Motion Control Systems
Some NC processes are performed at discrete locations on the workpart (e.g., drilling and spot welding). Others are carried out while the workhead is moving (e.g., turning and continuous welding). If the workhead is moving, it may be required to follow a straight line path or a circular or other curvilinear path. These different types of movement are accomplished by the motion control system.
Features of Motion Control Systems
Point-to-Point versus Continuous Path Control:
Motion control systems for NC can be divided into two types:
(1) point-to-point
(2) continuous path
Point-to-point systems, also called positioning systems, move the worktable to a programmed location without regard for the path taken to get to that location. Once the move has been completed, some processing action is accomplished by the workhead at the location, such as drilling or punching a hole. Thus, the program consists of a series of point locations at which operations are performed, as depicted in the following Figure.
Continuous path systems generally refer to systems that are capable of continuous simultaneous control of two or more axes. This provides control of the tool trajectory relative to the workpart. In this case, the tool performs the process while the worktable is moving, thus enabling the system to generate angular surfaces, two-dimensional curves, or three-dimensional contours in the workpart. This control mode is required in many milling and turning operations. A simple two-dimensional profile operation is shown in the following figure to illustrate continuous path control.
Interpolation Methods
One of the important aspects of contouring is interpolation. The paths that a contouring-type NC system is required to generate often consist of circular arcs and other smooth nonlinear shapes. Some of these shapes can be defined mathematically by relatively simple geometric formulas, whereas others cannot be mathematically defined except by approximation. In any case, a fundamental problem in generating these shapes using NC equipment is that they are continuous, whereas NC is digital. To cut along a circular path, the circle must be divided into a series of straight line segments that approximate the curve. The tool is commanded to machine each line segment in succession so that the machined surface closely matches the desired shape. The maximum error between the nominal (desired) surface and the actual (machined) surface can be controlled by the lengths of the individual line segments, as explained in the following figure.
Approximation of a curved path in NC by a series of straight line segments. The accuracy of the approximation is controlled by the maximum deviation (called the tolerance) between the nominal (desired) curve and the straight line segments that are machined by the NC system. In (a) the tolerance is defined on only the inside of the nominal curve. In (b) the tolerance is defined on only the outside of the desired curve. In (c) the tolerance is defined on both the inside and outside of the desired curve.
A number of interpolation methods are available to deal with the various problems encountered in generating a smooth continuous path in contouring. They include: (1) linear interpolation, (2) circular interpolation, (3) helical interpolation, (4) parabolic interpolation, and (5) cubic interpolation.
