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[DPRG] Fw: Cruise control on wet roads (Off-topic)

Subject: [DPRG] Fw: Cruise control on wet roads (Off-topic)
From: Chuck McManis cmcmanis at mcmanis.com
Date: Wed Dec 4 12:00:02 CST 2002

[ Snip, story about Cruise Control elided ]

On my car (Subaru) if the cruise control manages to create a sudden change 
in engine RPM it automatically disengages. This has been true on many 
cruise control systems for a while (consider what would happen if it didn't 
and you were sitting parked and hit "resume" ;-)) I think the bottom line, 
which is always good advice, is that when driving conditions are 
substandard, make sure it is the human who is driving not some circuit.

But seeing as most of my robots (BattleBots excepted) don't have a human in 
the loop, one factor that I learned when building the "Wild Cougar" (a toy 
conversion of a Radio Shack R/C car) is that if your encoders completely 
stop sending you signals, don't blindly keep increasing power to the wheels 
:-) I had the strange occurrence of finally getting the PID control so that 
it would move at less than 20MPH and it drove over a piece of paper that 
got picked up in the tread. The paper disconnected the wire that went from 
the hall effect sensor to the Miniboard (which was doing the PID) and my 
algorithm compensated for the lack of pulses by increasing the duty cycle 
of the PWM right up to full on. WHOOSH! The car jumped across the room and 
smacked right into the base of my bookshelf. It then proceeded to make a 
nice black mark on the carpet as it spun the tracks and I raced over to get 
it off the ground.

Since that time, I've done what my Subaru does, and that is if I get sensor 
readings (especially ones that are controlling my actions) that are more 
than a couple of sigma off the "norm" (and there are different ways to 
track this) then I stop using that sensor. If by stopping to use the sensor 
I can't do what I'm supposed to do (like modulate wheel speed) then I just 
stop and indicate an error condition.

--Chuck


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