Thread: Detecting and handling sensor failure gracefully
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Unread 20-04-2004, 01:27
Jay Lundy Jay Lundy is offline
Programmer/Driver 2001-2004
FRC #0254 (The Cheesy Poofs)
Team Role: Alumni
  
Join Date: Jun 2001
Rookie Year: 2001
Location: Berkeley, CA
Posts: 320
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Re: Detecting and handling sensor failure gracefully
We relied heavily on 3 sets of sensors this year: pots for our two arm joints, prox sensors to count wheel rotations (actually sprocket teeth), and a light sensor to detect the lines perpendicular to the ball tees. This was the first year we relied so heavily on sensors and we learned a few things as we tested.

We also had problems with sensor failure. When a connector for the pot came out whatever joint it was attatched to would go crazy, which was really dangerous since our arm was so long and powerful. We ended up just putting in a manual override in case the pots died. When the pots were disabled it made the arms much more difficult to drive and we lost all preset positions, but at least we weren't completely disabled. Luckily we only had 1 pot fail in at least 35 matches.

We relied heavily on the prox sensors to keep us going straight during normal operation and to track our distance in autonomous mode. At first, we had the prox sensors reading the sprocket connected to the wheel shaft. However that sprocket moved every time we tensioned the chain, and one time 60 forgot to make sure one of the prox sensors was reading before a match and they slammed into the platform at 15 ft/s in autonomous mode and a pneumatic fitting on one of their tanks cracked. They lost all pressure and basically all function for the entire match. We ended up moving the prox sensor to a shaft on the transmission, where we got higher resolution and much more reliability (never failed again). We still had a manual override for the prox sensors, but we never needed to use it.

We relied on the light sensor to tell us when we crossed the ball tees and the center of the field so we could do various things depending on which automode we were using (stop / lift the whip / turn). The problem was, if the sensor didn't work the robot would never stop and would continue into the other alliances ball corrall. We solved that by simply putting in a timer failsafe, if 3.5 seconds into autonomous we hadn't seen the lines, then jump into the next state anyway.