Investigation of control of the incremental forming processes

Abstract

Incremental Sheet Forming (ISF) is a new-emerging sheet forming process that promises high flexibility and formability because it does not need any dedicated dies or tooling. This makes ISF well suited for small scale and customised production. ISF uses a simple tool with a smooth end to deform sheets along a toolpath typically generated from CAM software. In ISF, the part is fabricated incrementally by orderly accumulation of plastic deformations localised around the ball end of the tool. Without using dedicated tooling, ISF can quickly adapt to new and modified product shapes via toolpath alteration. Hence, ISF is a promising sheet forming process for quick and low-cost production for small batch manufacturing. In spite of these outstanding characteristics, ISF still suffers from some drawbacks including long processing time and low geometric accuracy. The latter one is the major cause of low take-up of ISF in the forming industry. Therefore, the work presented in this thesis is mainly focused on the improvement of geometric accuracy of the formed parts via in-process toolpath correction using feedback control. In this thesis, Model Predictive Control (MPC), an advanced model-based control technology, was adopted to develop feedback control strategies for ISF toolpath control and correction to improve geometric accuracy. The first research aspect of this thesis is focused on the development of a simple MPC algorithm to optimise the step depth, a critical toolpath parameter defined by the user in the toolpath generation. Firstly, a parameter investigation was performed to experimentally study the influences of the step depth on geometric accuracy, surface finish, and thickness distribution. Two sets of experimental tests with varying step depth values were conducted in the typical Single Point Incremental Forming (SPIF) process that is without toolpath control. The results showed that a smaller step depth led to better geometric accuracy and part surface quality in the ISF process, while the material formability decreased with the step depth value. Too small step depth values would lead to material failure. The parameter investigation work provided significant fundamental information for the development of feedback control strategies for toolpath correction. Then, a simple MPC algorithm was developed for in-process toolpath control/correction in SPIF. During the forming process using the simple MPC algorithm, the step depth of the contour toolpath was optimised at each forming step by solving a receding optimal problem based on the measured shape feedback during the forming process. The developed algorithm was experimentally verified in two case studies to form two different shapes. Results show that the geometric accuracy in ISF with feedback control has been greatly improved (from ±3 mm to ±0.3 mm) at the bottom area of the formed parts compared with a standard ISF approach without control. Improved geometric accuracy has been achieved on the wall of the parts as well, but the errors in the wall areas are still relatively large.In the second research objective of this thesis, a two-directional MPC algorithm was developed by augmenting the simple MPC algorithm with a horizontal control module for horizontal toolpath correction. With two separate MPC modules in vertical and horizontal directions, the toolpath was corrected by optimising the step depth and the horizontal step increment at each step during the forming process. Two case studies were conducted to experimentally verify the developed two-directional MPC algorithm. In the first case study (a truncated pyramid), two control approaches with different assumptions for the horizontal springback distribution along the horizontal cross-sectional profile were tested and compared. Then, the developed MPC control algorithm was applied to form an asymmetric cone. Results show that the developed strategy can reduce the forming errors in the wall and base of the formed shape compared to the existing works. The SPIF process with two-directional MPC control leads to significant accuracy improvement (from ±3 mm to ±0.3 mm) in the base and most wall areas of the formed shapes in comparison with the typical SPIF process that is without toolpath control. The last research objective of this thesis is focused on an enhanced MPC algorithm specially developed for Two Point Incremental Forming (TPIF) with a partial die to conduct in-process toolpath correction. In the horizontal control module, dense profile points in the evenly distributed radial directions of the horizontal section were used to estimate the horizontal error distribution along the horizontal sectional profile during the forming process. The toolpath correction was performed through properly adjusting the toolpath in the horizontal and vertical directions based on the optimised toolpath parameters at each step. A case study for forming a non-axisymmetric shape was conducted to experimentally verify the developed toolpath correction strategy. Experiment results indicate that the two-directional toolpath correction approach contributes to part accuracy improvement of about 90% (from ±3 mm to ±0.3 mm) in TPIF compared with the uncontrolled TPIF process. The work in this thesis explores various aspects of ISF research, although the most important contribution is the feedback control of ISF processes and the experimental validation, which helps to achieve great improvement in ISF geometric accuracy and provides good approaches for in-process control and optimisation in the field of advanced manufacturing.