CC BY
3Panasenko S.P.
Vice Director for Research & Integration, ANCUD Ltd., Moscow, Russia
Syrchin V.K.
Leading Specialist, ANCUD Ltd., Moscow, Russia & Professor, National Research University of Electronic
Technology, Moscow, Russia
Zaytsev D.S.
Senior Engineer, ANCUD Ltd., Moscow, Russia
APPROACHES TO RFID READER CONSTRUCTION FOR SPECIFIC USE IN ACCESS CONTROL SYSTEMS
ABSTRACT
Currently we can see wide use and continuous development of access control systems that allow restriction or management of physical access to protected areas. Many of such systems are based on RFID tags as the primary or sole factor of user authentication. The use of RFID technology in access control systems imposes various specific requirements both to RFID tags and to readers communicating with them. This paper discusses approaches to the RFID reader development in order to meet these requirements.
Keywords: access control, radio-frequency identification (RFID), RFID reader, RFID tag, reader antenna.
1. Introduction
RFID tags are used widely in a variety of access control systems. Most, if not all, of RFID tags types can be used in such systems for user identification, for cryptographic keys and authentication data storage and so on. Examples of RFID tags applications in access control systems include the following:
- serial numbers in low-end RFID tags can be used in physical access control systems;
- high-end RFID tags can store keys for strong user authentication to provide authorized user access for computer resources.
Reliability and efficiency of RFID systems depend not only on the characteristics of RFID tags, but also on a reader that provides a radio frequency signal exchange with tags in a predetermined frequency range. The reader may have one built-in antenna or several connected antennas (a multichannel reader), each of them operates at a predetermined reading area.
Despite the fact that there are a lot or RFID readers in the global market, sometimes it is required to design a custom reader with specific characteristics and dimensions for its subsequent use in access control systems. This paper proposes a short report about our experience in the development of an RFID reader that meets the specific requirements.
dF
2. Reader Antenna Modeling
Antenna systems designing is based on electromagnetic theory that describes the relationships between a conductor, through which the electric current flows, and a magnetic field generated around the conductor. In general, the antenna is a conductor having a loop shape with a radius r and with the electric current I inside. For such configuration a magnetic flux density of the generated magnetic field Bf is defined by the formula (for an axis passing through the center of the loop perpendicularly to its plane):
Bf =
Vol Nr2
-,Wb/m2
(1)
2( r2+ a2)2
where fioo is the magnetic constant (H/m);
I - an electric current (A);
a - a distance from the center of the loop to a predetermined point on the axis (m), for which a magnetic field is to be calculated.
To calculate a generated magnetic flux Bf value should be multiplied by the loop's area.
While an RFID reader operates, an electromagnetic interference occurs between antennas of the reader and a RFID tag, i. e. between two loop conductors. In accordance to Faraday's law, magnetic flux F, generated by the reader antenna, induces a voltage U in the tag antenna; the voltage U is determined by the number of loops (turns) N in the receiving antenna and by variation of the magnetic flux, generated by the radiating antenna in time:
U = -N2^=-N2-(jBrdS) = -N2-(J
Voh r-2
3
2( r12+ a2)2
dS
(2)
or
U = —M — =
dt
where N, N2 and n, r2 - numbers of turns and their radii (m) in the reader and tag antennas respectively;
S - area of the tag antenna, which a magnetic flux, generated by the reader antenna, flows through (m2); a - distance between antennas (m); Ii - current in the reader antenna (A);
VpnNi N2 (riT2)2 dli --(3)
2( ri2+ a2)2 M
M - coefficient of mutual induction between antennas depending on the geometry and the relative position of antennas (H).
For a multi-turn antenna the voltage induced in it can be determined by the formula:
U0 = 2nfNSQB0cosa, (4)
where f is the radiating antenna signal frequency (Hz);
N - the number of turns in the receiving antenna; Q - the quality factor of the receiving antenna; Bo - the magnetic induction flux density of the radiating antenna signal (Wb/m2);
a - the angle between the flow direction and the receiving antenna plane.
By fixing the voltage required for the tag power supply we can define the sufficient Bo level for the reader antenna from (4); then we can calculate the required number of ampere-turns in it from (1). For example, let us consider a contactless card with standard dimensions 85.6 x 54 x 0.76 mm, the working frequency of 13.56 MHz, power voltage U = 4 V, and 4-turn antenna with the quality factor Q = 40. The reader antenna, oriented in parallel to the card, should create in the card the field Bo = 0.045 ^Wb/m2 approximately.
To generate the required level of magnetic flux the reader antenna with the size n = 5 cm and the communication distance of 10 cm should have about (N1I1) = 0.08 ampere-turns, i. e. it should provide the current of 80 mA for a single turn design or 20 mA for 4-turn respectively.
Another important parameter of an antenna is its inductance, i. e. the coefficient of proportionality between a magnetic flux and an electric current that flows through the antenna. It depends on antenna shape and size and on the cross-sectional area of the antenna conductor. In the considered case, the antennas of the reader and the contactless smart card are rectangular flat spirals of a film conductor having rectangular cross-section of width b and thickness h. For this conductor with length l the inductance is calculated by the following formula:
L = 0,002 {ln + 0,50049 + ^H
where the values b, h, and l are used in cm. The antenna design can be represented as a set of such rectangular segments, where each antenna turn consists of four segments. Any adjacent turns are located at a distance 5, measured between centers of the conductors. The full inductance Lp of the planar antenna contour is the difference between the sum of own inF = ln
(5)
ductances of all the antenna segments and the coefficients of mutual induction of the segments M+ (when the current in the segments flows in one direction) and M- (the current flows in opposite directions), which are determined by the formula:
M = 2ZF (nH),
where
= 'n{© + Mâ
1/2) / 2 1/2
I- l + (- D. +
(Î).
(6)
l and 5 values are used in cm in (6). The design scheme for two parallel segments of different lengths for multi-turn spiral antenna is shown on Figure 1.
MiM = + - (MP +
= + Mfc) -
= 1{(M7 + Mfc) - Mp)
M
2
= M
fc+p Mp
%fc = Mfc for p = q = 0 The antenna to be designed consists of 16 segments, therefore, to determine the inductance Lp of its contour it is required to use the formulae (5) to (7) to
Depending on the ratio of the dimensions and relative placement of the segments, one of the following formulae is used for the calculation:
(7)
for p = 0 (7, a)
for q = 0 (7, b)
for p = q (7, c)
(7, d)
calculate their own induction and to calculate 24 M+ and 32 M- coefficients to determine the mutual inductance.
h
j
i
Ô
p 1
k
i k
Figure 1 - The geometric model of inductive interaction between the segments of antenna
2
3. RFID Reader Development Process
According to the technical requirements, the reader being designed should communicate with cards of the ISO/IEC 14443A standard [4]. It should be designed as a compact device with an individual antenna.
The reader conceivably should be located somewhere near the computer that it is connected to, or embedded directly in the computer system unit. In the latter case, the reader antenna can be shielded by any metal block and/or placed in the vicinity of metal components that may cause a strong noise or reduce reliability of the communication channel.
After analyzing the existing market offers of chips on the market with ISO/IEC 14443A support, we selected the TRF7960 chip from Texas Instruments as the main element of the reader.
To increase the working distance of the reader it is desirable to use an antenna with circular polarization; two variants of reader antenna placement were possible:
- inside the metallic case of the device; a technological hole (or a non-shielding cap) had to be provided in this variant for communicating with cards (in the presumable direction of communication);
- directly on the device case.
The main requirement was to provide a stable working channel to exchange with RFID tags. 80 percent of successful communications was defined as a sufficient level of stability according to the technical requirements.
When designing the reader antenna we decided to rely on available research results and recommendations. So we used Application Notes from Texas Instruments and Philips companies. The device case dimensions and construction (see Figure 2, a) defined the initial size for the antenna.
The antenna topology was formed as a set of rectangles with the given diagonal d (see Figure 2, b) in accordance to the recommendations of Texas Instruments [5].
a) b)
Figure 2 - (a) Initial dimensions of the reader and (b) initial topology of the reader antenna
The given diagonal was d = 38.8 mm; it provided 2d = 77.6 mm communication distance, which was considered enough. The width of the segments w = 1.25 mm and the gap between them 5 = 0.55 mm were selected according to the literature's recommendations [1-3], [6].
Using the metallic case of the device led to noise occurrence and to reader performance reduction. To avoid negative influences of a metallic environment a
U1
ferrite shielding should be used [5]. However, there is no economic advantage in such solution for this type of devices. Also it is recommended that the distance between antenna and massive metal components is at least as large as the operating distance (on both sides of the antenna) [5].Therefore we decided to adjust the reader circuit in the device case directly by means of a variable capacitor (C5 on Figure. 3).
Figure 3 - The electric circuit of the reader
The electric circuit for the RFID reader (see Figure 3) was also developed in accordance to recommendations given by the chip's manufacturer - Texas Instruments [5].
The capacity of C5 was varied from 20 to 100 Pf. The noise level was fixed on -80 dBm, the signal level while communicating between the reader and a card
was varied from -58 to -49.8 dBm. The distance between the card and the reader during its adjustment was 8.5 inches.
Besides the circuit elements, the power consumption of the circuit played an important role. In the first iterations we decided to use a 3 volt power supply. Later we found out in experiments that such a supply did not provide an efficient power for communication with a card. Probably, it was caused by the metallic case, which stimulated a large noise. As a result, the power supply voltage was increased to 5 volt, and this
considerably raised the channel stability. We also decided to move the antenna to the device case to further improve the stability. At the moment, the antenna is glued to the dielectric substrate on the metallic reader case.
We chose -49.5 dBm as a reference power level for a signal, because it corresponds to the signal power level of the test board from Texas Instruments [5]. Signal power levels for several circuit variants with different power supply and C5 capacity are represented on Figure 4.
13:26:06 Saturday March 27,2014 Ref Lvl: -36,S dBm -36,!
-41,8 -46,8 -51,8 -56,8 -61,8 -66,8 -71,8 -76,8
i
i
i
! 4 -58.2 dBm
;
1
i
Start: 13,4614MHz Stop: 13,6614MHz
RBW: 3kHz Center: 13,5614MHz
VBW: 10 kHz Span: 0.2MHz Sweep: 100.0 mS
13:25:22 Saturday March 27,2014 Ref Lvl: -36,8 dBm -36,8
-41,8
-46,8
-51,8
-56,8
-61,8
-66,8
-71,8
-76,8
a)
: -53.8 dBm
!
Start: 13,4614MHz Stop: 13,6614MHz
RBW: 3kHz Center: 13,5614MHz
VBW: 10 kHz Span: 0.2MHz Sweep: 100.0 mS
13:25:42 Saturday March 27,2014 Ref Lvl: -36,8 dBm -36,
-41,8 -46,8 -51,8 -56,8 -61,8 -66,8 -71,8 -76,8
.....j -56.3 dBm
Start: 13,4614MHz Stop: 13,6614MHz
RBW: 3kHz Center: 13,5614MHz
VBW: 10 kHz Span: 0.2MHz Sweep: 100.0 mS
b)
13:25:06 Saturday March 27,2014 Ref Lvl: -36,8 dBm -36,8
-41,8
-46,8
-51,8
-56,8
-61,8
-66,8
-71,8
-76,8
4 -49.i dBm
1
Start: 13,4614MHz Stop: 13,6614MHz
RBW: 3kHz Center: 13,5614MHz
VBW: 10 kHz Span: 0.2MHz Sweep: 100.0 mS
c) d)
Figure 4 - Signal power levels; (a) with 3 volt power supply, (b) with 5 volt power supply and 120 Pf C5, (c) with 5 volt power supply and 20 Pf C5, (d) with 5 volt power
supply and 100 Pf C5
Finally, we selected capacitor's value to provide the maximum power of communications. The variable capacitor was replaced by a 100 Pf ceramic fixed capacitor, because its parameters drift was noticed due to thermal fluctuations.
Another aspect of the reader development was software adaptation for the Linux operating system. The SPI interface driver used for data communication
with TRF7960 chip did not meet the requirements to timing delays. Therefore it was modified to provide asynchronous reply to an event with a delay on the order of tens of microseconds.
As a result of the development, we created a device that allowed to communicate with ISO/IEC 14443A cards under the Linux operating sys-
tem. Our analysis shown that the channel operating stability for the described device configuration is about 90 % successful communications.
Conclusion
The performed research and subsequent development showed that the specific requirements for RFID readers imposed by access control system features can be fulfilled by following the recommendations of reader chip's manufacturers with subsequent steps of adjustment under experimental control of reader performance.
Acknowledgment
We would like to thank Alexey Gvaskov for his assistance while carrying out experiments and Alexander Domoratsky for reviewing and valuable comments on this paper.
References
1. Grover F. W. Inductance Calculations Working Formulas and Tables. Dover Publications, 1946.
2. Hardy J. K. High Frequency Circuit Design. Reston Publishing Company, 1975.
3. Henry K. Radio Engineering Handbook. McGraw-Hill Book Company, 1963.
4. ISO/IEC 14443-1:2008. Identification cards. Contactless integrated circuit(s) cards. Proximity cards. Part 1. Physical characteristics.
5. Schillinger J. Antenna Matching for the TRF7960 RFID Reader. Texas Instruments, May 2009.
6. Welsby V. G. The Theory and Design of Inductance Coils. John Wiley and Sons, 1960.
Rokochinskiy A.M.
National University of Water Management and Nature Resources Use, Professor, Doctor of Engineering,
Volk P.P.
National University of Water Management and Nature Resources Use, Senior Lecturer, Ph.D.
Koptiuk R.M.
National University of Water Management and Nature Resources Use, Associate Professor, Ph.D.
Pallu L.M.
National University of Water Management and Nature Resources Use.
THE TRADITIONAL AND OPTIMIZATION APPROACHES TO SUBSTANTIATION OF THE PARAMETERS OF AGRICULTURAL DRAINAGE AND THE RESULTS OF THEIR COMPARATIVE
EFFECTIVENESS
ABSTRACT
The traditional and optimization approaches to substantiation of the parameters of agricultural drainage and the results of their comparative effectiveness are considered.
Keywords: evaluation, approaches, justification, parameters, agricultural drainage.
For today massive development of reclamation associated with significant investments are very significant for the economy of any country, but the received effect is thus at best 60-70% of the project. One of the major reasons is the imperfection of existing methods of design and calculation of drainage systems [1].
In addition, together with necessity of increase the economic efficiency of drainage reclamation, today there is an extraordinarily acute problem of validity of reclamation activities by ecological requirements [2, 3].
That is, construction projects and reconstruction of reclamation facilities should provide immediate ameliorative effect of all aspects of its implementation. Therefore it requires new approaches and advanced methods substantiation of, especially construction and agricultural drainage parameters as defining regulatory element drainage system [3].
Theoretical foundations of the science of soil drainage works were laid by H.Darsi, J. Dupuis, J. Boussinesq and others. Subsequently, at different stages of development of melioration science, known
scientific schools were identified two basic methods of calculating the parameters of agricultural drainage: hy-dromechanical based on theoretical principles of the movement of water in natural and technical systems, empirical that based mainly on statistical data processing of numerous natural investigations. Each of them has its advantages and disadvantages.
Should be noted that the hydromechanical method for determining the distance between drains is the most reasonable in theory, but it does not consider economic, environmental, and some regime-technological aspects of drainage.
There are many received based on this method formulas, that do not take into account the presence of the initial pressure gradient which determines water movement [4]. Excluding this condition error of distance between drains can range from 3% to 40%, depending on the length of the period of drying.
A major disadvantage of hydro-mechanical formulas is also ignoring the conditions of formation of the drainage flow in the phase of raising the level of
