Motion Control and Motor Drives
NI Stepper Motor not putting out enough torque
So I have a motion control system with a P70360, UMI 7772, PXI 7332, and a NEMA 23 stepper motor with 380 oz-in. The purpose of the set up is to rotate horizontally a testing fixture and hold it in position (slowly). Previously the set up was using a motor with 180 oz-in but it was decided that it wasn't enough torque seeing as it wasn't able to fully rotate the fixture, also using a step resolution of 18000, current reduction is disabled, multistepping is enabled, dynamic smoothing is minimal, and load-inertia ratio is set to 0-1.
When the 380oz-in motor was plugged in it was still unable to rotate the platform completely but it wasn't even able to rotate it as far as the 180 oz-in which leads me to believe it was outputting less torque than it should have been. The only reasoning I could think of for this was that there was an issue with the settings somehow where it was limiting the power being fed into the motor for whatever reason. So I walked through the manual (http://www.ni.com/pdf/manuals/372324d.pdf) and it still wasn't giving enough torque.
I also read that decreasing the step size would increase the torque so I reduced it to 200 and it started making a grinding noise, which made everyone in the lab stare at me and start laughing... I immediately hit the kill motor button (I've been controlling it using NI MAX).
So far I can't seem to find anything that mentions the motor making a grinding noise or not pulling enough torque while using the P70360 so here I am asking the community. Why is the motor making the grinding noise (which also seems to become less the higher the step resolution is) and why isn't the motor putting out more torque?
Also of note, I'm an electrical engineer, meaning my understanding of mechanical terms is limited to the basics.
Thanks for any help you can give.
The grinding noise is the stepper "slipping". That means you are exceeding the torque abilities(and/or speed). What does your fixture look like? Is there a lot of rotary inertia? I suspect you need a gearbox.
The fixture is unbalanced, the mechanical who designed it is making a counter balance to level it out. But even without the counter balance he was saying that he measured roughly 180oz-in of torque.
Even if it is a lack of torque, shouldn't it have been more pronounced when I was using the smaller motor?
When I was using the stepper motor with a rated torque of 180oz-in it was able to rotate the fixture roughly +/- 20 degrees before swinging back to its "home position" (essentially where the fixture would hang when the motor had no current applied). When I got the 380oz-in motor it was only able to do roughly +/- 10 degrees before the fixture swung back.
Concerning the gearbox, those are used to reduce speed and increase torque or vice versa (I'm pretty sure that's what they're for), but based off of the measurement by my mechanical the rated torque of the motor should be more than enough to rotate the fixtures inertia. So the gearbox shouldn't be needed?
I was using the same step resolution with both motors and the only difference between the settings that I changed was changing the dial on the P70360 from "1" to "3" to signify the different motor being connected.
The 180 oz-in motor is T21NRLC-LDN-NS-00 from the NI site
The 380 oz-in motor is T23NRLH-LDN-NS-00 from the NI site
Don't confuse torque with inertia. Your motor could be sized correctly for torque(even oversized), but still not have enough rotor inertia. Gearboxes are also used for inertia matching, not just torque multiplictaion. What does your fixture look like? Dimensions? Weights?
I can't get too specific about the dimensions but it's a rectangular platform about 8 inches wide and 25 inches long (axis of rotation goes through the long way). There are also the UUT seating on one side which is what's causing the imbalance but by placing my lab notebook on the other side it balances out. I would estimate my book to weigh around 2 pounds.
The exact weight I'm not sure, the platform is attached to two stands through some type of bearings and the motor is turning a part of the shaft that is sticking out slightly.
On another note though, I decided to attach a oscilloscope to the motor to verify the signals were correct. I had made all the connections, double checked them to make sure all my connections were correct and weren't shorting then turned on the power. As soon as I did there was a white flash from the driver, I turned off the power, and I could smell something was burning. I took the lid off but couldn't see any immediate burn marks anywhere nor was there any visible smoke, just the odor. I decided not to try it again and my principal engineer has been trying to convince me to use another companies motion system. So sadly this may be the last well funded venture into NI motion control I take.
I'm still going to try to get in touch with a rep and see what I can do about it including taking into consideration the possibility of a gearbox but it's going to have to be on my spare time :'(
