Paul Tu

Temporized coloured Petri nets with changeable structure (CPN-CS) for performance modelling of dynamic production systems

One of the difficulties in modelling dynamic production systems is the representation of changes to the system along time. Coloured Petri nets with changeable structure (CPN-CS) are capable of modelling systems subject to change. However, this is restricted to functionality modelling of the system. In this paper, we extend CPN-CSs into temporized CPN-CS (TCPN-CS). The extension is carried out by associating relevant modelling elements with time parameters and modifying the mechanism for structure change of the CPN-CS model. TCPN-CS makes it possible to model a dynamic production system considering both functionality and performance. A case study is provided to show that TCPN-CS is effective for modelling dynamic production systems.

Adaptive production scheduling of virtual production systems using object-oriented Petri nets with changeable structure

Virtual Production Systems (VPSs) are logically constructed by organizing production resources belonging to one or more physical manufacturing systems. VPSs can enhance the agility of manufacturing systems. However, an effective scheduling approach is required to cope with disturbance and changes to these systems. An adaptive production scheduling method is proposed. Object-oriented Petri nets with changeable structure (OPNs-CS) formulate the scheduling problem of VPSs. To resolve resource constraints in a VPS, the OPNs-CS is modified by introducing limited token available time and by revising the enabling and firing rules. The artificial intelligent heuristic search (A*) algorithm is modified and applied to generate the optimal or near optimal schedule. When a VPS encounters any disturbance, an estimate of the effects of the disturbance can be estimated by simulation on the OPNs-CS model. If the scheduling target (completion time) is not affected, rescheduling is not required. Whenever there is a change to the VPS, the TOPNs-CS model is updated to refresh VPS schedule. A case study is presented to demonstrate the procedures for applying the proposed scheduling approach. The given case study shows that the proposed approach is capable of scheduling a VPS dynamically in response to disturbances and changes are involved.

Colored Petri Nets with changeable structures (CPN-CS) and their applications in modeling one-of-a-kind production (OKP) systems

In this paper, colored Petri Nets with changeable structures (CPN-CS) are defined. The mechanisms used for carrying out the structure changes include change-by-modification (CBM) and change-by-composition (CBC). CBM is for modification of the structure of the current CPN model while CBC is for the composition of another CPN into the current CPN. A case study is provided to show that CPN-CS is effective for modeling one-of-a-kind production (OKP) system that is characterized by product design uncertainty and changes in resource availability.

Image feature analysis of plasma spot produced from femtosecond laser ablation for silicon wafer

Abstract. When using a femtosecond laser to machine a single-crystal silicon wafer, it is accompanied with a diffraction spot of plasma. The existing literature reports that the brightness of the image of plasma can be used as an indicator to online measure the depth of the machined groove on a micrometer scale. Because the plasma spot is influenced by eruption and partial occlusion of ablated material, this method, which simply relies on the spot image brightness as a feedback parameter, is not reliable or accurate. The pixel area, perimeter, and brightness characteristics of the plasma spot image need to be comprehensively analyzed to provide a reliable and accurate feedback to establish close-loop micromachining technology. Therefore, we first analyze the chirped amplification principle of generating a femtosecond laser and the application of the diffraction spot of plasma during the micromachining processing using the femtosecond laser. Second, we experiment using femtosecond laser ablation with a piece of 10×10  mm and thickness of 650±10  μm single-crystal silicon wafer to obtain the corresponding relational data among parameters of laser processing power, processing speed, and laser spot image of plasma. Third, aiming at the characteristic of dim target of the laser spot image, the two-dimensional Otsu (maximum class square error method) is used to segment the laser spot image to improve the segmentation accuracy of the laser spot image. Finally, we analyze the relationship among area, perimeter of the laser spot image, and laser energy; the relationship among area, perimeter of the laser spot image, and the machined depth of groove; the relationship between brightness of the laser spot image and laser output power; and the relationship between brightness of laser spot image and machining speed.

An extension to timed colored Petri nets (TCPNs) for modeling manufacturing systems with limited resources availability

This paper presents an extension to timed colored Petri nets (TCPN) for modeling manufacturing systems subject to the limited availability of some resources. The limited availability of a resource refers to the situation that a resource can only be used within one or more time intervals, which cannot be modeled by the existing TCPN approach. The extension is made by defining the limited usable time stamp, and modifying rules for enabling and firing transitions that have input or output places representing the limited availability of resources. The extended TCPN can be applied in real time control and scheduling of manufacturing systems subject to the limited availability of some resources.