Stiffening Capacitors

[zachriggle]

Any chance we can use a stiffening cap (0.5F - 2F) to offset the immense instantaneous draw by the drivetrain and pnuematic system on our battery?

Looking at putting 2 one-Farad capacitors on the robot.

Is this legal?

[sanddrag]

I think that would violate the safety rule, so I would give that a "no."

[Gamer930]

This would fail rule R02 on Page 7 of the robot rules

<R02> Energy used by FIRST Robotics Competition ROBOTS, (i.e., stored at the start of a match),
can only come from the following sources:
 Electrical energy derived from the onboard 12V and 7.2V batteries
 Compressed air stored in the pneumatic system, and stored at a maximum pressure of 120
PSI in no more than four Clippard Instruments tanks. This compressed air must be supplied
by the compressor included in the Kit Of Parts, and can not come from any other source.
 A change in the altitude of the ROBOT center of gravity.
 Storage achieved by deformation of ROBOT parts. Teams must be very careful when
incorporating springs or other items to store energy on their ROBOT by means of part or
material deformation. A ROBOT may be rejected at inspection if, in the judgment of the
inspector, such items are unsafe

[Kris Verdeyen]

Originally Posted by zachriggle

Any chance we can use a stiffening cap (0.5F - 2F) to offset the immense instantaneous draw by the drivetrain and pnuematic system on our battery?

Looking at putting 2 one-Farad capacitors on the robot.

Is this legal?

Are you actually observing a significant instantaneous voltage drop? If so, you need to check your main power connections to be sure they are solid. If not, don't worry about it, the batteries themselves are very good at sourcing large instantaneous currents.

As for your cap idea, I'm afraid it is illegal, although awesome.

<R63> Custom Circuits can not... Directly alter the power pathways between the battery, fuse blocks, speed controllers, relays, or motors. Custom high impedance voltage monitoring or low impedance current monitoring circuitry connected to the ROBOT’S electrical system is acceptable, because the effect on the ROBOT outputs should be inconsequential.

[wireties]

It is a bad idea. Most of the 1F capacitors (usually called supercaps) have low voltage ratings (5V, 3.3V, etc) and they are extremely intolerant to overvoltage conditions. They'll blow up thus violating the safety rules.

[zachriggle]

Actually, the capacitors I had in mind are car-fi capacitors, designed to work at 14 volts, tolerant up to 20. Perfectly safe, aside from the potential to weld metal should they be shorted. Definitely no more dangerous than a lead-acid battery in that regard (the welding-when-shorted bit).

When hooked up to a high-resolution voltmeter, there is a significant drop in voltage with the motors and compressor running, down to about 10v on a full-charged system. Tests with two 1-Farad Rockford Fosgate capacitors from my car resulted in the voltage staying above 11.5V, thus ruling out wiring as a potential fault.

In accordance to <R02>, the energy would infact be derived from the batteries, since that is where it would charge from. I don't see how this effects <R63>, because it does not alter the pathway.

Regardless, the safety could be called into check, and that's a big one on FIRST's list. Just an idea :-D.

[Al Skierkiewicz]

Zach,
Yes they violate the rules stated above and therefore cannot be used. Capacitors to my knowledge have always been illegal. The drop you are seeing is the voltage drop across the internal resistance of the battery which is .011 ohms. When you draw large currents, a voltage drop will occur. The caps are holding up the power supply but only in a way that the current from the battery is reduced and so the voltage drop is reduced. All teams suffer this indignity and that is why the backup battery is reguired.

If this was an allowable device, I would recommend that they could only be used with a suitable bleeder resistor in parallel with each one.

You can minimize the drop in a variety of ways. A clean electrical design following the Tips and Guidelines is a good start. Short wires and a mechanical design that does not put significant loads on the drive train during turns are also a big help. Remember that any time the power supply falls below about 8 volts for a short period, the RC will go into standby and draw current from the backup battery. During this time, the PWM outputs will be disabled.

[Mike Betts]

Originally Posted by zachriggle

Actually, the capacitors I had in mind are car-fi capacitors, designed to work at 14 volts, tolerant up to 20. Perfectly safe, aside from the potential to weld metal should they be shorted. Definitely no more dangerous than a lead-acid battery in that regard (the welding-when-shorted bit).

When hooked up to a high-resolution voltmeter, there is a significant drop in voltage with the motors and compressor running, down to about 10v on a full-charged system. Tests with two 1-Farad Rockford Fosgate capacitors from my car resulted in the voltage staying above 11.5V, thus ruling out wiring as a potential fault.

In accordance to <R02>, the energy would infact be derived from the batteries, since that is where it would charge from. I don't see how this effects <R63>, because it does not alter the pathway.

Regardless, the safety could be called into check, and that's a big one on FIRST's list. Just an idea :-D.

I agree with Al and believe that most inspectors would rule against this. However, I would have your team's rules expert post your request to the FIRST Q&A.

You never know...

Mike

[zachriggle]

Originally Posted by Al Skierkiewicz

Remember that any time the power supply falls below about 8 volts for a short period, the RC will go into standby and draw current from the backup battery. During this time, the PWM outputs will be disabled.

Uh-huh. Reeeeaally. Did not know that. Hopefully the omni's will help when they get here.