Transmission of safety-related signals via standard field bus systems

Status:

completed 12/2001

Aims:

Standard field bus systems serve as intelligent networks for the transmission of data in machines. Such systems involve the transmission of data via two-core lines, over which all bus participants communicate with each other. Conventional safety controls employ separate lines for the transmission of safety-related signals. The objective of the project, carried out on behalf of the "Print and Paper Processing", "Iron and Metal II" and "Electrical Engineering" BG expert committees, was to study under what conditions standard field bus systems could be employed for the transmission of safety-related signals. Measures against the various mechanisms of signal corruption were to be developed further and assessed during the project. At the same time, the various levels of safety technology were to be related to the quality of data transmission, in order to enable the measures to be assessed and employed correctly.

Activities/Methods:

Transmission errors in bus systems were analysed based upon studies performed on safety bus systems in railway applications (prEN 50159-1, "Railway Applications: Requirements for Safety-Related Communication in Closed Transmission Systems"). Measures against such transmission errors, adapted to plant and machinery, were grouped and assessed. The mechanisms for assurance of data integrity are characterized by the residual error probability, upon which the quality of the transmission path has a major influence in the form of the bit error probability. Mathematical models were developed in consideration of the bus structure which enable the number of transmission errors per hour (residual error rate) to be computed based upon the transmission rate of safety-related messages. The residual error rate can then be related directly to the quantifiable safety level (safety integrity level).

Results:

The entire safety chain, from signal acquisition, through signal processing to the safety-related reaction, must be considered for the processing of a safety function. This means that measures can be implemented in the application layer without modification of the actual bus hardware and message structure. For assessment of the mechanisms for the safeguarding of data, the residual error rate was first correlated generally to the safety integrity level. This has permitted, for the first time, measures to be employed which are also tailored to a low safety level. Test principles were drawn up with all major manufacturers of standard field bus systems which draws together the experience gained in the project.

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