Tessellator: Space Shuttle Tile Inspector

The Robotics Institute, Carnegie Mellon University, Pittsburgh, PA 15213

Principal Investigators: Kevin Dowling, William L. Whittaker

Core Team: Kevin Dowling, Hagen Schempf, Mike Blackwell, Jesse Easudes, and Simon Gatrall

Students: Bob O'Toole, Jason Almeter

MSE Program: 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 details.

Tessellator , bytes.

Overall Description

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:

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.

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.

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 NASA.

Status

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.

Paper

A Mobile Robot for Ground Servicing Operations on the Space Shuttle by Dowling et al.

Stories

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: nivek@cmu.edu