Motion control with AC VFD inverter drive and encoder? (Theory)

At work we have a couple machines that use a large DC motor attached to a timing belt and then ballscrew. At the other end of the ballscrew is an encoder that provides the controller with positioning. Would this be possible with a VFD AC inverter and encoder?

Not with a VFD. You are talking about an AC servo and the encoders are specific to the motors/controller. DC motors are a LOT easier to use for position control, Simple optical rotary encoders and a decent DC servo driver and you have a closed loop system. You always have to worry about the "LOOP" (feedback loop) and position has to be stable and precise. Now if you just want speed control the VFD and encoder (or tach) feedback starts to make sense. It is used on things like spindles for constant torque/rpm. A good example would be to set the RPM of a lathe spindle to sync to the carriage feedrate to cut threads by measuring and adjusting the motor RPM. In that case one RPM is not a lot of error and the motor inertia and rather loose feedback loop to a VFD are okay. Go have a sit down with your control engineers and discuss closed loop control systems

Most high end VFD's, have the option of encoder feedback for precise control in place of the second best solution, Sensor-less vector type.
In commercial CNC machines, the encoder on the final spindle shaft goes to the controller, not the VFD as the Z axis is geared off of this encoder for precise sync control.
Al.

So something like a basic pid controller paired with a VFD inverter and encoder on the ballscrew wouldn't be possible for linear movement? I apologize if this is a dumb question in advance. Just reading about pid control and it has me curious.

Also, I wish we had control engineers here at the shop. They would never be rid of me. ;)

What are you planning to do and how much money are you willing to spend, its definitely possible. I just finished with and ABB ASCM1 drive an a 30hp ac motor with encoder, this is just very precise speed control but it will do position control as well with pulse input if you want.

In the early stages of brainstorming currently, researching options vs what components I have avaliable. I like the rpm of ac motors, as well as their durability. Possibly a single 1.5hp ac motor driving dual ball screws on a fixed bridge design? Machine would be plasma / drill / mill, maybe even a punch/stamp. One of the best machines we have here at the shop moves the material, not the machine, and runs a 33amp dc motor to drive the large axis. Very reliable and accurate, but a prototype from Ficep in the early 80's

May want to run some numbers. Plasma is typically done at high speeds (compared to milling and routing.100 to 350 IPM depending on the material. To get to those speeds with ballscrews with typical 5 TPI count you need to spin them at 500 to 1500 RPM. Depending on the length, diameter and mounting of the screws you may find that far exceeds the max RPM of the screws. Then there is all of the nasty plasma dust getting on the screws........If you intent is to move the tool instead of the work your Z is going to have to be really heavy duty for milling and drilling. That much mass won't accelerate very well with one small AC motor ; another important aspect of effective plasma cutting. The "All-in=one-do-anything sounds great but is seldom practical because of the wide range of Physics that are involved. I am not like the 1899 patient dept and declare "everything that can be invented has been invented." BUT you also have to ask why there are no products that do all of that in one machine.