Current Measurement

[writchie]

Originally Posted by DeepWater

Ok, after some more searching I think I have found a current sensor exactly like what I had envisioned but I still don’t know if it is legal per the wording highlighted above in <R44>.

Are these Phidgets line of current sensors legal per <R44>?
If they are legal then they seem like they would be easy to connect to the cRIO analog inputs and program to read the current draw thus exactly solving our problem.

These are available from several places including Trossen Robotics.

IMHO these would be legal, assuming the supplier qualifies as FIRST legal. IMHO such hall effect units are the best solution because they are electrically isolated from the power leads.

Note that they are good for 500% overload i.e. 250 amps.

50 amps is probably not enough range for all conditions. You could probably replace the 50 amp parts with the 200 amp allegro parts. That would enable you to measure currents near stall conditions.

[ChuckDickerson]

All, thanks for the info. I'm glad to hear that I am not the only one that would consider the Phidgets current sensors "low impedance" and thus legal.

Al, I as well can't make out the details of the schematic but it's no big deal. It looks like the Phidgets devices are a COTS solution to our problem.

Originally Posted by writchie

IMHO these would be legal, assuming the supplier qualifies as FIRST legal. IMHO such hall effect units are the best solution because they are electrically isolated from the power leads.

Note that they are good for 500% overload i.e. 250 amps.

50 amps is probably not enough range for all conditions. You could probably replace the 50 amp parts with the 200 amp allegro parts. That would enable you to measure currents near stall conditions.

I haven't gone back and double checked but I think Trossen Robotics was even listed as an official supplier in the Kit of Parts when they were flashing the suppliers and sponsors before the start of the Kickoff. The Phidgets devices are available as COTS from several internet suppliers so I don't see any reason they wouldn't be legal COTS from that persective.

As long as they meet FIRST's idea of "low impedance" then I think they are legal.

I'm not sure why we would ever need more than 50A. We can't have any motor on a breaker greater than 40A so why wouldn't 50A be enough. Same with the 20A and 30A Phidgets on 20A or 30A breakers. Please explain further if I am missing something.

[MrForbes]

The motors can draw a pretty big current spike under some conditions, without popping the breaker.

[writchie]

Originally Posted by DeepWater

I'm not sure why we would ever need more than 50A. We can't have any motor on a breaker greater than 40A so why wouldn't 50A be enough. Same with the 20A and 30A Phidgets on 20A or 30A breakers. Please explain further if I am missing something.

Motor currents can approach the stall current for short periods of time. You can have loads of 60 amps for 2 second and 100 amps for hundreds of ms without exceeding the capabilities of the battery, breakers, or speed controllers.

If you limit yourself to breaker amperage, your measurements will be clamped at 50 amps during these periods. This may or not matter to your application.

In this years game, friction may limit the maximum torques and consequent motor current for drives. But you could still have higher currents in your mechanism, especially shooters.

Also keep in mind that the speed controllers are PWM. Your current is pulsed (around 100hz for victors, 15kHz for jaquars). You need to integrate the current over time to have an meaningful values.

[ChuckDickerson]

Thanks, I wasn't really thinking about spikes. Where did you find that they are good for 500% over their measrement range?

We are interested in current measurement mostly for use in traction control so I am guessing large spike due to stalled drive motors may not be a huge problem this year. I am assuming that a jammed or locked mechanism driven by a CIM would stall the motor long enough for the breaker to kick out but I could see that brief large spikes would occur when say a CIM shifts from full forward to full reverse. Any thoughts as to if these Phidgets would smoke on a brief spike like this or just max their reading?

[writchie]

Originally Posted by DeepWater

Thanks, I wasn't really thinking about spikes. Where did you find that they are good for 500% over their measrement range?

We are interested in current measurement mostly for use in traction control so I am guessing large spike due to stalled drive motors may not be a huge problem this year. I am assuming that a jammed or locked mechanism driven by a CIM would stall the motor long enough for the breaker to kick out but I could see that brief large spikes would occur when say a CIM shifts from full forward to full reverse. Any thoughts as to if these Phidgets would smoke on a brief spike like this or just max their reading?

See the datasheet for the allegro sensors. 500% will protect them from damage. The way these work is that the sense terminals go through the IC package so that the magnetic current in the wire is sensed by hall effect inside the package. There are electically isolated. I think they can withstand 500% overload indefinitely.

[Al Skierkiewicz]

This will add to your confusion...
All motors are in stall when power is applied and the rotor is not moving...i.e. when starting.
Snap action breakers can withstand a 600% over current without trip for a second or two, 200% overload for up to 10 seconds.
Snap action breakers reset almost immediately.
CIM motor is 129 amps at stall,
Battery is capable of delivering 600+ amps at full charge.