Question about analog input on servo drive

[jsheerin]

I'm trying to get a Fanuc AC servo running using EMC2, the bldc component for EMC2, and an AMC brushless servo drive. The basic idea is that EMC2 takes the C1,C2,C4 and C8 commutation signals from the Fanuc encoder and outputs hall signals to the AMC drive. It can also take incremental encoder signals (but that's still down the road). The AMC drive takes +/-10V analog input, so I've built a pwm to analog circuit so EMC2 can give the drive an input signal. This sort of works and I've made a motor spin. However I currently only have it spinning in one direction whether I give it a positive or negative analog input voltage. So that is my question: Do I need to feed a positive voltage into the + input on the AMC drive for rotation in one direction and a negative voltage into the - input on the AMC drive for rotation in the other direction?

I ask because I can make the motor rotate the other way by putting a positive voltage into the - input on the drive, but that doesn't make sense to me... Simulating the input circuit in the AMC drive, I'd think I'd need positive and negative signals on their respective inputs to get reversing motor rotation.

[Al_The_Man]

The AMC has +ref and -ref input, it just need an alternate or reverse polarity between these two to run in either direction?
The correct commutation must exist of course.
Al.

[underthetire]

Some drives do use a plus and minus for direction, some use an input for fwd/reverse with a separate 0-10vdc for speed. a lot of drives have both options, set able from parameters or jumpers.

[jsheerin]

I'm leaving one of the inputs at ground and driving the other one with a voltage. So if I'm trying to go forward, I put up to +10V on the + ref input and ground on the - ref. If I want to go backwards, I leave + ref input at ground and put up to -10V on the - ref input. Is that how it's supposed to work?

[jsheerin]

This is the drive I'm using:
http://www.a-m-c.com/download/datasheet/bx15a20.pdf

The manual says: differential reference input (+/- 10V operating range)

[Al_The_Man]

It actually takes a differential signal but it will work single ended, IOW + on +ref and gnd on - ref, reverse is -on the +ref and + on the -ref.
I usually test separately first with a battery box.
2 9v batteries in series and a 5k or 10k pot across the outer 18v ends, the series point is common and the slider is ±9vdc.
With an normal applied signal, the gnd stays on -ref and the +ref goes pos or neg WRT gnd.
Al.

[jsheerin]

Thanks Al. That clears it up for me. So I'm currently doing it wrong, but the software portion of my setup is working correctly... I'll have to rearrange my circuit a bit.