Design and development of a RFID reader with carrier frequency of 125 KHz

Transcription

1 Design and development of a RFID reader with carrier frequency of 125 KHz Lucian - Nicolae Cojocariu, Octavian - Modest Manu Ștefan cel Mare University Suceava, Romania luccojocariu@stud.usv.ro, octavianm@eed.usv.ro Abstract: This paper presents the development of a low frequency Radio Frequency Identification transceiver system aimed at providing non-contact solutions for identifying, monitoring, and tracking people, animals, and objects. The communication between Radio Frequency Identification reader and transponder is done wireless. When the transponder is placed in the electromagnetic field emitted by the RFID reader antenna, it is activated and the information stored is transmitted in digital form by modulating the field emitted by the reader antenna. Various antenna configurations were analyzed and the optimal configuration was chosen for the implementation. Keywords: RFID Reader, Antenna, microcontroller system, transceiver, graphic interface. I. Introduction The function block diagram of the developed RFID reader is presented in Fig. 1 Radio Identification (RFID) systems received an increasing amount of interest during the last years due to the great potential for application applications in the areas of control access [1], indoor localization [2-5] healthcare monitoring systems [6]. Numerous international research centers and companies aim at developing smart electronic chip for RFID systems capable of operating in a variety of environmental conditions with a high level of data integrity. The main component of a RFID reader is the transceiver which must provide the carrier wave frequency and the modulation type suitable to the envisioned application. The developed RFID reader presented in this article is based on a MLX90109 integrated transceiver produced by Melexis Company which has an operating frequency of 125 KHz and uses Amplitude-Shift Keying (ASK) modulation. These characteristics are required for low frequency RFID applications [7]. The main parts of the developed RFID reader are an antenna, a MLX90109 integrated transceiver, an Atmega8 microcontroller and a FT232RL convertor. The communication at the functional block level consists of transponder tag interrogation by the RFID reader using radio electromagnetic wave, which reads the data contained in the transponder transmits them to a computer via USB. Computer USB FT232RL Convertor UART Atmega8L Microcontroller Transceiver MLX90109 Antenna FIELD Transponder Figure 1. RFID system block diagram R F I D R e a d e r

2 The MLX90109 Integrated Circuits is controlled by a microcontroller and is connected to a coil antenna and a capacitor which forms a parallel LC circuit tuned to a 125 KHz frequency. The transceiver transmits to the microcontroller the data read from the transponder memory which are already demodulated and decoded. The microcontroller Atmega8 switches from plus or minus some of the transceiver control pins in order to allow the identification of a large class of transponders. The data provided by the transceiver to microcontroller are transmitted via a Universal Asynchronous Receiver Transmitter (USART) interface to the FT232RL convertor. The FR232RL convert data received from UART interface to data transmitted via Universal Serial Bus interface (USB). Finally, the graphical interface allows the users to interact with the RFID reader and tag in an easy manner. II. Hardware Design The hardware architecture of the RFID reader is based on the MLX90109 transceiver, which is schematically presented in Fig. 2. In order to serve as a transceiver, the MLX90109 integrated circuit requires additional electronic components such as resistive voltage divider connected to pin MODU, low pass filter, antenna, fast decay circuit. The resistive voltage divider provides a voltage drop equals to 0.8 V on pin MODE of integrated transceiver when the terminal of R 4 resistor is switched to the negative pole of the power supply by the PC0 pin of Atmega8 microcontroller. Thus the antenna field is activated. If the microcontroller switches the terminal of R 4 resistor to the positive pole of the power supply the voltage drop across the MODU pin of the transceivers is higher than 0.8V and the antenna field disappears. By knowing the voltage supply and the resistance values, the voltage drop value is obtained from the formula for the resistive voltage divider as follows: VDD R4 VM O D E (1) R R 4 5 For the selected values of 30 KΩ for R 5 and 150 KΩ for R 4, the voltage drop on pin MODE is V MODE =0.83V when the resistive voltage divider is supplied at V DD =5V. A low pass filter is designed to stop stray signals with frequencies below 60 Hz, from electric power supply network, and it is compose of the capacitor C 8 and resistor network R 4 and R 5. Figure 2. MLX90109 transceiver with external components. The value of C 8 capacitor can be determined by using Equation 2. C8 R1 R2 (2) 2 R1R 2 f For a signal with a frequency of 60Hz and for the selected values of R 4 =150 KΩ and R 5 =30 KΩ, the value of condenser C 8 = 100 nf is obtained. The antenna connected to the integrated transceiver MLX90109 must have impedance between 1 KΩ and 5 KΩ. In order to obtain the 125 KHz carrier frequency a parallel LC oscillator circuit is used, including the antenna coil inductance and the C 7 capacitor as shown in Fig. 2. At resonance the capacitive reactance of C 7 and the inductive reactance of the antenna coil become equal leading to the following expression for C 7 : 1 C 7 2 (2 f0 ) L (3) where f 0 is the carrier frequency. For a resonant frequency f 0 =125 KHz and an inductance L=44 µh, a value of C 7 =37 nf was determined. The diameter of a single conductor insulated with used for the antenna coil is determined from the following current density formula: I ant d 2 (4) J Cu The electrical current density of copper is j Cu =11 A/mm2 and the maximum current which flows into the antenna coil at resonance is I ant =218 ma. Thus the obtained value for the

3 wire diameter d is 0.28 mm. The standardized value for wire diameters is chosen as d=0.3 mm for a wire without insulation, and d=0.337mm for wire with insulations. Next, the number of turns for the antenna coil can be calculated as follows: interface, decodes and sends them via UART interfaces to microcontroller and vice versa. N 2 L 8 D 0 D ln 2 d (5) were N represents the number of windings, L represents the coil inductance measured in H, D is the inner coil diameter measured in mm, d is the wire diameter measured in mm and µ 0 represents the magnetic permeability of air with a value of 4 π 10 7 [Wb/A m]. Various antenna configurations were designed and made based on the Eqs. 4 and 5. Two of them are presented in Fig. 3. In order to decrease the time decay of the antenna oscillations below the maximum recommended value, a fast decay circuit was developed with a P-MOS transistor noted with T and a Schottky diode noted with D in Fig. 2. The transistor works in switching mode and the diode D has the role of protecting the transistor T from the occurrence of reverse voltage on antenna coil. Figure 4. Atmega8l microcontroller external components and connections to the RFID transceiver Figure 5. FT232RL convertors with externals components Figure 3. RFID antennas with diameter of 20mm (left) and 135mm (right). The Atmega8L microcontroller handles the RFID transceiver via the followings pins: MODU, MODE, and SPEED. The data provided by the MLX90109 integrated transponder are read by the microcontroller via CLOCK and DATA pins. The Atmega8L microcontroller was chosen for its low energy consumption, the reduced number of pins and its high processing speed. Due to the powerful instructions, most of them executed in one single cycle, this microcontroller achieves throughputs close to 8MIPS at the working frequency of 8MHz. The design of RFID reader is oriented to low power consumptions such that the entire application can be powered from a USB port. The FT232RL convertor, presented in Fig. 5, receives data from a computer via USB Figure 6. Practical implementations of RFID reader designed. The decay time is given by the following relation: 1 tdecay Qant (6) f0 where Q ant represents the quality factor of antenna coil. The hardware part of RFID reader prototype developed according to the proposed design is shown in Fig. 6.

4 III. Software Components IV. Prototype testing The software of the developed RFID reader consists of the program implemented in the microcontroller that allows it to control and communicate with MLX90109 transponder and the user interface applications running on the computer which displays information received from transponder and allows user to configure the MLX90109 integrated transponder. The first step into programming the Atmega8 microcontroller was to choose the fuse bits because it uses an external quartz oscillator in order to ensure frequency stability of the clock signal. The microcontroller program was written in C and contains roughly 200 lines of code. An external interrupt service routine serves the data readings from the transponder memory. An internal timer interrupt is called times per seconds and sends data to user interfaces application via the USB. The graphical user interface running on the computer was developed using Microsoft Visual C++ integrated development environment. The source code for the interface project totals 1000 lines of code. It runs an event that reads from the USB port and the two threads that are called every 10 ms. One thread is dealing with the selection of the port to which the hardware is connected, and the other thread is dealing with the processing of data received from microcontroller via FT232L converter. Through the developed software one can manipulate the integrated MLX90109 RFID transceiver. By checking the appropriate radio button fields on the interface, it is possible to read a variety of RFID transponders operating on frequency of 125 KHz. Accessing the application entitled "RFID Reader" and connecting the developed RFID reader to the computer via USB cable determine the application to automatically choose the corresponding USB port. In order to read data stored in a RFID transponder, one must know the header of the tag, data transmission speed (bandwidth), data coding mode and the memory size of the tag. Numerous tests of the developed RFID reader prototype were performed by using various tags, antenna coils, geometries and environments. By connecting the antenna presented on the left side of Fig. 3, a range of 4 cm was determined while by using the antenna presented on the right side the range was extended to 15 cm. The received antenna signals were analyzed by using a LeCroy WaveSurfer digital storage oscilloscope, and sample results are presented in Fig. 8. The measured carrier frequency was khz which is very close to the target frequency of 125 khz. a) b) Figure 8. a) The carrier frequency, b) the modulated signal. Figure 7. Graphic user Interface.

5 understanding of the operation of RFID systems. Due to the fact that the integrated transceiver that can handle antennas with impedance ranging between 1 KΩ to 5 KΩ a wide range of antennas can be developed and the benefits and disadvantages of them can be analyzed. The presented RFID reader has been designed to work with power supply from the USB port of the computer that is connected to. Also, the small size of the RFID reader provides portability. Given the above considerations the developed RFID reader can be used as a study platform for low frequency RFID systems. VI. Acknowledgements Figure 9. Free decay of antennas oscillations. L.C. thanks Lucian Andrieș for his support during the development of software components. O.M. thanks CNCSIS-UEFISCSU PROJECT NUMBER RU-107/2010 for the financial support. VII. References Figure 10. The decay of antennas oscillations via the developed decay circuit. The efficiency of the decay circuit is apparent from Figs. 9 and 10. In the case of antenna coil with 130 mm diameter the decay time of the antenna oscillations is 280 µs when no decay circuit is used. This value is greater than the maximum imposed by the manufactures which is 200 µs. By using the decay circuit with the same antenna, the decay time of antenna oscillations drops to 71 µs. V. Conclusions The MLX90109 RFID integrated transceiver can be the basis of the development for many RFID applications that use a carrier of low frequency. Developing applications that use this integrated transceiver can lead to a better [1] AGRAWAL P., BHARGAVA N., CHANDRASEKHAR C., et al. The MIT ID Card System: Analysis and Recommendations, MIT, Tech. Rep., December 2004 [2] ROPPONEN A., RIMMINEN H., SEPPONEN R., Robust System for Indoor Localisation and Identification for the Health Care Environment, Wireless Personal Communications, Springerlink, 2010 [3] POPA V, COCA E., DIMIAN M Applications of RFID Systems - Localization and Speed Measurement, chapter 8 în Radio Frequency Identification Fundamentals and Applications, Bringing Research to Practice, Intech Publisher, Wien, (pp ), [4] WILLIS S., HELAL S., LFID information grid for blind navigation and wayfinding, In Proceedings ninth IEEE international symposium on wearable computers (ISWC 05) (pp ) Osaka, Japan, 2005 [5] HIHNEL D., BURGARD W., FOX D., et al. Mapping and localization with LFID technology. In Proceedings IEEE international conference on robotics & Automation (pp ) New Orleans, LA, [6] NGAI E. W. T., POON J. K. L., SUK F. F. C., NG C. C., Design of an RFID-based Healthcare Management System using an Information System Design Theory, Information Systems Frontiers, Springerlink 2009 [7] FINKENZELLER K., RFID Handbook (second edition), 2003, ISBN [8] MLX90109 Datasheets from MELEXIS available at:

6 [9] Atmega8L Datasheets from Atmel available at: ts/2486s.pdf [10] FT232RL Datasheets from FTDI available at: FT232R.pdf Lucian Nicolae COJOCARIU Msc student, Ştefan cel Mare University from Suceava, Faculty of Computer Science and Electrical Engineering, Msc thesis: Designing and developing a radio-frequency identification sensor network for healthcare application, supervisor: Prof. Eng. Mihai DIMIAN, PhD. Octavian Modest MANU PhD student, Ştefan cel Mare University from Suceava, Faculty of Computer Science and Electrical Engineering, PhD thesis: Contributions to the development of smart antennas and applications, PhD supervisor: Prof. Eng. Adrian GRAUR, PhD.