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Zhu Zhiyuan, Tan Jie, Zhao Hongsheng, Guan Qiang, and Li NaAbstract-in many RFID application scenarios the distance between reader and tags is constantly changing with a different velocity. And this change will affect the performance of RFID system. In this paper, a novel scheme named a dynamic RFID performance test system is proposed to find out how movement affects the performance of the RFID system. A new simplified model is proposed to describe these scenarios, and the architecture of a dynamic RFID performance test system is designed base on this model. Furthermore, the work flow of the system is introduced, and two pictures for sample signals captured by this system are demonstrated. The test system can complete the test task very well in dynamic scenarios.
Radio Frequency Identification(RFID) is a widely used technology. It is applied in areas like: precise location, security authentication, logistics management, traffic management and so on. It originates from military applications during World War II, when the British Air Force used RFID technology to distinguish allied aircraft from enemy aircraft with radar . A general RFID system consists of three parts: a tag, a reader with its antennas and a computer equipped with a middleware application .
A reader communicates with tags by electromagnetic waves. The reader sends a command to a tag by modulating a signal into the carrier. The tag not only draws operating energy, but also receives information from the reader's carrier. However, the reader receives information from the tag by transmitting a continuous-wave (CW) RF signal to the tag; the tag responds by modulating the reflection coefficient of its antenna, thereby backscattering an information signal to the reader .
Testing is necessary for any RFID system. For one thing, testing is an effective mean for RFID system developers. They use testing method to shorten the development cycle and improve the quality of products. For another thing, with the rapid development of RFID technology, the number of related products manufacturers increased gradually. So, there are a great variety of RFID products in market. Users need testing to choose the best product which can meets their needs.
There is a common feature in many RFID application scenarios that the distance between reader and tag is constantly changing. Besides, the speed of changing can not be predicted. Such as: RFID technology is utilized in an electronic material flow control system for improving production efficiency in integrated-circuit assembly industry . RFID-enabled healthcare management systems in a medical organization are introduced by writers in  . An electronic toll collection system in expressway based on RFID is designed in . RFID technology has been adopted in the supply chain management  . RFID technology has recently been favorably considered as a cost-effective alternative for indoor location tracking [9-11]. Users need testing to choose the best product in different scenarios. So testing is necessary for the RFID system in dynamic scenarios.
There has been prior study of RFID performance test in static scenarios, where both the reader and tags are all static. However, there is little research for RFID performance test in dynamic scenarios. For one thing, it is difficult to build dynamic test scenarios, because such scenarios occupy a big venue. At the same time, the implementation of these experiments is challenging. Operators are required to conduct the same experiment multiple times at the same condition. That means an operator with some RFID tags must tries to pass an antenna in the same way and at the same speed over and over again.
In this study, research for the RFID performance test in dynamic scenarios is conducted. A big plastic wheel is applied to resolve these problems which arise from dynamic scenarios: It takes a lot of space and time for the test, and operators must conduct the same experiment multiple times at the same condition. A dynamic RFID performance test system (DRPTS) has been developed using a rotation system, a signal generator, a spectrum analyzer and the controller of DRPTS.
The rest of the paper is organized as follows. In Section 2, the related work on simulating and testing of the RFID system is discussed. In Section 3, the design process of a dynamic RFID performance test system is introduced. Firstly, the basic principle of the RFID system and two simplified models for the dynamic scenario are described. The dynamic RFID performance test system is designed base on the second model. And then the architecture of the dynamic RFID performance test system is shown. The principle and the structure for rotation system which is a key subsystem in this scheme are introduced. Besides, the work flow of the system is described in detail, and two pictures for signals are demonstrated. Finally, conclusion is given in Section 4.
The previous study for RFID focuses on the device and equipment. However, much work has been done recently to simulate the practical scenario of a RFID system since the testing in a real scenario requires a great amount of time and high costs.
The simulation of an application RFID system is an extremely effective way in research and development, and it has been widely applied in various fields. Such as: The simulation of a RFID system which is used to find victims in an earthquake is discussed in . An agent-based simulation was adopted for such a test. An extendable platform based on The Math Works Matlab Simulink is developed in . This platform is based on a multi-processor hardware target. Besides, it can run Ultra High Frequency (UHF) RFID tag simulations of very high complexity. In , a RFID system is simulated by electrical models. The reader has a mono-static architecture. A wireless channel with path loss and variable environmental factors establishes the reader-tag link. The tag is represented by a simple model. System modeling can be optimized by modifying the parameters of the building blocks.
In addition to system simulation, some work has been done to develop some simple devices in debugging, measuring existing RFID installations. To meet the needs of industry, several implementations of UHF RFID tag emulators are discussed in . Tag emulator can be used as a general testing tool for designing, developing and evaluating RFID system. In , a new and economical design of RFID testing channel based on direct digital synthesis chip is discussed. It can reduce the cost of testing RFID chip, and bring huge profits to chip manufacturer and test equipment supplier.
In , an UHF RFID measurement and performance evaluation test system is presented, which allows for evaluations of designs for labels and ICs. Furthermore, it provides a method for quantifying tag performance in various applications through measurements and tests.
How mobility affects RFID performance in real applications is not know yet. Machine-aided experiments were conduct to study the effects of mobility on RFID in . The circular conveyor belt was used to perform these experiments, and the fastest speed of conveyor belt can be adjusted for 2.0 m/s. Those Machine-aided experiments need a lot of space and time to implement, and it takes a large amount of money to build this system.
DESIGN OF A DYNAMIC RFID PERFORMANCE TEST SYSTEM
Two models in dynamic scenario
There are many applications of RFID technology in dynamic scenario. Nine application examples were cited in [4-11]. A simplified model was proposed to describe the dynamic scenarios in , as shown in Figure 1, model one. We analyzed this model, and then propose another model base on it.
In model one, the antenna of reader is located in point. The tag under test moves from point A to point B at the speed of along the straight line c, which takes. The distance between point A and point B is. The angle between line and line is. The angle between line and line is. Since the value of is much smaller than the value of, the value of is small. is the wavelength of electromagnetic waves that is applied in RFID system. The electromagnetic waves which are emitted from point P will reach point A and point B respectively. Distance difference in these two ways is. The value of is small, so can be calculated by:
The phase difference caused by the change in distance as follows:
So the Doppler frequency shift in the model one is:
The dynamic test scenarios are not easy to build, because it needs to occupy a big venue and operators are required to conduct the same experiment many times at the same condition. A new model is proposed which can solve these problems base on model one. It is model two as shown in Fig. 1.
Fig. 1. two models in dynamic scenario
The basic principle of model two is replacing the linear motion with rotation. The length of âŒ’is almost equal to the length of when the value of is small, which will be proved in the next paragraph.
In Model Two, the antenna of reader is located in point. The tag under test moves from point to point at the speed of along the ArcâŒ’, which takes. The distance between point and point is. The angle between line and line is. The angle between line and line is. Both line and line are the tangents of the round. So it is proved that.is the radius of round . So the value ofcan be calculated by:
Besides, the length of âŒ’is equal to .So the following formula will be correct if the length of âŒ’is almost equal to the length of.
That means that the value of must be very small.
Since the value of is much smaller than the value of, the value of is small. The electromagnetic waves which are emitted from point will reach point and point respectively. Distance difference in these two ways is. The value of is small, so can be calculated by:
Similarly, the phase difference caused by the change in distance is as follows:
So the Doppler frequency shift in the model two is:
The architecture of DRPTS
DRPTS consists of a rotation system, a signal generator, a spectrum analyzer, antenna for Signal generator and spectrum analyzer, a controller of rotation system and a controller of DRPTS, as shown in Fig. 2.
Fig. 2. the architecture of DRPTS
The controller of DRPTS is mainly responsible for controlling and managing other equipments in this system. On the one hand, it sends control instructions to the controller of rotation system, and the speed of plastic wheel in the rotation system can be set in it. So the speed of the under test tag which is installed on the surface of plastic wheel can be set too. The controller of DRPTS communicates with the controller of rotation system through serial port.
On the other hand, the controller of DRPTS can trigger the spectrum analyzer, and record the experimental data and pictures which are captured by the spectrum analyzer. Besides, the working frequency and power of the signal generator can be set in the controller of DRPTS. And the signal generator simulates the signal of a reader by running a program which is edited in the controller of DRPTS. Both the signal generator and the spectrum analyzer are connected to the controller of DRPTS through the Ethernet. Two antennas are connected to the signal generator and spectrum analyzer through RF feeder lines respectively.
The structure of the rotation system
The rotation system includes the controller of rotation system, drive motor, plastic wheel and the under test tag. Detailed structure is shown in Fig. 3.
Two antennas for the signal generator and the spectrum analyzer are installed on the shell of the rotation system by plastic brackets. And all geometric centers for these two antennas and the under test tag are in a straight line which is the tangent for the plastic wheel. There is a rectangular opening in the shell, and the width of it is. The radius of the plastic wheel is. The under test tag is installed on the plastic wheel by a plastic rod.
Fig. 3. the structure of rotation system
The controller of rotation system is connected to both the controller of DRPTS and the drive motor. Firstly, it receives control commands from the controller of DRPTS. These commands include the speed of the wheel, direction of rotation and other information. And then the controller of rotation system calculates the rate of pulses that will be sent to the drive motor. So it can control the speed of the wheel. The speed of the under test tag is:
In (1), is the revolutions per minute, is the distance from center of plastic wheel to the tag.
All experiments use the Omron-V740-HS01CA circular polarization antenna with a gain of 6dBi as the antenna of Signal generator. AgilentE4438C Vector Signal Generator is applied in this system to simulate the real reader, which supports the EPC class 1 Gen 2 standards . The carrier frequency range of it is from 250K Hz to 3G Hz. The program which is used to generate the signal has been edited in the controller of DRPTS, and then downloaded to the signal generator via Ethernet.
In this system Tektronix RSA3308A Real-Time Spectrum Analyzers is applied to Capture and analysis the signal. The Real-Time Spectrum Analyzers can be triggered through detecting the signal of the reader, and then captures all signals after triggered in a period of time. It can capture the signal which changes from 0Hz to 8GHz and decode the signal automatically. Besides, the data will transferred to the controller of DRPTS through Ethernet. And all the experimental data are stored in the controller of DRPTS.
The controller of rotation system uses Panasonic PLC programmable controller AFPX-C30T +AFPX-COM1.There is a Touch Panel in the surface of controller, and it is used for Human-Computer interaction and displaying rotation system parameters.
The rotation system is mainly composed of three parts: drive motor, plastic wheel and the plastic rod that is used to install under test tag. Panasonic MINAS-A4 Series AC Servo System is applied as the drive motor. Built-instantaneous speed observer, so the speed of motor can be detected accurately and rapidly. Speed response frequency is as high as 1K Hz. The plastic wheel is another important part in the rotation system. The radius of the plastic wheel is 30cm, and the thickness of it is1cm. There are ten holes in the wheel, and they are used to install plastic rods. The under test tag is pasted on the plastic rods. There are special requirements for the material of both wheel and rod. Dielectric constant of the material must less than 1.5.There is a protective cover with the size of outside the plastic wheel. Besides, there is a rectangular opening in the shell with the width of it is.
The Work flow of DRPTS
The principle and the composition of the system have been described in the preceding section. And the working process of the whole system will be introduced in this section. The connection diagram for DRPTS is shown in Figure. 4. The workflow of DRPTS is as follows:
1) Initialization of the equipment: communication links were established between the controller of DRPTS and signal generator, spectrum analyzer, rotating system controller. Spectrum analyzer is set to frequency-domain templates trigger mode.
2) Set the velocity of the under test tag in the controller of DRPTS. The controller will sends a command with the value of speed to the controller of rotation system. The under test tag will move at the set speed.
3) Set the transmitting power of signal generator at maximum value which is permitted for the reader in this region through the controller of DRPTS. Set the carrier frequency of the Signal will launch at.Signal generator sends out read tag commands continuously.
4) The spectrum analyzer is set to frequency-domain templates trigger mode. And it will be triggered by the signal at the frequency of. Record the value of transmitter power for signal generator and the signal captured by spectrum analyzer at the controller of DRPTS.
Fig. 4. the connection diagram for DRPTS
5) The spectrum analyzer demodulates the captured signal, and analyzes signals after demodulation. Save the image of demodulated signal. However, if the signal can not be demodulated, back to step 4.
6) If the speed and the frequency for under test tag have changed, back to step 2. The speed and the frequency can be changed from the controller of DRPTS. Changes in frequency must be within the frequency range of the tag works.
7) Test completed, disconnect the communication links which have been established between the controller of DRPTS and signal generator, spectrum analyzer, rotating system controller. Close all test equipment.
The signal captured by DRPTS
In this experiment, the carrier frequency of those Signals which are launched by signal generator is set atHz. And a ticket of New China Science and Technology Museum was brought to test.
For a good description of signals in dynamic scenario, the acquisition time was set at 200ms in this experiment. So we can capture 40 signals every time. Two pictures for signals captured by DRPTS have been shown in Fig. 6. Picture 1 shows those signals which are captured with tag moving at the speed of 8.33m/s, and Picture 2 shows those signals which are captured with tag moving at the speed of 1.39m/s.
There is great difference in those two pictures. The value of power that spectrum analyzer received change over time.
Fig. 5 .signals captured by DRPTS
In this paper, a novel scheme named a dynamic RFID performance test system is proposed. We have considered the situation where the distance between reader and tag is constantly changing at a different velocity. How this movement affects the performance of the RFID system is a valuable but challenging problem, so the dynamic RFID performance test system was designed and developed. The advantage of this system is as follows:
1) The proposed scheme reduces the experiment required space, and takes less time to implement than the conventional schemes. That means that this system can build in an ordinary lab with less space and implement in a very short time.
2) The proposed scheme can conduct the same experiment multiple times at the same conditions. It is difficult to achieve in the conventional scheme.
3) The system is easy to build since the structure is simple. So it will reduce the experimental efforts and cost.
4) The carrier frequency of those signals can change from 250 KHz to 3GHz, so we can test all tags which work in this range. And the power of the signal which is launched by signal generator can be as large as 25w.
In the next phase, we will implement experiments in different dynamic environments by using this system, and find how movement affects the performance of the RFID system.
The authors would like to thank all the reviewers for their helpful comments. This project was supported by the National Natural Science Foundation of China grants U0935002/L05 and National High-Tech Research and Development Program of China (863) grants 2006AA04A103.