My machine is running on AC servos using Step/Dir (actually quadrature) signals from an Ethernet SmoothStepper (ESS) from Warp9TD and Mach4. I am extremely happy with the system. I think you will find AC servos to offer higher performance compared to DC systems.
I think JoeAverage has done an excellent job describing the system, so I won't get into that, but one thing to consider is selecting a servo drive which has dual loop control using load feedback. In this case, both the motor encoder AND the glass scales would be wired into the servo drive which handles the position, velocity, and current loops entirely on its own. You only need to give it a command (Step/Dir for example) and it handles everything else.
If you use an ESS, you can easily bring the encoder outputs from the drive back to Mach 4 for display purposes. As far as Mach 4 is concerned, it is driving an open loop stepper motor, but you really have full closed loop control at the drive.
Not every servo drive can do this, but I know the new Kinetix 5100 from AB has this feature as well as select drives from Delta, Omron, Baldor, and likely most big name suppliers. Give them a call and ask what their offering is for standalone drives with dual loop load feedback.
https://ab.rockwellautomation.com/Motion-Control/Servo-Drives/Kinetix-Indexing-and-Stand-Alone/Kinetix-5100https://www.deltaww.com/Products/CategoryListT1.aspx?CID=060201&PID=23&hl=en-US&Name=ASDA-A2+SeriesIf I had a nicer machine, this would be the route I would take!
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