Dave,
My multi-purpose machine has a PLC in it because it allows me to do things not easily done by Mach 3. I recently replaced the surplus DL06 I was using with a DoMore BRX because of all the stuff it brings to the table. It has high speed inputs and outputs with motion commands so it could control the motion of your knee lift. It talks ethernet, serial, and modbus so you can send the tool length to the BRX and let it position the table and tell mach 3 the position move is done. Mine runs everything including air blowoff, misting, flood, vacuum cleaner, lights, safety circuit, heaters for the 3d extruder, 3d bed heater, lathe spindle and index pulse for threading etc.