Cautionary Tale: All-in-one DC, high bus voltage, solid handwheels and Autotune

I recently had to relearn a lesson I should have remembered from many years ago.

If you run the torque-mode Autotune feature on a machine with a comparatively high DC bus voltage (e.g. from directly rectifying 120VAC), and which has significant rotational inertia (e.g. from solid aluminum handwheels on the ends of the ballscrews), you will blow the FETs on the drive.

This is because the high-speed move, which autotune makes to determine the achievable max rate and accel time, causes excessive regenerative voltage when the control tries to bring the axis back to a stop. The handwheels act like flywheels.

The machine in the attached pictures is a Sharp LMV-50 knee mill, which previously had an Anilam 3000 control. However, this type of handwheel is common on many semi-manual CNC knee mills.

If you are retrofitting this type of machine, you should:
- Remove the handwheels
- Install a step-down transformer to keep the bus voltage below 120VDC
- Not run Autotune

Any one of the three should be sufficient to avoid damage. Two out of the three would be safer.

Quote:

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Originally Posted by cncsnw

I recently had to relearn a lesson I should have remembered from many years ago.

If you run the torque-mode Autotune feature on a machine with a comparatively high DC bus voltage (e.g. from directly rectifying 120VAC), and which has significant rotational inertia (e.g. from solid aluminum handwheels on the ends of the ballscrews), you will blow the FETs on the drive.

This is because the high-speed move, which autotune makes to determine the achievable max rate and accel time, causes excessive regenerative voltage when the control tries to bring the axis back to a stop. The handwheels act like flywheels.

The machine in the attached pictures is a Sharp LMV-50 knee mill, which previously had an Anilam 3000 control. However, this type of handwheel is common on many semi-manual CNC knee mills.

If you are retrofitting this type of machine, you should:
- Remove the handwheels
- Install a step-down transformer to keep the bus voltage below 120VDC
- Not run Autotune

Any one of the three should be sufficient to avoid damage. Two out of the three would be safer.

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Wow, interesting advice. I'm doing a retrofit now of a 10x54 knee mill. It was previously fitted with a Mitutoyo Millstar III Control, using direct rectified 120VAC to the Glentek SM7115 servo amps. I am using direct rectification again with the All in One DC. I've been posting updates on my YouTube Channel, Martyscncgarage.

I'll discuss more with Centroid Support when the time comes.

Marc, thank you for the heads up.
Marty
Mesa, AZ

Happy to report my machine is tuned and running fine. I chickened out and asked Centroid Support for assitance. They remoted in, tuned the servos in about 20 minutes. I think it was nice that my servos were standard Glenteks so there were no surprises. The base PID figures in the AIO DC manual were a good starting point for these servo motors.

Marty