Case study: Using RFID for asset tagging

Dr. Robert Oberle, RCD Technology - July 14, 2010


Introduction

Many financial institutions have been retrofitting data centers with Radio Frequency Identification (RFID) systems and replacing barcode labels on their IT assets with passive RFID tags1,2.  This is in response to more demanding regulatory requirements mandated by the implementation of the Sarbanes-Oxley Act. Since the successful implementations of RFID in the financial sector, more companies are starting to employ passive RFID tags to better track and account for their IT assets.

As more companies employ passive RFID tags, they turn to IT Equipment OEM’ and request that RFID tags are place on assets at the point of manufacture. OEMs that take on “source tagging” initiatives are agreeing to adhere to the Financial Services Technology Consortium (FSTC) standard for tag performance and data content3. The standard requires that the RFID tag be read at 2-meters using a fixed RFID reader in all of the three major UHF reader bands; 865- 868 MHz in the European Union, 902 – 928 MHz in North America and 952 – 954 MHz in Japan. This requirement is driven by the diversity of frequency bands utilized for UHF RFID around the world, Figure 1. Ideally, a suitable tag would exhibit a uniform RF sensitivity response across a 100MHz bandwidth, 860 -960MHz.


Figure 1: Figure 1: UHF frequency bands utilized for ISO 18000-6C tags world-wide
(Click on image to enlarge)

Unlike RFID tags designed for application to cardboard boxes and packaging materials, IT asset tags are more challenging to design.  This is due to their size, RF performance and durability requirements. IT asset tags must be small in size, relative to widely deployed RFID tags, due to the footprint available for placement of the tags on the assets (often blade servers). The chassis is steel with the attendant RF interference considerations.  In addition, the IT asset tags must be durable enough to handle significant stresses that occur during moving or installation of the asset.

  Design Process

RCD Technology addresses all new applications through our flexible design process, which consists of assembling all known design constraints and performance criteria at the beginning of the development effort.  We then develop a design that meets those criteria and is compatible with our established production processes and design rules.

The major components of the functional requirement are summarized below.

·         The tag must function using the industry standard ISO 18000-6C passive tag protocol. The read range is ideally 9.5 feet and must be a minimum of 6.5 feet (2 meters) when read by an RFID reader with an output of 33dBm ERP across the frequency range 860 – 960 MHz.

·         Since most of the IT assets tagged are blade servers, which are closely spaced in a rack, the application requires a small tag; ideally with a footprint of no more than 1 square inch and an overall length of no more than 1.5 inches. Additionally, the tag cannot block air ventilation through the device where space is already at a premium. The allowable footprint is approximately one third that of the widely used dipole tag.  

·         The tag must function when mounted on a metal chassis, a feature that is impossible for standard dipole tags.

·         While the operating environment is fairly benign in terms of temperature and humidity, the tag must be durable enough to survive an impact that might occur during installation of the asset.

·         The data identifying the tagged asset is stored in the tag’s EEPROM, and a back up copy of that data in the form of a human readable number and a 2D barcode must be printed on a label on the exposed face of the tag.

·         Installation of the tag, whether on the deployment site or at the OEM must be done without the use of mechanical fasteners that would pierce the chassis. The preferred approach is the use of a durable pressure sensitive adhesive to secure the tag.

The functional requirements of the tag’s working environment and the use case guide the design process for the final object.  This requires an approach different from what has been typically used in RFID deployments, such as those for inventory tracking. Subsequently, the widely used dipole tags, shown in Figure 2, which exhibit exemplary performance on cardboard packaging materials, are not suitable.


Figure 2: Dipole RFID tag from Alien Technology Inc. Dipole length end-to-end is approximately 3.75 inches

An RFID tag is generally a single chip device in which the antenna is both a source of power for the chip and the communication antenna. As such, the ability to harvest energy from the incident reader signal is of paramount importance since the communication link signal budget is dominated by the requirement to power the chip first. The match between antenna and chip is critical and this is heavily influenced by the external loading of the antenna by objects in close proximity to the tag. Hence, the design of a mount on metal tag must take into careful consideration the presence of the metal mounting surface and the attachment mechanism.

In a departure from widely used RFID tag designs, RCD Technology fabricated a resonant structure which takes advantage of the metallic substrate to which the tag is attached, Figure 3.

The structure, Figure 3, with the FEM result S11 plot of Figure 4 using the simulation model of Figure 4a, is essentially a three-dimensional resonant cavity with a tuning structure on the free-air surface of the tag.


Figure 3: The final RFID tag


Figure 4: Finite Element Method (FEM) simulation plot of tag return loss, S11, of tag mounted on a conductive surface.
(Click on image to enlarge)


Figure 4a: The model tag used for the FEM simulation .

Connections between the conductive surfaces are made using conductive through-holes near the corners of the cavity. The resonant cavity couples with the metallic substrate to which it attached, though no direct connection is made between the tag and the substrate. The conformation of the folded elements on the free air face allows the center frequency of the tag to shift to maximize the response at the desired frequency.

Given the single chip design and the lack of an available matching network, geometry variation is the only available tool for manipulating the center frequency and bandwidth of the antenna. Finite element modeling software was used to simulate the full EM field solution in the vicinity of the tag, as well as plot the current distribution on the conductive surface and investigate the effect of geometric variation of the antenna elements relative to the response of the tag. Considerable effort was spent in variational analysis to insure that the final design was as stable as possible with respect to anticipated fabrication and attachment variability. Among the parameters considered were the positioning of the through-holes, the length and width of tuning elements on the free air face of the tag, thickness of the dielectric and variation of the dielectric constant (real and imaginary components).

Results and Validation

Design validation was performed in an anechoic chamber. Tags were mounted to a stainless steel sheet suspended in the chamber. The tag sensitivity was determined by variation of the incident power transmitted to the tag and monitoring the minimum incident power required for the tag to respond at frequencies that spanned the requirement; EU 865 – 867 MHz, US 902 – 928 MHz, and Japan 950 – 956 MHz,. All measurements were made using the EPC communication protocol, that is, with the tag actively communicating with the RFID reader.

Measurements were made with a fixed reader antenna, on axis and with the tag and substrate rotated on vertical and horizontal axes parallel to the substrate surface. Typical tag read ranges are 7-feet in the EU band, 12 – 14 feet in the US band and 7 – 8 feet in the Japanese band, all are referenced to reader output power of 33dBm ERP.

The final design is a tag 36mm x 10mm x 8mm affixed to the metallic surface of the chassis using a high reliability pressure sensitive adhesive. The active component is a single packaged passive UHF EPC class 1 Gen 2 IC from Alien Technologies, Inc. This chip was chosen to be the best impedance match for the antenna and to meet the data requirements of the FSTC specification. The packaging of the chip and the overall construction result in a highly robust RFID tag that tolerates thermal shock4 and physical abuse5 in a small package.

Sun Microsystems6 is first to implement this new addition to RCD’s Sentry Family of asset tags, Sentry-M (WW). This patent pending, asset tracking tag is unique in that maintains significant read range across the worldwide UHF frequency band and can withstand significant mechanical stress. Placing this tag on IT Assets at the point of manufacture not only meets customer demands but improves OEMs’ internal supply chain management.  Additional applications for the tag format and performance are found in the aerospace, defense, automotive and asset management industries where small, robust, mount-on-metal asset tracking tags that can be permanently and reliably affixed to an asset are required.

 

Conclusion

Sentry – M (WW) was developed by understanding OEMs’ needs and their use case scenarios. As demonstrated above, these tags specifications were determined by its use case and the environment the tag was going to operate in. With the worldwide tag gaining more popularity, new applications are starting to emerge and it is an RFID supplier’s job to understand these new applications and develop tags that meet specifications that emerge from these new applications.

Footnotes and References

  1. “Bank of America Deploys RFID in Data Centers”, http://www.rfidjournal.com/article/view/4426 .
  2. “Wells Fargo Banks on RFID”, http://www.rfidjournal.com/article/view/4456/1/1/.
  3. http://www.fstc.org/includes/download_file.php?f=FSTC - RFID Basic Functional Requirements - Data Center Assets - Final.pdf
  4. Thermal shock measurements -40 to 150C; isothermal dwell time 20 minutes, transition time < 1 minute
  5. As determined by a steel Ball Drop test, 1” diameter steel ball dropped from 24 inches directly on the chip area of the tag.
  6. Sun Microsystems was acquired by Oracle Corporation in early 2010; http://www.oracle.com/us/corporate/press/044428

About the author


Dr. Robert Oberle founded RCD Technology in 2000 and presently serves as Chief Technology Officer and as a member of its Board of Directors. Dr. Oberle is responsible for the design and development of RCD’s products and processes to meet the needs of its diverse customer base. He holds five issued US patents and numerous foreign patents as well as numerous patents pending in the US and internationally. Dr. Oberle has spoken at numerous conferences on the subject of RFID tag fabrication and assembly.

Prior to founding RCD Technology, Dr. Oberle held positions in Marketing and Product Development with Engelhard Corporation, now part of BASF GmbH, and Research and Development at Enthone Corporation, a subsidiary of Cookson Electronics. Bob holds a Ph.D. and MS in Materials Science from The Johns Hopkins University and a BS in Physics from Rensselaer Polytechnic Institute.


Introduction

Many financial institutions have been retrofitting data centers with Radio Frequency Identification (RFID) systems and replacing barcode labels on their IT assets with passive RFID tags1,2.  This is in response to more demanding regulatory requirements mandated by the implementation of the Sarbanes-Oxley Act. Since the successful implementations of RFID in the financial sector, more companies are starting to employ passive RFID tags to better track and account for their IT assets.

 

As more companies employ passive RFID tags, they turn to IT Equipment OEM’ and request that RFID tags are place on assets at the point of manufacture. OEMs that take on “source tagging” initiatives are agreeing to adhere to the Financial Services Technology Consortium (FSTC) standard for tag performance and data content3. The standard requires that the RFID tag be read at 2-meters using a fixed RFID reader in all of the three major UHF reader bands; 865- 868 MHz in the European Union, 902 – 928 MHz in North America and 952 – 954 MHz in Japan. This requirement is driven by the diversity of frequency bands utilized for UHF RFID around the world, Figure 1. Ideally, a suitable tag would exhibit a uniform RF sensitivity response across a 100MHz bandwidth, 860 -960MHz.


Figure 1: Figure 1: UHF frequency bands utilized for ISO 18000-6C tags world-wide
(Click on image to enlarge)

Unlike RFID tags designed for application to cardboard boxes and packaging materials, IT asset tags are more challenging to design.  This is due to their size, RF performance and durability requirements. IT asset tags must be small in size, relative to widely deployed RFID tags, due to the footprint available for placement of the tags on the assets (often blade servers). The chassis is steel with the attendant RF interference considerations.  In addition, the IT asset tags must be durable enough to handle significant stresses that occur during moving or installation of the asset.

  Design Process

RCD Technology addresses all new applications through our flexible design process, which consists of assembling all known design constraints and performance criteria at the beginning of the development effort.  We then develop a design that meets those criteria and is compatible with our established production processes and design rules.

The major components of the functional requirement are summarized below.

·         The tag must function using the industry standard ISO 18000-6C passive tag protocol. The read range is ideally 9.5 feet and must be a minimum of 6.5 feet (2 meters) when read by an RFID reader with an output of 33dBm ERP across the frequency range 860 – 960 MHz.

·         Since most of the IT assets tagged are blade servers, which are closely spaced in a rack, the application requires a small tag; ideally with a footprint of no more than 1 square inch and an overall length of no more than 1.5 inches. Additionally, the tag cannot block air ventilation through the device where space is already at a premium. The allowable footprint is approximately one third that of the widely used dipole tag.  

·         The tag must function when mounted on a metal chassis, a feature that is impossible for standard dipole tags.

·         While the operating environment is fairly benign in terms of temperature and humidity, the tag must be durable enough to survive an impact that might occur during installation of the asset.

·         The data identifying the tagged asset is stored in the tag’s EEPROM, and a back up copy of that data in the form of a human readable number and a 2D barcode must be printed on a label on the exposed face of the tag.

·         Installation of the tag, whether on the deployment site or at the OEM must be done without the use of mechanical fasteners that would pierce the chassis. The preferred approach is the use of a durable pressure sensitive adhesive to secure the tag.

The functional requirements of the tag’s working environment and the use case guide the design process for the final object.  This requires an approach different from what has been typically used in RFID deployments, such as those for inventory tracking. Subsequently, the widely used dipole tags, shown in Figure 2, which exhibit exemplary performance on cardboard packaging materials, are not suitable.


Figure 2: Dipole RFID tag from Alien Technology Inc. Dipole length end-to-end is approximately 3.75 inches
(Click on image to enlarge)

Figure 2: Dipole RFID tag from Alien Technology Inc. Dipole length end-to-end is approximately 3.75 inches

An RFID tag is generally a single chip device in which the antenna is both a source of power for the chip and the communication antenna. As such, the ability to harvest energy from the incident reader signal is of paramount importance since the communication link signal budget is dominated by the requirement to power the chip first. The match between antenna and chip is critical and this is heavily influenced by the external loading of the antenna by objects in close proximity to the tag. Hence, the design of a mount on metal tag must take into careful consideration the presence of the metal mounting surface and the attachment mechanism.

In a departure from widely used RFID tag designs, RCD Technology fabricated a resonant structure which takes advantage of the metallic substrate to which the tag is attached, Figure 3.

The structure, Figure 3, with the FEM result S11 plot of Figure 4 using the simulation model of Figure 4a, is essentially a three-dimensional resonant cavity with a tuning structure on the free-air surface of the tag.


Figure 3: The final RFID tag
(Click on image to enlarge)


Figure 4: Finite Element Method (FEM) simulation plot of tag return loss, S11, of tag mounted on a conductive surface.
(Click on image to enlarge)


Figure 4a: The model tag used for the FEM simulation .
(Click on image to enlarge)

Connections between the conductive surfaces are made using conductive through-holes near the corners of the cavity. The resonant cavity couples with the metallic substrate to which it attached, though no direct connection is made between the tag and the substrate. The conformation of the folded elements on the free air face allows the center frequency of the tag to shift to maximize the response at the desired frequency.

Given the single chip design and the lack of an available matching network, geometry variation is the only available tool for manipulating the center frequency and bandwidth of the antenna. Finite element modeling software was used to simulate the full EM field solution in the vicinity of the tag, as well as plot the current distribution on the conductive surface and investigate the effect of geometric variation of the antenna elements relative to the response of the tag. Considerable effort was spent in variational analysis to insure that the final design was as stable as possible with respect to anticipated fabrication and attachment variability. Among the parameters considered were the positioning of the through-holes, the length and width of tuning elements on the free air face of the tag, thickness of the dielectric and variation of the dielectric constant (real and imaginary components).

Results and Validation

Design validation was performed in an anechoic chamber. Tags were mounted to a stainless steel sheet suspended in the chamber. The tag sensitivity was determined by variation of the incident power transmitted to the tag and monitoring the minimum incident power required for the tag to respond at frequencies that spanned the requirement; EU 865 – 867 MHz, US 902 – 928 MHz, and Japan 950 – 956 MHz,. All measurements were made using the EPC communication protocol, that is, with the tag actively communicating with the RFID reader.

Measurements were made with a fixed reader antenna, on axis and with the tag and substrate rotated on vertical and horizontal axes parallel to the substrate surface. Typical tag read ranges are 7-feet in the EU band, 12 – 14 feet in the US band and 7 – 8 feet in the Japanese band, all are referenced to reader output power of 33dBm ERP.

The final design is a tag 36mm x 10mm x 8mm affixed to the metallic surface of the chassis using a high reliability pressure sensitive adhesive. The active component is a single packaged passive UHF EPC class 1 Gen 2 IC from Alien Technologies, Inc. This chip was chosen to be the best impedance match for the antenna and to meet the data requirements of the FSTC specification. The packaging of the chip and the overall construction result in a highly robust RFID tag that tolerates thermal shock4 and physical abuse5 in a small package.

Sun Microsystems6 is first to implement this new addition to RCD’s Sentry Family of asset tags, Sentry-M (WW). This patent pending, asset tracking tag is unique in that maintains significant read range across the worldwide UHF frequency band and can withstand significant mechanical stress. Placing this tag on IT Assets at the point of manufacture not only meets customer demands but improves OEMs’ internal supply chain management.  Additional applications for the tag format and performance are found in the aerospace, defense, automotive and asset management industries where small, robust, mount-on-metal asset tracking tags that can be permanently and reliably affixed to an asset are required.

 

Conclusion

Sentry – M (WW) was developed by understanding OEMs’ needs and their use case scenarios. As demonstrated above, these tags specifications were determined by its use case and the environment the tag was going to operate in. With the worldwide tag gaining more popularity, new applications are starting to emerge and it is an RFID supplier’s job to understand these new applications and develop tags that meet specifications that emerge from these new applications.

Footnotes and References

  1. “Bank of America Deploys RFID in Data Centers”, http://www.rfidjournal.com/article/view/4426 .
  2. “Wells Fargo Banks on RFID”, http://www.rfidjournal.com/article/view/4456/1/1/.
  3. http://www.fstc.org/includes/download_file.php?f=FSTC - RFID Basic Functional Requirements - Data Center Assets - Final.pdf
  4. Thermal shock measurements -40 to 150C; isothermal dwell time 20 minutes, transition time < 1 minute
  5. As determined by a steel Ball Drop test, 1” diameter steel ball dropped from 24 inches directly on the chip area of the tag.
  6. Sun Microsystems was acquired by Oracle Corporation in early 2010; http://www.oracle.com/us/corporate/press/044428

About the author





Dr. Robert Oberle founded RCD Technology in 2000 and presently serves as Chief Technology Officer and as a member of its Board of Directors. Dr. Oberle is responsible for the design and development of RCD’s products and processes to meet the needs of its diverse customer base. He holds five issued US patents and numerous foreign patents as well as numerous patents pending in the US and internationally. Dr. Oberle has spoken at numerous conferences on the subject of RFID tag fabrication and assembly.

Prior to founding RCD Technology, Dr. Oberle held positions in Marketing and Product Development with Engelhard Corporation, now part of BASF GmbH, and Research and Development at Enthone Corporation, a subsidiary of Cookson Electronics. Bob holds a Ph.D. and MS in Materials Science from The Johns Hopkins University and a BS in Physics from Rensselaer Polytechnic Institute.