# Enhancing Antifragile Performance of Manufacturing Systems through Predictive Maintenance
## 6 Method for Injection of Artificial Errors in a Virtual Environment  
Increasingly sophisticated methodologies are available nowadays to determine the causes of the failures of flexible manufacturing lines. Of these, the most widely used seems to be the failure mode and effects analysis (FMEA), most often associated with criticality analysis (CA). The use of FMEA allows both the identification of failures and the actions required to remove them, usually based on rules of priority in the execution of corrections. The testing and validation of the method was carried out on the testbed called SMART Flexible Assembly System within the research project CIDSACTEH ([[http://cidsacteh.upb.ro](http://cidsacteh.upb.ro)]([http://cidsacteh.upb.ro](http://cidsacteh.upb.ro)) accessed on 1 September 2022). The main objective of the project is the use of advanced modelling and simulation technologies for the performance assessment of manufacturing mechatronic lines. The logistic support for performing the tests is a laboratory model for a flexible assembly line of industrial products with five workstations (Stations 1 to 5), as presented in Figure 1.  
The technological flow consists of successive sequences of movement of a support on which the component elements of a product are mounted. The actual assembly is performed using a robotic arm in the central workstation (No. 3). Our method proposes the use of a virtual environment for injecting and evaluating the impact of artificial errors on the system's antifragility. This method uses FSM (finite state machine) representation of the process correlated with FMEA (failure mode and effect analysis) to dynamically quantify this impact. The method involves two steps: (1) building a behaviour model in the virtual environment, and (2) an analysis of the model under artificial error injection. Process modelling (Figure 2) is a DT representation of the system, with all of the phases, nodes, and dependencies. This can be performed through an FSM where different diagrams capture different detail levels of the plant as well as dependencies between different nodes, allowing a nested top-bottom approach. Each node represents either a phase, a branch, or a final element. A risk factor is assigned to each element, taking into consideration the severity of the possible failures, the occurrence, and detectability probability. The risk factor of a phase or branch consists in the sum of all of the included elements. According to this data, a criticality matrix is built to represent element failure along with the occurrence and severity. For each station of the manufacturing line, an FSM representation was built. For example, Station 1 (Figure 3) was modelled using 10 states and 12 transitions. The states are represented by: _nr_pf_, the number of products and _nr_pi_, the number of components for each product, given by the PLC; two inductive sensors, SP1 and SP2, showing the product entered or exited the conveyor belt; one RFID sensor, RFID1, to identify the stop position; one optical sensor, SO1, to check the pallet availability in the rack; two feedback sensors, SF1 and SF2, which confirm the element is in the correct position and can be released from the stack; a capacitive sensor, SC, for confirming the elem  
Figure 1: Block diagram of the mechatronic assembly line. B2_free, a parameter confirming the next station accepts the new elements. The transitions represented by elements P1 to P12 check the cumulative conditions required for each step for the element to be correctly processed until it leaves the station. The FMEA analysis was applied on Station 1, considering for each element possible the failure modes, and causes and effects, and assigning a risk ranking with a factor between 1 and 10 for the severity of the event, the probability of occurrence, and the ease of detecting it (measured by detectability). By multiplying all three indices, we obtained a risk factor which varied from 1 to 1000.