A Compact FMCW Interrogator of Microstrip Antenna for ... - IEEE Xplore

2016 Progress In Electromagnetic Research Symposium (PIERS), Shanghai, China, 8-11 August

A Compact FMCW Interrogator of Microstrip Antenna for Foot Pressure Sensing Jun Yao1, Saibun Tjuatja1, and Haiying Huang2 1Department

of Electrical Engineering University of Texas at Arlington, Arlington, TX, USA 2Department of Mechanical and Aerospace Engineering University of Texas at Arlington, TX, USA Abstract- This paper presents a compact antenna-sensor interrogator for pressure sensing. A Frequency Modulated Continue Wave (FMCW) generator was developed to detect antenna sensor's resonant frequency, which encodes the pressure information. The power consumption of this generator is less than 160mW and can be powered by a 3.7V portable lithium battery. In the paper, the operation principle of this FMCW interrogator is firstly explained. Subsequently, the design, simulation and validation of the FMCW generator are described. Finally, static pressure tests were performed to validate the performance of the proposed FMCW interrogator. The results were compared with those from Network Analyzer (VNA) measurements to demonstrate its accuracy. 1. INTRODUCTION

Diabetic foot ulcer is a result of peripheral neuropathy,i.e., the loss of protective sensation in the foot. Because of this condition,the patient may not feel the abnormal pressure experienced by the foot and will not seek treatment until the wound has reached advanced stages [1]. Various foot pressure sensing technologies have been developed for the purpose of solving this problem. One of them is laser optical sensor [2]. A laser light directs onto a pressure sensitive plate and the plate compression caused by the foot pressure can be measured by the interferometry technique. Fiber Bragg Gating (FBG) [3] sensor is also be used for foot pressure detection. Pressure changes the wavelength of the Bragg. Thus, by detecting the wavelength variation of the Bragg the applied pressure is achieved. But the costs of the mentioned pressure sensors are too high, which make them can be only used in the hospital or the medical center. Because of the low cost, compact size and passivity patch antenna has emerged as a promising sensor for biomedical behavior monitoring system [4]. In [5], a novel foot pressure sensor, which contains a microstrip patch antenna and a metal reflection plate, was represented. The metal plate was placed over the antenna sensor. Foot pressure applied on the reflection plate reduces the distance between the plate and the patch, which varies the resonant frequency of the antenna. Therefore,by interrogating antenna-sensor's resonant frequency the applied pressure on the pressure sensor can be determined. VNA and frequency spectrum analyzer are two common devices which are used for patch antenna-sensor interrogation [5,6]. However,the devices are inconvenient for the patients if they want to use them every day since both of them are too large to be carried with. In this study, a PCB-based antenna sensor interrogator was proposed. Linear chirp FMCW signal [7] was produced by a triangle wave generator and a Voltage Control Oscillator (VCO) chip. Then the signal was sent though a circulator to interrogate the antenna sensor. Finally, the antenna reflected signal was collected by a RF power detector which determines the resonant frequency of the antenna sensor. 2. OPERATION PRINCIPLE OF FMCW INTERROGATION CIRCUI T

In order to detect the frequency shift of the antenna pressure sensor a FMCW interrogation circuit was designed. The circuit consists of a PCB-based FMCW generator, a RF circulator and a RF power detector. The diagram of the interrogation circuit is shown in Figure 1. First of all, linear chip signal which has a carrier frequency sweeping from h to 12 is generated by the FMCW generator. The signal is sent to port 1 of the circulator and interrogates the antenna senor though port 2. Then the antenna reflected signal goes from port 2 to port 3 as the input of the RF power detector. The power detector converts the received RF power into DC voltage. The lower power it receives the high DC voltage it generates. If the interrogation frequency matches antenna's resonant frequency the reflected RF signal will has the smallest power. Thus,the RF power detector outputs the largest DC voltage. So by detecting the position of power detector's peak output the resonant frequency of the antenna sensor can be achieved. 2101


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In the FMCW generator, a periodic linear chirp is implemented by using a VCO controlled by a periodic triangle signal. As such, the instantaneous frequency of the chirp is swept through a frequency range continuously during each period of the triangle signal. The sweep rate and the frequency range of the chirp signal can be adjusted by changing the frequency and the amplitude of the triangle control signal. First of all, a triangle wave generator was designed and simulated by using a 555 timer (TLC555). The SPICE simulation schematic of the generator is shown in Figure 2(a) and the simulation results are demonstrated in Figure 2(b). The simulated triangle wave varies from 1.32 V to 3.3 V at a frequency of 52.8 Hz. Then the circuit was fabricated and the output was measured to validate the simulation results. As shown in Figure 2(c), the measured triangle wave is from 1.4 V to 3.6 V with a frequency of 52.1 Hz, which matches the simulated curve very well. The slight discrepancy is due to the tolerance of the lump components. 4.00

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In this study, a VCO (CVC055BHS-5600-5800) chip was used. In order to achieve accurate frequency sweeping range the relationship between VCO input control voltage and output RF frequency was calibrated. A DC power supply is used to send the control voltage from 0.2 V to 6 V with a step of 0.2 V to the VCO. The VCO output was collected using a high frequency oscilloscope and the scope's built-in FFT function was used to determine the dominant frequencies of the received RF signal at different input voltages. The experimental frequency-voltage curve is represented in Figure 3. As expected, the experimental curve has a high degree of linearity 2102


(coefficient of the determination R2 0.999). Base on the calibrated frequency-voltage relationship (as seen from the linear equation of the trend line) ,the frequency sweeping range of the VCO output is from 5.643 to 5.735 GHz if the input triangle voltage ranges from 1.4 to 3.6 V. Since the VCO input impedance is only 50 n which does not match the output impedance of the triangle wave generator there will be a big voltage drop during signal transmission. In order to solve this problem a voltage follower was developed which placed between the triangle wave generator and the VCO. The circuit schematic of the follower is shown in Figure 4. In the design,AD623 op-amp was implemented and servers as unity gain voltage follower. =

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In order to to make the FMCW generator fit in the shoe all the electrical components are surface mounted and soldered on the PCB board. RTjduroid 5880 Laminates are selected as the PCB material because of its low dissipation factor. The PCB layout is represented in Figure 5 and the fabricated circuit is shown in Figure 6. The dimension of the PCB circuit is 5 mm * 4 mm. In the circuit there are two DC sources: 5 V and 12 V. The 5 V DC source provides the power for the triangle wave generator and the VCO. The 12 V one powers the voltage follower. Both of the two DC sources can be powered by a 3.7 V battery. The power consumption of the whole interrogation circuit is around 160 mW. 4 . EXPERIMENT S A N D DATA ANALY SI S

Static pressure tests from 50 kpa to 550 kpa with a step of 100 kpa were conducted to characterize the pressure antenna sensor and FMCW interrogation circuit. First of all, the resonant frequencies of the pressure sensor at different pressure levels were measured using a VNA (ROHDE & SCHWARZ Z VA24) . As shown in Figure 7,the pressure sensor was clamped using a MTS compression machine and connected to the VNA using a coaxial cable. Return-loss Bu curves were recorded every static pressure levels. Then the FMCW interrogation circuit was implemented to replace the VNA. The system setup was demonstrated in Figure 8. The PCB-based FMCW generator was powered by 2103


a 3.7 V lithium battery. A RF circulator (DiTom D3C4080) and a power detector (Mini-Circuits Z X47-60LN_s+) were used for RF signal transmission and antenna return-loss measurement. The DC outputs of the power detector were collected by a high frequency oscilloscope (Lecory Wavepro 760zi) at different pressure levels. The sampling rate of the oscilloscope was set as 50 kHz. Since the frequency of the FMCW signal is 52.1 Hz and the carrier frequency sweeping range is from 5.643 GHz to 5.735 GHz the frequency resolution is (5.735 - 5.64) e9 /(50000/52.1) 96 kHz. =

Figure 7: Setup for static pressure tests using VNA.

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In Figure 9,the detector output voltages of single FMCW period are plotted at different pressure levels. The time scale (x-axle) was converted into the frequency scale based on the calibrated time frequency relationship of FMCW generator. To account for the amplitude variations, the output voltage was normalized respect to its maximum voltages. The antenna sensor's resonant frequencies are achieved at the normalized amplitude "I " and plotted in Figure 10. As comparison, the VNA measured antenna-sensor's resonant frequencies are also plotted. As seen from the figure, the resonant frequencies measured using FMCW interrogation circuit and VNA match every with each other. The discrepancies which are normalized respected to the VNA readings are very small which are within ±0.002%. It proves that the designed FMCW interrogation circuit is accurate enough for pressure antenna sensor interrogation and resonant frequency detection. 5. CONCLU SION S

In this paper,a compact FMCW interrogation circuit for antenna pressure sensor was demonstrated. The interrogation circuit can realize a dynamic interrogation up to 52 Hz with a frequency resolution of 96 kHz. The accuracy of the proposed interrogation circuit was also validated using static pressure testing. The normalized discrepancies between the measurements of FMCW interrogator and VNA interrogator are within ±0.002%. In future,Bluetooth function will be added into the interrogation circuit. Thus, wireless interrogation of the antenna pressure-sensor can be achieved. 2104

2016 Progress In Electromagnetic Research Symposium (PIERS), Shanghai, China, 8-11 August REFERENCES

1. Bus, S. A. and A. de Lange, "A comparison of the I-step, 2-step, and 3-step protocols for obtaining barefoot plantar pressure data in the diabetic neuropathic foot," Clinical Biome chanics, Vol. 20, No. 9, 892-899, 2005. 2. Hughes,R., H. Rowlands, and S. McMeekin, "A laser plantar pressure sensor for the diabetic foot," Medical Engineering & Physics, Vol. 22, No. 2, 149-154, 2000. 3. Lim, T., C. S. Tjin, L. Tay, C. Chua, Y. Wang, and J. M. Brownjohn, "Measurement of contact forces between human and vibrating floors using fiber bragg grating foot sensors," Smart Materials and MEMS, 161-172,2001. 4. Huang, H., "Flexible wireless antenna sensor: A review," IEEE Sensors J., Vol. 13, No. 10, 3865-3872, Oct. 2013. 5. Yao, J., C. Xu, A. Mears, M. Jaguan, S. Tjuatja, and H. Huang, "Pressure sensing using low-cost microstrip antenna sensor," SPIE Smart Structures and Materials Nondestructive Evaluation and Health Monitoring, San Diego, 2015. 6. Sanders,J. W., J. Yao, and H. Huang, "Microstrip patch antenna temperature sensor," IEEE Sensors J., Vol. 15, No. 9, 5312-5319, Sep. 2015. 7. Yao, J., S. Tjuatja, and H. Huang, "Real-time vibratory strain sensing using passive wireless antenna sensor," IEEE Sensors J., Vol. 15, No. 8, 4338-4345, Aug. 2015.

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