Re: Battery powered raspberry pi
 

I suspect that tolerance is more closely related to ambient temperature than their manufacturing capabilities.

Re: Battery powered raspberry pi
 

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?

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?

Re: Battery powered raspberry pi
 

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).

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.

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

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'?"

Re: Battery powered raspberry pi
 

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.

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.

Add a couple of capacitors in parallel to the raspberry pi's power input. If your problem was the rippled power, this should solve your problem.

Re: Battery powered raspberry pi
 

http://www.mouser.com/ProductDetail/...2fVA l1Xs8%3d

KA278R05CTU - Used this part to substitute my power supply in senior design. Low dropout, .5v @ 2amps. Basically, keep the input voltage to 5.5v or more and your pi should work. There's also an adjustable version if you're so inclined.


Good luck.

Re: Battery powered raspberry pi
 

I think it's probably important to frame this for the reader as to the advantages and disadvantages:

1. Use a portable computing device that is designed with an integral battery:

Advantages (limiting this only to the power issue):
A1. No effort to design additional power circuits required.
B1. Complete isolation from the battery used to power the robot motors.
C1. Battery life that will exceed 30 minutes.
D1. Easily replaced year on year (performance advantages outside scope).

Disadvantages:
A2. Limited opportunities to reduce the weight of the unit.
B2. Something else to remember to charge.
C2. Limited shelf life.

2. Create a battery powered COTS version of various boards like the Raspberry Pi, Arduino, BeagleBone, Panda...

Advantages:
A1. No effort to design additional power circuits required.
B1. Complete isolation from the battery used to power the robot motors.
C1. Slips past FIRST approval process if it is legal to do at all (unknown criteria).
D1. Allows weight tuning to optiminal levels.
E1. New market territory as not many things like this exist.

Disadvantages:
A2. We don't know how far you have to go to make something like this a COTS portable computing device. You might have to include parts at the time of sale that can be removed later to reduce the weight.
B2. You get whatever you get power wise. How long is long enough to run on battery? If your target market exceeds FIRST there's a real design issue in battery run time.
C2. Cost - you are bundling the cost of the computing device into the cost of the battery package. You must absorb both costs at the time of manufacturing and stocking.
D2. New market means uncharted territory and risk.

3. Create a super capacitor based circuit to provide isolated power to whatever you connect to it:

Advantages:
A1. No effort to design additional power circuits required.
B1. Potentially complete isolation from the battery used to power the robot motors.
C1. You can provide power to whatever works within those limits increasing the target market.
D1. If a new version of a computing device ships you don't care as long as it needs the same power.
E1. Super capacitors do not contain as many nasty or volitile chemicals.
F1. Bleeding edge market opportunity.

Disadvantages:
A2. Don't know if FIRST is willing to approve something like this.
B2. Approval is a process that commits the maker to certain requirements.
C2. The cost of the super capacitors mean this will likely cost at least $15 but more likely closer to $25.
D2. Super capacitors generally will provide less run time then a battery for the same volume.
E2. Cannot be expanded with a mere pass transistor - besides it stores power not merely regulates the power.
F2. Bleeding edge markets might leave the seller bleeding.

4. Use a step-up then step-down power supply:

Advantages:
A1. Limited effort to design the power circuits.
B1. Cheap - There are piles of Chinese boards for this on E-bay and through North American / EU supply houses.
C1. The power supply maker doesn't lock you into a computing board as part of the package.
D1. When the robot is off this will not hold power very long.
E1. Can retain operation at proper output voltage down around a 3V robot battery.
F1. Someone could mitigate many of the disadvantages below just be bundling these items into a COTS item.
G1. If FIRST requires approval for this at least it has wide potential market.
H1. At least 85% efficient and easily past 90% efficient.
I1. It is possible to exploit existing power supply circuits in the PDB to achieve the first stage regulation.
J1. Good isolation but when the input from the battery is charging the output filter of the first stage noise can pass.
K1. The odds of 2 separate regulators synchronizing with the noise on the robot battery are very low.

Disadvantages:
A2. Very few COTS step-up then step-down modules prefabricated (boost - buck)
B2. Generally single stage (boost - buck) converters have lower power limits than 2 seperate systems chained.
C2. The circuits themselves are much more complex than a 7805.
D2. End user packaging is probably required.
E2. If someone bundles this into a COTS item FIRST may require approval for it.
F2. Regulation can not be expanded with external pass transistor without inheriting the disadvantages from next choice..

5. Use a simple 7805, 7809, 7812 or adjustable reference regulator:

Advantages:
A1. Simple.
B1. Generally a small circuit.
C1. Cheap - most of the car power supplies that output 5V are 7805.
D1. A low dropout version will work to fractions of a volt at the input over the output voltage.
E1. Great for limited currents.
F1. Low weight when the current limits are low but high currents require heat sinking.
G1. If you use mica insulators you might be able to heat sink to an aluminum or copper robot component.
H1. Available in various packages with various current limits: surface mount, TO92, TO220, TO3
I1. It is possible to exploit existing power supply circuits in the PDB to provide initial regulation to reduce heat.
J1. It is possible (but counter-productive) to make a circuit to bypass the regulator if the input voltage drops lower than the regulator input voltage requirements.

Disadvantages:
A2. Gets rid of excess energy as heat.
B2. Increased current demands increases the heat generated.
C2. At increased currents needs a heat sink so weight increases unless you can manage to use the robot components as a heat sink.
D2. Does not deal with highly inductive or capacitive loads well.
E2. Can sustain physical damage under high loads.
F2. Can not produce a higher voltage than available at the output.
G2. Versions that are not low drop out require at least 1V more at the input than the regulated output.

Comments or suggestions let me know.

[quote=

3. Create a super capacitor based circuit to provide isolated power to whatever you connect to it:

Advantages:
A1. No effort to design additional power circuits required.
B1. Potentially complete isolation from the battery used to power the robot motors.
C1. You can provide power to whatever works within those limits increasing the target market.
D1. If a new version of a computing device ships you don't care as long as it needs the same power.
E1. Super capacitors do not contain as many nasty or volitile chemicals.
F1. Bleeding edge market opportunity.

Disadvantages:
A2. Don't know if FIRST is willing to approve something like this.
B2. Approval is a process that commits the maker to certain requirements.
C2. The cost of the super capacitors mean this will likely cost at least $15 but more likely closer to $25.
D2. Super capacitors generally will provide less run time then a battery for the same volume.
E2. Cannot be expanded with a mere pass transistor - besides it stores power not merely regulates the power.
F2. Bleeding edge markets might leave the seller bleeding.
[/QUOTE]

I think a capacitor bank is the best idea. It is easy to wire up as well. As far as the previously mention of legality goes, I'm pretty sure it is legal. I remember team 118 used some capacitors in one of their circuits on their 2012 robot. I forget what it's purpose was. It doesn't appear to me to be illegal, so I would recommend it.

As referred in C2, what do you mean by super capacitors? Why would these super capacitors cost $25? That is a lot of money for a capacitor. In Houston I can grab a handful of these guys (picture) for $5-10. That should be enough to stabilize the power input.

Like you said there are some drawbacks to having a capacitor bank, but the issues don't strike me as major ones.

Attachment 15133

Re: Battery powered raspberry pi
 

Super capacitors are generally a class of capacitors using double layers and gold to produce a large capacitance with a high internal resistance. They are suitable to provide power to digital circuits. They are not so suitable for things like locomotion.

In this case many of these capacitors are $5 and many rated at less than the voltage required by the load. So you need to put them in series. Putting them in series works like resistors in parallel so your capacitance divides. Then you need more capacitors because your capacitance is being divided down to reach your voltage requirements.

If this is legal then there a few people that have spoken to me in the past that have been asked at inspection to remove capacitors incorrectly.

The risks of a bank of capacitors are that the current to charge it when discharged can be high. If you limit the input current then it takes a while to reach the target output voltage. Plus if you turn off the robot with a large capacitance there the power will remain.

All of this can be eliminated with a good circuit design.
However between the cost of the capacitors and extra circuitry the cost as a whole will climb.

Keep in mind that large capacitances like this were not available at these prices until recently (last few years).
It's a big problem when people say....just put some capacitors in parallel on something.
You could have 82F capacitors....are you sure you want hundreds of Farads sucking power from your battery hard when discharged?
There really is such as thing as too much or too little with this.

Ok. That is different. That way might not be legal like you said. I was just thinking of a couple if smaller capacitors in parallel to the input power. I have seen small capacitors like that on robots before. Would a small device like the raspberry pi even need a large bank like that though to stabilize the power? It seems like a lot for something so small.

Re: Battery powered raspberry pi
 

Up to about 100uF (way less than even one super capacitor) you are not proposing something unusual.

What is required depends on how badly the power from the battery is being hammered. In a well designed fully operational robot the battery might never drop below 7V and 100uF would be just fine. Actually in that case even 10uF is more than enough.

However there is the not so unusual case of a robot with some design issues or damage that draws too much on the battery in random and unpleasant ways.

Then you get into the territory where you want storage of power segregated from the main battery. You can't do that with a small number of typical capacitors. You could store this kind of power in super capacitors. However then the design requirements go up.

With super capacitors it is possible to make a circuit that could have the robot battery pegged at 1V for 15 seconds and still provide power to the computing device like nothing unusual is going on. Why? Perhaps the extra computing device is monitoring that system or taking a hit like that will make that computing device do strange and unpredictable things that could complicate troubleshooting.

Re: Battery powered raspberry pi
 

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.