Dynomotion

Group: DynoMotion Message: 6385 From: azubar@ymail.com Date: 1/5/2013
Subject: Snap amp specs
Hello Tom,

I'm looking at using a Snap amp on a 2 axis stepper motor system that is rated at 1.95 amps at 170 vdc max. It will also be using the encoder feedback.

I'm concerned that the minimum current setting on the SnapAmp is the maximum for my motors. Is there a way to set an idle current on the Snap Amp so the current at idle is 1/2 the running current? i.e. 2 amps running, 1 amp idle?

A question about the encoder input specs. I see they are differential inputs.
Do the signals actually go into a hardware differential receiver chip to get the common mode noise rejection? If so what is the receiver chip model?
Are they spec'd at the rs422 or rs485 signal standard for differential voltage?

If I have a single ended encoder with A, B, no Z should I set up a resistor divider to set the voltage on A/ B/ at 1.5 volts so the receiver will see the differential with only A and B transitioning?


Thank you
AZ
Group: DynoMotion Message: 6388 From: Tom Kerekes Date: 1/5/2013
Subject: Re: Snap amp specs
Hi AZ,

SnapAmp's peak current limits are not what is used to control the Stepper's Coil Currents.  The Peak current limits are only for short circuit protection and such.

SnapAmp outputs have a 10-bit (with sign) setting for current that corresponds to the maximum range of +/-35Amps.  So +/-1024 counts corresponds to +/- 35Amps.

So to set for 1.95A set the Microstepping Amplitude for 1.95/35x1024=57.

To reduce the current in half while idle change the setting to 28.

The differential receivers on SnapAmp use AM26LS32ACD devices.  They have a max threshold voltage of +/- 0.2V.  There are no termination resistors on SnapAmp's differential inputs.  You should be able to tie the unused input to 1.5V.  I would include a filter capacitor.  Or KFLOP has single ended inputs that you might use.  Or you can purchase 3rd party cables/modules to convert single ended signals to differential. 

Regards
TK