Quote:
Originally Posted by Mike Betts
I normally respect your opinion but let's look at engineering and physics for a minute...
How many of those "small and light" minibots are using limit switches rated for AC only? Answer: All of them.
From an engineering perspective, this is reckless and stupid. AC current switches through zero 120 times a second and the arc created when one tries to open a circuit is extinguished.
This does not happen for a DC current. It is much harder to switch... The arc tries to bridge the gap resulting in contact pitting and/or welding. This arcing becomes even larger when switching inductive loads (like DC motors).
To pick an example, the Honeywell microswitch in the KOP is rated for 11 amps AC only. An electrical engineer would never use it for DC currents except at milliamp (logic signal) levels. And in the extreme cases where expected lifetime is measured in thousands of cycles, never at all...
Now, you can specify DC rated microswitches but they ain't so micro...
So... We have teams who use NXT controllers and NXT touch sensors and they end up with large, non-competitive but better engineered minibots. And then we have poorly engineered but competitive minibots whose mentors have turned a blind eye to good engineering and are just hoping that they get though the season without failures setting in.
The correct way is not necessarily the competitive way...
Just who is ignorant of physics or of the world around them?
Regards,
Mike
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This is FIRST. People shouldn't give a hoot about lifetime ratings in this case; the part needs to last a grand total of about 1-2 minutes max in competition. I'd take something even lighter than the KoP Honeywell switches if I could get my hands on them. It is
better engineering to realize that the task at hand requires performance wayyyyyy over survivability.