Improvement of a delayed velocity reference control (DVRC) for machining operations

Abstract

This paper deals with a non-time-based controller for machining operations, suitable to control the interaction force between the tool and the workpiece. In the traditional approach usually the tool feed velocity (on which the force depends) is predetermined: it is chosen on the basis of some important parameters, such as tool dimensions, shape and material, the depth of cut, the workpiece material, etc. In this traditional approach, the reference function is calculated off-line, and during the task execution, at each instant, the control module is required to track the input reference; the drawback of this approach is that the system is not able to modify its velocity if unpredictable events occur or if side conditions change; this could produce high forces that could damage the tool or the workpiece. This paper presents a non-time-based controller for machining operations. Basically, in this control, the desired input reference is calculated on-line according to the measured force signal in such a way to improve the interaction. The controller allows to set the optimum interaction force F max. In addition to this the same control is able to move the tool, during free motion paths, with a constant customizable velocity V max, thus implementing a velocity control. As a third feature, it is possible to set the maximum allowed acceleration A max of the end effector during the motion. This control scheme, called delayed velocity reference control (DVRC) belongs to the category of non-time-based controllers, since the reference signal is not directly related to time. The effectiveness of the controller has been proven by means of its implementation on a three-axes CNC center for EPS foam cutting.