Design Objective

1
Design Objective

• Design an autonomous free flying robot with the
  capability of locating air leakage from outside
  the spacecraft

• Major Goals
• Design of Robust Sensors
• Design of Autonomous Flight System
• Design of Safety Securing System

2
Diode-Laser Based Sensing

• The flyer will utilize the relative attenuation
  of a scanning beam in order to detect surface
  defects.
• Similar types of laser/detector arrays have been
  successfully employed in surveys of the high
  atmosphere and in investigation of combustion
  flows.

• Typical oxygen/air sensing applications employ
  Aluminum Gallium Arsenide (AlGaAs) diode
  lasers, tuned to approximately 760 nm.
• Diode lasers, along with all ancillary
  equipment (current injection sources, etc.), are
  widely available from commercial sources.

Image Credit Sigem Plus

• This type of sensing arrangement is based upon
  attenuation of the supplied beams intensity as
  it propagates across the flyers probe length.
  The degree of relative beam degradation is given
  by the familiar Beer Lambert law.

3
Leak Detection via Laser Measurement

• In the case of a defect free hull section, the
  detector will measure no attenuation of the
  scanning beam (assuming a perfect measurement
  model).

• If a leak exists, the resulting beam absorption
  is given by the Beer Lambert relation. In
  the case of the flyer, the extent of absorption
  will also be a function of the scanning height
  necessitating accurate trajectory control.
  

4
Trajectory / Attitude Control System

• The position and velocity of a free-flyer will
  be identified with the aid of GPS satellites. Its
  trajectory will be controlled by the closed loop
  system shown below.

Relative position velocity
Actual position velocity
Free-flyer Navigation Software
Actuator Dynamics
GPS Satellites
Telecommunication
External disturbance
ISS Navigation
ISS position velocity
Uncertainty
Signal Noise

• The attitude of the free-flyer will be sensed
  and controlled by control moment gyros (CMGs).
  Paired thrusters will also be used for attitude
  control to ease the burden required of the CMGs.
  A similar type of closed loop system will be
  applied to the attitude control.

Actual attitude
CMGs
Desired attitude
Desired attitude
Thrusters
5
Background of Leakage Problem in Space

• In June1997, Russian space module Mir collided
  with space freighter Progress 234 due to an error
  made during the manual docking procedure. As a
  result, control of the Progress 234 was lost and
  imminent collision with the Mir space module
  became inevitable. Following impact with Mir,
  Progress 234 lost all vital communication and
  life support systems, placing the crew and their
  craft in a grave situation. From this one
  example, it is clear that the development of
  autonomous navigation and control systems will
  become an integral part of both manned and
  unmanned missions in outer space.
• In Dec 29 2003, air leakage was reported aboard
  the ISS. In ISS, ultrasonic sensors, module
  isolation, and alternate operating modes were
  tried to detect the air leakage. After two
  weeks, the source of air leakage was found and
  repaired. At present, ISS fully operational.

Air leakage and collision are serious problem for
space structure.
6
Gas Leakage Model

• In space, the continuum flow assumption breaks
  down each individual particle needs to be
  considered. Thus, our model is constructed based
  on an isentropic flow assumption inside of the
  spaceship and rarefied gas dynamics theory in
  space.

L slit length D slit width (r,?) location of
particle
?
r

• As shown in the figure above, the density of the
  gas diffusing through a crack drastically
  decreases as it goes further in radial direction
  approaching a magnitude of 10-3 in the limiting
  case.

7
Hardware / Software Configuration
The following are the devices that will
be on a free-flyer to locate air leakage from the
outside of a space module.

• Hardware
• Main Processing Board
• GPS Receiver for Navigation and Safety
• Diode-Laser Based Sensing Unit for Leakage
  Location
• High Resolution Standard Proximity Sensor for
  Safety
• Xenon Based Propulsion Unit for Navigation and
  Control
• Lithium-ion Battery Unit for Power Supply
• Control Moment Gyros (CMGs) for Attitude Control
• Software
• Flight Navigation Process
• Proximity Sensing Process
• Air Leakage Sensing Process
• Management of Memories and Data

8
Background of free flyer
Design Objective
Unmanned and autonomous operating system save
astronauts taking the task of free flyer and
supervising free flyer.

• Remote viewing and inspection
• Remotely piloted operations
• Supervised autonomous operations

Specification
From http//aercam.nasa.gov/
The nanosatellite-class spherical Mini AERCam
free flyer is 7.5 inches in diameter and weighs
approximately 10 pounds. AERCam has complete
miniaturized avionics, instrumentation,
communications, navigation, video, power, and
propulsion systems, including two video imagers
and one higher resolution still-frame imager.
AERCam is designed for automatic stationkeeping,
point-to-point maneuvering, and automated docking
approaches. In fact, this AERCam is
still under development.
9
Free-flyer Trajectory

• The free-flyer does not randomly fly around in
  space. It will take a specified path to maximize
  the efficiency of its operation.

A space module is modeled as a cylinder for
simplicity. We concluded that straight paths
along the cylinder length would be the most
efficient way to cover the surface of space
module.
By www.thespacestore.com
The free-flyer will fly the order of 10-2
10-3 m off the surface of a space module. Most of
the thrust will be applied when the free-flyer
turns at the edges as is shown in the figure on
the left. Occasional thrust will be applied to
maintain a straight path and specified altitude.
Trajectory model
10
Safety

• To maintain the safety of the ISS, the free-flyer
  will be equipped with several safety features.
• Collision Avoidance
• High Resolution Standard Proximity Sensor (HRSPS)
• Global Positioning System (GPS)
• Reference Positioning
• Backup systems will be required for some of
  the critical components such as propulsion,
  power source, and hardware / software
  (communication device, GPS and MEMS gyros, and
  sensors) using the following procedure.
• Monitor system
• Detection of Failure
• Using backup procedure