Fail-safe Circuits Research Articles

A Simple Strongly-Fail-Safe Circuit Used as Basic Cell for Designing Safety Architectures

Abstract As part of critical application used in railways transportation, space, chemical and nuclear industries, the processing part which controls the actuators of the electromechanical part is realized with fail-safe circuits. But these give use to the following problems. Complexity of design in case of redundant solutions, done till now, which require conventionnal fail-safe circuits. Necessity of off-line test sequences in case of non redundant solutions involving “strongly-failsafe” circuits. This paper ailns to provide a practical solution using “strongly-fail-safe” circuit fordesigning dependable COMputer systems aimed at critical processes. The goal is to make use of suchsystems easier, to avoid the drawbacks generated by off-line test phase and to obtain the best “costsafety”compromise. First of all, we introduce the scheme of a “strongly-fail-safe” basic cell without any off-I.ine testequipnents. The advantage is to obtain a “strongly-fail-safe” circuit which requires fewcOlnponents. Then, we suggest an architecture of a “strongly-fail-safe majority voted output” circuit designed from the “strongly-fail-safe” basic cell. The reliability, the safety and the availability of this ⤜majority voted output” mechanism is evaluated. In conclusion a triple modular redundancy computer system including this last mechanism iscOmpared with well known dependable cOmputer systems. The good results obtained show thatsuch a solution may meet a wide range of safe applications because of its low complexity and itsfitness to be easily implemented.

VLSI implementation for control of critical systems

Self-checking integrated circuits, i.e. able to detect its own errors, are not adequate to control the actuators of critical systems used in railway transportation, space, chemical and nuclear industries. Only the conventional fail-safe circuits can apply these needs, unfortunately they cannot be implemented in integrated circuits technologies and thus they cannot be used to realise complex systems.This paper deals with a method to solve this problem. We present briefly the theory of generalized fail-safe systems which includes new, more general definitions, construction conditions and basic properties for fail-safe systems. Then, in order to take into account the problem of undetectable faults, we define strongly fail-safe circuits who achieve the totally fail-safe goal. As an application we propose a fail-safe integrated interface which transforms the outputs of self-checking systems into signal adequate to drive electromechanical actuators and such that the whole system, self checking processing part and interface, implemented in VLSI is strongly fail-safe. Therefore this system can be used for safe drive of complex critical processes.

CONTROL OF POSTOPERATIVE PAIN BY INTERACTIVE DEMAND ANALGESIA

A demand analgesia apparatus is described. It communicates with the patient by means of spoken messages in any desired language, so that preliminary instruction is unnecessary. Dosage is limited by a reduction in respiratory rate, and by a series of electronic fail-safe circuits. Operation of the system is illustrated in a series of 10 patients given fentanyl "on demand" for the first 12 h after operation. The mean dose rate 0.73 +/- 0.49 microgram min-1. There was no evidence of cumulation or tolerance during the period of study.

Negative failsafe sequential circuits

The letter presents a new method of designing failsafe sequential circuits by using appropriate negative forms for the next state functions and a proper state assignment of a k-out-of-n code. The use of negative monotone functions makes the circuit oscillate between two states which are outside the normal states of the circuit when the circuit, by any reason, assumes some state outside the normal ones.

Overview of Hardware-related Safety Problems of Computer Control Systems

The main hardware failure types of computer systems are classified. The risks and the difficulties in the safety demonstration of different fail-safe computer systems are compared. Such computer systems are: Multi-computer with hard- or software comparator, with equal and diverse processing; single computer with checking program or with diverse multi-processing. The problems of computers with fail-safe circuits or with processing in coded form are discussed. A structure of the safety demonstration is recommended. In order to achieve a low probability of multiple failures a fast failure detection is necessary. In practise it is not possible to analyse the internal failures of a microprocessor in detail. An operational method of failure detection using a second processor and a function test program is outlined. Advantages and disadvantages of hardware diversity are discussed.

Fail-Safe Asynchronous Sequential Machines

Fail-safe circuits are designed to assume a 1 (1-fail-safe) or a 0 (0-fail-safe) output state upon failure. This correspondence extends fault detection techniques previously presented [1] to include the design of fail-safe asynchronous sequential circuits. Faults causing failures in the internal state logic and the output state logic circuitry are treated. These failures are assumed to be symmetric and the resulting circuit realizations require less hardware than realizations derived from previously presented techniques.

Railway signaling

MANY NEW AND USEFUL TECHNIQUES have become available in the communications field in recent years. There is a natural and quite proper desire to employ these new techniques for the control of railway traffic. A modern railway signaling system must have as its foundation properly designed fail-safe circuits; but under the careful guidance of a signal engineer, the more conventional communications circuits may then be superimposed and the remote control of railway traffic achieved with complete safety.