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Unread 16-05-2015, 13:50
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Joe Johnson Joe Johnson is offline
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AKA: Dr. Joe
FRC #0088 (TJ2)
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Re: Brownout behavior - alternative design goals

Quote:
Originally Posted by Greg McKaskle View Post
Well now that we have the emotional venting out of the way, let's see if we can look at this in a more productive way.

<snip>
Insults ... I've been called worse, but they aren't so productive.

<snip>
Greg McKaskle
Greg,

Was I being hyperbolic with the "criminal" comments? sure. Calling it "really dumb"? No, not at all. If anything I would use stronger language.

I am not a EE by training but I have years and years of robotics experience both inside and outside of FIRST, as a hobbyist and as a robotic professional. These years of experience have taught me that you just don't scrimp when it comes to protecting the power supplies that keep your processors/coprocessors, sensors, and communications link alive.

That is a general rule. When it comes to FIRST FRC control system, where we know there is a battery that can be easily overwhelmed in terms of amps it can source compared to the amps requested by the motors allowed, the designers should take even more precautions when it comes to protecting control system from the inevitable low battery conditions. FIRST can put as many "buyer beware" signs up as it wants. It doesn't absolve them from the responsibility to provide a control system worthy of the name.

Based on the results I have seen using the RoboRIO and the discussion about the trade offs people made during the design, it is obvious to me that FIRST (and NI) did not take power management as seriously as they should have when as they were designing the new control system.

All the defense of 7V Brownout is missing the point. As are discussions about what last year's system would have done. This was a chance to design a system from scratch.

Yes, once you decide that it is okay for the RoboRIO to reboot at 4.5V and that you are not going to keep the 5V and 3.3V rails alive that it is okay to power them off the 6V line, then yes, you eventually get to a Brownout Voltage of 7V. Sure, a brownout is better than a reboot, but can we all agree that few brownouts are better and no brownouts are best?

For all that, I really don't mind that the PWMs are disabled at a certain voltage. Do I wish it were a bit lower? Sure, but I bow to the reality that ultimately it is better to have the motors turned off than to lose more critical thinking and sensing functions. I said brownouts but I was really talking about the larger issue of preserving higher brain function down to a voltage sufficiently low that it never happens.

The original sin in all this is that FIRST didn't take power management on critical systems as seriously as it should (as evidenced by the initial choice to not to even protect the 5V and 3.3V rails which power sensors and co-processors).

Yes, I am calling FIRST (and NI) out here. This could have and should have been a complete non-issue. It was foreseeable and it was avoidable.

I have a question that I will pose to every person that defends the current status quo: How much would it have cost if FROM THE START OF A CLEAN SHEET DESIGN to have the RoboRIO, the Radio, 6V*, 5V & 3.3V rails all stay alive down to a Battery voltage of 3.3V?

Saving a few bucks by not providing this feature on a control system that costs literally 100s of dollars that 1000s of teams will use to control robots that each team will invest many 1000s of hours and many 1000s of dollars making is... well... I don't want to be called out for making insults again so I'll leave it to the readers to decide what they would call it once they understand how easily and inexpensively this feature could have been provided.

Sincerely,

Dr. Joe J.

*I am not 100% what the 6V supply powers. If it's just the servo power, we could I would be willing let the 6V go -- servos going dark are no worse than motors turning off.
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