Analysis of the antenna circuit and its influence on the wheel unit s RF performance

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1 Scientific Bulletin of Politehnica University Timisoara TRANSACTIONS on ELECTRONICS and COMMUNICATIONS Volume 60(74), Issue 2, 2015 Analysis of the antenna circuit and its influence on the wheel unit s RF performance Silvia V. Botea 1 Abstract The purpose of this paper is to investigate the performance of the antenna used in the TiS (Tire Information System) module and to find a possible improved solution. A series of simulations of the loop antenna integrated in the TIS module is presented. There are considered 3 different cases: the standalone simulation of the module, the version where the module is placed on the rim, and the last one is the module simulated together with the rim and the tire. The position of the module on the rim and the influences of the tire based on the simulations results are presented by evaluating the impedance and the radiation efficiency. Different dimensions are considered for the rim and the tire. Keywords: TiS, antenna, HFSS I. INTRODUCTION TiS (Tire Information System) measures the inflation pressure. The systems developed by Continental permanently monitor the tire's inflation pressure and reliably alert the driver in the event of a loss of pressure. It consists of a central receiver (e.g. remote keyless entry) and four rim modules. The modules measure the current tire pressure, taking into account also the temperature of the tire. In this way, our Tire Information Systems help eliminating a frequent cause of accidents to enhance driving safety. In addition to this, they lower carbon dioxide emissions and reduce fuel consumption. [1] One of the Tire Information Systems designed by Continental Automotive is called TG1C. This product is followed by the TG1D version. The frequencies required by TiS are 315 MHz in the U.S., Canada and Japan and 434 MHz for the rest of the world. The physical size of the TG1C module is very restrictive and it is much smaller than the wavelength of interest. As reminded above, the module operates at a frequency of 315 MHz (λ ~ 1 meter) or 434 MHz (λ ~ 0.7 meters). They are usually not structurally selfresonant at these frequencies and have a low radiation resistance and large reactance. In order to improve the radiation efficiency, the module usually utilizes a matching network to cancel the reactance and transform the low radiation resistance to a larger input resistance. [2] For the simulations, the HFSS program has been used. HFSS is a commercial finite element method solver for electromagnetic structures from Ansys Corporation. The acronym stands for High Structural Simulator (HFSS). It is one of several commercial tools used for antenna design, and the design of complex RF electronic circuit elements including filters, transmission lines, and packaging. II. EVALUATION OF THE ANTENNA IN THE TIS SENSOR The module was analyzed in 3 different cases: first in the standalone version, then placed on the rim and lastly placed on the rim and tire. A. Case 1: Standalone antenna structure The HFSS model used for the analysis is described in the picture below. It is a simplified version used in order to reduce the simulation time. The TG1C module dimensions are: 48 mm x 27.6 mm x 12.5mm (L x W x H). In this case the antenna is placed on the PCB. Also the battery and the housing are present in the simulation. They are modeled after the physical model. In HFSS the antenna is made out of cooper and has a width of 5 mm, a height of 7.3 mm and a total length of 39 mm. Fig. 1. TG1C product 1 Faculty of Electronics and Telecommunications, Communications Dept. Bd. V. Parvan 2, Timisoara, Romania, Silvia.Botea@continental-corporation.com 3

2 XY Plot $rel_perm='3' Phi='0deg' Theta='0deg' $rel_perm='3.2' Phi='0deg' Theta='0deg' $rel_perm='3.4' Phi='0deg' Theta='0deg' $rel_perm='3.6' Phi='0deg' Theta='0deg' $rel_perm='3.8' Phi='0deg' Theta='0deg' $rel_perm='4' Phi='0deg' Theta='0deg' Fig. 2. TG1C HFSS Model In Table 1 the real and the imaginary parts of the impedance and the radiation efficiency are shown for both 315 and 434 MHz frequencies. Table A.1. Antenna on PCB (without housing) Fig. 4. for TG1C module, with variations of the relative permittivity of the housing between: 3 and 4, with a step size of XY Plot $rel_perm='3.4' $tg_delta='0' Phi='0deg' Theta='0deg' $rel_perm='3.4' $tg_delta='0.01' Phi='0deg' Theta='0deg' $rel_perm='3.4' $tg_delta='0.02' Phi='0deg' Theta='0deg' $rel_perm='3.4' $tg_delta='0.03' Phi='0deg' Theta='0deg' $rel_perm='3.4' $tg_delta='0.04' Phi='0deg' Theta='0deg' $rel_perm='3.4' $tg_delta='0.05' Phi='0deg' Theta='0deg' Fig. 5. for TG1C module, with variations of the dielectric loss tangent of the housing between: 0 and 0.05, with a step size of It is clear from figures 4 and 5 that the housing material is very important and it does influence the results. This needs to be taken into consideration when the materials for the series production are chosen. Fig. 3. HFSS model: antenna on PCB (without housing) First the antenna alone on the ground plane with the battery is analyzed to get a clear image of the housing influence presented in the next paragraph. Below, in table 2, we can see the results. Table A.2. Antenna on PCB (with housing) In the next 2 figures, the influence of the housing parameters is shown. The relative permittivity and the dielectric loss tangent were taken into consideration. The defaults values, that are currently in use are: r = 3.4, tan = A.3.Analysis of the module position on the wheel The TG1C module is fixed on the rim through the valve. Investigations were made to see if there is possible to find new positions that could lead to better results XY Plot Phi='0deg' Theta='0deg' var_y='-85mm' var_z='13mm' 1 Phi='0deg' Theta='0deg' var_y='-72mm' var_z='8mm' 1 Phi='0deg' Theta='0deg' var_y='-49mm' var_z='8mm' 1 Phi='0deg' Theta='0deg' var_y='2mm' var_z='-1mm' 1 Phi='0deg' Theta='0deg' var_y='25mm' var_z='15mm' Fig. 6. for different positions of the module along the rim s width 4

3 B. Case 2: Antenna and rim investigation C. Case 3: Antenna, rim and tire investigation Fig. 9. Component parts of the tire Between the tread inner liner and outer rubber there is a simplified metal steel belt. Fig. 7. TG1C module and rim HFSS Model In the table below the results of the simulation of the wheel unit with the rim are presented. There is no direct connection between the module and the rim, the module is placed 8 millimeters above the rim. The rim geometry is based on an actual wheel structure. The module is placed near the location of the tire valve. The metal rim of the wheel plays an important role in the overall performance. We can see in the table below a clear improvement of the efficiency, comparing to the standalone version. The dimension for the rim used in this case is R15 (Table 3). In figure 8, a variation of the rim dimensions is shown and the way it influences the efficiency. Table G_RIM_DIAMETER='14mm' Phi='0deg' Theta='0deg' G_RIM_DIAMETER='15mm' Phi='0deg' Theta='0deg' G_RIM_DIAMETER='16mm' Phi='0deg' Theta='0deg' G_RIM_DIAMETER='17mm' Phi='0deg' Theta='0deg' G_RIM_DIAMETER='18mm' Phi='0deg' Theta='0deg' G_RIM_DIAMETER='19mm' Phi='0deg' Theta='0deg' G_RIM_DIAMETER='20mm' Phi='0deg' Theta='0deg' XY Plot Fig. 8. for TG1C module, with variations of the rim dimensions between: R14 and 20, with a step size of Fig. 10. TG1C HFSS model: the module, the rim and tire section For the investigation of the antenna together with rim and tire, the following tire configurations were used in simulation: 175/70/R14, 195/65/R15, 215/55/R16, 215/50/R17, 235/40/R18, 235/55/R19, 255/50/R20. Below, in figure 11, the results are presented. It can be seen that there are some maximum efficiency points. The values at 315 and 434 MHz are highlighted. Y XY Plot 3 Phi='0deg' G_RIM_DIAMETER='14mm' G_TYRE_HEIGHT_PROC='70' G_TYRE_WIDTH='175mm' Theta='0deg' _2 G_RIM_DIAMETER='15mm' G_TYRE_HEIGHT_PROC='65' G_TYRE_WIDTH='195mm' Phi='0deg' Theta='0deg' _3 G_RIM_DIAMETER='16mm' G_TYRE_HEIGHT_PROC='55' G_TYRE_WIDTH='215mm' Phi='0deg' Theta='0deg' _4 G_RIM_DIAMETER='19mm' G_TYRE_HEIGHT_PROC='55' G_TYRE_WIDTH='235mm' Phi='0deg' Theta='0deg' _5 G_RIM_DIAMETER='17mm' G_TYRE_HEIGHT_PROC='50' G_TYRE_WIDTH='215mm' Phi='0deg' Theta='0deg' _6 G_RIM_DIAMETER='20mm' G_TYRE_HEIGHT_PROC='50' G_TYRE_WIDTH='255mm' Phi='0deg' Theta='0deg' _7 G_RIM_DIAMETER='18mm' G_TYRE_HEIGHT_PROC='40' G_TYRE_WIDTH='235mm' Phi='0deg' Theta='0deg' _ Fig. 11. for TG1C module with rim and tire, with the following variations of the tire dimensions: 175/70/R14, 195/65/R15, 215/55/R16, 215/50/R17, 235/40/R18, 235/55/R19, 255/50/R20 5

4 III. 3. POSSIBLE IMPROVEMENTS IFAs (Inverted F Antenna) usually have a very good behavior with gain values that ensure adequate performances taking into account the standard values of the output power and receiver sensitivity of short range radio devices. That is why I investigated a new possible improved solution consisting in an IFA antenna. Below, in fig. 12, it is pictured the HFSS model. A simplified housing was used for the investigations. The height of the antenna is 7 mm. Fig. 13. Comparison between loop antenna and IFA antenna: gain total, xz plane Fig. 12. TG1C HFSS model: IFA antenna Next, the performances of this antenna are shown. The efficiency comparing to the loop antenna is increased and when increasing the length of the antenna even better performances should be obtained. Further investigations with this antenna placed on the rim and including the tire still need to be made to see if it could lead to a better behavior in all cases. Also, the dependence and stability of the antenna parameters is very important and needs to be analyzed. Table 4 Fig. 14. Comparison between loop antenna and IFA antenna: gain total, yz plane A comparison between the TG1C with loop antenna and the module with the IFA is presented. They were only compared in the standalone version. The graphs below are only for 434 MHz frequency. Fig. 15. Comparison between loop antenna and IFA antenna: gain total, xy plane 6

5 IV. MATCHING NETWORK Antenna s input impedance is strongly dependent on the environment. If metallic parts or even dielectric bodies are positioned close to the antenna, a strong mutual coupling between antenna and the environment can appear. Typically close means a distance less than λ/10 (λ is the free space wavelength at operating frequency) between the antenna and a metallic or a dielectric body. The coupling is strongest in the case of metallic bodies. In order to improve the efficiency TIS antennas utilize a matching network to cancel the reactance and transform the low radiation resistance to a larger input resistance. [3] For maximum power transfer to the antenna we use a certain matching topology. This matching is filtering the harmonics and favoring the fundamental frequency. It is made from LC components with small tolerances. For each PCB supplier it is necessary to redesign the matching accordingly. V. CONCLUSIONS Automotive industry is in a continuous and very quick development and so are the radio systems integrated in the cars. This paper tries to show how to do an investigation on the parameters that influence the antenna performance of the TG1C module. These results are used for future improvements of the new products that are developed. Some other investigations with a IFA antenna and were made, but so far the loop antenna was kept on the module because of better stability of electrical parameters. The purpose is to improve the overall performance. Further investigations can still be made in order to find the best solution. From November 2012 it is mandatory that all new cars produced in EU are equipped with the tire information system, so we get the conclusion that this is an important car service and a lot of engineering in the development process is needed to introduce on the market new improved solutions. VI. REFERENCES [1] Continental website: /body_security/pi_tire_information_en.html [2] Hua Zeng and Dr. Todd Hubing, Technical Report: CVEL , Investigation of Antennas used in Tire Pressure Monitoring Systems, 2010 [3] F. Lathiere, Synthesis of measurements and simulations for ITEMS WU positioned in wheel, Continental intern,