motor stall current vs Victor ratings

[Tristan Lall]

Originally Posted by dawilliams

1. Are FIRST robots allowed more than one battery?

Yes, but not in the way you're thinking. Only one 12 V battery (of the Exide ES18-12 or EX18-12 types) and one 7.2 V battery (any NiCd pack) may be used as described in the rules.

Originally Posted by dawilliams

2. Can we have more than one main breaker in parallel off the battery (if only one is allowed)?

This one should be a no, per <R54>, but there's a little bit of a catch here. If you read <R54> as meaning "use these devices in this general arrangement," (there's a diagram) "but don't worry about making the connections in exactly this way", then you've probably satisfied the rule as it was intended to be followed. But the loophole is, of course, that a 120 A circuit breaker (even a 2^nd one) is a legal additional electrical part*. If you're reading the rule to mean that the connections don't have to be made in exactly this manner, then you have not restricted the nature of those connections in any meaningful way. You could therefore claim, that having two parallel 120 A breakers (in the usual place in the circuit) satisfies the diagram (because a subset of the electrical system is exactly as required, and the diagram does not preclude the possiblity of different or additional connections).

Now, to anyone who's been in FIRST a while, that's obviously not the expected conclusion—but how can we interpret the rule to make this configuration impossible? It seems to me that we would have to read the rule as meaning "use these devices in this exact arrangement, including making the specified connections". This, means that you must not add anything extraneous to, or remove anything from the specified portions of the circuit. All this is fine, until you run into the fact that the Maxi fuse panel and both ATC panels are in the circuit as drawn, and connected in a particular fashion. That implies that all must be present and connected exactly this way, even if not used. The Maxi block is heavy, and teams might understandably want to be rid of it, to make weight (sacrificing maximum power output in the process); similarly, it's likely that one of the ATC panels is redundant. The rule certainly wasn't interpreted and enforced according to this reasoning last year.

We could look for a middle ground (where the extra main breaker is illegal, but the rest of the diagram is less strictly defined), but no such thing was proposed during the season—indeed, it seems to have been overlooked by all.

I should note that depending on the circumstances, it may well be ruled as unsafe per se, and therefore in violation of <S01> and/or <R40> (which pertain to unsafe design, operation or additional part usage, as judged by referees and inspectors, respectively), but without direction in that regard, various officials will have different feelings on the matter, depending on their perspectives and knowledge. (And indeed, if the parallel arrangement is unsafe, consider that the very same reasoning might lead a single 120 A breaker to also be considered unsafe—I can think of some arguments which might lead down this road....)

I don't recommend trying to exploit this potential loophole, as it would probably be quashed very quickly upon being submitted to the Q&A.

Originally Posted by dawilliams

3. Are the circuit breakers provided in the KOP auto resetting? Main breaker auto resetting?

The individual 20, 30 and 40 A breakers are self-resetting; the 120 A main breaker is not.

All of the preceding applies to the 2006 game, and may not be true in 2007.

*It is legal to have on your robot (and not just as a decoration), because it satisfies the additional electronics rules and is not specifically prohibited anywhere. Its precise function (beyond its mere presence) is the issue.

[eugenebrooks]

R54 refers to the battery and the 120 amp breaker, directly,
in the singular. That, in combination with the diagrams and the
fact that the breaker clearly protects the 6 gauge wire, battery
connector, and battery from overcurrent, makes the intent of
R54 clear. Seeing this as a loophole is quite a stretch. I don't
think that you would get this one past any technical inspector.

Eugene

Originally Posted by Tristan Lall

But the loophole is, of course, that a 120 A circuit breaker (even a 2^nd one) is a legal additional electrical part*. If you're reading the rule to mean that the connections don't have to be made in exactly this manner, then you have not restricted the nature of those connections in any meaningful way. You could therefore claim, that having two parallel 120 A breakers (in the usual place in the circuit) satisfies the diagram (because a subset of the electrical system is exactly as required, and the diagram does not preclude the possiblity of different or additional connections).

[Richard Wallace]

Originally Posted by eugenebrooks

I don't think that you would get this one past any technical inspector.

Well, maybe someone could get it past one -- Tristan is an experienced FRC robot inspector.

But I agree with Eugene about the intent of the rule.

[dcbrown]

The EX18/ES18 battery specification lists a maximum rating of 230A for 5 seconds. Various estimates of a 120A load based upon the the battery specification chart and different derating values yields between 20-40 seconds of rated battery life. Real life experience may differ, but the these are the rated values from the specification.

So on the practical side, pushing the battery beyond 120A for all but short brief periods will mean the 'bot will die before the round is over. Running two 120A main breakers would seem to indicate the design only wants the robot to run a few seconds before completely draining the battery AND it would allow the battery to be run outside of its specifications.

Running the battery anywhere up around the maximum rated value of 200A+ for any time at all is flirting will damaging the battery at least or causing some safety issue like melting or worse. Also, the number of discharge/charge cycles the battery can go through is highly dependent upon the rate and depth of discharge. Draining capacity at high loads can severely shorten the lifetime of the battery. Some data on AGM/GEL cells indicates as few as 10s of cycles under these extreme circumstances.

So even if you wanted to run multiple 120A breakers, there is no practical reason to do so -- the battery would at best drain very quickly and not last through the competition. All such a design would be doing is causing a potential risk to the robot as well as others.

Of course there have been rumors of changing the battery for this upcoming season, so there may be a whole different set of issues in two weeks to discover.

On the practical side, the motors when geared down seldom fully stall but instead start spinning the wheels even with grippy treads.

Bud

[dawilliams]

Thank you all for your replies. This is certainly a credit to the FIRST gracious professionalism creed.

Still one curiousity comparing my BBIQ experience with FIRST robots. Most teams at BBIQ used 24v systems for drive, motors from 1-3 hp PER SIDE (or 2-6 hp total in the differential drive), and batteries with 5-6 AH ratings. The playing surface was sandtextured paint on steel, which ended up fairly slick by the third day of competition. If two bots pushed against each other, one would move or the wheels would spin. This lack of traction compared to rubber on carpet as with FIRST would protect BBIQ bots from burning up Victors or motors. But I still saw several burned motors or Victors (pink/grey smoke) each year.

The FIRST motors are approx 1/2 hp for the largest ones. The battery has higher AH rating letting it keep up with whatever the motor draws. The playing surface with rubber tires is less forgiving (no slipping to protect the Victors and motors). To my thinking, this means robots pushing against each other won't be able to move each other. Thus more likely to stall the motor. Even geared down, if you push against the side of the opponent, you won't be able to move it, and that is still a stalled motor.

The 4 largest motors draw 133, 96 or 63 amps at stall. That exceeds the 40 amp rating of the Victor 884. The Victor 884 doesn't list a higher current for any short time (as the Victor 885 does).

So do you suggest a single Victor 884 for one of these large motors, or two in parallel for each motor, each with an independent 40A circuit breaker?

Thanks again!

[Joel J]

What I usually see the robot designers do is to select a traction material for their wheels, then calculate the maximum torque they can get with those wheels, given the robots weight, as a first step. So lets say there is a 6 inch tire, a 150lb robot, and the coefficient of friction between the wheel and the floor is 1.3 (IFI roughtop tread). The gearbox designers would calculate the max torque at the wheel to be: 150*1.3*3(tire radius) = 585 in-lbs. Convert that to the oz-in units that's given on the CIM motor chart, and you get 9360 oz-in's at the wheel before it starts slipping. I've typically seen FIRST drivetrains use the 4 identical CIM motors, two per side, meaning that the load would be evenly shared across these motors. So each motor needs to provide 9360/4 oz-in's of torque, or 2340 oz-in. The designer would then determine the amount of torque they would get when pulling 40 amps from the CIM motor, as opposed to the 133 amps at stall (let's say its 105.43 oz-in's, as opposed to the stall torque of 343.4 oz-in). Divide the torque needed from the individual motor by the max load you want it to see, and you get the reduction needed between the motor and the wheel. In this case 2340/105.43 gives a reduction of 22.1948 minimum. When you reduce the free speed of the CIM motor (5310 RPM) 22:1, you get a final RPM of 239.245, which translates to a ground speed of 6.2634 fps. I didn't factor in losses (4.5253 fps, with losses, on most one speeds.. 4.1fps on a typical 2 speed). At this reduction, the idea is that the wheels would start spinning before the motors begin to pull more than 40 amps, preventing stall. I guess the idea is similar to having a low traction surface to play on. If the final speed you get after doing these calculations is too low, then you just lower the "grippiness" of the wheels you are using. If you want to be a strong pusher, you can only lower the grippiness so much before you start losing traction advantage. You'll start getting into the whole tradeoff between speed/torque/current pull that I just can't bother with; I'll stick to programming.

This was a bit rushed. If you want a detailed description of this idea, you can search around the forums for topics on drivetrain design, or gearbox design, or something..

[dcbrown]

At River Rage, drivers of our team bot which used two CIM motors, 6 wheel drive with 2" wide IFI roughtop tires side pushed all the robots it came up against to prevent shooters from scoring. Motors never stalled, breakers never popped. The robot used the KOPS transmissions and weren't geared down very far from there - that is top speed was still acceptable.

Bud

[eugenebrooks]

Unless the rules change this coming season, relative to last year, I suggest a single 40 amp breaker, feeding a single Victor 884, connected to a single CIM motor.

Yes, the Victor 884 is rated at 40 amps continuous, and the breaker is rated at 40 amps, but both of these parts will sustain higher currents for short time periods.

Eugene

Originally Posted by dawilliams

The 4 largest motors draw 133, 96 or 63 amps at stall. That exceeds the 40 amp rating of the Victor 884. The Victor 884 doesn't list a higher current for any short time (as the Victor 885 does).

So do you suggest a single Victor 884 for one of these large motors, or two in parallel for each motor, each with an independent 40A circuit breaker?

Thanks again!

[JBotAlan]

Originally Posted by dawilliams

Thank you all for your replies. This is certainly a credit to the FIRST gracious professionalism creed.

Well, the official hint has been released, so traffic here is picking up.

Originally Posted by dawilliams

To my thinking, this means robots pushing against each other won't be able to move each other. Thus more likely to stall the motor. Even geared down, if you push against the side of the opponent, you won't be able to move it, and that is still a stalled motor.

Two things:
1) the drivers are trained not to keep pushing if we're not moving--that would release magic smoke for sure if the wheels weren't slipping which brings me to:
2) our wheels start to slip when pushing against something relatively static at somewhere around half power, so our motors don't stall unless the driver stalls them on purpose (holds them at quarter power for a long time). We still have a strong pushing force; ask anyone who has played against us.

Originally Posted by dawilliams

So do you suggest a single Victor 884 for one of these large motors, or two in parallel for each motor, each with an independent 40A circuit breaker?

At first, I thought that having two victors on one motor was against the rules, but I re-read last year's rules and the only slightly relevant rule is <R86> in section 5:

Originally Posted by 2006 rules

<R86> No more than one motor may be connected to each Speed Controller.

That doesn't prohibit two Victors on one motor. However, I can see this being frowned upon during inspection. One victor is all that's needed to white smoke a motor; two seem like they would cause problems for sure. I just don't see the need for that much power. I don't think we've ever tripped a breaker. Just plan for the wheels to start slipping just as the motors start to stall.

JBot

[eugenebrooks]

Refer to "FIRST Guidelines, Tips and Good Practices"
and the "Robot Power Distribution Diagram." These
documents are updated each season. R80 refers to
the robot power distribution diagram.

I see our robot is traveling in circles...

Eugene

Originally Posted by JBotAlan

At first, I thought that having two victors on one motor was against the rules, but I re-read last year's rules and the only slightly relevant rule is <R86> in section 5:

That doesn't prohibit two Victors on one motor. However, I can see this being frowned upon during inspection. One victor is all that's needed to white smoke a motor; two seem like they would cause problems for sure. I just don't see the need for that much power. I don't think we've ever tripped a breaker. Just plan for the wheels to start slipping just as the motors start to stall.

JBot

[Alex698]

Now i have to ask:
How long before we melt the anderson power-pole connectors? them seem to be the weakest factor. Just a thought.

[Gdeaver]

Keep in mind that while a robot is in a pushing match they are not scoring points. Scoring point wins matches. Yes, defense comes into play, but a well coordinated offense can neutralize a defense with picks and such. First play is different than battle bots and the rules more limiting.

[RogerR]

Originally Posted by Gdeaver

Keep in mind that while a robot is in a pushing match they are not scoring points. Scoring point wins matches. Yes, defense comes into play, but a well coordinated offense can neutralize a defense with picks and such. First play is different than battle bots and the rules more limiting.

this isn't always the case; in 2000, 2001, 2002, and 2003 you got points based on where you or you're goals where (granted, in '01 you were fighting gravity, instead of opposing 'bots). in 2004, when most 'bots hung from the upper platform, being able to hold one's position while attaching to the bar was often dependent on how your drive train measured up to your opponents. likewise, the triplets' beefy drivetrain, coupled with effective programming, helped them to hold their positions while under attack from opposing defenders, which allowed them to unload massive quantities of balls into the high goal.

while the specifics depend on the game and startegy, an effective and powerful drive train can be a potent offensive weapon.

[Al Skierkiewicz]

DA,
Maximum rated current for a Victor was published a few years ago but can be derived from the current specifications for the MOSFETs used. Keep in mind that each FET is in parallel with two others for current sharing but that there is derating due to lack of heatsink. The FETs have a very low "on" resistance and so very little power is dissipated within the device which keeps internal heating to a minimum. However, there are other factors...usually at maximum current, a) the speed controller is no longer using a PWM output, it is at full "on", b) the motor is at or near stall, c) the motor may be back driven by a pushing robot, d) bad electrical connections at the Victor will raise it's temperature due to heating of the contact area, e) the fans become inefficient due to the lower battery voltage. Keep in mind that about 90% of all speed controller failures are due to foreign and conductive material inside the controller. The remaining failures are a result of incorrect wiring, over heating, repeated high current due to stalled motors or rapid direction changes or defective mechanical systems.
As pointed out, many teams design for at least 6 ft/sec but speeds from 8-12 ft/sec are acceptable. Higher speed designs will result in mechanical systems that will easily stall and draw excessive currents when starting or accelerating.

[dawilliams]

Originally Posted by Al Skierkiewicz

DA,
Maximum rated current for a Victor was published a few years ago but can be derived from the current specifications for the MOSFETs used.... The remaining failures are a result of incorrect wiring, over heating, repeated high current due to stalled motors or rapid direction changes or defective mechanical systems. .

Can you direct us rookies to this published info? We're not electronics engineers and able to calc from the MOSFET ratings. Its too bad IFI doesn't publish these ratings on the Victor 884's like they do on the Victor 885's.