The time is gone when machines, controls, and drives were ‘‘add-ons,’’ as shown in Figure 1. Early machines used add-on drives. These drives had one thing in common: There was no interaction among the control, drive, and machine. To a large extent these machines were open loop. Today’s numerically controlled machines are tightly integrated machine systems. There is a great deal of interaction between the machine, drive, and control. It is this interaction that results in problems with stability, surface finish on the work part, and accuracy. The actual block diagram for today’s machine systems looks more like the block.
