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What really happened? BLACKBOX clarifies equipment trouble

10 September 2008

Gearbox damage on a press! Faulty batch in cosmetics production! A bottling system produces excessive waste! Who is at fault? Who is going to have to pay for the damages?

Autem's 'Black Box'
Autem's 'Black Box'

Long-term recording of PLC signals is not only a good idea but is mandatory for some industrial processes. Besides tracing sporadic errors, the archived measurement data can clarify other essential questions as well.

Increasing complexity of automated systems as well as increased time pressure during installation and maintenance are common these days. Excellent availability and productivity is only guaranteed if errors can be detected and rectified quickly.

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Image 1: Blackbox: a complete computing system recorder in a massive aluminum housing.

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Service personnel normally do not have real time signal waveforms of PLCs, yet this information is needed to diagnose complex logic errors or analyse sporadic errors. Sporadic errors can only be detected efficiently if all signals are recorded in real time and can be displayed at any time. The production planner also needs a precise overview of the timing for the progression of functions in a system, to detect and eliminate unnecessary downtimes in the machine.

CLASSIC SOLUTIONS

Often, programming tricks are attempted to solve these types of sporadic problems. However, changing the existing PLC program may introduce a new source of trouble—and may cost additional time. Line recorders or logic analysers are also insufficient, because they are limited to PLC inputs and outputs. Internal variables, such as flags, data records, counters and timers cannot be analysed.

Modern PLC programming software offers the user support in diagnosing errors using status displays. However, the user cannot usually detect what the cause of the error was when the machine is at a standstill, because the system was probably in another state immediately before that time. Many users are also not able to leave their programming devices or notebooks attached to the system while unattended for weeks, or even months, on end.

A MORE ELEGANT SOLUTION

A special software logic analyser called ‘PLC-Analyzer pro’ made by Autem (www.autem.de) is designed for recording PLC signals. It is normally installed on a PC or PC-based programming device under MS Windows.

Process data are recorded through standardised PLC interfaces or automation networks such as MPI interface with Simatic S7 or Ethernet TCP/IP. A programming device or an operational PC with a connection to the PLC can therefore be used immediately. The recorded data can be evaluated or prepared for use in documentation with a PC license.

To inspect the recorded data, the program has extensive search and time measurement functions. The search functions can be used to quickly find important trigger events, certain bit patterns or special edges. By changing the time-base, the signal waveform can be expanded in certain areas to make details visible. Export functions to CSV or HTML format enable further data processing with spreadsheet programs.

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The AD_USB-Box
The AD_USB-Box

Image 2: The AD_USB-Box is used to capture external signals and synchronise them with the recorded PLC signals.

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LONG-TERM RECORDING

In practice the signals must often be recorded for a long time (several months) with no interruptions until the respective error occurs. A special recording computer is required, which can remain attached to the system for weeks or perhaps permanently. To satisfy this need Autem offers an ultra-compact, fanless mini-PC (195 x 268 x 80 mm) called ‘Blackbox 2’ that can be integrated directly in the switching cabinet.

‘Our need for the present project is for long term archiving of multiple Siemens S7-300F PLCs over Profinet. The program must run continuously. When an incident occurs, the maintenance personnel would then look at the files created by the PLC-Analyzer. They are not necessarily looking for program faults but also mistakes made by the operators’ says Mr. B. Huge, of Siemens Transportation Systems.

The data recording is done with a special version of the PLC-Analyzer pro 5, which has been expanded with several special functions for full self-sufficiency in long-term recording. The connection with the PLC is done via the respective programming interface or that of an automation network. The process data can be recorded and archived continuously with no interruptions over a very long period of time. The recording capacity of the hard drive is normally 3 years.

An integrated ‘watchdog’ ensures that the recorder is automatically rebooted in case of a recording fault. After a reboot, it immediately continues with recording. When the capacity has been reached, the oldest signal file is automatically deleted to create space for a new signal file. The operating status can be requested at any time from an external device.

A new type of remote service allows remote control via network connection (LAN) from any PC. A dial-up connection (modem) or Internet (VPN) are already integrated. The user can access the archived data at any time, even while recording is underway.

Complex production systems often have mixtures of PLCs, such as Siemens S7 and Pilz safety control systems. Since properly timed processing of these individual controllers must be guaranteed throughout the production process, Autem’s analyser records signals from different PLC systems simultaneously. The Blackbox has numerous interfaces to simultaneously record signals from different PLCs.

Sometimes there are external signals (voltages and currents) that are related to the PLC problem, so in order to capture these, Autem has a special measurement adapter, the ‘AD_USB-Box.’ It connects to the main analyser through the USB port and synchronises the external signals with the PLC signals it is recording.

There are many possible areas of implementation for long-term process data archiving with the BLACKBOX: It is also relevant for system and machine manufacturers and users because the data obtained is applicable for localizing errors as well as documenting operating behavior and system operation. Maintenance, construction and quality assurance receive valuable information concerning the system. The user therefore has a ‘PLC tachograph’.

Dipl.-Oec Joerg Brahms: AUTEM, Emden, Germany

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Image 3: Time difference measurement at analysis of signal file recording.

Time difference measurement
Time difference measurement

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SIDEBAR: QUESTION OF GUILT

The Blackbox can be used to clarify questions of guilt in system failures.

Machine and system manufacturers have to be prepared to face claims from their customers that the equipment is not performing according to the guarantee. Their claims may or may not be valid.

If, for instance, the gearbox in a press malfunctions, is it because the manufacturer didn’t conform with the specifications for the parts, or was it because of operator error? Often, it is hard to prove.

Recorded PLC data are unerring proof and provide valuable support in researching the cause of the problem.

For industrial insurance companies, the advantages are obvious. In disputes, it may be sufficient so that the innocence of a customer can be proven so that liability is not necessary.


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