current limit output to DC motor

[mfs]

Hello,
I am a student and I have to install a simple motion system (1 axis) for my current project.
Before I start programming I would like to limit the current output to the DC motor from the drive to <1 amps to avoid any fast motion.
It there a way to set a limit through LabVIEW and the motion controller? Or do I have to find a hardware limit on the drive?

The system:
LabVIEW 7.1
PCI 7344 Motion Controller
UMI 7764 Interface
Emerson LX 700 brushless servo drive
Emerson DXM 208 servo motor


Thanks a lot!

Malte

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[AnalogKid2DigitalMan]

Typically it is the drive that sets up the max current, have you look at teh manual for the Emerson LX 700?

[mfs]

Thanks for the response!

I had a look in the manual. There are some dip switches for the "nominal current adjustment". However, the lowest value is 3.78 amps (rms) and we want less to avoid damage in the development stage.
So maybe I have to talk to Emerson but perhabs there is a way to limit the voltage output the driver through the controller?

Thanks!  

[AnalogKid2DigitalMan]

I haven't had any experience with the hardware, but typically the motion controller will output a +/-10V signal to the drive. The drive then regulates current to the servo motor to alter/hold position. Since both your drive and motor are from Emerson, it may be best to talk to them regarding compatibility. Hope this helps.

-AK2DM

[Jochen]

Malte,

the option that you are looking for is a bit hidden, but it's available in NI-Motion. I don't know your drive, but typically the drive's output current is proportional to the input voltage. This is true, if the drive is configured in current mode or torque mode, which is the recommended mode if you are using a 73xx motion control board.

In MAX there is an option to limit the output voltage of the 7344, which also limits the output current of your drive:



So e. g. if your drive outputs 3,78 V when the input voltage is + 10 V and you want to limit the output current of your drive to +/- 1 A, you should set the torque limits in MAX to +/- 2,65 V.
As I have said, current mode is the recommended mode for your drive. But even if your drive has to be configured in velocity mode for some reason, limiting the output voltage of your 7344 will help, as this would limit directly the velocity of your motor.

I hope this helps,

Jochen Klier
National Instruments





Message Edited by Jochen on 08-01-2008 10:35 AM

[mfs]

Thank you, I guess that helps!

Malte