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Identification system for filling station on base of UHF RFID

11 июн 2013

Научная публикация нашего сотрудника - Ларистова Д.А., к.т.н.

Abstract—An identification system for diesel locomotives filling stations on base of passive UHF RFID is presented. Construction of a reader antenna and tags location for proper installation of a filling pistol in a fuel tank is proposed. The proposed system allows control of fueling operation and prevents theft of fuel. Theoretic calculations and experimental results are presented.

Keywords—automatic filling station; loop antenna; UHF RFID.

I. INTRODUCTION

Radio Frequency Identification (RFID) is in active development stage of new applications in many areas of our today life. One of key factors for such rapid development of RFID systems is possibility of contactless exchange of data between a RFID tag, placed on an identifiable object, and a reader – a device that reads information about an object from memory of the RFID tag [1, 2]. This possibility allows using RFID systems for building of automatic identification systems that increase speed and accuracy of information processing in production and delivery of goods and services and also prevent theft.

Advantages of contactless identification are also used in fueling systems. There are several patents, describing using of contactless identification technology for fueling systems [3 - 7], and there are also commercial systems using RFID for automation of gas stations [8]. Such systems allow identifying a car, determine required type of fuel and make automatic payment for fueling. But they don’t decide problem of fuel theft during filling process. With this challenge we faced in our practice. Below a structure of an automatic filling station for diesel locomotives using UHF RFID is described. The main attention is paid to a sensor for a fuel tank and a reader antenna for a refueling nozzle. Proposed designs ensure installation of a refueling nozzle in an identifiable refillable fuel tank and prevent unauthorized discharge of fuel during refueling.


II. GENERAL PRINCIPLES OF THE SYSTEM WORK 

The basic integration scheme of RFID system with filling station control system is shown on Fig. 1. 

In proposed identification scheme special designs of a sensor with passive RFID tags and a reader antenna are used. The sensor is fixed on a fuel tank neck and the reader antenna is placed on a refueling nozzle. 

The integration scheme with filling station control system
Fig.1. The integration scheme with filling station control system

Turning on of a filling station is possible only during simultaneously registration of all tags in the sensor. Using described below designs of the reader antenna and the sensor with RFID tags registration of all tags is possible only with normal location a refueling nozzle in a fuel tank and is not possible with any other locations of a refueling nozzle near a fuel tank. Thereby fuel theft is excluded. In addition, identifying of known and registered in the system tags in the sensors on the neck of fuel tank allows for reliable accounting refueling operations.


UHF RFID EPC Class1 Gen2/ISO 18000-6B tags and a reader are used in the system. In the pilot project Impinj R420 RFID reader was used [9]. This reader has 4 antennas ports. So, up to 4 refueling nozzles can be connected to one reader.


Proposed scheme works as follow:

1) A fuel tank of a diesel locomotive is identified in time of installing of a refueling nozzle;
2) A refueling nozzle is identified by the reader serial number and serial number of the refueling nozzle connected to the defined reader;
3) Data are transmitted to filling station control system via Ethernet or Wi-Fi;
4) Identification number of the fuel tank is verified in filling station control system. Then command is sent for fuel feeding of required type and quantity through the refueling nozzle.

As a result of work using this scheme the user receives the following benefits:

• Automatic identification of a fuel tank (a diesel locomotive), quantity of fuel, location of a filling station and time of filling.
• Protection against theft of fuel.

Automatic identification of elements of the filling system is easily done by processing software data from the reader. Less trivial task is developing of the reader antenna design and the sensor for a fuel tank neck that allow identify proper installation a refueling nozzle to a fuel tank. Such constructions were developed and are presented in the following sections.


III. DESIGN OF THE SENSOR AND THE READER ANTENNA

A. The main principles of interaction

For proper detection of mutual position of the reader antenna, placed on a refueling nozzle, and tags in the sensor, placed on a fuel tank neck, inductive interaction between antennas in the near field is used. Loop antennas are most suitable for this type of interaction. Therefore we used small loop tags and a loop antenna design for the reader in our project.

The main idea of the construction which identifies correct installation of a refueling nozzle into a fuel tank is the following. The sensor case that fastened on a fuel tank neck has a structure in form of torus with flat faces and four hollows for the tags (Fig. 2). Small loop tags are placed in the hollows on orthogonal axes of the case perimeter. Then the hollows are sealed. The reader antenna is placed in a case of similar design (Fig. 3) that fastened in a refueling nozzle. When the refueling nozzle is proper installed in the fuel tank the reader antenna and the tags are placed in parallel planes, which is optimal placement for interaction between them. Read range of the tags depends on chip sensitivity, size of the tags and the reader antenna as well as output power of the reader. For specific applications this parameter can be varied by output power of the reader. 

B. The reader antenna design 
The reader antenna is designed as a printed loop antenna on FR-4 substrate (εr = 4.7, tgδ = 0.025) with 1 mm thickness (Fig. 4). Overall dimensions of the antenna were determined by the refueling nozzle construction for diesel locomotives. The antenna circuit is placed close to outer boundaries of the substrate to maximize distance from metal parts of the refueling nozzle. 

To maintain the same read range of all tags of the sensor above the antenna circuit, uniform magnetic field distribution is needed. This requires uniform current distribution along the loop antenna contour. For loop antennas with electrically large perimeter this condition can be satisfied by incorporating blocking capacitors into the antenna structure and thus segmenting the loop contour on electrically small parts [10]. For convenience of value selection of the segmented capacitors and for reducing of numbers of operations during the antenna assembly the segmented capacitors were implemented in planar form (Fig. 4). 

Design of the sensor case for RFID tags
Fig. 2. Design of the sensor case for RFID tags (dimensions in mm)

 

Design of the reader antenna case

Fig. 3. Design of the reader antenna case (dimensions in mm)

 

The reader antenna design

Fig. 4. The reader antenna design (dimensions in mm)

 

For initial calculation dimensions of the antenna elements analytical formulas from [11] were used. Subsequent refined calculations with influence of metal parts of the refueling nozzle and the fuel tank were carried out using finite element method (FEM). On Fig. 5 the antenna model with surrounding metal parts is shown. To reduce calculation time only one part of the symmetrical model divided by perfect electric boundary are used in calculation.

The calculations model of the loop antenna

Fig. 5. The calculations model of the loop antenna (dimensions in mm)

 

Calculated input impedance of the loop antenna is shown on Fig. 6. For reducing of the antenna Q-factor and widening of frequency range an 80 Ohm resistor is in-series in the antenna circuit (see Fig. 4 and Fig. 6). It should be noted that the analytical model gives comparable with the FEM model results from point of view of resonant frequency. Reason of observed difference in active resistance may be the fact that only influence of the metal plate under the antenna board is taken into account in analytical calculation. For matching high input resistance of the loop antenna with 50-Ohm coax line LC-balun was used [12]. 

Calculated input impedance of the loop antenna

Fig. 6. Calculated input impedance of the loop antenna

 

Fig. 7 and Fig. 8 show current distribution along the loop antenna contour and magnetic field in the near-field of the antenna. These diagrams indicate that the loop antenna provides uniform distribution of magnetic field above the antenna plane. Thus we can expect that all tags placed in the near-field over the antenna will be read at the same power of the reader.

Amplitude of surface current on the loop conductors
Fig. 7. Amplitude of surface current on the loop conductors

 

Magnetic field distribution in the near-field of the loop antenna

Fig. 8. Magnetic field distribution in the near-field of the loop antenna


IV. EXPERIMENTAL RESULTS 

A. Measurements of prototypes 
Before installing the tags and the antenna in sealed cases, measurements of the input antenna parameters and the tag read range were made. 

Measurement of reflection coefficient at the antenna input was carried out using network analyzer OBZOR-103 (Planar). Location of the antenna prototype is shown on Fig. 9. Fig. 10 shows measurement results of reflection coefficient for different distance h between the antenna board and the metal plate. Comparison of measured results with calculation data is given on the same chart. Presented results demonstrate significant dependence of reflection coefficient from height h of the antenna board above the metal plate. Calculation model provide close similarity of the obtained characteristics in terms of the resonant frequency. But for obtaining acceptable reflection coefficient experimental tuning required. For further testing the antenna board was located above the metal plate at height h = 10 mm. 

Measurement of the loop antenna prototype
Fig. 9. Measurement of the loop antenna prototype

 

Reflection coefficient at the antenna input
Fig. 10. Reflection coefficient at the antenna input

For estimation of distribution uniformity of magnetic field in the antennas near-field the loop tags were placed on a round metal surface on fixed distance 35 mm over the antenna. The tags were placed on orthogonal axes of the metal surface with 10 mm dielectric layer for minimizing influence of metal on the tags characteristics (Fig. 11). 

Measurement setup
Fig. 11. Measurement setup

During measurements the metal surface with the tags was rotated around central axis with 90° step. On each step the minimum output power of the reader (Impinj R420) at which the tags answered was measured. Three different types of near-field tags were used: Lab ID UH180 (Monza 3), UPM Trap (Monza 3) and TagItron RF-Loop Tag (Monza 4). The measurement results are presented on Table 1. These data indirectly indicate that non-uniformity of the antenna field in the tags reading zone is no more than 3 dB. It should also be noted that RF-Loop Tags have highest sensitivity. Therefore this type of tags been used for the sensor later. 

Additional tests were carried out to check the tags registration in different incorrect positions of the refueling nozzle relative to the fuel tank neck (that is in other possible positions when planes of the antenna and the tags are not parallel). This experiment shown that in case of using only 2 tags there are positions where both tags are read. To except this situation we decided to increase number of tags in the sensor to 4.

B. Measurements of the antenna and the sensor in real enviroment

In the final part of testing the antenna and the sensor with the tags were placed in leakproof polyurethane cases.

On Fig. 12 the antenna on the refueling nozzle and the sensor on the fuel tank neck in real environment are presented. The antenna was connected to the reader with 10 meter RG-58 coax cable installed in metal protective braiding for explosion protection. 

Measurements in real enviroment
Fig. 12. Measurements in real enviroment

During the experiment distance between the reader antenna and the sensor with RFID tags was varied from 28 mm to 32 mm. This is minimal and maximal possible distance between these elements for different depth position of the refueling nozzle in the fuel tank (the refueling nozzle is twisted in the fuel tank neck). The measurements showed that within the specified distance limits all tags in the sensor are registered with 30 dBm output power of the reader. Thereby power of the reader was increased approximately by 15 dBm in comparison with the first measurements of the antenna prototype. The reason for this can be losses in the long coax cable (approximately 5.5 dB on 10 meters), change input impedance of the antenna placed in the polyurethane case and also degradation of the tag characteristics in the sensor case. Further study for clarification of cases influence on the antenna and the tags performance and minimization of this effect is needed.


V. CONCLUSIONS

Constructions of the sensor and the reader antenna for automatic identification system on base of UHF RFID for diesel locomotives filling stations are proposed. Presented results show that using of the proposed constructions allows identifying correct installation of a refueling nozzle to a fuel tank of diesel locomotive and prevent theft of fuel.

The proposed constructions are adapted for fuel tanks and refueling nozzles of diesel locomotives. But in case of need they could be modified for using on filling stations for another type of transport.


REFERENCES

[1] K. Finkenzeller, “RFID Handbook. Fundamentals and Applications in Contactless Smart Cards and Identification,” 2nd ed. John Wiley & Sons, 2003.
[2] D. M. Dobkin, “The RF in RFID: passive UHF RFID in practice.” Elsevier Inc., 2008. 
[3] B. G. Bates, “Communications system and method, fleet management system and method, and method of implementing theft of fuel,” U.S. Patent 6 024 142, Feb. 15, 2000. 
[4] D. Kelerich, D. Talmor,”Fueling system,” U. S. Patent 5 857 501, Jan. 12, 1999. 
[5] R. Pinkus, “Automatic payment method using RFID tags,” U. S. Patent 7 565 307, Jul. 21, 2009. 
[6] R. G. Barrett,”Fuel delivery information system,” U. S. Patent 2011/0035049 A1, Feb. 10, 2011. 
[7] S. Inhoffer, “Method and system for preventing misfueling,” U. S. Patent 2009/0315729 A1, Dec. 24, 2009. 
[8] Fuelomat Gold – Vehicle identification system [Online]. Available: http://www.orpak.com/index.php?option=com_content&view=article&id=175&Itemid=41 
[9] Speedway® Revolution UHF RFID reader [Online]. Available: http://www.impinj.com/Speedway_Revolution_UHF_RFID_Reader.aspx 
[10] D. M. Dobkin, S. M. Weigand and N. Iye, “Segmented Magnetic Antennas for Near-field UHF RFID,” Microwave Journal, vol. 50, No.6, June 2007. 
[11] A. L. Popov, and O. G. Vendik, “A Loop Antenna Model for UHF RFID Systems,” IEEE 2012 International Conference on RFID – Technologies and Applications, pp. 113 – 116. 
[12] “AVR2004: LC-Balun for AT86RF230,” Atmel Application Note, July 2004.

Автор: Ларистов Дмитрий Александрович, к.т.н.

 

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