pegasus emblem in
gold-on-black (now lovingly referred to as the ‘pegasus egg’ for its egg-like shape). The new box
design was constructed in-house, and lead us intuitively through the remainder of the
mechanical modifications.
Other new additions to the platform this year include a stereoscopic camera system, an
active ventilation system in the new electrical box as well as a unique custom hinge for the box,
a translucent removable fiberglass stand-off board for the electronics, ‘on when open’ interior
lighting, new pontoons, welded aluminum bumper brackets, and a safety mechanism to keep
the boat from sliding off of its trailer.
Mechanical Design
The mechanical structure and design of the robotic boat at the beginning of the the
2014-2015 school year was, indeed, a complete working package with good working concepts.
However, there were a few areas where the team felt a redesign would allow us to have a
better mechanical advantage, increasing maneuverability while still maintaining
interchangeability of components, and keeping a stable center of gravity.
Pontoons
The previous pontoon design features a hexagonally flat bottom with angular geometry.
This works somewhat well with strictly forward or reverse trajectories, but performing lateral
and rotational movements requires more power due to the sharp angles on the bottoms of the
pontoons. This hinders the platform’s ability to make small and precise angular or lateral
corrections to the trajectory of the platform while it is moving forward or backward. Thus, if the
platform drifts slightly away from its goal trajectory, small corrections fail and steady state error
increases until the platform is forced to make a large correction. These large corrections often
result in an expensive loss of momentum, or even oscillations.
To improve on this, pontoons with rounded bottoms were designed which allow the
water to slip more freely under the boat as it strafes side to side or rotates, without affecting
the stability of the platform. Rounded pontoons will hand more control over to the motors so
that smaller changes in thrust can affect the trajectory of the platform while it is on a forward
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path. This results in smoother navigation, the steady state error of the trajectory is reduced,
and the platform is able to maintain its momentum.
The pontoons were initially designed using 3D cad software to find the correct sizes and
curve shapes to maximize their effectiveness. The construction of the pontoons will be
accomplished using the natural stiffness properties of ⅛” plywood clamped in precisely built
frames, bent into the correct shape as it fits a line through a clamped spline. The plywood is
shaped into an oval cylinder with a piece of plywood on the inside and on the outside with a
half inch gap between them to make a form. Next, expandable 4 lb two-part urethane foam is
poured into the form which, upon curing, is then wrapped in a layer of fiberglass for a hard shell
on the outside. In order to mount the pontoons to the frame, brackets will be added to the
mold and cured inside of the expandable foam, thus securing them firmly.
Electrical Box
The previous version of the
electrical box was precisely that: a large box,
with very little ventilation. In redesigning
this aspect of the boat the team chose what
can be best described as a flattened egg
shape, the shell of which is made entirely
from fiberglass. The design has many
advantages over the previous design, plus
some added functionality to make it an
overall better electronics housing:
Fig. 2 “The Pegasus Egg”
➢ The wider footprint allows for the electronics to be mounted spread out, ensuring direct
access to all electrical components and connections with little or no effort.
➢ The electronics are now mounted all on one board, and the board can be removed to work
on the electronics outside of the boat.
➢ The shape allows for thinner sidewall material while still maintaining its strength, resulting
in a lighter box than the original, which was made of thick plastic.
➢ The box is far more aerodynamic than the previous cubic one, and therefore it is less
affected by wind that could push it off its course.
➢ Since the new box is lighter, we were able to raise it a few inches above the frame while
maintaining the same center of gravity, in turn making the bottom of this box useful for
waterproof electrical plugs and other features.
➢ The rounded and spacious interior of the box allows for better airflow through the
electronics.
On that last note, one added feature of the new box is a forced convection cooling
system which takes air from underneath the box (where it is cooler due to the proximity of the
water), and blows it through the box to keep it cooled even when it is in direct sunlight for long
periods of time. With this cooling system, two safeguards were put in place: a filter that helps
dry the air, and a back-flow that only allows air to flow in and not any water that may splash
directly on the vents.
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Frame
A 1-inch square aluminum-tubing frame holds the two pontoons and the electronics box
together. The frame is designed with four mounting points for the electronics box, slotted to
ensure adjustability for optimal weight distribution across the length of the vehicle. The tube
lengths were cut on a horizontal miter saw, professionally TIG welded together, and then
anodized to reduce corrosion and extend longevity. The mounting plates for the electronics box
and L-channel were milled in-house using a CNC machine to ensure proper alignment.
Aluminum spacers reinforce areas of the frame with bolts through them, to prevent permanent
deformation. One eye-bolt is located on each pontoon mounting section (four in total) for easy
hoisting during water entry. The frame is kept the same as it was for the 2013-2014 year with
only a couple minor modifications:
➢ This year, the sensor mast was relocated from the second to the front-most laterally
opposed bar of the frame in order to make room for the longer electrical box.
➢ After an unfortunate accident, a hook was added to the back of the frame for
connection to a safety harness which keeps the platform securely on its trailer.
Motors
The motors are configured for a vector thrust design that easily
allows the boat to achieve forward, backward, rotational, and
lateral movement. The motors are angled 30 degrees inward
resulting in an approximate 13% overall reduction in forward
and reverse thrust when compared to a fully forward facing
motor setup. This sacrifice does however allow for 50%
Fig. 3 Motors in Diamond Configuration
of the thrust to be applied to lateral motion. In the event that
power is lost to the front or rear sets of motors, the vehicle is configured to maintain reliable
movement in the forward, backward, and rotational aspects of motion. The symmetry of the
motor/pontoon configuration gives the platform exceptional levels of redundancy.
Sensor Mast
The vehicle’s aluminum mast hosts the majority of the vehicle’s external sensors. The mast is
designed with ¼ inch holes spaced one inch along the height of the mast. These holes allow for
easy adjustment of the mounting position of the sensors and other hardware affixed to the
mast. The mast holder, which connects the mast to the frame, is bolted through the frame. At
the top of the sensor mast sits GPS antenna, the E-Stop button and a strobe light which is active
when the boat is in autonomous mode. Directly below is the stereo camera mount, and the
LiDAR.
Fig. 4 Bumper Mounts

Welded Aluminum Bumper Mounts
Previously, the front bumper was held in place by 3-D printed
brackets (as was the case with a great number of other parts on the boat).
Those bumper brackets both fractured and broke during an accident with
the platform. As a result, these brackets along with a number of others
were re-fabricated in-house out of aluminum using a tig welder and angle
bender.
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Electrical Design
The goal of the electrical team this year was to create a simplified yet modular electrical
system which promotes accessibility, and uses space efficiently. The layout was designed to
provide immediate access to every one of the important components, while minimizing the
length of wires and creating a tidy appearance. In order to do this, the team decided on a
compact power bus system, opting to scrap the existing DIN rail.
The use of power busses reduced both the footprint and weight of the previous year’s
industrial DIN rail by more than 95% while still providing modularity and capacity appropriate
for a platform this size. The new power bus system also made <Insert Boatname>’s electrical
system far easier to navigate and intuitive to troubleshoot.
Fig. 5 <Insert Boatname>’s Electrical Block Diagram
Propulsion
Four Sevylor SBM 18 motors power the propulsion system [2]. 12 volt rated models
were chosen by a previous team given the desired specifications of thrust, form factor, and
compatibility. The vertical tiller of the motor houses the wires and allows for mounting to the
vehicle’s frame. The motors are rated for up to 18 lbs of thrust, but testing of the motor’s thrust
output revealed to average around 13 pounds in the forward direction at 13 amps, and around
11 pounds at 13 amps in the reverse using the included propellers. The two motors in the front
of the vehicle and two in the back are wired to a set of two Roboteq motor controllers. These
motor controllers receive serial RS232 signals from a custom PSoC microcontroller board in
which the commands for motor speeds and direction are packaged.
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Batteries
The boat contains two main power rails which represent a logical and physical
separation between the ‘logic’ and ‘motor’ power. The logic power rail is responsible for
providing power to computing systems and associated electronics. This contrasts with the
motor power rail that is liable for keeping power running to the hardware components which
move the platform. Power isolation has two major benefits: preventing interference between
different systems and allowing for reliable emergency cut-off of power to actuating devices per
competition requirements.
A 4-cell LiFePO4 battery with a 6.6 Ah rating provides a nominal 12.8 volts from which
the motor rail receives its power. 3 additional batteries may be placed in parallel to extend the
runtime of the motor system for a total capacity of 26.4 Ah (approximate runtime of 3 hours
under normal circumstances). The logic components of the vehicle (e.g. onboard computer and
MCU) are powered from two 6-cell Li-Po batteries providing a nominal 22.2 volts each. This
logic rail is also fed to a vehicle power supply which provides both a regulated 12 volt and 5 volt
power source where necessary. Charging of all the batteries is achieved via an external
Hyperion EOS0615IDUO3 battery charger [3]. Two sets of batteries for each rail are available to
maximize vehicle runtime during practice.
Printed Circuit Board
The custom printed circuit board (PCB) integrates all analog and digital communication
signals for this year’s vehicle. The PCB contains an ARMv7 based PSoC5 microcontroller/system
on chip that handles several responsibilities including:
➢
➢
➢
➢
➢
➢
Interpret RC receiver signals.
Command motor controllers
Monitor vehicle status (Battery voltages, temperature, etc.)
General Purpose Input Output (GPIO).
High current output.
Serial communication to main computer.
The high current output on the PCB consists of a high power N-channel MOSFET in an
open drain configuration. This allows for switching of devices up to one amp. This is utilized on
the vehicle to switch high current components including the autonomous indicator strobe, side
lights, and low battery alert indicators. Commercial Molex connectors and many open GPIO
buses on the board enabled our reuse of the PCB in this year’s platform.
Shore Power
Shore power is delivered through the use of an external power supply fixed to the
vehicle’s trailer and encased in a NEMA 6P enclosure. The power supply uses a 120 volt AC
power supply providing 24 volts at 15 amps. A shore power relay triggered by the 24 volt signal
itself switches from battery power to the power supply. This effectively replaces the batteries
with the power supply when an AC current is supplied for additional testing and development
when not in the water.
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Emergency Stop
Safe platform operation is maintained with the implementation of an emergency stop
button (mounted atop the sensor mast) and circuit. Using a 200 amp 12 volt coil contactor
triggered by a single pole double throw relay, full stop of the vehicle is readily available. One of
the throws triggers the contactor and the other is used to control the relay itself. The vehicle
can also be made immobile remotely with the RC controller. When the Remote E-stop is
triggered, all communication to the motor controllers ceases.
Wireless Networking
A high power wireless networking solution provides software developers the ability to
debug the vehicle’s code remotely. This ability is a key factor in efficient use of testing time on
the course supporting remote configuration and adjustment of parameters on the fly for
optimization. Ubiquiti Networks airMAX Rocket M wireless base station is a powerful 2x2 MIMO
modem that has incredible range, speed, and RF performance in outdoor environments [4]. The
network is configured as a point-to-point link bridging the vehicle and operator command
center networks.
Software Design
This year’s software team transitioned us from our old code base, Zebulon, to a similar
albeit more widely supported type of message based architecture, ROS (Robot Operating
System). Much of the boat’s software was written from scratch, but some of the most difficult
tasks, such as sensor fusion with Kalmann filters, are already written into ROS. Some of the
software from Zebulon also had to be leveraged in order to fully utilize all of our sensors, and
even our MCU.
Fig 6. File structure for cross-Implementation of ROS and Zebulon Libraries
with new code base
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Architecture
<Insert Boatname> is built on a hybrid architecture which relies very little on local (close
range) localization, opting to use global (e.g. GPS) localization primarily, fused with reactive
local navigation. In other words, small scale mapping, path planning and localization are
minimally integrated, but global mission planning still plays an integral part in accomplishing
the platform’s objectives. In place of small scale path planning, reactive PID visual servoing and
3D object detection and avoidance takes place, courtesy of the platform’s calibrated
stereoscopic vision setup. The input from large scale path planning and local navigation is fused
together in a manner similar to a reactive potential fields architecture, but the weights of the
vectors are adjusted programmatically depending on a number of factors. The boat’s state
machine is in charge of these weights, and will ultimately decide how the robot reacts to the
environment ahead of it. What results is a less computationally expensive but still effective
method of achieving mission objectives.
The robot fuses Stereoscopic odometry, GPS, compass and Imu data to determine its
bearing in relation to a goal location. Planar distance detection also takes place, using a Hokuyo
lidar unit. However, the lidar information has proven to be far less useful on the water than the
stereo image data. In fact, the stereoscopic vision has an extremely useful tendency to filter out
the water as nondifferentiable noise, leaving almost nothing but solid objects remaining in the
3D point cloud. The lidar is most useful at close distances when the 1-dimensional points are
densely packed (and thus will play an important role in the docking challenge), whereas the
stereoscopic vision is able to reliably discover objects at a distance by detecting blobs and
contours which can easily be cross-referenced by the stereo image processor to extract
distance information.
Each detected object adds a component to the platform’s velocity commands by
weighted vector addition, with closer objects necessitating more urgency. With an absence of
obstacles (or channels in the case of the start gate/speed gate challenge), the robot will
naturally tend toward its current waypoint destination. Once it reaches the destination its state
changes, and the weights of certain types of navigation commands will begin to take
precedence. The course is broken down into missions with one for each challenge. Each mission
is broken down further into states and sub-states in which discrete actions are taken based on
sensor data.
Motor Control
The unique motor configuration of <Insert Boatname> allows for unrestricted general
plane motion along the water’s surface. In other words, the platform can combine
forward/reverse plus lateral and angular motion all in a single maneuver. This setup equips the
boat with the power necessary to provide effective propulsion for time-sensitive tasks without
sacrificing the versatile movement needed for completing those tasks. Reliable and smooth
lateral control will give the vehicle a time sensitive edge during the automated docking
challenge (due to forward facing sensors) and will also be used for advanced maneuvers around
targets and challenge course buoys.
The platform’s velocity controller utilizes the GPS and IMU sensors in a finely tuned PD
controller, ensuring that the boat actually performs its intended actions within a low margin of
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error, and carries them out smoothly without oscillation. Any navigational commands sent to
the controller are adjusted accordingly, before motor mixing occurs.
Low level motor control requires mixing three channel movement commands to four
motors. Symmetrical positioning of motors simplifies the mathematics needed for this process;
Each motor can be placed in a quadrant where each quadrant is a unique linear combination of
the three channels.
Object and Buoy Detection
Two Logitech webcams equally spaced
from the center of the sensor mast are
used in all missions of the course.
Computer vision algorithms are used
to filter live frames from the webcams
by color using the HSV color space.
This space separates channels
differently than RGB and helps
account for changing brightness due
to weather conditions.
Fig 7. ROS Rviz 3D RGB point cloud
Rectified images from the calibrated stereoscopic vision setup are compiled into 3D
point clouds with RGB, which can be visualized in ROS’s Rviz program (Fig. 7). After color
filtering of 2D images is completed, the point cloud from the stereoscopic setup is examined to
further filter colored blobs that are detected. Blobs which have no range associated with them
are discarded and only those blobs with distances within a certain range are counted as buoys.
Conclusion
The Robotics Club at UCF’s ASV team has devoted a year of hard work to innovate and
ensure the success of <Insert Boatname> at the 2015 RoboBoat competition. Care has been
taken to utilize the best aspects of previous years’ designs, while improving on as much of the
vessel’s functionality and robustness as possible. Important changes were made to the
electrical and mechanical components of the platform, and a new approach to the ASV’s
software is being implemented and used in competition for the first time.
Rebuilding and testing the autonomous platform has tried our abilities as problem
solvers, roboticists, programmers, and especially as control engineers as much of the boat’s
reactive components require skillful adjustment of PD and PID parameters. Though a few of the
ASV team’s members have been a part of the Robotics Club at UCF since before the 2014
RoboBoat competition, a majority of the teammates are new to AUVSI’s competitions, and
some even new to robotics. As such, many of the designs and solutions created for the platform
were done so with a fresh perspective, and therefore this platform is vastly different from its
predecessor, Classic Boatname, in many ways.
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Acknowledgments
The team would like to thank our sponsors at Army Research Labs (ARL), Simulation and
Training Technology Center (STTC), the University of Central Florida’s Student Government
Association (SGA), the Institute for Simulation and Training (IST), Advanced Navigation, and
Turbine Technology. We’d also like to say thanks to Chris Sprague, our industry advisor, and Dr.
Stephanie Lackey for their support, interest and involvement with the club.
References:
[1] About ROS, The Robot Operating System <
http://www.ROS.org/about-ros/
>
[2] 12 V Electric Trolling Motor, Sevylor
<
http://www.sevylor.com/12V-Electric-Trolling-Motor-P2063C45.aspx
>
[3]
Hyperion EOS0615i DUO3 Charger
<media.
hyperion
.hk/dn/eos/
EOS0615iDUO3
-MAN-EN31.pdf>
[4] Ubiquiti Networks - Rocket M airMAX base station
<https://www.ubnt.com/airmax/rocketm/>
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