Range Measurements

In general, the road width of a freeway including the shoulder of the road is less than 30 meters, and then the maximum range of measurement is specified 30 meters. The frequency of the triangular waveform is 1000Hz to adapt the round-trip time of signal propagation and the sampling rate of DSP chip. Hence the frequency resolution is obtained 2 KHz. In other words, for each 1 meter of the range resolution is corresponding to each 2 KHz of the frequency resolution.

The range of 30 meters is corresponding to the beat frequency of 60 KHz.

Therefore, the bandwidth of the IF filter can set to more than 60 KHz and may extend to 100 KHz for the environmental detection.

The sensor located in the roadside was vertical to the multiple-lanes, since a vertical direction to the target results in a zero-frequency Doppler shift, whether the vehicle is stationary or moving. Range measurements were performed to detect vehicle occupancy in the field tests using the setup in Fig.

6.6. The output signal of the IF amplifier was obtained by the commercial digital signal processor TMS320C6701. The following digital processor parameters were set when performing the signal processes. The pulse-repeated-frequency (PRF) was set to 500.0Hz, and the period of the triangular wave (tm) was 1ms.

According to the curve in Fig. 3.8 (b), the amplitude of the triangular wave required to control the CMOS FMCW transceiver with a modulation bandwidth (BW) of 150.0MHz was 42.5mV. Since the maximum distance between the vehicle and sensor was 30.0m according to Fig. 6.6, the maximum round trip time (τ) was 0.2μs. Therefore, with reference to (2-3) and (2-7) in chapter 2, the maximum beat frequency (fb) and range resolution (R₀) were 60.0KHz and 1.0 m, respectively. A digital signal processor was adopted to obtain the distance

between the vehicle and the sensor according to (2-4), after obtaining f_b from the digitized IF signal. Figure 6.9 plots both the measured and the theoretical beat frequency. The measured beat frequency is a function of the distance calculated by the digital processor: the gradient of the curve was nearly a constant. Since the sensor height was 3m, the maximum linear detection range was 8m, as calculated by trigonometric identity, and the maximum linearity was 4%, as calculated from Fig. 6.9. Therefore, the linearity of range measurement partially corresponded to the linearity of VCO. These measurements confirm that the operation of the proposed CMOS-based FMCW sensor, including the CMOS transceiver and leaky-mode antenna arrays, follows the principles described in chapter 2. An X-band FMCW sensor can thus be feasibly realized by CMOS technology for TMS applications.

5 10 15 20 25 30

D (m) 0

10 20 30 40 50 60 70

Beat frequency (kHz)

Measured fb Theoretical fb

Fig. 6.9 Measured and theoretical beat frequency vs. distance calculated using digital processor.

In theory, the bandwidth of 100 KHz is still far less than the maximum

caused by the system setup. Figure 6.10 also demonstrates the beat frequencies (fb) varied with the widths (D) of the freeway and the real distance (R) from the sensor to each lane is denoted by dashed-line. However, nonlinear effects at less than 8m were caused by the system setup. In particular, at less than 3m, the nonlinear characteristic cannot be compensated by data processing. Fortunately, a range of less than 3m includes only the shoulder of the road. In general, the width of each lane of the freeway is about 3 meters and the transverse width of most of the vehicles is less than 2.3 meters. The two cars can’t occupy in the same lane at the same segment and a vehicle can’t always run on white zig lines of road dividers by the rules of the road. In case of the above assumption, the range resolution of 1 meter is enough to discriminate two cars in neighboring lanes.

Fig. 6.10 System setup of FMCW sensor of the range is less than 11 meters.

The sensor was used to construct a range measurement of TMS, the csc²θ pattern is the key factor that supported a near uniform SNR for measures region.

The near uniform SNR is described that the difference of maximum and minimum echo power was less than 30 dB in the measured region. Since the vary vehicles drive on the different lanes of the highway that include trucks, bus, cars, and so on. The sensor is applied to TMS include the traffic flow analysis

3m

Roadside Shoulder of the road 1st lane 2nd lane

6KHz 8KHz 8KHz 10KHz 12KHz 14KHz 16KHz 18KHz 18KHz 20KHz 22KHz

(D) (fb) R

and the vehicle classification. The range measurement of this sensor is used to analyze the traffic flow from the nearest lane to the farthest lane and the spatial power of range measurement is frequency domain that has no relationship with the amplitude of the echo signal. But the sensor is also applied to vehicle classification that requires a near uniform echo power at the measured region.

For example it can distinguish trucks from cars in same lane with the intensity of echo power. Another focal point is improvement of the dynamic range from IF circuit to digital signal processor unit. In one word, the csc²θ pattern of the leaky-wave antenna just provides a near uniform SNR and compensates some loss of spatial power of 1/R⁴ distribution.

CHAPTER 7

Conclusion

The study presents a CMOS-based FMCW sensor system for the vehicle detection in TMS. The sensor comprises a CMOS transceiver, two planar leaky-mode antenna arrays and the signal-processing unit. The transceiver is fabricated by standard 0.18μm CMOS 1P6M technology with a size of 1.68 mm×1.6 mm.

The electrical performance of both individual blocks and transceiver is experimentally characterized and reported, revealing two significant issues in FMCW sensor design. First, the on-chip VCO achieves a linearity of 3.0% with the modulated bandwidth against the triangular wave amplitude for a frequency modulation of 500MHz at 10.5GHz. The linearity dominates the accuracy of the range measurements in vehicle detections. Second, the on-chip isolation between the transmitting and receiving paths is 55dB. The isolation directly influences the SNR of the sensor system. Additionally, the antenna array in the sensor system operates in leaky mode. Base on measurement results, the antenna gain and the isolation between two adjacent leaky-mode antenna arrays with a spacing of 5.0mm are 18.0dB and 42dB, respectively, at 10.5 GHz. The reported performance measurements indicate that the proposed system has a better isolation than a sensor system with a single antenna and a circulator.

For multiple lanes measurement of TMS, the function of the roadside unit is similar to the csc² type radar, which has been nearly uniform spatial echoes power between detection ranges. The csc² type antenna pattern generally applies to surveillance radar of military application. Such antennas are always

large-scale, high-power and long-distance, and are designed with reflected antenna technology. However, the antenna arrays presented to herein adopt short-range detection and planar antenna. The special pattern of antenna arrays is specific to leaky mode, and conditionally compensates for the space loss of 1/R⁴. In other words, the function of the presented antenna pattern is similar to the sensitivity time control (STC) of the IF filter, which provides the spatially uniform SNR of range measurement.

The future works of the thesis will use the CMOS integrated circuit technology to miniaturize and integrate all the circuits of all parts, such as radio frequency circuits, analog circuits and digital circuit and so on. The miniaturized radar system integrates with RFIC, AIC, and DIC into a system on chip (SOC).

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博士候選人資料

姓 名 ：張繼禾

性 別 ：男

出生年月日 ： 民國 52 年 6 月 15 日

籍 貫 ： 四川省自貢市

學 歷 ： 中正理工學院 電機系 學士 (72.07～76.07) 中山大學 電機研究所 碩士 (78.09～80.06)

經 歷 ： 陸軍第十軍團七四通信兵群排長 (76.08～78.08) 陸軍第一 O 九機械化師通信排長 (80.07～81.03) 陸軍通信基地勤務處通材修護官 (81.04～82.01) 中山科學研究院 副工程師、工程師(82.01～今 )