A non-time based force control system for CNC applications

In this work, an innovative force control module is presented, which can be employed in CNC end milling machines in order to maintain a constant cutting force in the presence of parametric uncertainty for a time varying end milling process. The controller is a non-time based one; in fact, the desired tool position reference is delayed according to the integral of a proper function related to the on-line cutting force measurement. Such a controller, which is referred to as delayed reference control (DRC), consists in a outer force feedback loop around an inner position feedback loop. Two main advantages make this controller suitable for a commercial diffusion: it avoids complicate and time consuming algorithms for re-planning the path in response to cutting force and it does not require to design a new position feedback loop. Good experimental results are obtained by implementing the controller on a wood milling machine prototype. The system has shown a remarkable robustness in terms of immunity to the mechanical and electrical noise, working properly also with unfiltered control signals.