Performance analysis of cross-coupled controllers for CNC machines based upon precise real-time contour error measurement

Experimental results from the implementation of a cross-coupled control scheme on a 3-axis CNC mill governed by an open-architecture software controller are presented. Unlike prior cross-coupling schemes, which depend on osculating-circle approximations to the commanded toolpath for real-time contour error estimation, the proposed scheme is based on essentially exact contour error computations for free-form curved paths. The implementation illustrates the feasibility of precise real-time contour error computation with a modest (300 MHz) CPU and a 1024 Hz sampling frequency. For a P-type controller and paths with strong curvature variation, the new scheme ensures much better diminution of contour error than earlier methods. The contour error is observed to decrease monotonically as the relative gain is increased, up to a critical gain value that incurs instability. For PI-type controllers, the comparative contour error reduction is more modest, due to their effective suppression of steady-state position error along the path. In practice, the exact contour error measure permits accurate tracking of strongly curved paths using a simple P-type controller, with smaller feedrate fluctuations than a PI-type controller incurs.

► Algorithms proposed for real-time measurement of path contour error.
► Feasibility of exact real-time contour error measurement demonstrated.
► Performance analysis by implementation on open-architecture CNC mill.
► With P-type controller, exact contour error measurement gives better accuracy than osculating-circle approximation at high feedrates.