CIM Motor Strength

We currently are using 1 CIM motor to operate our lift and elevate our robot to the needed height without any problems and in approximately 5 seconds or less. I’m wondering how easily the motor would be able to lift our robot and a second robot connected to us at the same time or if it would put too much strain on the motor.

“If it ain’t broke don’t fix it”

Seriously leave it alone if it works, certainly there is something else that you could work on, even if it is just fabricating spare parts or doing something whimsical, like underglow on the bot. From what I am hearing 5 seconds is plenty fast, and if your not blowing fuses the motors will be just fine.

This is something you can mathematically calculate. How big is your winch?

OR, they could see about making their robot better. That’s no crime.

To answer your question, it depends on the gearing and efficiency of your current system. Gearing is easy to determine, efficiency much less so. Luckily there’s a simple way for you to answer your question experimentally.

Grab a clamp on Ammeter and measure the max current draw on your CIM while you’re lifting. Whatever current you’re currently drawing, a second robot of equal weight will likely double it. So if you’re currently pulling 30 amps, a second robot would make 60 amps or so. So then it’s a question of if you can actually give the CIM that much current. The Jaguar will happily pump 40 amps to your CIM for quite a while. 50 amps for much less so. 60 amps for not very long at all.

So, one measurement and a little math should give you an answer.

Great suggestion.

If they don’t have access to a clamp-on ammeter, they can measure how fast the bot is lifting when the motor sounds like it is under the most load, then calculate how fast the motor is turning. Then use the CIM torque vs speed curve to estimate the torque, then use the CIM torque vs current curve to estimate the current. This assumes, of course, they are applying 100% command to the motor, and that there is 12 volts at the motor. The actual voltage at the motor will be somewhat less than 12V of course, because of voltage drops across battery resistance and external resistance - which is why this is only an estimate.

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Oooohhh. Good point. That’s a reasonable assumption to make in absence of a clamp-on ammeter. In fact, if they’re using a straight up winch style lift, it should have a reasonably constant load on it after the first second or so. You could probably just figure the motor speed on the average lift time and go with it. It’d give you a built in bump to the low-speed, high-torque side so it should be pretty safe.

If you can afford a clamp-on ammeter, though… they are really, REALLY useful tools.

Make sure the one you get measures DC amps. We found a nice one for less than $100 that has two ranges for DC Amps… 4A and 80A. It is PERFECT for FRC work. We use it many times each year and it has got rid of a lot of “guess work” for us.

Jason

Please provide brand and model number? Available at Home Depot or Lowe’s ? Or did you get it on-line ?

Thanks.

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It is the JET 310021, as shown on page 112 of http://www.jetequipment.com/products/HT/CAT08-HT_109-113.pdf

I think we bought it at KMS Tools a few years ago… which doesn’t do you a lot of good as far as a “lets just run out and get one” point of view*… but here is the closest thing they have to it http://www.kmstools.com/darson-instruments-ac-dc-small-clamp-meter-1868

I notice it says it is manufactured by “Darson” rather than JET and has a different part number, but it looks exactly the same and has similar specs.

I don’t remember paying anything near that much for it, though… KMS Tools has awesome sales for a couple days every month, though, so it is possible that I could have picked it up at 50% off.

Hope you’re able to find something useful in your price range… I’d recommend all my “local” sources… Lee’s Electronics has one that costs less, but is less sensitive (http://www.leeselectronic.com/catalog/product_info.php?cPath=15_16&products_id=7847) and RP Electronics (http://www.rpelectronics.com/) has a selection, too… but the JET/Darson one is nice because of it’s lower 4A/80A settings. These stores do sell on-line but maybe you can access similar stores nearer to home?

Jason

*after being sent out in search of various parts essential for the construction of the robot field that are only available at American Home Depots, or at non-existent chains like Lowes, or where the part number is different in Canada than the states, I know how this feels… believe me!

One problem with that is that you can only have the jaguar on a 40 amp fuse, nothing larger and only one. We actually tripped our 120 amp at competition this last weekend. Four cims at full power and a vacuum drawing 30 amps from two fisher prices. Unfortunately we didn’t listen to the mentor who said “you better put the cims on 30’s because you could blow the main breaker and that doesn’t reset”…ooopps. :slight_smile:

It is also possible to purchase current sensors from digikey. I can’t remember the part numbers that we used last year, but we ran our motor leads through the sensor, and got an analog output that we could plug in to the control system. (Note that these aren’t clamp-on meters… they are in series with the wire/controller/motor)

That way we had real time current draw data for each motor appearing in a labview graph on our driver station. Analyzing the data allowed us to write code that identified when the motor was generating too much torque (assuming current is proportional to torque) and dial back the Jags a bit. It provided a fairly simple traction control system.

If you got one of the 200A current sensors (I think they had them…) then you could monitor your entire robot’s draw, and shut down non-essential systems when you are concerned about tripping the main breaker.

Jason

Clamp on Ammeters are great (I’ve solved many a problem with their aid). However, if you want to save some money and get current readings constantly from multiple motors on your robot both on the field and in the pit, you can use the current and voltage readings built into the all the Jaguars. In order to get these readings you do need to hook up the Jags via the CAN. They can also tell if their breaker has tripped (they were just reset). I realize this is moderately difficult and may not be something you can change now. At least it is something to consider next year when deciding how to wire your motors.

Hey thanks for taking the time to share all that. I’ve been meaning to pick up a clamp-on ammeter for years and it’s about time I did something about it.

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Excellent point. If Jags are still in the KoP next year I’m definitely going to push for getting the CAN bus set up.

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Yesss… but those 40 Amp breakers don’t trip at 40 Amps. Take a look at the Data Sheet. 40 amps is the must hold rating. That means that the breaker will never trip at 40 amps. Meanwhile, 135% of rated amperage is the “must trip” level. This means that at 54 amps, the breaker is guaranteed to trip… eventually. Within the two minutes for sure, even if you get a bad breaker on a cold day. Other trip values are similar. 200% rated amperage gives you a trip time of 1.5 - 3.9 seconds. And faster for ever increasing currents. Interestingly, even at 120 amps (around the max a CIM can pull), you’re looking at a trip time of 0.5 - 1.1 seconds. So don’t count on that 40 amp breaker to save your bacon.

I don’t think I would leave two points up to whether a breaker trips or not because you are drawing too much current. I would much rather gear it down so the cim draws less than 40 amps because everything you stated is what is SUPPOSED to happen, don’t forget murphy’s law. The one time it doesn’t work the way you thought it would is in the final match when the other team is ahead by one point.