Developing a Mechatronic System Model for Self-Pierce Riveting SPR Tools

Self-pierce riveting (SPR) is a complex joining process where multiple layers of material are joined by creating a mechanical interlock via the simultaneous deformation of the inserted rivet and surrounding material. Inertia-based servo SPR systems are commonly used in producing joints for automotive applications.

Due to the large number of variables which influence the resulting joint, finding the optimum process parameters has traditionally posed a challenge in the design of the process. Furthermore, there is a gap in knowledge regarding how changes made to the system may affect the produced joint.

In this thesis, a new system-level model of an inertia-based SPR system has been proposed, consisting of a physics-based model of the riveting machine and an empirically-derived model of the joint. Model predictions have been validated against extensive experimental data for multiple sets of input conditions, defined by the setting velocity, motor current limit and support frame type. High levels of accuracy have been achieved in the predicted response of the system as well as the head height of the joints.

A model-based case study has been conducted to identify changes to the system which enable either the cycle time or energy usage to be reduced. It is shown that the system configuration and parameter settings can be optimised to achieve significant savings in cost or energy consumption, without compromising the overall quality of the produced joint.

In addition, global sensitivity analysis methods have been used to identify the factors with the most influence on the joint via two distinct examples. In the first example, the Elementary Effects method is used to explore the plausible design space of the SPR system. In the second example, a variance-based method is used to understand how the variation in the outputs of a specific SPR process is affected by uncertainty in its inputs. The relative importance of three factors is highlighted: the friction in the planetary roller screw mechanism, the maximum available spring compression in the clamping mechanism, and the length of the rivet.

The current work is the first to develop a mechatronics system-level model of the SPR process. The extensive and systematic validation of the model gives confidence to the model-base analyses performed. The examination of the effects of system-wide process factors on the produced joint via a global sensitivity analysis forms an important contribution to knowledge. The usefulness of the model is demonstrated in identifying areas of improvement for the SPR process, such as significant reductions in the cycle time or the energy usage. The predictive capabilities of the model may be further leveraged to reduce the costs involved in the design and validation of SPR systems and processes. Additionally, it may serve as a tool for exploring further avenues of research, such as co-simulation with a finite element model of the joint in order to achieve high-fidelity representation of the full riveting process.