Comparison of online identification methods of material parameters in press straightening process
More and more automatic pressure straighteners are used on the manufacturing industry because of their high productivity and good quality. When the technologic parameters are calculated, the precision of results will be influenced by the part's material performance parameters. In this paper, three online identification methods of performance parameters were studied. They were BP neural network method, RBF neural network method and the support vector machine method. Then their identification effects were compared. It was shown that all three methods can be used to identify material performance parameters in straightening process. But later two methods have better identification precision and shorter response time than BP neural network method.
ANN-BASED REAL-TIME IDENTIFICATION OF MATERIAL PROPERTIES AND FRICTION COEFFICIENT
In order to avoid defects in intelligent control of sheet metal deep drawing,the control technological parameter should be changed adaptively,which can be predicted from material properties and friction coefficient.An artificial neural network model for identification of material properties and friction coefficient is established according to deep drawing characteristics and more automation requirements.Real\|time identification of material properties and friction coefficient is realized with neural network,which is a new tool to process information. It lays basis of intellectualization in sheet metal deep drawing.
Establishment and Application of Load-Deflection Model of Press Straightening
It is important to calculate straightening stroke precisely for automatic straightening machines. But, the existing mathematic models for calculating straightening stroke can not solve this problem well. In this paper, the mathematic load-deflection model, which reflects elastic range, elastic-plastic range and elastic spring back of press straightening process, is developed. By using the model, the straightening stroke can be calculated directly according to the initial deflection of the bent part. By simplicity, the rectangular section part is selected for study and is assumed to be the ideal elastic-plastic material. Based on the elastic-plastic mechanics theory, through deriving relationships among the load, bending moment, deflection and elastic region length, the theoretical model of press straightening process has been established. Finally, two examples are calculated. The results calculated by the model are in good agreement with the results computed by FEM. Experimental data also show that the calculation results by the model is exact and the error is small. So the validity and the accuracy of the model are verified. It will be helpful to the development of automatic straightening machines. Introduction To meet the expanding production requirements, more and more automatic straightening presses are used on the manufacturing industry because of its high productivity, good quality and economical efficiency. In keeping with the automation of straightening presses, researches on straightening technology theory are necessary. Specially, the calculation of straightening stroke is important because it influences the straightening precision directly. Although several varieties of good automatic straightening machines have been developed by different manufacturers in few developed countries, their design development seems not to be backed up by a detailed theoretical analysis. So, some domestic scholars [1~3] have done some works on it, and some mathematical models of calculating straightening stroke have been established. However, we notice that all these models are based on straightening curvature equation and have two main defects. First, before calculating the straightening stroke, we must fit the axial curve equation of straightened part and compute initial curvature. As a result, measurement errors and fitting errors are inevitable. Second, these models can not reflect the process of straightening and then is unsuitable to be used on the straightening control. Otherwise, T. Katoh [4] and S.C. Kin [5] used assumed load-deflection model of straightening process in their studies on straightening control, but they have not expounded and derived the theoretical model. In this paper, mathematical load-deflection model of straightening process is studied. It can overcome the shortages of the existing models and can be used on straightening control. Load-deflection Model and Application Key Engineering Materials Online: 2004-10-15 ISSN: 1662-9795, Vols. 274-276, pp 475-480 doi:10.4028/www.scientific.net/KEM.274-276.475 © 2004 Trans Tech Publications Ltd, Switzerland All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications Ltd, www.scientific.net. (Semanticscholar.org-13/03/20,17:14:13) Press straightening is a three point reverse bending process actually. In straightening process, the part with initial bending deflection 0 δ is simply supported at two points while a load F is applied at the mid-point. The part will generate reverse bending under the load F, and reverse bending deflection is marked as w δ , meanwhile elastic-plastic deformation is occurred. Unloading F, the part will spring back, and permanent plastic deformation is residual in the part. If the spring back amount equals to reverse bending deflection w δ , the part is straightened exactly. At this time, the total press down amount, i.e. straightening stroke, is w δ δ δ + = Σ 0 . The whole straightening process can be described by the load-deflection model, shown in Fig. 1. It includes three stages: elastic deformation line AB, elastic-plastic deformation curve BC and elastic spring back line CD. AB and CD have same slope coefficients k. Point B is the yield point. Utilizing the model, if the initial deflection of the bent part is known, spring back line can be gotten and will intersect the elastic-plastic curve at one certain point. Then corresponding reverse bending deflection of the intersecting point can be calculated. Finally the needed straightening stroke is obtained. Establishment of Load-deflection Model Assumptions. Comparing the length between two support points, the deflection of the part is small, so press straightening is a small elastic-plastic deformation problem. Two basic assumptions are reasonable: plane section assumption and uniaxial stress assumption, and Bauchinger effect is neglected. By simplicity, the rectangular section part is selected for study in this paper, and it is simplified to be the ideal elastic-plastic material. Elastic Deformation Stage and Spring Back Stage. Deformation deflection of straightening at the elastic deformation stage can be calculated according to material mechanics: F EI l EI l F 6 48 ) 2 ( 3 3 = = δ (1) where, E is Young’s modulus, I is inertia moment against neutral axis of the cross-section, 2l is the length between two support points. Yield point B is the end of elastic deformation, and corresponding load l M F t B 2 = . Here, t M means elastic limit moment of the section at middle point. s Z t W M σ = , where, Z W is section modulus in bending, s σ is yield limit of material. As to rectangular section with height H and width B, 6 2 BH WZ = . According to simple unloading law, spring back process is linear when unloading, and spring back line has same slope coefficients with loading line at elastic deformation stage: 3 6 l EI k = . Elastic-plastic Deformation Stage. At this stage, plastic deformation occurs from the outer fibre in the material. The dashed area in Fig. 2 is the plastic region, whiches boundary isξ-x curve. The length of the plastic region is 2xt .ξ is elastic region ratio, which is defined as the ratio of the thickness of elastic deformation fibre and the whole height of the part at certain section. The bending moment diagram M-x is also shown in Fig.2, and the bending moment can be written as: ) ( 2 x l F M x − = (2) Fig. 1 Load-deflection model of press straightening process 476 Advances in Engineering Plasticity and Its Applications