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Unread 29-07-2013, 13:21
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Re: Battery powered raspberry pi

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Originally Posted by ohrly? View Post
This year, we had problems with the unstable power supply for our onboard raspberry pi. I'm a little stuck as to how to solve that:
What power supply did you use last year?
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Unread 06-08-2013, 11:56
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Re: Battery powered raspberry pi

Has anyone considered using one of these for your power needs from the robot battery?

http://www.dfrobot.com/index.php?rou...product_id=752
(That is the source in China)
http://www.robotshop.com/productinfo...222&lang=en-US
(They carry their products here in the U.S. and Canada)

I have several Chinese manufactured PCB with National Semiconductor regulators and when I have some time (probably in the next week or so) I will power them up and test them at 1A from 7VDC to 15VDC.

I stumbled across that because I bought one of their Relay Shield V1.2 to update my ever growing pile of Arduino swag.

Last edited by techhelpbb : 06-08-2013 at 12:14.
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Unread 06-08-2013, 14:17
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Re: Battery powered raspberry pi

We used a second 12v-5v power converter (http://www.andymark.com/product-p/am-0899.htm). It was a problematic because if the compressor and shooter motors were both running at the same time, there wouldn't be enough power left for anything else.

I realize now that was because we chose our gear ratios terribly and the shooter motors were running very inefficiently, but all the same I wanted to try to isolate the RPi's power from the rest of the robot so it wouldn't cut out.
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Unread 06-08-2013, 14:35
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Re: Battery powered raspberry pi

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Originally Posted by ohrly? View Post
I realize now that was because we chose our gear ratios terribly and the shooter motors were running very inefficiently, but all the same I wanted to try to isolate the RPi's power from the rest of the robot so it wouldn't cut out.
The problem ultimately will be that you can't really store energy for a protracted period with the FRC rule limitations on custom circuits.

The allowance of a laptop and it's battery I think were more intended to allow teams to not have to make more electronic modifications to the laptops which usually come neatly packaged.

I suspect if you put any largish capacitor or battery on the robot within a custom circuit specifically to provide 0.X seconds of storage you'll start to have issues getting through inspection.

I would be interested to know if anyone has managed to pull off a storage capacitor or battery in a custom circuit and made it through an actual FRC inspection. You can get high Farad value capacitors at 5V that would easily provide a robust filter against short drops in the battery. However those capacitors do not generally discharge themselves and for a relatively short period of time after a power off power will remain. It is possible to make a discharge circuit for a capacitor after power off but I'm not even sure the capacitors or auxillary battery would be allowed.

Last edited by techhelpbb : 06-08-2013 at 14:37.
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Unread 06-08-2013, 16:02
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Re: Battery powered raspberry pi

So I guess this is out of the question?
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Unread 06-08-2013, 16:13
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Re: Battery powered raspberry pi

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Originally Posted by FrankJ View Post
So I guess this is out of the question?
LOL I actually have a 3.5F capacitor for automotive electronics on my test robot frame at home. It basically comes with it's own welder.

I was thinking something more like this:
http://www.karlssonrobotics.com/cart...Fcef4Aodwy0AYQ

I love that datasheet.
For tolerance it lists: -20%〜+80%
So I guess it's perfectly okay to send out a 15F capacitor instead of a 10F capacitor?
Might make the RC time constants *just a bit* longer.

BTW: With a capacitance this large one really needs to consider the currents that will flow if that capacitance is entirely discharged and someone connects a power source to it. Luckily with control system boot and robot setup times of at least 1 minute there's time to charge it slowly. Though I still doubt it is legal in FRC.

Last edited by techhelpbb : 06-08-2013 at 17:13.
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Unread 06-08-2013, 17:04
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Re: Battery powered raspberry pi

How about a BEC used for RC cars and planes?

http://www.graysonhobby.com/catalog/3ampbec-p-786.html
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Unread 07-08-2013, 16:18
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Re: Battery powered raspberry pi

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Originally Posted by techhelpbb View Post
I love that datasheet.
For tolerance it lists: -20%〜+80%
So I guess it's perfectly okay to send out a 15F capacitor instead of a 10F capacitor?
Might make the RC time constants *just a bit* longer.
I suspect that tolerance is more closely related to ambient temperature than their manufacturing capabilities.
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Unread 07-08-2013, 16:30
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Re: Battery powered raspberry pi

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Originally Posted by efoote868 View Post
I suspect that tolerance is more closely related to ambient temperature than their manufacturing capabilities.
Well Digikey sells the Nichicon super capacitors and they list +/-20% as does Murata.
BTW if you look through the high end capacitance in the Digikey component search it's pretty interesting.

A few thousand Farad capacitor anyone?
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Unread 07-08-2013, 17:08
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Re: Battery powered raspberry pi

maybe I read the rules incorrectly, but wasn't there an exemption on battery power when in came to on board computing? Laptops can use a battery. what's the significant difference in that RPI and a laptop?
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Unread 07-08-2013, 17:20
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Re: Battery powered raspberry pi

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Originally Posted by wilsonmw04 View Post
maybe I read the rules incorrectly, but wasn't there an exemption on battery power when in came to on board computing? Laptops can use a battery. what's the significant difference in that RPI and a laptop?
Quote:
R34

The only legal source of electrical energy for the ROBOT during the competition, the ROBOT battery, is one of the following 12VDC non-spillable lead acid batteries:
A. MK Battery (P/N: ES17-12) or
B. EnerSys (P/N: NP 18-12)

Exception: Batteries integral to and part of a COTS computing device or self-contained camera are also permitted (e.g. laptop batteries), provided they’re only used to power the COTS computing device and any peripheral COTS USB input devices connected to the COTS computing device and they must be securely fastened to the ROBOT.
You can probably squeeze a real time clock battery under this.
However the Raspberry Pi does not ship with a battery to power it.

I've sent a question over to FIRST in private to see if they'll let a battery or large capacitor integral to an approved power conditioning device onto an official competition field. Of course getting approval is a process in itself.

Fundamentally I think the answer to why integral batteries to COTS devices are allowed but not custom circuits has to do with the chance that someone builds something strange or unsafe that leaks power into unexpected places with the potential to cause movement when disabled or powered off. Also it goes back to the rules about devices that store energy of any type. Though I grant that custom circuits are not supposed to be connected to actuators of any sort so how exactly the power gets to movement must be left to the imagination (many laptops have fans that spin in them as well but again...they are within the laptop itself).

Last edited by techhelpbb : 07-08-2013 at 17:31.
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Unread 07-08-2013, 22:09
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Re: Battery powered raspberry pi

So you build a small battery into a case that holds a raspberry. Make enough to sell to other teams that might want one. create a little LLC to make/sell it. Presto a cots device. Fits the letter & spirit of the rules.

I think the main reason for no batteries in custom circuits is there is just not enough time to adequately inspect them & it greatly simplify the rules in this area.
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Unread 07-08-2013, 22:25
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Re: Battery powered raspberry pi

There is, as far as I can tell, no reason you can't put some significant capacitance across the power input to the Pi, so long as the capacitors are charged by the robot battery. (Dropped to 5V, of course...) The rule doesn't say that you cannot store electrical energy, only that the electrical energy has to come from the battery.

The standard way to power a Pi from a 12V battery is to simply use a car "cigarette lighter" charger. Most of them should contain a 5V switching power supply capable of delivering at least one amp.

I'd say, find an old car charger that you've got kicking around, hook it up to a variable power supply and your Pi, and slowly turn down the voltage on the power supply until the Pi stops.

Jason

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"Mum, why did that old guy call the power port a cigarette lighter?"
"Because people used to use that to light cigarettes, dear."
"Yeah, but what's a cigarette?"

Although more likely it will be...

"Mum, did you really have to plug your chargers in to something and use wires?"

or...

"Mum, what is 'USB'?"
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Unread 07-08-2013, 22:29
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Re: Battery powered raspberry pi

Quote:
Originally Posted by FrankJ View Post
So you build a small battery into a case that holds a raspberry. Make enough to sell to other teams that might want one. create a little LLC to make/sell it. Presto a cots device. Fits the letter & spirit of the rules.

I think the main reason for no batteries in custom circuits is there is just not enough time to adequately inspect them & it greatly simplify the rules in this area.
That is something worth testing the waters with as well.

The question is what a 'laptop' consists of.
Is a tablet a laptop?
If it is there goes the mouse and keyboard.
Does it really need a display?

I know we removed the display and keyboard from our laptop and it passed inspection.
However both the keyboard and display were present when we bought that netbook.

Quote:
Originally Posted by dtengineering View Post
There is, as far as I can tell, no reason you can't put some significant capacitance across the power input to the Pi, so long as the capacitors are charged by the robot battery. (Dropped to 5V, of course...) The rule doesn't say that you cannot store electrical energy, only that the electrical energy has to come from the battery.
Well outside of the rules I can think of several reasons to limit the capacitance between the power supply and something like the Raspberry Pi.

The current that a high value capacitor will draw when deeply discharged needs to be limited. If that current is drawn through a 7805 regulator as discussed earlier in the topic it could cause some damage unless there is at least a series resistance between it and the regulator output. If you simply use a resistor like that it will increase the amount of time it will take for the voltage on the capacitor to reach say 5V. If the capacitor is plugged directly into the input of the Raspberry Pi that will create a strange situation where you rely on the reset control circuitry to hold off the chip operation until the input power is at the proper voltage. That could cause some issues.

On the other hand if the power supply from the battery to the capacitor is a switching power supply then you have the problem that happened other years (regarding the radio DC-DC converter) where there is a finite upper limit of capacitor you can place there before it dramatically alters the filter at the final stage of the switching regulator altering the quality of regulation.

So there are a few reasons to properly size the capacitor or use especially high value capacitance in circuits that manage their behaviors. Not to say it can't be done if in fact no one says no to it.

Last edited by techhelpbb : 07-08-2013 at 23:32.
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Unread 08-08-2013, 15:58
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Re: Battery powered raspberry pi

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Originally Posted by techhelpbb View Post
...Well outside of the rules I can think of several reasons to limit the capacitance between the power supply and something like the Raspberry Pi.

The current that a high value capacitor will draw when deeply discharged needs to be limited. If that current is drawn through a 7805 regulator as discussed earlier in the topic it could cause some damage unless there is at least a series resistance between it and the regulator output. ....
Good point. I didn't really explain my circuit well. Putting the capacitors downstream of a 7805 would definitely present the problems you have illustrated. Rather what I meant was that I'd have a diode (a fairly skookum one) coming off +12, leading to a capacitor bank. Upon start-up the caps would charge rapidly. The diode might exceed it's rated specs for constant current flow, but could likely handle the brief burst of current needed to charge the caps. It would also keep the capacitor bank from being "drawn down" when the main battery voltage slipped below the capacitor bank voltage.

I'd have the voltage regulator (either switched or linear) run off the 12V capacitor bank. The capacitor bank would often be drawn down below 12V, but every time you stopped the motors and the battery voltage went back up again it would recharge. So long as the caps stayed above about six volts, the Pi would be fine.

I'd also put a capacitor across the 5V leads on the output of this circuit, of course, just to deal with any ripple from the regulator.

The main reason I'd tend to not use the caps downstream of the regulator as my "backup supply" is that they would drop below 5V very rapidly as they would only be charged to 5V to begin with!

The main point that I wanted to make, however, was that a capacitor bank would be legal, so long as it was charged by the robot battery.



Jason
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