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This paper presents an improved direct torque control (DTC) based on fuzzy logic technique. Two major problems that are usually associated with DTC drives are the high torque ripple and switching frequency that varies with operating conditions. To overcome these problems, fuzzy controllers are proposed and associated with DTC scheme. The proposed approach is shown to be able to reduce the torque and flux ripple and to improve performance DTC. The control algorithm is based on the SVM (space vector modulation) technique to provide a constant inverter switching frequency. Furthermore, to verify the effectiveness of this model, a DSPbased experimental induction motor DTC drive system is built.

Safety Instrumented Systems (SIS) are electric/electronic/programmable electronic safety related systems which are intended to achieve or maintain a safe state for the Equipment Under Control (EUC), in respect of a specific hazardous event. A well designed system can increase plant availability by reducing the number of spurious « Trips » caused by an SIS that fails to properly evaluate a safety situation and unnecessarily shut down a process. Data suggest that over 90% of problems in SIS can be attributed to the sensors and final elements. This paper presents the results of a simulation and experimental study based on fault diagnostic of SIS components (especially sensors). The EUC is a furnace in gas process industry. It has been shown that adding a second sensor to the SIS may improve its performance (Safety Integrity Level: SIL).

Purpose The purpose of this paper is to show the impact of operational and environmental conditions (risk influencing factors) on the component criticality of safety barriers, safety barrier performance and accidents frequency and therefore on risk levels. Design/methodology/approach The methodology focuses on the integration of criticality importance analysis in barrier and operational risk analysis method, abbreviated as BORA-CIA. First, the impact of risk influencing factors (RIFs) associated with basic events on safety barrier performance and accident frequency is studied, and then, a risk evaluation is performed. Finally, how unacceptable risks can be mitigated regarding risk criteria is analyzed. Findings In the proposed approach (BORA-CIA), the authors show how specific installation conditions influence risk levels and analyze the prioritization of components to improve safety barrier performance in oil and gas process. Practical implications The proposed methodology seems to be a powerful tool in risk decision. Ordering components of safety barriers taking into account RIFs allow maintenance strategies to be undertaken according to the real environment far from average data. Also, maintenance costs would be estimated adequately. Originality/value In this paper, an improved BORA method is developed by incorporating CIA. More precisely, the variability of criticality importance factors of components is used to analyze the prioritization of maintenance actions in an operational environment.