Fieldbus & Industrial Networking


Reliably determine actual fieldbus availability

June 2014 Fieldbus & Industrial Networking

Communication via fieldbus is highly valued in process automation due to its reliability. Like any installation technology, however, it is not exempt from faults and failures. But what is the actual risk of failure and what availability can be realistically assumed? Calculations often provide an unsatisfactory reflection of the real situation. This results not only in incorrect assumptions, but often in protective measures that are expensive and barely effective.

In practice, to what degree does availability affect whether a fieldbus installation is commissioned? You need to understand what can cause problems and how best to protect against them, so that you can take preventive measures and ensure increased fieldbus availability. However, availability calculations are based on descriptions, assumptions, and observations from the theory of probabilities. Here at Pepperl+Fuchs, we have discovered from many years of interaction with users that these calculations are based on assumptions that are sometimes unrealistic, and sometimes just plain wrong. Of course, the results of such calculations do not really reflect reality.

What do we mean by actual availability?

The International Electrical Vocabulary (IEV) has 47 different definitions of the term availability and, accordingly, different ways of calculating it. Stationary availability is usually called availability (A) for short. It is defined as the mean value of current availability in a time interval. The mean time to failure, called MTTF, and the mean down time, called MDT, can be used for a simplified calculation of the stationary availability, provided these values are constant. In this case, the following formula is used for the calculation:

Often, the inverse of the Lambda failure rate for a product or series of products is mistakenly used to calculate the MTTF, such as for a fieldbus segment. But this procedure reflects only the random failure of the component(s). This means that important systematic criteria are not being taken into account. In practice, these criteria play a crucial role in availability: When environmental influences as well as the mode of operation and its effect are not accounted for in the calculation, a significant discrepancy arises between the mathematical theory and the effect in practice when it comes to process automation. However, a brief glance at alarm and failure statistics makes it very clear that it is precisely the effects of the mode of operation and environmental conditions that are responsible for faults much more frequently than random events that cause component failure.

An example shows the difference

The extent to which this type of one-dimensional view can distort reality when calculating availability is shown by a simple example: If a person in his role as employee is used instead of the process system, the result is an MTTF of 1401 years or 72 800 weeks according to the above-mentioned procedure. But this value takes into account only the ‘total failure’ i.e., possible death of an employee, which certainly does not accurately reflect the employee’s actual availability in his professional life. If you further assume that a replacement is found for a failed employee after six weeks (=MDT), the availability is calculated as:

Of course, important aspects of everyday working life remain unaccounted for. Much more frequent causes of absence from the workplace are vacation, illness, doctor’s appointments, or business trips, which can occur several times a year. If you assume that a failure of on average two weeks occurs twice a year for these reasons, the availability is calculated as:

Reduction of failure risks

Finding the right method of calculating availability is just the first basic step. Once you have calculated reliable figures for the actual availability of a plant, it follows that you must take measures to reduce the failure rates effectively and thus increase availability. Systematically, there are four methods to protect against a component or part of a system failing and thus positively influence the MTTF.

First: Preventive measures and procedural instructions must be given. The correct and protective handling of technology is often enough to help reduce failures significantly.

Second: The predictive, automated handling of faults. With this method, techniques and components are used that have been specially developed to detect and isolate typical faults in a targeted and proactive manner, before they can spread. The impact of the fault remains limited to a deactivated device, while the plant itself remains in operation. For example, if a measuring device connection is deactivated in the case of a short circuit, but the fieldbus segment remains unaffected, because the failure of an individual measuring part is tolerable.

Third: The detection of faults through diagnostics. With this method, discrepancies between the actual status and the best possible status are detected through monitoring and reported to the control room. Before this can have a negative impact on the overall function, proactive intervention can be taken. For example, if you measure a change in the frequency of filling level sensors using a tuning fork, this indicates that the sensor has become jammed. The problem can then be corrected.

Fourth: Redundancy. This protects against failures whose causes have to be investigated in the device itself. Redundancy is indispensable if these faults cannot be avoided in any other way, but must absolutely be controlled to ensure safety or plant availability. This is the case for power supplies and control technology boards or for field devices where the measuring circuit is required to have a very high level of availability.

For more information contact Mark Bracco, Pepperl+Fuchs, +27 (0)87 985 0797, mbracco@za.pepperl-fuchs.com, www.pepperl-fuchs.co.za



Credit(s)



Share this article:
Share via emailShare via LinkedInPrint this page

Further reading:

EtherCAT interoperability removes industrial networking barriers
Fieldbus & Industrial Networking
Selecting the right communication technology is one of the most important decisions engineers make, and interoperability helps with that decision. Key development tools and standards ensure interoperability among many EtherCAT devices and manufacturers.

Read more...
Condition monitoring to go
Turck Banner Southern Africa Fieldbus & Industrial Networking
Anyone who wants to efficiently monitor the climate in control cabinets will find a comprehensive range of control cabinet monitors for the DIN rail in Turck Banner’s cabinet condition monitoring family.

Read more...
A new standard in high-speed Ethernet communication
Fieldbus & Industrial Networking
The TXMC897 module from TEWS Technologies supports a range of Ethernet standards and speeds, making it suitable for diverse applications, including the defence, industrial, and IIoT markets.

Read more...
Data-driven battery production
Turck Banner Southern Africa Fieldbus & Industrial Networking
The availability of high-performance batteries at moderate prices is one of the most important factors for the success of electromobility. As a long-standing automation partner to the automotive industry, Turck Banner supports the major battery manufacturers with its know-how.

Read more...
Bring critical temperature data to your condition monitoring system
Turck Banner Southern Africa Fieldbus & Industrial Networking
Data conversion just got easier. Turck Banner converters are compact, simple add-ons that seamlessly fit into your factory applications. You can take various types of signals such as discrete, analogue and many others, and convert them to protocols like IO-link, PICK-IQ, PWM/PFM, and Modbus.

Read more...
RFID made simple
Pepperl+Fuchs Industrial Wireless
Pepperl+Fuchs now offers a practical solution for users looking for an easy entry into the world of RFID with all its possibilities. The new F191 RFID read/write device combines the advantages of sophisticated industrial UHF technology with a standardised interface for IO-link communication.

Read more...
Case History 190: Measurement problem ruins level control.
Michael Brown Control Engineering Editor's Choice Fieldbus & Industrial Networking
The widely held belief in many plants that tuning will solve all base layer control problems is completely fallacious. Bad tuning is generally not the main reason for loops to perform badly. It is important when performing optimisation that all elements in a loop are considered, in addition to the control strategy, before even thinking of tuning.

Read more...
Precise part machining
Beckhoff Automation Editor's Choice Fieldbus & Industrial Networking
toolcraft manufactures on behalf of its customers using its 60 CNC machines, and designs, plans and builds turnkey production systems for companies in various industries, having added injection moulding, mould making and additive manufacturing technologies along the way. Robotics is the company’s newest technology division. This is why toolcraft relies on PC-based control, including in the production cells, which have seven-axis milling robots for machining components at CNC level.

Read more...
Fully digital vertical roller mills
Loesche South Africa Fieldbus & Industrial Networking
Vertical roller mills are found in mining and cement milling operations worldwide. They require complex technology for their operation. As a leader in this field, LOESCHE has proven software solutions for digitalisation that can optimise each piece of milling equipment for energy efficiency, and increase its availability and output, making the plant easy to operate.

Read more...
CNC cycle package for triple-axis milling and drilling
Beckhoff Automation Fieldbus & Industrial Networking
Beckhoff provides a comprehensive package of different cycles for triple-axis milling and drilling machines with the TwinCAT 3 CNC milling base.

Read more...