Tessellator: Space Shuttle Tile Inspector
The Robotics Institute, Carnegie Mellon University, Pittsburgh, PA 15213
William L. Whittaker
Kevin Dowling, Hagen Schempf, Mike Blackwell, Jesse Easudes,
and Simon Gatrall
Bob O'Toole, Jason Almeter
During the Tessellator development we engaged the services of the Masters of
Software Engineering Program at CMU's Software Engineering Institute. Two
teams with overlapping sixteen month studio courses, took on the
challenge of forming, designing and implementing software systems for
the Mobile Base and Manipulator Systems in addition to the High Level
Controller and additional sub-systems. See the paper below for more
Tessellator inspects and waterproofs each of the 17,000 tiles that
coat the space shuttle's underside, saving humans a laborious task
that lasts from the time the shuttle lands at Kennedy Space Center
until just before liftoff. By inspecting tiles more accurately than
the human eye, Tessellator reduces the need for multiple
reinspections. It also injects into each tile a toxic waterproofing
chemical, which prevents the lightweight, silica tiles from absorbing
water. Human workers have had to wear heavy suits and respirators to
inject the chemical, all the while maneuvering in a crowded work area.
Tessellator is designed to meet all the constraints of the job:
Before each inspection shift, a supervisor enters instructions into
Tessellator about shuttle position and inspection sequence via an
off-board computer, the Workcell Controller. Tessellator then uses a
rotating laser to position itself under the shuttle; the robot's
camera locates the exact tile to be inspected. Since the shuttle's
belly is not flat, Tessellator customizes its upward movement to each
tile: Two vertical beams on either side of the robot raise the
manipulator arm, which holds the injection tools and camera; a smaller
lifting device raises the arm the rest of the way.
- During inspection, the Orbiter Processing Facility is
crowded. The shuttle is raised on platforms. Cords, cables, and hoses
hang from above and cross the floor. Some internal doors to the
facility are only 1.1 meters wide, and some spaces beneath the shuttle
are no more than 2.5 meters long and 1.75 meters high. Tessellator can
maneuver in these spaces and also raise its inspection and injection
equipment up to heights of 4 meters to reach individual
tiles. Omnidirectional wheels move the robot smoothly in any direction
and over cable covers on the floor.
- Because there are so many tiles, Tessellator divides, or
tessellates, its work area among uniform work spaces, inspecting tiles
in each area with as little overlap between work spaces as possible.
- To isolate electrical components from the chemical, which is
flammable as well as toxic, Tessellator's electrical compartments are
sealed in aluminum enclosures and purged with nitrogen. Special heat
pipes cool the electrical compartments and the injection tool plate
without circulating chemical-laden air through the electronic parts.
- Tessellator's chassis and actuators are stiff. The robot's
manipulator arm is counterweighted and preloaded to keep the
inspection and injection tools steady. Tessellator's high-stiffness
design ensures the robot's accuracy.
By comparing the current state of each tile with the state of the
tile at previous inspections, Tessellator's database characterizes
anomalies in tiles as cracks, scratches, gouges, discoloring, or
erosion. The robot also indicates when it is unsure what's wrong with
a tile, so the supervisor can re-analyze the tile on the screen of the
Workcell Controller. At the end of a shift, Tessellator's updated tile
information is entered into existing NASA databases.
On board, a computer controls Tessellator's high-level processing
tasks; a low-level controller and amplifiers direct arm and wheel
motions. Two more computers control the robot's vision and injection
systems. If anything goes wrong-rising compartment temperatures, low
battery level, or other changes-safety circuits will shut the robot
down, and Tessellator will correct the problem.
Tessellator's vision system is made by SRI International and its
injection system by Rockwell International. The project is funded by
In spring 1994, integration of mechanics and electronics of the vision
system and rewaterproofing system occurred at CMU. Tessellator was
delivered to Kennedy Space Center in June 1994. Final integration with
the is scheduled to occur this fall at KSC.
A Mobile Robot for Ground Servicing Operations on the Space Shuttle by Dowling et al.
These are a couple of stories that Kevin wrote up during the course of the project. Hope you like the stories.
The Great Drivetrain Robbery is the
funny (in retrospect) account of a stolen component of the robot.
The June 1992 Trip to Kennedy Space Center is the account of an exciting trip to demonstrate the robot at NASA.
Funded by NASA's Telerobotics Program
Dave Lavery, NASA Telerobotics and Rover Technology Program Manager
Last-modified: Mon Jul 7 16:30:50 1997
Kevin Dowling, email: