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#1
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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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#2
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Re: Battery powered raspberry pi
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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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#3
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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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#4
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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 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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#5
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Re: Battery powered raspberry pi
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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:
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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#6
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Re: Battery powered raspberry pi
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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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#7
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Re: Battery powered raspberry pi
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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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#8
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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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#9
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Re: Battery powered raspberry pi
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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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