Hello. My team was having a little trouble at the West Michigan regional with the main CIM motor’s breakers tripping towards the end of the match. My mentor tried to explain that as the victors heated up, resistance would rise causing a higher current draw, but that seems backwards to me.
My thought was that maybe 2 victors weren’t enough to push our bot and the CIMs were being over-taxed. The bot weighs 119.2 pounds(without batteries) We are using a two wheel center drive system, 1 motor per wheel using standard toughboxes with direct drive to the wheel (no further reduction after the toughboxes). Would adding another CIM to each wheel remedy this situation? Thanks alot!
[quote= Would adding another victor to each wheel remedy this situation? [/QUOTE]
Each motor should only be powered by one victor, period. You can’t add another one per the rules and adding one in the circuit would most likely not fix your problem. Make sure that the fans are running on the victors because they can and will over heat with extended use. The fact that it is happening at the end of the match rules out a short unless it is just bad luck…[/quote]
Assuming you are in complaince with the rules (it’s hard to not be given inspections) you should only have 2 CIM motors if you are only using 2 speed controllers. Each speed controller is protected by (at most) a 40A breaker. You can’t drive a (single) motor with more than one victor under the rules (this also presents some coordination problems), and the victors can handle more than the 40 A (or so) that the breakers blow at.
Keep in mind that using two CIMs as drive motors gives you half the power of using 4 CIMs. This presents a possible solution, but it is too much weight given your margin.
Since you are only using two CIM motors, you have a limit to the force they produce as well. This depends on the reduction, but if the motors are loaded too much, they will draw excess current, and thus blow the breaker.
If these problems occur late in matches, look for something heating up or binding which would present mechanical resistance. Also, look for ways to eliminate friction within the drivetrain.
However, I would suggest you see if it is merely the time of the match that coincides with the breaker blowing. Consider what the robot was doing at the time. If it was pushing into a robot (or a wall) and you have high traction wheels, this would easily cause a breaker to blow.
EDIT: Almost forgot…
If a victor overheats, its internal resistance increases. However, increased resistance would cause less current draw. If the victor was overheating, you’d have a wonderful burnt plastic smell in the vacinity of your robot. Also, you would see decreased power at the motor without the breakers tripping. If it were to overheat too much, you’ll melt the transistor packages, then cause all sorts of thermal effects that would alter the doping of each transistor, rendering it unusable. (but you’d probably stop before this when you saw the smoke and (potentially) fire.)
Part of what the rules do is prevent this. FIRST knows that a properly cooled victor handling 40 A, as expected in a match, would most likely not overheat. Safety is built into the rules
How hot are the motors getting? 120lbs. robot, one CIM per side, direct drive off of tough box. Sounds to me like the two CIMs are having to work too hard. After they heat up, they do draw more current, and could be causing your breakers to trip.
I have seen teams get into pushing matches with only two CIMs and no gear down and do fine and not blow breakers. Check for shorts and make sure that the connections on the power distribution block are tight because they come loose easily. Did it happen at the same time during every match? Did you ever regain power to that motor? and was it both motors or just one?
I really have nothing to compare the heat of the motors to, being on a rookie team and all, but The CIM’s were hot. If adding more motors is a necessity, we can always shave a bit off of our 30 lb counterweight.
It seemed like both motors, and I suspected breakers because if I let the robot idle for a few seconds, it would regain full power. The breakers seemed to trip always towards the middle to the end of the match.
A 40 Amp breaker doesn’t magically break at exactly 40 Amps; the trip time is an imperfect function of its internal heating, which is a function of current. When the metal strips inside reach a certain temperature, they bend and push away from each other.
CIMs can draw upwards of ~120Amps if you stall them. At that draw, the breakers should pop within a few seconds. But you aren’t stalling them, you are simply asking a lot of them. It is very possible you are drawing 60-80 amps many times a match.
The first time you do this, your breakers won’t trip because they are cold. Each time after that, you run an increasing risk of popping as they heat up.
Adding two more CIMs will definitely help the situation. In the mean time, you can reduce the load on your motors by reducing any friction you may have in the transmission.
It sounds like you (the O.P.) have a very similar drivetrain to ours this year, except that we used two CIMs per wheel. Based on our experiences, I suspect that you’ve got a combination of issues:
With the low reduction ratio you’ve got, the robot is probably nominally operating at 50% or less of the no-load speed of the motors. In that scenario, you are dumping a lot of current (more than 40A) into each motor in order to quickly accelerate the robot. If you were to nominally run at a higher fraction of the motor no-load speed (most experienced teams seem to recommend about 75%-80%) then the robot would reach it’s top speed quickly, causing the motor input current to drop quickly as well. Less time would be spent drawing over 40A. The drawback is that the top speed isn’t as high. In our situation, we found that we could never use the top speed we’d planned on because of the small playfield size. Translation: gear it down if you can, or add a second pair of motors (as others have suggested).
The other possible issue is your control scheme. At one point for us, the encoders which measured our drive wheel velocity failed. That resulted in our velocity-based PID control loops getting very, very confused, and as a result our PWM values would essentially change from 0 to 254 (full rev/full fwd/repeat) at the slightest change in joystick position. You couldn’t tell by looking at the robot, since it still accelerated and decelerated normally, but in reality the motors were being asked to change direction 38 times per second. As a result they saw average currents much, much higher than 40A. In our situation, the breakers didn’t trip, but the main battery overheated and dropped to about 4V (according to the IFI rep’s logged data). Once we switched back to open-loop control, the PWM values changed in a more controlled manner, and we never overheated anything again. If your control system is doing something similar to your PWM values then the problem your are experiencing might be fixable with just a software (or driving style) change. Hope that helps…
When we changed out the final pair of gears in our transmissions to increase our speed, we had issues with the breakers popping on our right-side gearbox CIM motors, and it did seem to happen more frequently as the playoffs progressed. Running 4" wheels (4.5" with the tread), we also noticed that the right-side motors were running hotter than those on the left. We analyzed the issue, and after testing showed us the issue was not in the servo-shifters and was in fact the breakers, we came up with several correctable fixes:
Frictional losses: upon inspection, we noticed more “slop” in the spacing of the gears in the problem gearbox, which allowed the gears to potentially rub. We disassembled the gearbox and added shims to tighten up the gear spacing. We also notice the bearing support of the direct-drive output axle was not as firm as we would have liked. A space was designed in for an additional roller bearing at the inner wheel support and the , but it wasn’t installed due to redundancy. Despite the rule-of-thumb (do not support a shaft at more than 2 points due to potential binding), we added the bearing to better support the drive shaft. These changes added up to an improved drive train.
Driving style: our driver was used to running both joysticks full forward (tank steering) down the straight-aways, then throwing both stick backward for a half-second to power brake. We believe this jacked up the amps with the backdriving (brief as it was) and helped to incur the breakers popping. We had him concentrate on his approach to each corner, driving smoother through the corner, and less “back-braking”.
Motor temperature: despite a reduction in the drive motor temp after matches with the other changes, we added the use of a clip-on fan aimed directly at the drive motors as a precaution (better safe than sorry) to help cool the motors between matches. If possible, get one that’s battery-powered–always safer when there’s no cords to get tangled, and a power strip may not be available to plug into during the playoffs.
Perhaps one or more of these ideas could help with your situation.
You probably need to gear down and/or use lower traction wheels. We are running on dual Andymark wheels from this year’s kit, with 12:1 reduction in our transmission, and 15:36 sprockets from the transmission to the drive wheels, with csingle CIMs. If you are six wheel drive, the center wheels should be lower than the front/rear wheels by about 1/4 inch for easier turning. Compare your drive ratio’s to ours. Too much traction, or too high a ratio for speed, will pull more amps and trip the 40 amp breakers. You are using 40 amp breakers, I hope, to the CIM’s ? PM me if you have any further questions.
The average current draw on your motors is a function of the torque seen by the motor and the average voltage from the Victor.
The torque seen by the motor depends somewhat on the friction in your drive train but mostly on the force against your wheels.
The coefficient of friction of the kit wheels (on carpet) is about 1.0 and for traction wheels is closer to 1.2. This means that your wheels can produce tractive force of up to (1.0-1.2) * the normal force (the downward force on your wheel, usually due to gravity.) If your 120 lb mass is evenly distributed on four wheels, the normal force on each wheel will be 30 lb and the maximum force on each wheel before slipping can be 30 - 36 lbs (normal force times coeefficient of friction). HOWEVER, with a center wheel drive and depending on how your other wheels are configured, you could have most or all of the mass on your two drive wheels. This raises the maximum force before slipping to 60 - 72 lbs per wheel. This maximum is important because the maximum torque seen by the motor occurs just before the wheels slip.
The force on your wheels and their radius determines the torque. With 6 inch wheels the radius is 3 inches so the torque on the wheel drive shaft will range from 7.5 to 18 ft-lbs. (with 30lbs - 72 lbs of normal force).
The standard toughbox gearbox has a ratio of 12.75. So the torque on your motor before accounting for the losses in transmission is on the order of 0.59 ft-lbs to 1.41 ft-lbs which is 113 oz-in to 270 oz-in. The normal losses in the transmission will be on the order of 15 - 30% so the actual torque as seen by the motor will be in the 133 to 318 oz-in range (with 15% losses). Now if you look at the torque curves for the CIM motor you will see that the expected currents (at 12Volts) at these torque levels are about 50 amps and 120 amps respectively. In fact, if all your weight is on your center wheels you are pretty close to stall on the motors.
If your robot has 1/4 of its weight on each wheel and you are pushing against the wall you will draw at least 50 amps. If 1/2 your weight is on each wheel (due to your CG or the geometry of what you are pushing against) your motors could draw as much as 120 amps each without slipping. Similar forces can be seen during maximum acceleration and reverse braking. You can’t actually draw 120 amps from each motor at 12V because the internal resistance of the battery will cause a voltage drop at the battery. (Your actual voltage will be more like 9 Volts under such heavy loads)
As others have pointed out, the 40 amp breakers have a time delay characteristics (and so does the 120 amp breaker). Auto resert breakers typically hold at 100% and trip at around 135% of rated current. However, it can take as long as 20 seconds to trip at 135%. So you can run at about 54 amps for this long. You could typically run at 200% (80 amps) for 2 - 6 seconds. Your popping breakers are a sign that you are probably averaging well above 40 amps for several seconds. The longer you play while pushing the motors above 40 amps average, the closer you are to popping the breaker.
The toughbox is designed for operation with one or two CIM motors with a chain stage of 22/15 to 6 inch wheels. You are missing the 1.5 torque multiplier from the chain drive which is leaving you a bit outside the envelope. The center wheel drive may be compounding the problem because you can end up with nearly all of the weight on these two wheels, especially during accelleration and pushing. To answer your question,if your design allows, a second CIM will halve the torque load and put you well into the pink. With 4 CIM’s fused at 40 amps, you do have some risk of popping the main breaker which is not-resettable leading to game-over. This is particularly true if the root cause of your problem is mechanical as discussed below.
Another consideration is your final drive shaft. The above calculations assume a fully working drive train that does not have extraordinary friction due to binding and/or misalignment. If you are driving the wheels directly - is your shaft supported at the far end? If so is the axis through all three bearings precisely aligned? If your shaft is binding (under load) the torque seen by the motor can be considerably higher. This might not show up when you are up on blocks but only when the wheels are loaded. Again, the center drive can compound the issue with greater weight on the shaft.
If your wheels are just hanging off the toughbox shaft this may actually be the root cause of the issues you are having. This might show up if you measure the force required to pull your robot manually (with the victors jumpered for coast). This gives you a measure of the friction through the drive train when the wheels are loaded.
For reference, we are using two CIM’s per drive with rear wheel direct drive using a modified toughbox transmission geared for 6.4 to one. This is similar to your configuration except with 1/2 the gear ratio. With two CIM’s on each wheel, the torque on each of our CIM’s is about the same as yours. Our wheels, however, will spin (by design) before we pop the breakers because our CG is well forward of the drive wheels and our normal force is on the order of 25lbs.
Just to add a little fuel to the fire here…
The CIM motors achieve 133 amps in stall, the 40 amp breakers can sustain a 600% overload for a few seconds without tripping and can withstand 120-140% over load without tripping indefinetely. All of that being said, direct drive on these transmissions run the current requirement in the motors pretty high. Since the breakers essentially have some resistance, high current does cause them to heat a little. Once they trip, there is additional heat and no way to remove it. This additional heat then in turn derates the breaker such that it may now trip right at 40 amps. Each subsequent trip adds more heat so that at some point, the trip is now significantly under the 40 amp point. Ambient heat from motors, shrouded electronics, high resistance crimps and loose electrical connections all conduct heat to the operating material inside the breaker. Since teams frequently fail to get a good connection at the 40 breaker panel, they should check the temperature of the block when it comes off the field. If you search CD you will find a picture of burned #6 wire from a loose power input to the 40 amp panel. I would suggest you check (remove and inspect) your #6 connections, and consider a little additional reduction in the output of your transmissions. Adding motors may help but you really need to know the true cause before you can come up with a solution.
BTW, when breakers trip they get hot. If you suspect your breakers have been tripping, don’t touch them when your robot comes off the field. You will get burned. Once you find the problem, consider replacing the breakers.