Design And Implementation Of The Real-time Motion Controller ASIC For Pythagorean Hodograph Curve

In the high-speed and high-precision CNC machining, the fluctuations of feed rate will have a serious impact onthe quality of machining. Accordingly the arc length along the curve traversed by tool in a unit time is required to be constant value in order to maintain a constant feed rate. In addition, the cutter compensation plays an important role in planning the tool paths for CNC machining, and it is closely related with offset curve. However, currently there are still many problems to be solved in the real-time computing of arc length and curve offset, such as low accuracy, large computation etc.Pythagorean Hodograph curve (referred to as PH curve) was proposed by the the American scholar R. T. Farouki and T. Sakkalis to solve the above problems. PH curves, which are compatible with the Bezier/B-spline representations of CAD systems, are a special class of polynomial parametric curves.The rapid calculation of arc length and offset curve can be achieved by using PH curve.The feed rate control can be achieved more easily by using PH curve.In order to further improve the accuracy and velocity of PH curve CNC interpolation, the novel architecture of the real-time motion controller ASIC for Pythagorean Hodographcurve built on a single FPGA chip is proposed in this paper.The ASIC designed in Quartusâ…¡9.0 consists of a soft-core Niosâ…¡processor and multiple functional modules. It uses a two-stageinterpolation scheme to reduce the computational burden of PH curve interpolation. Firstly, master control program and the 1st-stage interpolation algorithm are executed by the Niosâ…¡processor.The FPGA hardware logic performs the 2nd-stage interpolation and outputs two groups of pulses for the motion control of actuators (two stepper motors).The dedicated serial communication software between PC and the ASIC is developed in Visual C++6.0. Users can enter the data and control instructions related to PH curve interpolation in PC (such as data of initial coordinates and destination coordinates, Bernstein coefficients, feed rate and tool radius etc). The data and control instructions are sent into the ASIC via serial communication interface.Experimental data show that the average time of each step of PH curve interpolation including the tool compensation algorithm is less than 2 ms when the feed rate is constant.Benefiting from the advantages of FPGA hardware, the real-time motion controller ASIC for PH curve has the excellent characteristics of high speed, high accuracy, high integration etc. The ASIC is able to meet the needs of high-speed and high-precision CNC machining.