Estimating the Availability of the Interaction Man – Machine in Control Systems CALIN CIUFUDEAN, CONSTANTIN FILOTE Faculty of Electrical Engineering and Computer Science Stefan cel Mare University 9 University str., 720225, Suceava ROMANIA Abstract: - When conducting an availability analysis we cannot verify an assigned probability, as it expresses the analyst’s uncertain prior observation. We only review the background information used as the rationale for the assignment but, generally, it will not be possible to explicitly document all the transformation steps from this background information to the assigned probability. There are various algorithms that have been used to perform this task. This paper explores Markov chains techniques for searching a given problem probability for a particular target. The work in this paper has two parts. In the first part, a theoretical approach for determining the availability of interacting system man-machine is presented. In the second part an illustrative example of the presented approach is given. Results have shown that this approach allows the estimation of availability for complex systems, such as those involved in railway transport, without performing laborious calculus, e.g. large Markov chains. Key-Words: - Availability, coverage, Makov chains, failure rate, repair rate, man-machine interaction. 1 Introduction A characteristic of the railway transport system is that an answer to a flaw that makes the man-machine system go to a lower level of security is considered a false answer, namely a dangerous failure, while an answer leading to a higher level of security for the man-machine system is considered an erroneous answer, namely a non-dangerous failure. That is the reason for the inclusion of some component parts with maximum failure probability towards the erroneous answer and parts with minimum failure probability towards the false answer [1, 2]. One must notice that the imperfect functioning states of the components of the man-machine system imply the partially correct functioning state of the railway system. In the following pages the notion of imperfection will be named imperfect coverage, and it will be defined as the probability “c” that the system executes the task successfully when derangements of the system components arise [3, 4]. The imperfect reparation of a component part implies that this part will never work at the same parameters as before the derangement; in other words, in this paper, the hypothesis that a component part of the man-machine system is as good as new after the reparation will be excluded. We will show the impact of the imperfect coverage on the performances of the man-machine system in railway transport, namely we will demonstrate that the availability of the system is seriously diminished even if the imperfect coverages are a small percent of the many possible faults of the system. This aspect is generally ignored or even unknown in current managerial practice. The availability of a system is the probability that the system is operational when it is solicited. It is calculated as the sum of all the probabilities of the operational states of the system. In order to calculate the availability of a system, one must establish the acceptable functioning levels of the system states. The availability is considered to be acceptable when the production capacity of the system is ensured. Taking into account the large size of a railway transport system, be it only a local one, the interactions between the elements of the system and between the system and the environment, one must simplify the graphic representation. For this purpose the system is divided into two subsystems: the equipment subsystem and the human subsystem. There are various algorithms that have been used to perform this task [5-7]. This paper explores Markov chains techniques for searching a given problem probability for a particular target. The work in this paper has two parts. In the first part, a theoretical approach for determining the availability of interacting system man-machine is presented. In the second part an illustrative example of the presented approach is given. 10th WSEAS Int. Conf. on AUTOMATIC CONTROL, MODELLING & SIMULATION (ACMOS'08), Istanbul, Turkey, May 27-30, 2008 ISBN: 978-960-6766-63-3 200 ISSN: 1790-5117