CIM Motor Strength

[robself705]

Originally Posted by Kevin Sevcik

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.

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.

[dtengineering]

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

[The Lucas]

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.

[Ether]

Originally Posted by The Lucas

you can use the current and voltage readings built into the all the Jaguars

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


~

[Kevin Sevcik]

Originally Posted by robself705

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.

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.

[robself705]

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.