Originally Posted by Gary Dillard (Post 1299405)

Our controls team is looking at integrating a raspberry pi with a vision system for a fall project. One question they have is on boot up time - the specs are quoting about 45 seconds. Has anyone had any issues with this, does it determine how it needs to be powered (breaker or direct), do you power up the robot in queue to give it time, and any other advice on integrating it to maintain connection through the match?

Originally Posted by philso (Post 1286812)

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.




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





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.

Originally Posted by Gary Dillard (Post 1299405)

Our controls team is looking at integrating a raspberry pi with a vision system for a fall project. One question they have is on boot up time - the specs are quoting about 45 seconds.

Originally Posted by wireties (Post 1306819)

To solve the boot time issue - perhaps put a circuit inline with the Pi's power that will use a dedicated battery until the robots mains come online? Two diodes and a capacitor will do or one can get fancy with a circuit based on something like the MAX6236.

HTH

Originally Posted by techhelpbb (Post 1306833)

That addresses the problem but unless said battery was bought enclosed with the Raspberry Pi complete and off the shelf it violates FRC rules. There was a whole topic on ChiefDelphi about it this year (this topic actually so this is a restatement)

Originally Posted by wireties (Post 1306858)

"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:"

You missed the point of SWITCHING power sources. I'm not talking about storing energy anywhere illegal during competition. The battery (or whatever power source) used to boot the Pi is not on the robot "during the competition". You remove it after turning on the robot mains. If necessary to comply with the rules a second approved battery can be the source during boot time, similar to how many of us power the driver station or Ethernet switches (for debugging) between matches.

Originally Posted by techhelpbb (Post 1306867)

I wonder if that would be considered legal. I can not say I have seen anyone do that on a competition field. Worth asking in the official Q&A though. Does address the boot time issue but not the potential for brown out.

Originally Posted by yash101 (Post 1306990)

It would be nice if we could make the Pi boot a read only image, to a RAMDISK, and make sure that the Pi doesn't get corrupted on brownout. That way, just like the cRIO, just turn it off by flipping the switch!