Control design of a reciprocating high-speed wire feed system for 7-axis robotic electric discharge machining

For decades, researchers have struggled to solve 6-axis robotic vibration while machining hard-to-cut materials. On the other hand, wire electric discharge machining (WEDM) stands out as a non-conventional machining process able to cut large and complex profiles of any conductive hard-to-cut material with minor non-contact forces. Thus, WEDM is a promising process to be combined into a robot to overcome vibration and low accuracy. However, the robot characteristics of a high degree of freedom combined with payload limitation oblige to separate the heavy wire winding system from the robot end-effector, demanding an equally high degree of freedom and unconventional solutions to feed and control the wire electrode. This study designs and reports experimental findings of the first robotic WEDM apparatus based on a high-speed winding system with 600 m of wire length, capable of controlling the wire speed from 1 to 10 m/s and wire tension from 0.1 to 10 N. The system adopts flexible outer cases to travel and reciprocate the wire into a 7-axis robotic system composed of a 6-axis robot and an external rotating axis. The proposed design is a highly dynamic process whose wire tension and speed are achieved by a hybrid controller to cope with the non-linear relation of speed and tension provided by the magnetic clutch. It combines a regression open-loop control for optimality and wire breakage avoidance with a closed-loop control to guarantee admissibility while coping with wire friction disturbances. The findings review a novel wire winding system capable of controlling usual wire disturbance and stepped surface of reciprocating high-speed WEDM as well as additional friction and elastic behaviour of the flexible case, delivering wire tension of ± 12% along with stable EDM process and uniform surface roughness between wire reciprocation areas with a Ra of 2.94 μm. Potential adoption of the method can finally make 6-axis robots a feasible and advantageous technique compared to computer numeric control (CNC) while shaping monolithic and complex workpieces of conductive and hard-to-cut materials.