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Reversing motor direction with resolver feedback
I have GEO Brick with the resolver feedback option controlling a gantry that has 2 motors with resolver feedback as the parallel axes. Due to the nature of the mechanics, I need the 2 motors to spin in opposite directions to move the bridge.

When I do this with quadrature encoders, I change Ixx70 to a negative number, swap I7mn0 (from 3 to 7 or vice versa) and "invert" the ixx91 phasing word. Using this method, I can easily swap the direction a motor spins without having to change any hardware. (I prefer to do it all in software so that if a customer ever needs a replacement servo cable, they are always all the same).

Is there a similar way to change the direction of a motor with resolver feedback (without modifying the cable)? Can I simply check the box in the PEWIN encoder conversion table (as seen on page 70 of the GEO Brick User manual)? Does this need done in conjunction with swapping any 2 of the power wires?
Ben Adams
Yes, changing between the "clockwise" and "counterclockwise" conversion of the resolver inverts the direction sense of the resulting position, without any cabling changes. Internally, this changes the value of bit 19 of the first setup word for the entry. It is the equivalent of changing I7mn0 between 3 and 7 for a quadrature encoder.

You will have to evaluate the effect of this on the absolute power-on phasing read as well as on ongoing position.
I did change that checkbox, and I did see the bit change. Without changing my cabling though, the motor goes unstable and has a tendency to run away. I tried various different parameter changes (Sin/Cos bias word (ECT), making Ixx70 negative, etc) but I didn't come across a solution that would cause the motor to run correctly (but backwards).

I guess I'm not exactly sure how the phasing works with the resolver. I followed the instructions in the GEO Brick User manual, and it gets the motor spinning, but I don't know where to look to change the phasing. It's clearly not all the same variables as I use for the Hall phasing.

At the end of yesterday, I swapped two motor power leads (as well as the CW/CCW bit in the ECT) and that works, but I was looking for a purely electrical way to do it, and would still prefer that method.
The thing you have to watch out for is what happens to the zero position of the resolver when you change direction. I think the Ixx75 phase position offset will be your key variable here.
I understand the gist of what I'd be trying to achieve with Ixx75, but I'm a little confused by the SRM entry for Ixx75 in relation to resolvers. It mentions power on phasing set by Ixx81 and Ixx91, but those are both set to 0 for my machine.
Where I set up the resolver (encoder conversion table) there is an entry for SIN/COS bias word. Does this function in the same way? If so, how is the word formatted, it looks like it is 6 hex digits long.
I ask because my motors spin freely, and I can get them going opposite direction (by switching a power wire) but they have some oscillation to them under load and they don't drive smoothly. I wonder if I'm getting some torque ripple from bad phasing information. I'd know what to do with an encoder, but I'm not familiar enough with resolvers to start playing around with them.
I solved my problem by buying new motors with encoders. I think the resolver option in the GEO Brick is half baked. I'll just avoid them in the future.
I am sorry for Ben's dossapointment. I have commented to Charles recently that the resolver information in the GEO BRICK USER MANUAL is not as comprehensive as say the endat or SSI, especially in regard to power on and ongoing phasing. Could this oversite be addressed as soon as possibly.
By the way we are successfully using resolver feedback on stepper motors in UHV high radiation environments, but as ever system wiring is a key contributor to a stable system.

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