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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?
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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.

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P.S. My hope for the year 2040...

"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

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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.



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Unread 08-08-2013, 16:40
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Re: Battery powered raspberry pi

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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.
That works fine as long as the capacitors in question are capable of withstanding 15V. The super capacitors I linked earlier would have to have at least 7 in series to make that work. Assuming a voltage rating of 2.5V to 2.7V.

Quote:
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!
Ideally I agree you'd want to regulate post storage as well but the issue with that is the Raspberry Pi in particular out of the box have RG1/RG2 post regulators on the board. I just used a 7805 as an example it could have been a 7809 or a 7812 or an LM317. With just a linear regulator (7805) outputing 5V 2 2.7V max super capacitors is possible but of course there is the in-rush current to consider so it would be a bad idea. One of the supplies on the Raspberry Pi produces 3.3V and the other 5V. The on-board 3.3V linear regulator is by default using up the extra energy as heat anyway (some people replace that for just that reason). You can operate a Raspberry Pi down to 4.75V usually but that's pushing it.

If your capacitors are not super capacitors or have really high capacitance then this becomes no issue. There are plenty of capacitors in the 25V range (just in case) you could put on the robot like that. If the robot is not really drained a short ride through should be fine. However because you have an upper limit (cost wise / design wise) on the amount of capacitance that can be used like this there is a limit to how much reserve you can really store. So if there's a serious protracted draw down on the robot battery this might not be enough.

Plus as I said I've heard people complain they were asked to remove largish capacitors from robots during inspection.
So I am in the camp that if there's an issue here let FIRST officially settle it.

Last edited by techhelpbb : 08-08-2013 at 16:49.
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Unread 09-08-2013, 00:31
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Re: Battery powered raspberry pi

One of the big problems of powering the pi from the robot battery is that the pi needs to be shut down properly. You cannot just cut the power. What I am thinking is that have the Pi connected to the battery directly (through a regulator), and have the Pi watching using it's IO, the status of the battery through the switch. When the power goes down, the pi needs to shut itself down. I forgot to add, you would need to add a reset switch to the power supply of the pi to allow you to turn it back on. NOTE: I DO NOT KNOW WHETHER THIS IS FRC COMPETITON LEGAL, so if someone knows, if they verify, it will be great. We were going to place a pi. For basic I/O, the propeller quickstart beats the pi in many ways. If being used for vision processing, why not just keep the pi with the driver station and use the network connection to function. You can bridge the connection of the laptop ethernet with the pi to get wifi.
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Unread 09-08-2013, 05:46
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Re: Battery powered raspberry pi

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Originally Posted by team1165wins View Post
One of the big problems of powering the pi from the robot battery is that the pi needs to be shut down properly. You cannot just cut the power. What I am thinking is that have the Pi connected to the battery directly (through a regulator), and have the Pi watching using it's IO, the status of the battery through the switch. When the power goes down, the pi needs to shut itself down. I forgot to add, you would need to add a reset switch to the power supply of the pi to allow you to turn it back on. NOTE: I DO NOT KNOW WHETHER THIS IS FRC COMPETITON LEGAL, so if someone knows, if they verify, it will be great. We were going to place a pi. For basic I/O, the propeller quickstart beats the pi in many ways. If being used for vision processing, why not just keep the pi with the driver station and use the network connection to function. You can bridge the connection of the laptop ethernet with the pi to get wifi.
Depending on how you configure the Pi you could cut the power.
Otherwise send the shutdown from the driver's station at match end.

You can add switches to custom circuits not to locomotion circuits or FIRST control system power inputs. Required FIRST switches like the master breaker are the exception.

Feel free to confirm this with FIRST but I have seen it done and pass inspection.

Personally if I am gonna go through the headaches revolving around all this the last thing I would want is to add the headaches of the Raspberry Pi using the field Wifi with any camera. At that point just use the driver's station laptop.

I am generally against sending critical video through the field. It has been a headache for too many people. It even previously could impact field operations. People do this every year and every year someone struggles with it. If you can process the video on the robot you should.

Feel free to disagee with my take on this but that is the benefit of what I have seen.
It is my opinion as an engineer and I am entitled to it.

Last edited by techhelpbb : 09-08-2013 at 05:48.
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