Study On The Twin-screw Compressor Rotor Profile Based On NURBS

Screw rotors are the key part of twin-screw compressor. The performance of the twin-screw compressor is determined by the rotor profile. Reverse engineering technology provides a new method for introducing, digesting advanced screw profile from abroad and developing new products. The reconstruct of screw profile is the key technology of screw profile for the reverse design. This paper studies reverse design of screw profile based on the NURBS which can represent the free form curves and conic curves accurately.This paper studies the technology of acquiring accurate data of the screw rotor profile and pretreating the clouds data. To fit screw rotor profile measured data, this paper researches fitting method based on the least square of the conic curve. For the ellipse and arc data in the screw profile, this paper develops the quadratic fitting method based on the least square of the conic curve fitting which the fitting precision is very high when the data is the ellipse or arc alone. But there are some problems in this method in fitting of the complicated screw profile which must be divided into lots of segments and the connections of fitting curves are discontinuous and not smooth. In order to solve these problems, this paper further proposes a method of reverse design of screw profile based on NURBS, it keeps the maximum fitting errors which are below given by using NURBS self-adaptive interpolation method on the basis of extracting the feature points from the screw profile data. The friendly interactive interface is developed. A new nodes vector algorithm is raised to solve the problem of bigger fitting error accrued in the area where the curvature changes largely while using the B-spline method based on the least square.At first, we properly select dominant points from the screw profile measured data and take the calculated parameters which corresponding to the dominant points in the original data as the knots vector of the B-spline bases. Then fit it with B-spline curves based on the least square. Although it is simple, this method greatly improves the fitting precision without increasing the control points. For the weights of the NURBS curve, genetic algorithm is considered to search the optimum ones of every data points which global fitting error is the least under the conditions of keeping the fitting precision. For the arc points of the screw profile, this paper brings forward an easy algorithm which can be applied directly to represent the arbitrary arc by using cubic NURBS based on the algorithm proposed. Besides, numerical solution of the conjugate profile of the screw in the form of NURBS and some calculations about geometric characteristics of the screw rotors are researched, the meshing condition is also simulated.The reverse design method and the curve reconstruction algorithms of screw profile based on NURBS which are proposesd in this paper are realized with the matlab language and the efficiency of the algorithm is validated by real examples.