In the summer of 1999, we had used the bottom two layers of 3T to provide autonomous control for a single subsystem--a second-generation biological water processor--during a 450-day, 24-hour-a-day, 7-day-a week test. Then in January 2000, the Advanced Water Research Group received ALS funding for the year-long IWRS test, involving four advanced water-recovery subsystems (Bonasso 2001) (see Advanced Water Recovery System sidebar).BuildupUsing 3T allowed us to develop the control for the IWRS in a modular fashion in two ways. First, moving from bottom to top (figure 4), each layer has its own data structures, timing constraints, and development tools that allow for parallel development of the software. Thus, we were able to develop skills sets based on the evolving hardware specifications and simultaneously develop the sequencer procedures. Early on, as the water research team developed the design for each subsystem, one part of the 3T team wrote the sequencer procedures for each subsystem in the RAPS language (which, in turn, is written in Lisp) (1) using virtual skills, that is, Lisp skills connected to a Lisp simulation of the expected hardware. A virtual simulation of, say, the reverse-osmosis subsystem could then be shown on a laptop to the WRS engineers and the control design refined in an iterative process even before the actual hardware was available. The primary result of this process was a set of skill specifications for each subsystem (figure 5).
The controls engineer shrugged, spreading his hands. "Sure, but with the single transducer to monitor eight tubes, we won''t know for three to five minutes after the pump command is sent.""Can we live with that?" asked the manager, glancing around the table at each member of the assembled group of microbiologists and chemical engineers.One of the engineers tapped at his personal digital assistant, then spoke up, "Even at 32 mils a minute, the pressure buildup from the recirculation pump won''t be enough to trigger the relief valve. I think it''s in the noise."
"Okay," said the manager. "We go with the two heads."The time frame was the winter of 1999, and this exchange was typical of many conversations that the Al controls team from the Automation, Robotics, and Simulation Division (AR&SD) at Johnson Space Center (JSC) would have with the advanced water-recovery personnel as the two groups prepared for a year-long test of a new Integrated Water-Recovery System (iWRS), slated to begin in January 2001. We were building an AI control system for this test that had to handle upward of 200 sensors and actuators grouped among 4 water processing subsystems. The control system would run 24 hours a day, 7 days a week, and be completely autonomous. It was an applied AI engineer''s dream, and in the end, we were extremely successful. However, there were events that happened for which we were ill prepared, and we would come away with a much better appreciation for the difficulties involved in controlling long-duration life-support systems.
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