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Unread 01-09-2018, 11:44 PM
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Inexplicable Current Draw?



This is the log for a kitbot with the default DriveTrain. It can't move without jittering and browning out.

Battery was tested with battery beak and resulted in "Status: Good"

The wiring was checked and no visible shorts were found.

Does anyone know why this is happening and how to fix it?
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Unread 01-09-2018, 11:53 PM
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Re: Inexplicable Current Draw?

What does your code look like?

Put the robot up on blocks and see which way the wheels drive. Try disconnecting all but one motor, drive it, then disconnect it, connect another motor, try driving it in the same direction, etc, for all the motors. This will show you whether any motors are driving against each other.
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Unread 01-10-2018, 12:15 AM
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Re: Inexplicable Current Draw?

4 CIMs, right? Standard drivetrain troubleshooting steps:
  1. Power off robot, disconnect all motors from speed controllers, lift wheels off the ground, Spin each side by hand. Wheels should be reasonably easy to spin in high gear. If one or both sides have significant resistance, check for binding, misassembled gearboxes, etc.
  2. With wheels off the ground, power on. Move stick(s) forwards, backwards, left, right. Use speed controller lights to verify speed controllers are moving in correct directions. I might have directions reversed, but the general logic is: Stick Forward, L1/2 forward, R1/2 reverse. Stick Back, L1/2 reverse, R1/2 forward. Stick Left, all reverse. Stick right, all forward. L1/2 should always match, ditto R1/2. If not, you have PWMs/IDs confused. If you notice ANY current draw at this point, you have a short somewhere after/inside the speed controller.
  3. One motor at a time plug motor into its speed controller. Verify wheels turn in correct directions. (It helps to make a table with joystick directions, and recorded direction for each motor.) Unplug motor and go to next. When your table's done, verify you don't accidentally have motors fighting. If motors are fighting, you have +/- swapped somewhere or a motor plugged into the wrong controller. Fix that.
  4. During the above you should observe about the same speed and current draw for each motor. If a motor is slower or has more current draw, it may be bad, shorted, smoked, etc. or there may be a mechanical problem isolated to that motor.
  5. If no motors appear to be fighting, plug them all in at once, and with wheels STILL off the ground, try driving the robot. You hopefully don't observe and significant current draw while the wheels are up. If you DO, either you missed two motors fighting, or you actually do have a mechanical problem in a gearbox or drive side that you missed.
  6. Finally, put the chassis on the ground. Drive forwards/backwards. That hopefully works correctly at this point. Try spinning in place. If that causes the current draw, you have a 6WD robot without enough center drop, and too much scrubbing is stalling the motors. Or possibly you're geared too fast/too big of wheels. Though you said kitbot, so I'm doubtful on this score.

Try all that, and if something works, do please update the thread.
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Unread 01-10-2018, 07:50 AM
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Re: Inexplicable Current Draw?

A total current draw of ~50A should not drop your voltage down to 6V. If this is truly the case, that means you have a very high resistance somewhere*. Check all of your electrical connections (especially those between battery and PDP) to make sure everything is tight. If you can move the connector without disconnecting it, it's too loose. Also make sure you're using fully charged batteries that aren't more than a few years old.


*Think Ohm's law: V=I*R. If a low I gives a high V**, your I must be very high.
**(edit) V here being the voltage drop, not the total voltage
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Unread 01-10-2018, 08:01 AM
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Re: Inexplicable Current Draw?

With a current draw like that, the location of the draw should be pretty warm after a minute or so.
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Unread 01-10-2018, 08:04 AM
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Re: Inexplicable Current Draw?

Quote:
Originally Posted by animanny View Post


This is the log for a kitbot with the default DriveTrain. It can't move without jittering and browning out.

Battery was tested with battery beak and resulted in "Status: Good"

The wiring was checked and no visible shorts were found.

Does anyone know why this is happening and how to fix it?
There's a lot of good information in the above, but your problem isn't limited to the motor-related paths, so I think motor-related current paths are unlikely to be the only root cause of the problem.

Why do I say that? If I'm reading the above chart correctly, your "resting" battery voltage, as reported on the PDP, is only 11.3 volts or so. That is when essentially no current is being drawn from the battery. That number should really be around 12.4 to 12.8 volts for a good, fully charged battery which has good electrical connections between the battery and the PDP.

You need to fix that problem first -- indeed, it may be the only problem. Put a voltmeter on the PDP input terminals to confirm the "resting" voltage.
If that's not up in the range of 12.4+ volts with a fully charged battery, something is wrong with one of the following: battery, cable connections to battery, cables themselves, the Anderson connector on the battery, the mating of the battery Anderson connector to the robot Anderson connector, the on-robot cabling from the Anderson connector to the PDP, the wiring from the Anderson to the main breaker, the cable connections onto the main breaker, the main breaker itself, or the connections from the main breaker to the PDP. You either have a bad battery or a high-resistance path (loose connection, poor connection, or something) in one of the above items.

All of the motor-related symptoms you are seeing are follow-on symptoms of the poor "resting" voltage and are not the (only) primary root cause.

Please let us all know what you find out!
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Unread 01-10-2018, 08:09 AM
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Re: Inexplicable Current Draw?

Quote:
Originally Posted by AriMB View Post
A total current draw of ~50A should not drop your voltage down to 6V. If this is truly the case, that means you have a very high resistance somewhere*.

*Think Ohm's law: V=I*R. If a low I gives a high V, your I must be very high.
A high resistance limits the current flow, not the inverse. Low resistances are characteristics of a high current-draw system.

V = IR
I = V/R
therefore, as R->inf, I->0. Likewise, as R->0, I->inf.

To think of this better, the voltage drop across a small section of wire is almost 0 since its resistance is negligible (current can flow). Across an air gap, the resistance is almost infinite, so no current can flow.
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Unread 01-10-2018, 08:49 AM
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Re: Inexplicable Current Draw?

+1 to a dead battery. I won't let my tean go out with a battery reading under 12.5 V.

In addition to that, you say "default drive train". Can you elaborate so we all know we're talking about the same thing? 6 wheel drive with a dropped center on 6" wheels, right? And the robot rocks forward and back on its center wheel, correct?. You properly greased the gearboxes? How many CIMs per side? Have you verified the CIMs are rotating in the same direction?
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Unread 01-10-2018, 09:06 AM
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Re: Inexplicable Current Draw?

Another thing to consider is the age of the PDP. If it's an older model, we ran into an issue where it would show things like that due to the firmware within the PDP. A CTR person at States checked it out with us to determine the issue. I don't remember the exact specifics, but I do remember I was pretty surprised by this...
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Unread 01-10-2018, 09:33 AM
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Re: Inexplicable Current Draw?


Quote:
Originally Posted by Jon Stratis View Post
+1 to a dead battery.
Quote:
Originally Posted by animanny View Post
Battery was tested with battery beak and resulted in "Status: Good"
...
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Unread 01-10-2018, 09:37 AM
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Re: Inexplicable Current Draw?

The term "high resistance" is being used a lot here. Just to clarify, in this context, we are talking relative to the resistances of thick wires, beefy connectors, fairly-powerful motors, etc.

All these should probably have a resistance in the range of 0.01-1.0 ohms. A very light-duty motor with a dirty commutator might get up to 1.5 or 2ish, maaaybe...

"High resistance" just means some current-carying element with an abnormally elevated resistance - 10 to 30 ohms, I'd guess.

Note all of these resistance values are going to be hard to measure accurately with your average multimeter - I wouldn't trust the results. Measuring voltage drops across components while under load will be more indicative.

+1 to the suggestion of per-motor testing while on blocks. Make sure the motors aren't working against each other. Also, if the wheels can't rotate while on blocks, no hope they will start rotating on the ground.

11V at the PDP is definitely low. Check with a multimeter both at that input (cross-check the reading) and at the battery terminals (right where they go into the plastic) - it should be nearly identical.

Last edited by gerthworm : 01-10-2018 at 11:08 AM. Reason: A wise man suggested I revise some of my estimates :)
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Unread 01-10-2018, 11:27 AM
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Re: Inexplicable Current Draw?

Quote:
Originally Posted by animanny View Post


This is the log for a kitbot with the default DriveTrain. It can't move without jittering and browning out.

Battery was tested with battery beak and resulted in "Status: Good"

The wiring was checked and no visible shorts were found.

Does anyone know why this is happening and how to fix it?
Do all of the above, but before doing that, a simple check of chassi frame grounding or the metal frame being a part of the electrical circuit (it is never supposed to be, and should be completely insulated from carrying any load or current whatsoever). I have witnessed it 3 times in the past, and exhibited same behavior you described all 3 times, resulting in damage or potential damage each time or extreme jerkiness and large current drops. EACH TIME there was a different cause, same frame grounding type of issue.

All 3 times were experienced using a C-RIO and not a robo rio.

1. 2011 A staple used on bumper fabric covering was dislodged on the practice field at Champs, said staple flipped up and into our sidecar crossing about 6 or 7 pins, this caused a frame grounding short after it actually shorted out said sidecar board. We do not know how.....a frame short was seen first with a multimeter test, only after turning the robot upside down, pounding itvon the concrete in our pit, and ridding it of all foreign matter. Near end of season I was amazed at what was in there. That act did not dislodge the now virtually welded in staple. Further very close inspection revealed the staple.

Replacement of the sidecar via a Champs provided spare parts sidecar also got rid of the frame grounding, later a full inspection of the burned up sidecar showed the board was complete burned toast. Best guess is that it was feeding current back through something that had to be connected to the frame, IE: A motor, something in our pnumatic system, or something else.

2. An older camera (for the life of me I cannot remember the model camera that came in KOP), that was grounded through the mounting base in Rebound Rumble (there was a warning posted that the base was the ground on that specific model camera and it needed to be mounted either away from frame structure or fully insulated when mounting, but the info was overlooked during the build, and it was mounted to a part of the frame and caused same type issue. Simply unplugging the camera, fixed it. Removed camera, insulated the mount, issue resolved.

3. Cannot remember the year (location Las Vegas a few years back ).A hasty replacement of a C-RIO on Friday late after last Q match by 2 students requesting zero help or direction, that was mounted too close to the frame, that still had the default but completely unnecessary and unused stock factory nice shiny chrome grounding screw down low on 1 end of the CRIO contacted the frame directly and went unnoticed or inspected closely (the removed/replaced one was missing the screw, so the issue did not originally exist as designed and built and used at a prior event and through the first 8 matches Friday.

The students went out Saturday morning for first of last 2 Q matches, came back mad or very upset, handing me a very warm, no, very hot battery that I set down on the bench as soon as I could....too hot to hold long, and loudly complaining that I gave them a bad battery after a disabled aborted match.....I said oh no I didn't (I had personally load checked and hand checked that battery for proper charged voltage before I handed it to a pit runner just minutes earlier), I ripped off the insulating tape from the battery posts & they both looked like a new copper penny each bright clean copper colored, (not usual at all as they usually have a silver colored coating on them that dulls as they age), the leads from the Anderson clip battery side were extremely hot, and very pliable and partially melted where they rejoined, but not yet shorted, still needing immediate replacement though. I immediately asked for the multimeter and stated, you have a grounded frame, this isn't a battery issue, but it came within seconds of being 1!. It took a while but we found the culprit after disconnecting many circuits and items. CRIO grounded directly to frame by that unnecessary in FRC ground screw.

Long winded I know....but in each case, Drive motors are connected to the frame along with gears, bearings, grease, etc. And they make a very weak connection, but a connection all the same, and an intermittent connection. Frame ground faults in each case showed exactly the same types of behavior you say your robot is showing, by feeding current into the drive motors in an incorrect and very dangerous pathway.

Grab that multi meter and make sure your frame is not in that power circuit (Any juice flowing through that frame is A NO GO....find that short, do not apply power again until you do once you know it exists!) Of course that needs to be done power on, so be quick about it. Then check the robo rio and PDB for debris or even 1 loose wire strand. Then move on to everything else listed by others.

I do not remember anyone mentioning motor speed controllers (though removing one drive circuit at a time would point you there also to the offending circuit), double and triple check to make sure they are all wired correctly and no loose strands, all connections tight, are they calibrated? A completely dead speed controller will also cause issues in a 2 or 3 CIM/MiniCIM gearbox combo also as 1 motor of 2, or 2 motors of 3 will not spin properly with the dead motor in the mix leading to herky jerky. It is as bad as wiring one of them backwards in the mix from others in the same gearbox set.

As will gearbox bearings installed backwards on the kit bot gearboxes....installed outside in, instead of inside the gearbox pointing out towards the wheels....But, that current draw and drop points a different direction if you don't really feel the gear and bearing binding in the robot on blocks, power off wheel spin by hand test. If the upper short shaft bearings are installed backwards, the largest gear inside rides against the inside frame face, as the lip on the bearing actually provides the gear/frame clearance (when installed correctly, you will not see the lip, it is inside the gearbox), and keeps that largest gear away from the frame. Gearbox apart you would see a big circle score the exact size of the gear face on the frame face, if the bearings have ever been installed incorrectly. Binding internally in the gearbox is much more evident as the bearings get pushed further out of the frame.

1 additional item, if students ever lift the batteries by the attached cables, it doesnt take much to loosen the bolts under those insulated connections. They are hidden, and forgotten. You would think the battery beak would show that though. Maybe not until under a load.

Take my $8.50 for what it is, a long winded 2 cents, and a bunch of bad luck. Good luck, as said issue will drive you nuts I know.
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Unread 01-10-2018, 11:47 AM
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Re: Inexplicable Current Draw?

The reason I stated CRIO, and not RoboRIO was when our issues arose, but the RoboRIo was reworked (I have read), to allow brownout conditions under high load over current draws and conditions to allow the non-disconnection during play and not forcing the RoboRio to reset completely like the CRIO used to have to do, allowing time for recovery without robot dead conditions ocurring. Is that causing the voltage drop I question due to a possible frame grounding? It sure looks like it is switching on and off voltage at a fast clip repeatedly.
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Unread 01-10-2018, 12:19 PM
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Re: Inexplicable Current Draw?

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Originally Posted by Ether View Post





...
So it won't quote quotes, but my experience is that a battery beak telling you a battery is good means its internal resistance is in an acceptable range, it is not referring to the state of charge.
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Unread 01-10-2018, 12:24 PM
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Re: Inexplicable Current Draw?

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Originally Posted by Mr V View Post
So it won't quote quotes, but my experience is that a battery beak telling you a battery is good means its internal resistance is in an acceptable range, it is not referring to the state of charge.
The user manual states:
Quote:
The battery status is calculated using the results from the Rint calculation. A set of predefined ranges are used to determine if the battery is “Good”, “Fair”, or “Bad”. The predefined ranges are dependent on battery Chemistry, Voltage and Ah rating.
So a "Good" battery may be 50% charged, if the Rint is in an appropriate range at that Voltage.
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