How to limit the maximum current a miniCIM can draw?

[MAXOFLIFE]

As we're nearing the end of our final build design, we need a way to "cap" the current of our miniCIM that's controlling our arm mechanism for opening the portcullis to avoid accidentally damaging our motor on the field. Everything we've done so far has been fairly basic, so a simple solution would be the most helpful.

We have a miniCim with a 333:1 ratio (connected to a Spark, but a Talon SR is available). The JVN calculator says we'll have a current draw of 16.65 amps and a stall load of 90.13 lbs. We know this is overkill, but we wanted to be 100% sure we wouldn't have any issues lifting. The motor's only task is to open the portcullis, so the rotational speed of our arm will be static.

Are the stall current and stall load proportionally related? If we limited our current to 8.325 amps would the motor automatically stop when trying to lift more than 45.065 lbs.?

Also, how should we go about limiting the current of the motor? I would think this is a programming related problem, but I couldn't find any clear guides or tutorials online in LabVIEW for limiting the current of a motor.

Thank you for taking the time to read this! I hope we can figure out a doable solution to save our driver(s) the worry of making sure not to overload our arms against a wall or something on the field.

[Ty Tremblay]

In the JVN calculator, the current draw is how much current your motor will be drawing to lift the specified load at the specified arm length. Stall load is the maximum load the motor can lift at the specified arm length. To find out what your stall current would be, plug the stall load into the "Arm Load" box (F8) and look at what your current draw changes to.

Assuming your 333:1 reduction is already built and you're moving the arm around, I wouldn't worry about limiting the current of the motor by any means other than a fuse in the PDB (which you have to do anyway). Choose a fuse that is less than the stall current determined in the method above and you'll be ok.

[MAXOFLIFE]

Originally Posted by Ty Tremblay

In the JVN calculator, the current draw is how much current your motor will be drawing to lift the specified load at the specified arm length. Stall load is the maximum load the motor can lift at the specified arm length. To find out what your stall current would be, plug the stall load into the "Arm Load" box (F8) and look at what your current draw changes to.

Assuming your 333:1 reduction is already built and you're moving the arm around, I wouldn't worry about limiting the current of the motor by any means other than a fuse in the PDB (which you have to do anyway). Choose a fuse that is less than the stall current determined in the method above and you'll be ok.

Thank you!

[Greg Needel]

I agree with what Ty said, you don't need to limit your current for this setup. Those numbers you are getting in the spreadsheet are fully loaded current, a situation that you should never be in since the portcullis is much less than 90lbs (even if you had horrible mechanical advantage I see it never reaching that kind of load).



Now that we have established that you don't need to limit your current, if you wanted to do so anyway, you could do it by limiting the PWM value you send your SPARK.

When you send a SPARK (or any other motor controller) a value of 1 (full forward) you have full power. If you send it less than that you have effectively limited your current via lowering your available power. Assuming your motor had a perfectly linear power curve (which they don't) if you gave a 50% PWM signal you could have 50% power, which would also be less maximum current. There is a chance you get into trouble with this method as you may fall on a bad spot in the power curve, and the fact that motors are not exactly linear so you might have to play with the value you send the controller.

[Chak]

Kind of irrelevant, but I'm curious: how did you get 333:1? 333 factors to 3*3*37, so you would need a 37:1 stage somewhere. Who in the world makes 37:1 ratios?

[cgmv123]

Originally Posted by Chak

Kind of irrelevant, but I'm curious: how did you get 333:1? 333 factors to 3*3*37, so you would need a 37:1 stage somewhere. Who in the world makes 37:1 ratios?

My guess is they're using a chain reduction with a non-integer (when reduced to n:1) ratio.

[GeeTwo]

Originally Posted by Chak

Kind of irrelevant, but I'm curious: how did you get 333:1? 333 factors to 3*3*37, so you would need a 37:1 stage somewhere. Who in the world makes 37:1 ratios?

Originally Posted by cgmv123

My guess is they're using a chain reduction with a non-integer (when reduced to n:1) ratio.

I agree. A simple way to get there would be a VP with two 10:1 stages, then a 10:3 in chain (18t driving 60t).

[FrankJ]

Could you monitor the current from the PD board and cut back the command if the current is high? We did something similar to that last year using current feed back from the Talon SRX. Years before current feedback we limited the command to a value that limited the current to an acceptable level. We had a turbo button to enable full power & it was on the driver not to smoke the motor with the turbo button.

[MAXOFLIFE]

Originally Posted by Chak

Kind of irrelevant, but I'm curious: how did you get 333:1? 333 factors to 3*3*37, so you would need a 37:1 stage somewhere. Who in the world makes 37:1 ratios?

Originally Posted by cgmv123

My guess is they're using a chain reduction with a non-integer (when reduced to n:1) ratio.

Originally Posted by GeeTwo

I agree. A simple way to get there would be a VP with two 10:1 stages, then a 10:3 in chain (18t driving 60t).

We used two 10:1 stages in a VP and a chain (18t driving 60t)

GeeTwo hit it on the nose.

[Chak]

Originally Posted by MAXOFLIFE

We used two 10:1 stages in a VP and a chain (18t driving 60t)

GeeTwo hit it on the nose.

Ah, that makes sense.

But if you're using 2 10:1 stages on a VP on a miniCIM, according to the recommended values in the Versaplanetary Loading Guide, you would break your gearbox before you stall your motor. Alternatively, you can use 105:1 or 108:1 with 3 stages, which is allowed by the loading guide (somehow... I hear it's because some gear stages are stronger than others).

Of course, since you're limiting the current the gearbox shouldn't break. But it doesn't hurt to be safe if you have space for an extra stage.