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Ensuring reliable identification

14 April 2015

Having the ability to track and trace products throughout the entire supply chain can offer huge benefits and this is where UHF RFID systems display their strengths, says Ingo Hecker, product manager RFID systems at Siemens Process Automation.

The use of ultra-high frequency radio frequency identification (UHF RFID) systems possess a major advantage compared to LF (low frequency) or HF (high frequency) systems. Objects can be captured from just a few centimeters all the way up to distances of up to, typically eight metres. 

Continuous identification and complete tracking & tracing along the entire supply chain – beginning with the production of vendor parts to the further processing at OEMs, the transport to dealers and the point of sale can be achieved with a single technology. This creates transparency, contributes to the increases in quality and helps ensure the optimal utilisation of resources.

To meet all the challenges of the many possible applications, sophisticated evaluation algorithms and commissioning and diagnostic tools tailored to the UHF RFID system are necessary. With RFID read/write devices, antennas and transponders the RF600 series and the Simatic RF-DIAG software tool, Siemens can provide all the necessary components.

High process reliability is an essential requirement of identification systems and especially in the production area, the challenges for UHF RFID systems should not be underestimated. The capture takes place with the aid of electromagnetic waves, over which information between the transponder – attached to the object to be captured – and the reading device is exchanged. In the UHF frequency band, electromagnetic waves are reflected by metallic surfaces, such as transport systems, machines and products. The reflected waves can overlap the original ones, leading to a so-called cancellation or overshooting.
The position and direction of the RFID antennas greatly influence the quality of the reading results. Finding a suitable position for the antennas and optimally aligning them are crucial steps in the commissioning of an RFID reading point. 

Compared to optical identification systems, RFID has the added complexity that instead of an image, all that is received is the result of the one only receives the result of the ‘radio capture’: ‘read’ or ‘not read’. With an image, it will be quickly apparent whether the focus needs adjustment or the illumination is insufficient; or the printed data matrix code is smeared or the print quality is bad. 

However, things are not so straightforward with UHF RFID and this is where Simatic RF-DIAG comes into play. The software offers different graphical views, which make the events between the read/write device (‘reader’) and the transponder (‘tag’) ‘visible.’ This simplifies and accelerates the setup of a reading point. 

RSSI is a depiction of the signal strength received from tag responses. In the RSSI view of the tool the ID of the transponder is captured as well as the signal strengths which are shown live in a table following the start of a diagnostic reading. On the large bar and number display it is can clearly be seen, even from a distance, the effect on the signal of moving/swiveling the antenna. This direct feedback from the display allows best alignment of the antenna can be completed in a few minutes.

Setting radiant power 
For the lasting, robust operation of a reading point in an industrial environment, however, it is not enough to just find the optimal antenna position and alignment. The proper parameters for the read/write device and antennas also need to be determined, for example, the radiant power. As a major factor for a low-interference communication over the medium of air, the radiant power should be set to enable a reliable communication between transponder and reader, while not interfering with neighbouring reading points. 

With RF-DIAG and the PowerRamp algorithm of the RFID devices Simatic RF620R/RF630R, the minimum pull-in power necessary to detect a transponder can be automatically determined automatically which eliminates time-consuming trial and error. 

BecauSe the pull-in power for transponders depends on their position, orientation and manufacturing tolerances, it is actually not correct to talk about a single minimum pull-in power. It can only be used as a reference value. This is where the PowerRamp algorithm demonstrates its strengths. If activated, it will automatically adjust the power while the UHF system is in operation, starting from the configured minimum pull-in power, and if necessary, all the way to the maximum power.

What happens if, once the main parameters have been determined and the antennas aligned, yet the expected reading results do not materialize? In this case, the Simatic RFID readers intended for a PLC integration offer a set of different algorithms that ensures a reliable capture – even when faced with the special challenges of a production environment.

In addition to the PowerRamp algorithm, the RSSI-Threshold and Blacklist algorithms can help filter out unwanted tag readings which can be caused by overshooting or by tags already processed. With RF-DIAG, the reality always stays in view because all tags captured with the active parameters remain visible. Tags filtered out are separately marked. The effect of the special UHF algorithms can therefore be easily and directly followed.

There are many reasons for an RFID reading point to fail or the reading rate to decrease. The most common reason is that an antenna has been moved and needs to be realigned. However, other changes made to the RFID process can also cause undesirable effects – such as the use of another transponder type or switching the production line over to another product. 

The different graphical views of RF-DIAG allow users to quickly determine whether adjusting the filter mechanisms and device parameters or using a special algorithm can return the reading performance to the desired level. In addition, firmware updates or the reading and writing of RFID data round out the functionality.

Easy commissioning and maintenance 
With its different graphical views, RF-DIAG is able to visualise tag signals which facilitates the commissioning of reading points. The direct feedback by the tool provides support in determining the optimal parameters and suitable algorithms, which improve the long-term quality of the reading point. 

Error diagnosis is also made easy with a portrayal of the reality and guess work is not needed as it is possible to gain a variety of views of the RFID parameters and results provided by RF-DIAG. 

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