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
|
1) although it may be common for teams to use AC microswitches, not all teams do. A small minibot could be engineered very well.
2)Engineers solve problems to be effective and efficient. The problem at hand is triggering the target before the other minibots. If you are too busy trying to extend the lifespan of a $1.50 (more or less) switch at the cost of abandoning the overall goal, you are not engaging in good engineering practices either. If an NXT gets smashed you are out $150.00, for just 5% of that cost, you could replace a microswitch 5 times. In addition, the initial cost for a microswitch based system is about 0.6% of the initial cost of an nxt based system if you include the touch sensor and the motor controller. Thus, the most effective system in terms of how fast the goal is reached and cost is the small minibot. The big minibot just has too many disadvantages.