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Unread 08-08-2013, 11:31
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Re: Battery powered raspberry pi

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Originally Posted by Nathan4567a View Post
True, It would be better to use the super capacitors I guess if the robot's power is really unstable. On the other hand though if the power is truly that unstable, it might be better to concentrate on fixing the power issue before working on a power supply for the raspberry pi.
I don't entirely disagree. However there are many cases where because of division of labor the programmers (who we all know are *always* at fault...I am a programming mentor just so you know) might want a hedge against strange issues caused by the power to their computing equipment or mechanical issues they can't deal with.

Having a COTS device capable of eliminating this issue basically transparently might be worth having. At least then if they need to use it they can put it in and see the problem vanish. Take it out and see the problem happen. If it's COTS they don't need to design it or construct it just install it. Plus it makes life easier for FIRST because it can be sealed such that the risks from the stored energy are very tiny.
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Unread 08-08-2013, 12:29
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Re: Battery powered raspberry pi

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Example 3: A Team obtains openly available design drawings from a professional publication during the pre-season, and uses them to fabricate a gearbox for their ROBOT during the build period following Kickoff. The design drawings would be considered a COTS item, and may be used as “raw material” to fabricate the gearbox. The finished gearbox itself would be a FABRICATED ITEM, and not a COTS item.
According to the glossary, drawings/plans can be a COTS item. So if the battery was integral to the COTS computing device plans (provided by some *non-team* source for free), then would the battery fall under the exception for R34?

If using a COTS battery, COTS computing device and COTS USB devices, without influencing any actuator, this would fall under the spirit of the exception as I see it.

Last edited by ohrly? : 08-08-2013 at 12:32. Reason: Adding clarity.
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Unread 08-08-2013, 13:18
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Re: Battery powered raspberry pi

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Originally Posted by ohrly? View Post
According to the glossary, drawings/plans can be a COTS item. So if the battery was integral to the COTS computing device plans (provided by some *non-team* source for free), then would the battery fall under the exception for R34?

If using a COTS battery, COTS computing device and COTS USB devices, without influencing any actuator, this would fall under the spirit of the exception as I see it.
I disagree. R34 says no batteries except those integral to COTS devices and even that is questionable. So as long as the constructed gearbox in your example is not COTS the result of the constructed plans is not COTS either. The plans would be.

Ultimately the people to provide definitive resolution to this are at FIRST HQ.
I advise as I have done. To ask them.

If they say go for it I certainly will not disagree. Best ask early.
Things that are not specific or overlooked often take time to get answered.
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Unread 08-08-2013, 13:19
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Re: Battery powered raspberry pi

Your drawings/plans would be a COTS item. The battery box made from them would be a fabricated item & not qualify as a cots item.

The only way this would work under the rules would be having a company build a computer, ("COTs computing device with an integral battery"), using a raspberry PI with a power supply enclosed together. The overhead would not be that high since you could use off the shelf parts for the most part and order them as you need them. Of course you wouldn't know if it was legal until the new rules came out.
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Unread 08-08-2013, 14:15
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Re: Battery powered raspberry pi

I agree that lawyering wouldn't be very effective, because the inspector can disallow anything he deems unsafe. I would prefer to have this explicitly allowed or disallowed directly by FIRST.

@techhelpbb Just so I can communicate more effectively with FIRST HQ, you mentioned you already asked them. Have they sent you a response? If so, did they give a clear indication of whether they were supportive or against the idea?
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Unread 08-08-2013, 14:27
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Re: Battery powered raspberry pi

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Originally Posted by ohrly? View Post
I agree that lawyering wouldn't be very effective, because the inspector can disallow anything he deems unsafe. I would prefer to have this explicitly allowed or disallowed directly by FIRST.

@techhelpbb Just so I can communicate more effectively with FIRST HQ, you mentioned you already asked them. Have they sent you a response? If so, did they give a clear indication of whether they were supportive or against the idea?
The people I sent the request to are currently at the NI event.
Some of them doing Q&A.
So understandably they have their hands full.

I would give this a couple of days.

If we get specifics I'll be glad to toss a bit of resources at some solution to this issue. Actually I already did seeing as DigiKey is shipping super capacitors to me for later this week. If they say no I'll add it to the pile with the FIRST specific parts of the control system I designed for that same bid.

BTW the name is Brian.
Like the brain in the jar as my avatar with the i & a backwards.

Last edited by techhelpbb : 08-08-2013 at 14:43.
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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, 16:16
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Re: Battery powered raspberry pi

Supercaps would be the cool, but I was originally thinking of using something like this: http://www.ianker.com/product/79ANS1052-BA.

Should I send this as a separate proposal?

Pros:
- Only needs to be replaced/recharged every few matches, in case one forgets.
- Totally isolated power supply
- Technically very easy (plug into RPi and you're done, use the other port for whatever USB peripherals you need)
- Much lighter than a laptop
- Should be quite safe, with unmodified COTS cables

Cons:
- Not as cool or fun as SuperCaps
- Obviously needs to be exempted from R34
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Unread 08-08-2013, 16:40
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Re: Battery powered raspberry pi

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Originally Posted by dtengineering View Post
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 08-08-2013, 16:45
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Re: Battery powered raspberry pi

Quote:
Originally Posted by ohrly? View Post
Supercaps would be the cool, but I was originally thinking of using something like this: http://www.ianker.com/product/79ANS1052-BA.

Should I send this as a separate proposal?

Pros:
- Only needs to be replaced/recharged every few matches, in case one forgets.
- Totally isolated power supply
- Technically very easy (plug into RPi and you're done, use the other port for whatever USB peripherals you need)
- Much lighter than a laptop
- Should be quite safe, with unmodified COTS cables

Cons:
- Not as cool or fun as SuperCaps
- Obviously needs to be exempted from R34
I think you should ask about it separately. It really is 2 different potential solutions but towards the same goal. If FIRST is okay with a COTS battery supply for the boards that is another simple and effective way to solve the problem. Plus items like that are available in existing local stores which is a nice bonus. The only thing I will caution about items like that is that some of those battery packs have a button on them that you need to press to get output and if the load is disconnected for a short period the output will turn off until that button is pressed again.

Besides something like that needs little explaination. There's really no serious custom circuit there.
It would be easy to ask an official question in the official question forum when the season opens.
FIRST would really only need to say yes or no to answer it like that.
That would be one not to miss.

Last edited by techhelpbb : 08-08-2013 at 16:53.
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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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Unread 11-08-2013, 12:44
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Re: Battery powered raspberry pi

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Originally Posted by techhelpbb View Post
3. Create a super capacitor based circuit to provide isolated power to whatever you connect to it:
This will not work due to the high ESR inherent to Super Caps. They are only meant for applications where the current draw is much less than 1 mA. The datasheet gives values ranging from 14 to 600 Ohms. If the supercap is charged to 5 Volts, a 300 mA input current (into the Raspberry Pi) through the 14 Ohms would cause a 4.2 Volt drop giving an input voltage of about 0.8 Volts at the Rasperry Pi). We made this mistake at work when they first became available and we were able to verify this. Note that the ESR will limit the charging current just like it would limit the discharge current so there is no concern about large charging currents.



Quote:
Originally Posted by techhelpbb View Post
5. Use a simple 7805, 7809, 7812 or adjustable reference regulator:
This will work the best if you use a low-dropout 5V regulator.

The regulators that are not low-dropout (78xx, LM371, etc.) require that the input voltage be at least 2-3 Volts higher than the selected output voltage. When the input voltage drops too low, the output voltage also drops, with the voltage across the regulator being in the 2-3 Volt range, depending on the regulator type and the output current.

With low-dropout types, the input voltage can be as little as 0.5 Volts above the selected output voltage.

It would still be best to install a capacitor bank and an input diode on the input of the regulator to provide "ride through" time. The anode of the diode would be connected to the battery. The cathode would be connected to the capacitor and regulator input. The diode becomes reverse biased when the battery voltage drops and "disconnects" the regulator input from the battery leaving it to be powered from the capacitor. Without the diode, the other loads on the battery will discharge the capacitor and you are no better off. It would be best to use a schottky diode such as a STPS1545FP from ST since it has lower forward voltage drop than the normal rectifier diode. With the maximum input current of 0.7 Amps for the Raspberry Pi, a battery voltage of 11 Volts, a forward diode drop of about 0.3 Volts, a minimum input voltage of 6.0 Volts to the regulator and a ride through time of 1 second, one would need a 150,000 microFarad capacitor (C = current x ride through time/ voltage change). Nichicon makes a suitable part that is 51 mm in diameter and 120 mm long and is rated at 6.3 Volts. This capacitor is rated for 15.3 Amps so one would not get very much voltage drop when discharging it. One should use an oscilloscope to monitor the battery voltage, the regulator input voltage and the regulator output voltage during high load to verify that the capacitor chosen is large enough.

http://www.digikey.com/product-searc...ds=LNR1E154MSE

There is still the problem of getting this past inspection. Allowing for a lower battery voltage, lower regulator input voltage and shorter ride through time will reduce the capacitor value required and the stored energy in the capacitor. A smaller part would be:

http://www.digikey.com/product-detai...579-ND/1882056




Quote:
Originally Posted by techhelpbb View Post
4. Use a step-up then step-down power supply:
You will still have to deal with the ride through time issue with this scheme.

You would want to choose a step up converter with a very minimum input voltage (at the desired load current) yet still has a high enough maximum input voltage that will accept the maximum battery voltage without damage. The output voltage should be as high as possible (check with next year's rules). You may want to choose a converter that has a maximum output current rating that is several times higher than the maximum load current. The rated minimum input voltage is usually with the maximum output current and will be lower with lower output currents. To get the most out of your system, it would be best to actually measure the maximum current draw of your Raspberry Pi while running the software you want it to run since it may be quite different than what the maximum current in the specification.

You would want to choose the step down converter with a maximum input voltage that is equal to or just greater than the output voltage of the step up converter. You would also want the minimum input voltage to be as low as possible at the desired output current. Again choose a converter with a maximum output current that is several times higher than your maximum load current.

You may need to connect a capacitor on the output of the step up converter to get the ride through time you want. The step down converter may have an input capacitor that will help with this. Use the same calculation shown above. You will not need the series diode since the step up converter will isolate you from the battery already.

It would be best to use an oscilloscope to monitor the battery voltage, the output voltage of the step up converter and the output of the step down converter during high load conditions to ensure that the system is working correctly and the ride through cap is sufficiently large.

When choosing the step up and step down converters, you do not need to choose isolated types since they are usually more expensive than the non-isolated types. The input reference terminal (0 Volt) of the isolated type is not connected to the output reference terminal but is connected in the non-isolated type.
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Unread 11-08-2013, 13:06
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Re: Battery powered raspberry pi

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Originally Posted by philso View Post
This will not work due to the high ESR inherent to Super Caps. They are only meant for applications where the current draw is much less than 1 mA. The datasheet gives values ranging from 14 to 600 Ohms. If the supercap is charged to 5 Volts, a 300 mA input current (into the Raspberry Pi) through the 14 Ohms would cause a 4.2 Volt drop giving an input voltage of about 0.8 Volts at the Rasperry Pi). We made this mistake at work when they first became available and we were able to verify this. Note that the ESR will limit the charging current just like it would limit the discharge current so there is no concern about large charging currents.
I agree but I am not sure about the relativity of large currents.

The Nichicon super caps that arrived yesterday are rated for 200mOhms ESR.
I could have gotten them with 100mOhms at higher cost.

My intention was to charge them higher than the 5V and then accept the loss that will be present anyway because of the on-board Raspberry Pi linear regulators.

My previous example feeding from a 7805 was merely an example.

The rest is a great contribution to the topic. Thank you.

Last edited by techhelpbb : 11-08-2013 at 13:15.
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Unread 13-08-2013, 19:01
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Re: Battery powered raspberry pi

I heard back from FIRST and they are willing to review a product using super capacitors. Of course you'd need to meet the same requirements with this project to get it on the approved hardware list as any other. Further they won't confirm or deny the rules in the future.

So there's the matter of making 10 of them.
Disclosing the testing.
Figuring out the costs.
Determining distribution so everyone can get one.
Potentially being asked to give FIRST enough for everyone free.
Explaining who your company is and what resources it has.

Of course the other take was see about making something like a Raspberry Pi laptop. Since there's no official approval for that I will now see if I can figure out how to approach that question.

In the meantime this does not solve the issue but it is neat (the plans are COTS what you build would not be):
http://blog.parts-people.com/2012/12...ble-rpi-to-go/

Further communication from FIRST representatives suggests that one could create a Raspberry Pi portable device that integrates a battery and a Raspberry Pi and it would likely fall under COTS rules of previous years. Again rules change so I am not sure how much risk someone is willing to take with this financially. I do have an idea of how to start.

Last edited by techhelpbb : 14-08-2013 at 15:57.
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