Control systems for machine axes

  • Autor:karl
  • View:4409
  • Time:2013-12-18 13:46:49
Many machining applications involve the generation of complex profiles or, at the very least, need to synchronize the motions of the different axes to generate the required machined surface. In a CNC machine tool, this is usually done with the help of a multi-axis servo control system that consists of an interpolator and several machine axis controllers. The interpolator generates the desired tool motion relative to the workpiece and then decomposes the desired motion into the reference position commands for the separate driving axes. Actual positioning of the axes is done by the individual axis controllers. The function of an axis controller is to provide the appropriate drive signal to the actuator, usually in the form of an electric motor, so that the actual position of the axis accurately tracks the desired axis reference command to eliminate the position along each driving axis.
The purpose of axis control systems is to control the position and velocity of the particular machine tool axis. In general, each machine axis in a CNC machine is separately driven and follows the command signal produced by the respective interpolator. Controllers are of two types: the open-loop or the closed-loop. Open-loop controls use stepping motors to drive the machine axis while in the closed-loop control systems, a position feedback element, such as an encoder or a resolver, is used to provide information of the actual position of the axis during motion.
There are two types of closed-loop position control systems, namely the point-to-point and the contouring systems. In point-to-point systems, the path of the machine tool axis from one point to the other and its velocity during motion are not significant. This is because, in point-to-point motions, the tool is not in contact with, or machining, the workpiece during the motion. What is of interest is only the accuracy of positioning the axis to the desired end point. In contouring systems on the other hand, the path that the cutting tool takes, relative to the workpiece, called the tool path, is of paramount importance. This is because the  contour,  or profile, of the machined part depends primarily on this path.