Development of a Low Cost Autonomous Indoor Aerial Robotics

Development of a Low Cost Autonomous Indoor Aerial Robotics
Development of a Low Cost
Autonomous Indoor Aerial Robotics
System with Passive Stabilization
28 July 2010
Frank Manning
Pima Community College
Christopher Miller
Pima Community College
[1] Abstract
The Pima Community College UAV Club has designed an air vehicle system to
compete in the International Aerial Robotics Competition (IARC). The rules
require an autonomous air vehicle to fly through an open portal into a cluttered
indoor environment, search for a small flash drive and exchange the drive with a
decoy while evading or deactivating various security systems. The mission
deadline is between 5 and 10 minutes, depending on whether security alarms are
triggered. The team designed a low cost air vehicle with a jellyfish configuration,
on which a balloon stabilizer provides passive stability. Twin propellers
suspended beneath the balloon provide lift, and a separate modular 2D thrust
vector control system provides precise horizontal positioning, allowing the
vehicle to respond rapidly to changes in HVAC air movement.
[2] Introduction
[2.a] Statement of the Problem
The overall objective is for an autonomous aerial robot to covertly retrieve a flash drive located
in a cluttered office environment. The robot gets access to the building through an open window,
and must evade various security elements on ingress and egress.
The mission begins from a starting point at least 3 m from the building, where the air vehicle
searches for an open window. Next, the vehicle searches for an LED on a security camera near
the window. When the camera becomes inactive, as indicated by the LED, the vehicle enters the
building through the window.
While navigating the confined environment, the vehicle searches for the office of the Chief of
Security, as defined by various signs posted on interior walls. During the search, a laser intrusion
detector must be either avoided or deactivated. Floor sensor alarms must also be avoided. Once
the security office is found, the vehicle searches for a flash drive resting on a stack of papers.
The drive is swapped with a decoy.
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At this point the vehicle exits the building through the window, delaying egress until the security
camera is off. The vehicle then delivers the flash drive after flying a minimum of 3 m beyond the
building. The time limit for delivery is 10 minutes maximum, which may be reduced depending
on whether alarms are triggered.
[2.b] Conceptual Solution to Solve the Problem
PHASE 1 -- Pre-position vehicle. Initiate hover, orient sensors toward approximate window
location. Search for window opening while maintaining 3 m minimum distance from building.
On finding window, hold position and search for blue LED.
PHASE 2 -- Ingress. Wait for falling edge on blue LED. On edge detection, record time and enter
window within 30 s deadline.
PHASE 3 -- Search for security office. Search for "Chief of Security" sign. Also search for
switch plate label on laser barrier. If label is found, either avoid barrier or deactivate it by
applying force to pressure plate.
PHASE 4 -- Enter security office. Search for office entryway, enter office.
PHASE 5 -- Search for flash drive. Search for drive, swap with decoy when found. During the
swap the vehicle positions itself to keep the propeller downwash away from the stack of papers.
PHASE 6 -- Egress. Reverse course, fly to window. Hold position just inside window until blue
LED turns off, as determined by integer multiple of 60 s period after falling edge recorded in
Phase 2. Exit window, fly minimum 3 m beyond window to deliver flash drive.
Notes:
[1] Vision-based SLAM handles high-level navigation.
[2] During all phases, keep track of alarm activation and elapsed mission time. Determine
deadline as function of alarm activation. If deadline passes without egress, trigger selfdestruction of vehicle.
[3] This paper describes a conceptual solution that is intended to perform the full IARC mission
at a future date. Only a small part of the solution has actually been implemented in hardware and
software as of this writing.
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[2.b.1] Figure of Overall System Architecture
Figure 1. Overall system architecture.
[2.c] Yearly Milestones
Air vehicle development is scheduled for 2009/10. Low-level obstacle avoidance and altitude
control will be emphasized in 2010. SLAM, optical flow and camera imaging in 2011.
[3] AIR VEHICLE
Figure 2. Air vehicle.
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The air vehicle uses a jellyfish configuration, with lift fans suspended underneath a balloon
stabilizer. The hybrid design combines the inherent stability of balloon with payload capacity of
quadrotor. Thrust Vector Control near the CM allows precision stationkeeping while minimizing
attitude transients. Ideally vehicle Z axis is always vertical.
[3.a] Propulsion and Lift System
Figure 3. Propulsion thrust vector control.
The propulsion system consists of a thruster with a 2D thrust vector control (TVC) mechanism.
The thruster is comprised of two propellers driven by brushed electric motors (see Figure 3
above). The motors are separated by a pylon. By varying the roll and pitch angles of the device, a
force can be generated in an arbitrary horizontal direction relative to the vehicle-fixed coordinate
frame. The thruster roll angle modulates side force (Fy), and the pitch angle modulates
longitudinal force (Fx).
The thruster is cannibalized from an R/C blimp. Motor controllers are cannibalized from servo
amplifiers.
Ideally the thruster is intended to produce a purely horizontal force. However, in order to reduce
mechanical complexity, the pitch and roll angles are limited to ±45° from vertical. Consequently
a significant vertical component exists, which contributes to lift and reduces the load on the lift
motors.
The thruster is located near the center of mass of the vehicle in order to minimize attitude
transients when the thrust vector changes in either magnitude or direction. Attitude transients are
further reduced by using counter-rotating propellers, which minimize gyroscopic moments that
are otherwise caused by rapid slew rates of the TVC mechanism.
The thrust vector can be changed rapidly without rotating the vehicle as a whole. This allows the
vehicle to respond rapidly to changes in air movement caused by HVAC or other sources.
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Figure 4. Gondola with lift fans and vanes.
The lift system consists of two counter-rotating propellers driven by E-flite Park 250 brushless
outrunner motors. Both motors run at the same speed.
[3.b] Guidance, Navigation and Control
[3.b.1] Stability Augmentation System
Pitch and roll stability -- a balloon provides passive stability in pitch and roll. When the vehicle
tilts to one side, the lift vector has a nonzero horizontal component, which causes the vehicle to
accelerate horizontally. The resulting aerodynamic drag on the balloon generates a moment
about the vehicle center of mass. Since the CM is well underneath the balloon, the moment acts
in such a way as to drive the tilt angle toward zero.
The balloon is filled with helium, which has only a minor effect on stability. Although helium
increases the payload capacity slightly, helium's main practical advantage is to keep the vehicle
upright when it's shut down and sitting on the floor.
Yaw stability -- vanes control the yaw rate. An off-the-shelf R/C rate gyro damps the yaw rate
and is quite simple to implement.
Altitude is controlled by a conventional throttle PID loop. Altitude is measured by an IR
rangefinder altimeter.
[3.b.2] Navigation
A scanning laser rangefinder was considered for a SLAM implementation, but a planar scan
pattern was not considered a good fit in an environment of complex, 3D clutter. Singularities
were a concern, in which small displacements of the scan plane potentially cause large changes
in data. A vision-based SLAM approach was thus judged to be a better fit to the environment.
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[3.b.3] Figure of Control System Architecture
Figure 5. Control system architecture.
The autopilot is a mulitcore BasicX system with three processors. A BX-01 master
communicates with the ground station through a radio modem. One of the two slave BX-24
processors handles fast control loops for the flight sensors, lift control, thruster control and yaw
control. The other slave BX-24 handles the flash drive manipulator.
[3.c] Flight Termination System
Figure 6. Flight termination system.
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The following section was copied from reference 5.
The purpose of the flight termination system is to cut off the supply power to the ISV
immediately by a radio transmitter in an emergency situation to prevent the ISV from injuring
people. A detailed schematic of the flight termination system is included. Our team’s flight
termination system is comprised of two components. The first component is a Spektrum DX6i
RC transmitter, the second a Spektrum AR6300 RC receiver and a Texas Instruments MSP430
microcontroller connected to a field effect transistor that acts as a switch. Flight Termination can
be achieved by simply flipping the kill switch on the transmitter.
1) A Spektrum DX6i RC transmitter sends a 2.4 GHz Pulse Width Modulated (PWM) signal
that represents the state of the kill switch
2) This signal is received by a Spectrum AR6300 RC receiver and is recreated at the output of
the receiver.
3) Next, a TI MSP430 microprocessor is used to detect and analyze the PWM signal, the PWM
signal is fed in to the timer_A module of the MSP430 where the microcontroller examines the
rising and falling edges of the PWM signal to determine the duration of the pulse.
4) When the when the kill switch is in the up position the PWM signal transmitted is longer than
1.5 milliseconds, when the microprocessor detects this the output on pin P1.7 is set high (3.3
volts). When the kill switch is in the down position a PWM signal of less than 1.5 milliseconds is
transmitted and P1.7 is set low (0 volt).
5) Finally, an N-type MOSFET is connected to act like a switch between the batteries and speed
controllers for the UAV. P1.7 is connected to the gate and of an N-type MOSFET and acts as the
control for the switch, when P1.7 is High electricity flows from the battery to the speed
controllers and then through the MOSFET back to the negative terminal of the battery. If P1.7 is
brought low then the circuit is opened and electricity can no longer flow to the speed controllers
thus terminating flight.
Figure 7. Flight Termination System board (left). Board with radio receiver (right).
[4] PAYLOAD
[4.a] Sensor Suite
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[4.a.1] GNC Sensors
Figure 8. Autopilot, based on NetMedia BX-24.
The autopilot is a multicore BasicX system with three processors. Figure 8 (above) shows an
single core version of the autopilot. At this writing a dual core version has been tested in a
breadboard configuration.
Sharp IR rangefinder -- measures altitude above the floor.
Sonar rangefinder -- for obstacle detection.
Magnetometer -- for heading.
Camera -- for vision-based SLAM. The camera also detects floor clutter, defined as obstacles too
low for the sonar detector, but high enough to confuse the IR altimeter. A small trash can is an
example of floor clutter.
Optical flow sensors -- for odometry and stationkeeping.
[4.a.2] Mission Sensors
[4.a.2.1] Target Identification
Cameras identify the flash drive as well as wall signs during the search for the security office.
The machine vision system uses SIFT (Scale Invariant Feature Transform) for pattern
recognition.
[4.a.2.2] Threat Avoidance
Threat
Sensor
Laser barrier
Camera identifies label on pressure plate.
Floor sensor
alarms
IR rangefinder measures altitude above floor.
Video surveillance
Camera identifies blue LED.
Air movement from Optical flow sensors sense air movement indirectly from vehicle motion.
HVAC, windows, etc.
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Security guards
Mission clock keeps track of mission time in order to avoid the security
guards, who patrol at 10 minute intervals.
Obstacles (walls,
furniture, etc.)
Sonar and camera sense obstacles.
[4.b] Communications
A radio modem allows two-way data communications between the vehicle and ground station.
The modem operates on 900 MHz spread spectrum.
Video is transmitted over 2.4 GHz.
[4.c] Power Management System
A 7.4 VDC lithium-polymer battery powers the propulsion and lift systems, as well as all
electronics on the vehicle.
Twin motor controllers drive the lift fans. Each controller has a regulated 5 VDC BEC output
that powers the UAV autopilot, sensors, servos and data links.
[4.d] Sub-Vehicles
No sub-vehicle is used.
[4.e] Effector Suite
The Flash Drive Manipulator (FDM) exchanges the flash drive with a decoy. The FDM has a
lateral offset from the propeller downwash in order to minimize disturbance of the stack of
papers on which the flash drive is located. The FDM also deactivates the laser barrier.
[5] OPERATIONS
[5.a] Flight Preparations
[5.a.1] Checklists
Mechanical
Gondola
Check for damage
Balloon stabilizer
Attachment points
Check for leaks
Lift propellers
Propeller integrity
Vane/pylon
Pylon damage
Vane control horns
Vane pushrods
Thruster
Servo condition
Control horns
Propeller integrity
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Communications
Check Data link integrity
Controls
Exercise controls in sequence
Check for proper operation
[5.b] Man/Machine Interface
Since the vehicle spends most of its time hovering or flying at low airspeeds, it's not required to
have a low drag coefficient. Therefore the structure of the vehicle is open, with equipment easily
accessible for operation, maintenance and replacement.
[6] RISK REDUCTION
[6.a] Vehicle Status
The following real time sensor data transmitted from vehicle to ground station over the radio
modem:
IR altitude
Sonar range
Camera video
Voltage of flight battery
[6.a.1] Shock/Vibration Isolation
The structure of the gondola is mainly styrofoam with a small amount of reinforcement by
carbon composite. The styrofoam inherently damps vibration. In addition, all propeller blades are
balanced in order to reduce vibration. The large lift motors use an outrunner configuration with
no mechanical gear reduction, which further reduces vibration.
[6.a.2] EMI/RFI Solutions
RFI and EMI are minimized in the following ways (this section was copied from reference 5
with minor modifications):
o Both the FTS and video system operate around 2.4 GHz. Because of this care must be taken
when selecting the appropriate frequency over which to transmit video. Video is transmitted on
2.432 GHz while the FTS control signals are transmitted at 2.402 MHz allowing roughly 30
MHz of bandwidth between the signals. The video bandwidth is 20 MHz and the bandwidth of
the control signal is 2 MHz. To avoid interference the space between the signals must be greater
than at least half of the bandwidth of the signals.
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o Physical proximity between transmitters and receivers can also contribute to RFI in the form
of front end overload. Front end overload occurs when a strong signal (or a weak one that
originates close to the receiver) is demodulated directly in the receiver. This can occur even
when the receiver is not tuned to the frequency of the unwanted signal. This problem can be
avoided by placing the R/C transmitting antenna as far away as possible from the video receiver.
The extra distance will reduce the chance that the control signal will overload the video
receivers’ front end.
o Ferrite beads are installed on all wires that connect to the video receiver. These devices
essentially turn the wire in to a one-turn inductor. If a signal of infinite frequency is passed
through an inductor it will appear as though there is an open circuit. This principle is used to
block interference from entering our video receiver. Any RF that is traveling down these wires
will be choked out at this point and will not pass in to the receiver.
o All cables used to connect the receivers and transmitters are made of the highest quality
shielded cable available. The metal braid surrounding the conductors in a shielded cable act in
the same way that a Faraday cage does. Any RFI or EMI that strikes the cable is converted in to
an electric signal that is dissipated to ground thus keeping the wires from carrying the
interference further in to the radio.
o A band pass filter is connected inline with the video receiver so that only the desired signal
approximately 2.415 – 2.430 GHz to pass. All other signals (such as our control signal) will be
greatly attenuated thus reducing their effect on the receiver.
[6.b] Safety
Safety is enhanced due to the low weight of the vehicle at 270 g maximum. The low weight is
partly due to helium in the balloon, accounting for 50 g due to buoyancy. Light weight allows
low power motors, which minimizes injury potential from spinning propellers. The primary
structure of the gondola is relatively soft, consisting mostly of styrofoam. Also, the large balloon
stabilizer intentionally has high drag, thus limiting airspeed if the vehicle goes out of control.
[6.c] Modeling and Simulation
A software test harness was utilized to accomplish unit tests of software components.
[6.d] Testing
Flight testing lends itself to an academic lab environment, since testing can occur indoors in
cluttered environments. Large outdoor flight test areas are not required.
A BasicX Development Station is used as a breadboard setup for the vehicle electronics (see
Figure 9 below), which lends itself to easy testing. The vehicle itself can be connected to the
breadboard through flexible wires, allowing limited hovering capability. This test configuration
makes it easy to tune various PID control loops.
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Figure 9. Autopilot breadboard with sensors.
[7] CONCLUSION
The Pima Community College UAV Club has designed an air vehicle system to navigate in a
cluttered indoor environment and swap a flash drive while evading security elements. For the
task the team designed a low-cost air vehicle with a jellyfish configuration, with lift fans
suspended underneath a balloon stabilizer. The hybrid design combines the inherent stability of
balloon with payload capacity of quadrotor. Thrust Vector Control near the CM allows precision
stationkeeping while minimizing attitude transients.
Acknowledgements – the team wishes to thank Tony Pitucco for his support of the project as
faculty advisor, David Coombs of Desert RC Model Sports, for his invaluable hardware and
software development expertise, and Robert Wagoner for sponsorship of Electric Jet Factory.
[8] REFERENCES
[1] Michelson, R., Rules for the International Aerial Robotics Competition 6th Mission,
http://iarc.angel-strike.com/IARC_6th_Mission_Rules.pdf
[2] Lowe, David G., Object Recognition from Local Scale-Invariant Features, International
Conference on Computer Vision, Corfu, Greece (September 1999), pp. 1150-1157.
[3] Se, Stephen. Lowe, David. Little, Jim. Global Localization using Distinctive Visual Features,
Proceedings of the 2002 IEEE/RSJ Intl. Conference on Intelligent Robots and Systems, (October
2002), pp. 226 – 231.
[4] Manning, Frank. Barrigah, Tete. Kuang, Huihong. Nelson, Tyler, Han, Chien-Wei.
Development of a Low Cost Autonomous Aerial Robotics System V4.0, 2008, Pima Community
College, Tucson, Arizona.
[5] Jarrett, Zack. Miller, Christopher. Barrigah, Tete. Kuang, Huihong. Nelson, Tyler. Manning,
Frank. Development of a Low Cost Autonomous Indoor Aerial Robotics System V1.0, Pima
Community College, Tucson, Arizona, 1 June 2009.
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