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:
Option a) get a battery for the raspberry pi -> illegal under R34 because it is not an integral battery. Can I get around this rule, assuming it doesn’t change?
Option b) use a laptop -> laptops are usually comparatively heavy and big. Does anyone know a lightweight, cheap laptop with at least 2 USB ports and Ethernet? [a 3rd USB port for an Ethernet dongle is fine]
Option c) somehow guarantee the availability of 1 amp for the raspberry pi?
Option d) can anyone else think of another option?
My advice is a step-up switching regulator like in the FIRST PDB.
Going into a step-down switching regulator sort of like inside the cRIO.
National Semiconductor makes parts that will switch battery voltage up to roughly 20V.
Obviously because look at the cRIO input voltage.
National Semiconductor also makes parts that will switch roughly 20V down to 5V.
Each stage of regulation is both a noise filter and complements the other.
If you switch up to 20V and the battery caves to 8V the 20V suffers.
However the next stage just needs to get 5V so that 20V quality issue is damped.
Each stage will be probably about 87% to 95% efficient with switching regulation.
On the other hand a 7805 as pictured will remove the excess energy as heat.
If the battery is 14.3V depending on the current enough heat for a heatsink.
Sure you can get a TO3 metal can version of the 7805 or make an external pass transistor but it still just burns energy off as heat.
If you do build that 7805 circuit you should install blocking and bypass silicon diodes.
We have had pretty good luck with using one of these (http://www.andymark.com/product-p/am-0899.htm) to power an onboard RPi. It is the switching power supply that comes in the KOP for the wireless bridge but it works great for powering the RPi. Another bonus is you probable have one left over from a previous year.
(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.
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.
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.
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.
If you want ARM:
I have never dealt with them but there are netbooks like this (runs Windows CE you might be able to put Linux on it you’d have to try):
If you want something with more performance:
Search some place like Amazon or Walmart minding the $400 limit.
You should stay away from the refurbished or open box sales because they are not available consistently to everyone.
That might put you in violation of an FRC rule as well.
I advise you to use flash storage and not a mechanical hard drive.
It’s not just lighter it will not get trashed when you smash into things.
I can tell you from experience that if you remove the display and keyboard (which you do not need if your portable computer has a display output connector…the little ARM netbooks I linked earler do not) you can get a larger netbook down around 3lbs.
The specs for the power converter says it only operates down to 10v. I think almost any robot will draw the battery down below 10v at some point. The 7805 circuit that was posted will work down to a little more then 7 volts, which may be good enough. You can also use a low dropout linear regulator with the same footprint as the 7805 which will lower the required input voltage a little more. Both of these have the heat caveat that Brian mentioned. The 5v supply on the PDB will operate down to 5.5v and supply 3amps. This is probably the easiest, if you aren’t already using it.
The gold standard is the boosting type of converters that Brian recommended, however they are probably unnecessary.
You can use the Driver Station Log Viewer to see what the battery drops to, which will help determine the required minimum voltage.
The current logic power converter works for the radio because the 12v power supply on the PDB for the radio is a boosted power supply, and will stay at 12v until the battery dips below 4.5v. The power converter probably isn’t a great solution for other devices on the robot, however.
I think you’re probably right. We also were using somewhat older batteries, which would contribute to the lack of voltage. I will try to keep the 5v supply on the PDB open for the Raspberry Pi, if not I’m sure the 7805 or a switching regulator will work.
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.
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?
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).
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.
