Severe voltage drop

[ctt956]

Originally Posted by ctt956

Do the motors slow down/struggle at all when driving/turning? If you just run one side at a time, does that cause the same problem? Test this on the floor and propped up. I would also check the temperature of the motors(do they get hotter than they should?) as well as the fuses.

One thing I forgot: check all wiring from the battery to the motors and make sure there's no short. Check for metal shavings in/on the ports/boards/contacts of the motor controllers, roboRIO, and anything else that's somehow related to the motors. Make sure none of the ports are in contact with the frame or other metal. Just be sure there's no short anywhere.

[pilleya]

Originally Posted by Jon Stratis

Yup, that is probably the issue. The CIMple box is a 4.67:1 ratio, while the toughbox mini could have been anywhere from 5.95:1 to 12.75:1, depending on the configuration. It sounds like the easy solution is to either go back to the mini's or add another 2:1 or 3:1 reduction somewhere (look at the monk's to see what reduction you previously had!)

If they reverted back to the Mini's I think there would be serious issues with the CIM's clashing with the ground/defenses. However it is much better to be able to go over a few obstacles vs none at all because of brown-outs.

Looking at the amount of space you have, you should be able to fit a http://www.andymark.com/product-p/am-tbmicrooptions.htm enabling you to get the necessary reduction, and still have the clearance(in fact they are more compact than the CIMple box).

In regards to accomplishing the reduction outside the gearbox, your drive belt seems to have quite a lot of slack in it anyway. The easiest fix would be to just use #35 chain. Change the pulley on the output shaft for a small hex bore sprocket say 12 or 15 tooth and a change the pulley on the the drive wheel with a larger one say 30t( check that the sprocket is smaller with chain then the wheel diameter), some spacers can then be used to make up for the decreased width of the sprocket vs pulley

Here are a couple or resources, which should assist you in fixing your problem and prevent similar problems in the future.

http://www.wcproducts.net/how-to-belts/

http://www.chiefdelphi.com/media/papers/3188?

[Al Skierkiewicz]

Originally Posted by CalTran

In case anyone was wondering, (with the assumptions that those pullies are 4" tall and the tread has a similar COF to Blue Nitrile), your drivetrain had a free-speed of 19.93ft/s and a "pushing" current draw per motor of 265.15 Amps.

Just to remind everyone, the stall current on CIM is 131 amps. So yes, you are drawing max current as demonstrated above.

[RyanN]

Per everyone else's thoughts, one CIM per side on a tank drive is probably your issue.

Each CIM, as Al and other have stated, stalls at 131A. Two motors at stall is 262A.

Our typical battery has an internal resistance of 0.08 Ohms. Say a fully charged battery is 12.8V.

At 262A, you'll see a calculated (not real) voltage drop of 20.96V from the battery. Given the battery voltage is 12.8V, and full stall on two motors will drop the voltage, if given the opportunity, 20.96V, there is a very good chance that the stalling is your issue.

However, I won't stop there...

Check all of your big 8AWG or 6AWG wiring from the Anderson connector to the power distribution board. Are all of the crimps well done? Give them a good, firm tug. Are all of the bolts and nuts that fasten those ring terminals on tight? Don't overtighten (break) them, but make sure they don't move left and right if you try to move them. What we are looking for here is resistance through all of your connection points. Are any of the Anderson PowerPole connector you are using charred up on the inside from people testing motors with them? That'll cause some resistance.

Adding an additional CIM will likely NOT fix your issue. You need to figure out a way to gear it down (higher gear ratio). You can try a different gearbox, add a planetary gear set with a CIM output, or change the pulley's you are using.

[Al Skierkiewicz]

Ryan,
You misplaced a decimal...
Typical battery is 0.011 ohms internal so 262 amps will drop 2.88 volts on a fresh charge. Adding up other typical losses, a team might actually see a 3.5-4 volt drop at the input to the PDP under stall conditions.
Your other recommendations are perfectly stated.

[pilleya]

Originally Posted by RyanN

Adding an additional CIM will likely NOT fix your issue. You need to figure out a way to gear it down (higher gear ratio). You can try a different gearbox, add a planetary gear set with a CIM output, or change the pulley's you are using.

By using a Versaplanetary with CIM output shaft and 3:1 stage you should be able to gear your chassis down suffciently( that's if gearing is actually the problem). It should be relatively simple as well, as you can literally attach the entire CIM+planetary assembly as you would a normal CIM. The most difficult bit being reducing the length of the CIM shaft.

[RyanN]

Originally Posted by Al Skierkiewicz

Ryan,
You misplaced a decimal...
Typical battery is 0.011 ohms internal so 262 amps will drop 2.88 volts on a fresh charge. Adding up other typical losses, a team might actually see a 3.5-4 volt drop at the input to the PDP under stall conditions.
Your other recommendations are perfectly stated.

Ha... going off of memory here. It was either 0.08 or 0.008. I should have looked it up, the MK ES17-12 has a typical internal resistance of 0.012 Ohms (12 mOhm). Other batteries are likely similar (as you stated 11 mOhm).

So yes, if it is 0.008, the voltage drop of 20.96V goes to 2.096V. That would be fine for the battery, but I'm betting the resistance of the main breaker gets higher as the internal element heats up, as does every other part of the robot's backbone electrical distribution system. Resistance adds up in series.

[Kevin Sevcik]

Originally Posted by pilleya

By using a Versaplanetary with CIM output shaft and 3:1 stage you should be able to gear your chassis down suffciently( that's if gearing is actually the problem). It should be relatively simple as well, as you can literally attach the entire CIM+planetary assembly as you would a normal CIM. The most difficult bit being reducing the length of the CIM shaft.

That a brilliant solution, actually, but Vex is out of 3:1 stages until after the season. You have to have 2-4 3:1 stages already in stock, or know some teams that have them,

[Monochron]

Originally Posted by Al Skierkiewicz

Ryan,
You misplaced a decimal...
Typical battery is 0.011 ohms internal

We recently did a survey of all of our batteries (testing involved removing them from chargers for half an hour prior, then taking 5 readings between 10 seconds or so of connecting them to an unloaded motor). The batteries that we had purchased brand new all had an internal resistance of 0.016 ohms, our average batteries (owned for a year+) had around 0.018 - 0.019, and our older batteries that we have seen brown outs with were around 0.021-0.024.

Is the 0.011 ohms number an ideal case or should we really expect to purchase batteries with that resistance? Or could our testing method have been bad?

[Al Skierkiewicz]

Brian,
Measuring internal resistance takes some specialized gear to get it right. The 0.011 ohm specification is what is published for most of the batteries. It is however most often termed "internal impedance". For simple resistive loads, that is sufficient to use as a resistance in your calculatons. However, with complex loads like the PWM motor controllers and motors, the impedance would enter into more complex calculations.
The Battery Beak makes a quick calculation by switching in two resistive loads and making the calculation based on those readings. My examples all use worst case currents based on resistive loads.
Please understand that terminals, wire and connectors all will add to that resistance and the value is only valid on new, fully charged batteries. The one foot length of #6 wire allowed by FRC rules will add 0.001 ohms, the bettery terminals and Anderson connector will add another 0.002-0.004 ohms. So if you are measuring at the SB50, your reading of 0.018 ohms is accurate.

[Ether]

Originally Posted by Monochron

We recently did a survey of all of our batteries (testing involved removing them from chargers for half an hour prior, then taking 5 readings between 10 seconds or so of connecting them to an unloaded motor).

What readings were you taking and what equipment was used to take the readings?

[philso]

Originally Posted by team3192

Driving forwards and backwards drops the voltage from 12ish to about 9, and turning the robot (driving either side in the opposite direction) drops the voltage from 12 to 6 on average.

Originally Posted by team3192

Here are three pictures of the CIMple box coupling itself: http://imgur.com/a/jj0OB

Are those black rectangular pieces on your treads rubber or some other grippy material? It looks like you have a very long piece of tread in contact with the carpet on each side. Perhaps some of the other people who have direct experience with treads (we don't) can offer an opinion on whether or not this will be too much traction to allow turning.

[Monochron]

Originally Posted by Ether

What readings were you taking and what equipment was used to take the readings?

We used the CTRE Battery Beak measuring at the SB50 connectors with about 2' total of wire. Basically the setup that Al guessed above.

[Ether]

Originally Posted by Monochron

We used the CTRE Battery Beak measuring at the SB50 connectors with about 2' total of wire.

Beware of this issue.

[Kevin Sevcik]

Originally Posted by Ether

Beware of this issue.

There's lots of awful can hide under those mounds of heatshrink and electrical tape. Including terrible crimps, loose connectors, and varying amounts of corrosion. Mind you the worst battery read at a whopping 0.1 ohms. That one had corrosion covering one of the terminals, and really was dead. Dropped out to less than 10.5V after 3 seconds under a 7.5A load on the CBA.