Deflection detection and curve fitting in three-roll continuous straightening process for LSAW pipes

Aiming at the low efficiency of three-point mold bending straightening, a new method of three-roll continuous straightening is proposed in this paper, which is full in consideration of the geometric features and bending characteristics of LSAW pipes. Using the characteristics of the roll straightening device, a straightness detection method for large pipes is proposed just by adding a laser displacement sensor, and the calculation model for overall deflection based on local deflection is established. Based on the detected deflection data, a piecewise fitting algorithm with constraints by introducing the Kuhn-Tucker condition is proposed for straightness calculation, and a simple polynomial fitting method with fourth order is determined for the calculation of curvature and straightening moment. Further, using the semi-automatic experimental prototype for small pipes, the deflection detection experiments show that the overall deflection error is less than 3.5% compared with that of CMM, the piecewise fitting results show that the ideal order of the mid-segment is 10-th, and the three-roll continuous straightening experiments show that the novel method can correct the straightness to less than 0.2%, meeting the standard requirements.