How much current should be drawn while disabled?

Hi, we are finally to the point where we can test out our mechanisms and we have come across some strange problems. The symptom that got us started down this trail is when we have our "PID"s (they’re just P drives) hold our lift and wrist in place, they occasionally drop for a moment and snap back up. If they happened at different times, I would think the P constants needed tuning, but both the wrist and the lift drop and spring back up simultaneously. Sometimes the lift only drops an inch before it picks back up, but other times it’s almost 6 inches, which is another reason why a PID issue seems unlikely to me. I checked how much current was being drawn from the ports on the PDP powering those mechanisms. The lift motors float around 6 amps when stalled and peak at about 13 amps when moving. The wrist motors float around 3 amps when stalled and 20 amps when moving. There are two motors for the lift and two motors for the wrist. The speed controllers for the lift (one Talon SRX, one Victor SPX) are using 40 amp fuses and the speed controllers for the wrist motors (both Victor SPXs) are using 30 amp fuses.

The weirdest part is that when the robot is disabled, something is drawing a static ~40 amps. When the robot is enabled with moving mechanisms, the total current is that number plus the current drawn for the moving parts. I’ve checked the current draw for each individual port of the PDP during disabled periods and nothing adds up to nearly that much. The total count of everything hooked up to the PDP is the RoboRIO, PCM, VRM, 12 speed controllers, and a Limelight. I compared the disabled current draw on this robot to our other robots and none of the others are remotely close to that number.

I made sure every connection and joint was secure and that there were no exposed wires. Looking back at previous log files it looks like it has been a problem since that robot has first been powered on (Feb. 8th). Any ideas about anything else we should check out?

First of all, make sure your PDP has the most recent firmware updates. I vaguely remember there was a bug where the PDP current sensors reported an inflated value that was fixed with a firmware update.

If something were actually drawing a constant 40A for multiple minutes consecutively, that thing would heat up. Your battery voltage would also drop steadily over time without the robot being enabled. If you aren’t seeing either of these things, I’d bet its a bad sensor reading rather than an undiscovered short. You can always use a handheld ammeter to confirm the PDP readings, but that might take some time if you don’t have a clamp ammeter.

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Thank you for your reply. I checked the firmware version earlier today and I thought it was the most recent, but I will check again tomorrow. I never felt the PDP to see if it was hot, but over the course of an hour while disabled the battery voltage dropped from 12.5v to 12.0v, so it does look like the PDP readings might be off. To see the total current draw, I would clamp the ammeter around one of the battery cables, right?

If you have a clamp ammeter, then yes. If you have a normal ammeter, you need to insert the ammeter into the PDP input pathway.

or you could use a multimeter on the ammeter setting…

A disabled robot should draw less than 10 amps unless you have led lighting. How are you measuring the 40 amps? I suggest you remove all breakers and then measure the current. If it drops to 3 or 4 amps (roborio, radio, rsl, & pdp) then simply insert one breaker at a time until current spikes. Suspect that the meter is lying to you if nothing changes. Most clamp on as ammeters measure AC current only.

Sorry, forgot to give an update. I was wrong about the firmware version on the PDP, so I updated that and the current readings are now normal. I also found an update for the Talons and Victors, so I did that and now the stuttering is gone.

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