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View Full Version : Why MUST the 7.2v battery be attached to operate servos on the RC?


Mike Rush
02-05-2004, 02:14 AM
I've searched the threads and have found information on the 7.2v battery requirement but noone has explained WHY the battery must be connected. I understand the reason for it's existance (no loss of program or reset on battery voltage drops due to current demands) but not why if there is no current demand the system cannot be run on the main battery alone. Any ideas??
:confused:

FotoPlasma
02-05-2004, 02:49 AM
The Victor speed controllers are given their logic supply directly by the 7.2V source. My team learned this the hard way while trying to get servos working, early in the build period. I agree that a more ideal design would have used either the 12V source or the 7.2V source as the logic supply (and main drive for servos, uck).

That's a low-level "why". You'd probably have to talk to IFI for a higher-level answer to the "why" question. :)

Mike Betts
02-05-2004, 05:13 AM
That's a low-level "why". You'd probably have to talk to IFI for a higher-level answer to the "why" question.

This is a guess.

I imagine that the speed controllers do not behave themselves under a low voltage condition and powering off the 7.2 was the lesser of two evils. With so much current being sucked from the main battery, low voltages are almost guarenteed. The speed controllers are driving the most powerful motors on your machine. Keeping the internal logic alive and stable when the main 12V power dips is a primary safety concern.

KenWittlief
02-05-2004, 09:03 AM
we've been using our RC without the 7.2 volt battery connected and have not had a problem yet. So far we have 4 victors and 4 spikes connected

are you saying a Hitech servo will not work on the RC unless you have both the 12V and 7.2V batteries connected? that would really surprize me.

Mike Betts
02-05-2004, 09:20 AM
are you saying a Hitech servo will not work on the RC unless you have both the 12V and 7.2V batteries connected? that would really surprize me.
Ken,

All PWM outputs get their power from the 7.2V source.

Dave Flowerday
02-05-2004, 09:26 AM
All PWM outputs get their power from the 7.2V source.
The 7.2v battery provides PWM power, but the logic signals on the PWM ports come from the regular 12v source. Victors don't use power off the PWM ports (since they have their own), so this is why Victors work on the PWM ports but servos don't.

KenWittlief
02-05-2004, 12:11 PM
so then the 7.2V battery is NOT really a backup, it is required to get full functionality from the RC.

That also means, if you use servos alot, like having them constantly scan back and forth looking for something like, oh I dont know, an IR beacon

then you might need to charge the 7.2V battery between matches, or eventaully it will go dead on you.

Matthew_H
02-05-2004, 01:02 PM
so then the 7.2V battery is NOT really a backup, it is required to get full functionality from the RC.

That also means, if you use servos alot, like having them constantly scan back and forth looking for something like, oh I dont know, an IR beacon

then you might need to charge the 7.2V battery between matches, or eventaully it will go dead on you.

That is what we are running into. Our team is going to have to buy a new battery and charger to have on backup for the backup battery. Innovation First is the cheapest place I have found but it cost more to ship the battery than to buy it.

KenWittlief
02-05-2004, 02:53 PM
I would have set it up (the RC design) so the 12V battery would trickle charge the 7.2 V battery while it was connected - I dont understand why they arnt running the servo power from the main battery with the 7.2 wired as a logical OR supply.

Joe Johnson
02-05-2004, 03:51 PM
I would suppose that they decided to have the servos get power from the 7.2 battery because then they could take a relatively high current 5V regulator off the RC board. It saves IFI money, which in the long run will save FIRST money.

On a side note, this seems to be a case damned if you do, damned if you don't.

The backup battery is more or less a reaction to everyone giving FIRST a lot of grief for "brown outs" causing their robots to malfunction.

Now that FIRST has come up with a solution to keep your controller alive during such brown outs, a bunch of people are giving them grief for requiring a second battery (or for not powering off the main battery until the backup is need, or not trickle charging the backup battery or whatever...)

Bottom line, IFI has done a pretty good job with the controllers over the years. In general, I like what they have done to improve the system over the years. Kudos to IFI.

Joe J.

KenWittlief
02-05-2004, 03:59 PM
I agree - they took a big step to solve a major problem: having your robot reset itself in the middle of a match

and like everything else in engineering, now that they have put down a baseline, its much easier for the couple thousand engineers on teams to look at what they have done and say "Hey! this is great! and you know what would make it even better? ........"

sometimes its amazing the fuctionality you can add with a 10 cent diode, or 300 ohm resistor in just the right place :c)

Astronouth7303
02-05-2004, 05:16 PM
Huh? I've run the FRC just fine without the backup bat.

Dave Flowerday
02-05-2004, 05:24 PM
Huh? I've run the FRC just fine without the backup bat.
Yes, no one disputes that the RC will run without the backup battery. However, without it, you cannot use any servos. Try it if you don't believe us. At any rate, I think the consensus is that the rules require the use of the backup battery.

Joe Ross
02-05-2004, 09:55 PM
I would have set it up (the RC design) so the 12V battery would trickle charge the 7.2 V battery while it was connected - I dont understand why they arnt running the servo power from the main battery with the 7.2 wired as a logical OR supply.

I thought the trickle charge is what made the most sense originally, but today, I realized that these are NiCad batteries, and IFI may be worried about the "memory" effect.

Kevin Watson
02-05-2004, 10:06 PM
are you saying a Hitech servo will not work on the RC unless you have both the 12V and 7.2V batteries connected? that would really surprize me.Yep, it's true that both batteries need to be attached. The Victors and Spikes don't derive their power from the PWM cable.

-Kevin

KevinB
02-05-2004, 10:39 PM
Yep, it's true that both batteries need to be attached. The Victors and Spikes don't derive their power from the PWM cable.

-Kevin
No, that's incorrect. I have been driving our robot all week with no backup battery and it works fine. Servos won't move unless the backup battery is connected, but Victors and Spikes do work.

Kevin Watson
02-05-2004, 11:11 PM
No, that's incorrect. I have been driving our robot all week with no backup battery and it works fine. Servos won't move unless the backup battery is connected, but Victors and Spikes do work.Umm, please go back and have a look at the entire message. You'll see that I was replying to KenWittlief's statement: "are you saying a Hitech servo will not work on the RC unless you have both the 12V and 7.2V batteries connected?". In this context, I am not incorrect.

-Kevin

KevinB
02-06-2004, 06:57 PM
Umm, please go back and have a look at the entire message. You'll see that I was replying to KenWittlief's statement: "are you saying a Hitech servo will not work on the RC unless you have both the 12V and 7.2V batteries connected?". In this context, I am not incorrect.

-Kevin
Whoops -- I was reading so fast last night I totally misread your post. Please forgive me. I should've noticed who's post I was reading. :D