Voltage Drop problems

[philso]

Check the health of your batteries with a load tester such as the Beak. It is possible that the output impedance of the battery has increased with age/abuse.

Check that all the connections between the battery and the PD Board are tight. A loose connection will have higher resistance leading to increased voltage drops.

The starting current for the motor can be as much as 5 X the running current so take Ryan's advice to ramp up the large motors especially if they have high inertia loads. A large percentage of the (3-phase AC) motor controlers that I work on are used for doing just this.

[AlexH]

Sounds to me like your drivetrain is drawing too much and causing the voltage drop. Try adding 550 size motors with cim standin gearboxes or try a lower gear ratio.

[Al Skierkiewicz]

John,
Are we reading you right, the shooter is a direct drive CIM? I would expect that to spike motor current significantly when starting the shooter or when ejecting a ball. Remember that the CIM motor is speced at 133 amps at stall. While these two motor together shouldn't draw the battery down that much, there is a possibility that you have loose connections as described above by phil. A drop in voltage is normal, but on a fully charged battery running only these two motors I would expect it to only draw down to 10 or a little under during the first part of the match.

[kramarczyk]

Do your mechanical systems spin freely when powered down?

A direct drive CIM and a banebots motor on a 1:64 reduction should both be easily turned by hand. If not I would look into the possibility of any mechanical binding causing an increased load on your system.

If you have access to a clamp on DC current meter it may help your troubleshooting. I find chasing the current is much faster and more objective than any other means of troubleshooting binding on a FRC robot.

[philso]

"chasing the current is much faster and more objective than any other means of troubleshooting "

This is much easier than looking at voltages since you do not have to actually make a connection. You just clamp the meter around a wire in the circuit in question. If your meter has a "peak hold" function, this is the time to use it.

Thanks for the link, Mark. My old Craftsman clamp on meter is dying and I cannot justify paying for a Fluke.

[Kevin Sevcik]

Ditto the suggestion of chasing the amps to figure out where your major current draw is coming from. If you have a philosophical objection to Craftsmen, here's the Northern Tool version. I personally have a Klein CL2000 for work since I get to deal with lot of amps with funky waveforms on our welding equipment.

At any rate, figure out what's drawing all that current first. Then you can start checking for big voltage drops going into that motor. Just set your multimeter to DC Volts, clamp a lead on a motor terminal, and start working your way back along that wire to motor controller, touching your other lead anywhere there's bare metal to touch. If you see a big jump in volts from one point to another, you know there's a large resistance there where you're losing lots of power.

Also, if the problem does turn out to be your BaneBots 64:1, and you're running that with an RS-550, or one of the Fisher Price motors, consider switching to a higher torque motor. Specifically, either the BaneBots RS-775-12 or RS-775-18. Either of those motors is going to give you more torque per Amp than the RS-550. Mind you, both of them have a lower free-speed than the RS-550, so your system is likely to run some what slower. But at least it won't be completely draining your battery, right? Just make sure to look out for case shorts on the RS-775-18, as they're notorious for that.