The application of frequency selection surfaces(FSSs)is limited by large area,narrow bandwidth,low stopband inhibition and large ripple in the passband.A method for designing high-order wide band miniaturized-element frequency selective surface(MEFSS)with capacitance loading is introduced.The proposed structure is composed of multiply sub-wavelength interdigital capacitance layer,sub-wavelength inductive wire grids separated by dielectric substrates.A simple equivalent circuit model,composed of short transmission lines coupled together with shunt inductors and capacitors,is presented for this structure.Using the equivalent circuit model and electromagnetic(EM)model,an analytical synthesis procedure is developed that can be used to synthesize the MEFSS from its desired system-level performance indicators such as the center frequency of operation,bandwidth and stopband inhibition.Using this synthesis procedure,a prototype of the proposed MEFSS with a third-order bandpass response,center frequency of 2.75 GHz,fractional bandwidth of 8%is designed,fabricated,and measured.The measurement results confirm the theoretical predictions and the design procedure of the structure and demonstrate that the proposed MEFSS has a stable frequency response with respect to the angle of incidence of the EM wave in the±30°range incidence,and the in-band return loss is greater than 18 dB,and the rejection in the stopband is greater than 25 dB at the frequency of 3.2 GHz.
The frequency-dependent electrical properties and strain self-sensing behaviour of ultra-high performance concrete(UHPC)as cement-based stress/strain self-sensing(CBSS)smart materials were investigated in the frequency range from 100 Hz to 300 kHz.By using the electrical parameters of the equivalent electric circuit model,the quantitative relations of capacitance and conductance of CBSS with the measurement frequency were derived.The capacitance and the conductance exhibit power-law type dependence on the measurement frequency.The calculated capacitance values at frequencies beyond 2 kHz and conductance values are consistent with the experimental results.The sweep-frequency test and the fixed-frequency test were performed to examine effects of the excitation frequencies on strain self-sensing properties of CBSS.The fractional change in capacitance(FCC)and resistance(FCR)of CBSS are frequency-dependent in the frequency range from 100 Hz to the f_(B),but frequency-independent in the frequency range from the f_(B)to 300 kHz.The f_(A)and the f_(B)are 1.7-4.0 kHz and 11-78 kHz depending on the fiber dosages,respectively.FCC and FCR reach their maximum at the f_(A)and 100 Hz,respectively.The responses of capacitance and resistance of CBSS to strain show good repeatability during cyclic loading.As the fiber dosage increases,capacitance-based sensitivity to strain increases initially and then decreases at the f_(A),and resistance-based sensitivity to strain of CBSS increases with increasing fiber contents.
The congenital yield reduction caused by miss-seeding in spoon-type seed-metering device of small and medium-sized potato planter is huge.Based on the physical mechanism of different measured capacitance values between two fixed capacitor plates with different media,a miss-seeding detection scheme based on a spatial capacitance sensor is proposed first.A simple and efficient spatial capacitance sensor that can obtain as large capacitance measurement value as possible is designed,and a dual CPU coordinated seed-monitoring and compensation control system architecture is adopted.AD7745 is selected for the capacitance measure of the spatial capacitance sensor,and the code of grating encoder is also recorded at the same time.Thereby,when each potato spoon passing through the space surrounded by the capacitor plates,the maximum net capacitance fluctuation and its corresponding position can be acquired.A suitable threshold can distinguish between normal-seeding and miss-seeding effectively.Moreover,it should be emphasized that,this monitoring system only requires one monitoring point.Then,based on obtained information,an improved miss-seeding catching-up compensation plan is put forward.By utilizing the powerful memory capability of the CPU,this system does not need to complete compensation immediately after the miss-seeding identification.Instead,the miss-seeding information and the location of the accident can be just marked in advance,and only when the opportunity arrives,can the miss-seeding catching-up compensation be truly executed.In this way,the position of the seed-monitoring points can be free from restriction,and the control strategy can therefore be significantly simplified.The soil tank test data showed that,the identification accuracy of the miss-seeding detection system was not less than 94%.When the seed-metering chain speeds are 0.2,0.3,and 0.4 m/s,the average success rates of the miss-seeding compensation system are 94.32%,83.65%,and 75.00%,respectively.The final miss-seeding rate can be below 3%,and t
Guanping WangXiaoping YangWei SunYan LiuChengjiang WangHua ZhangXiaolong LiuBin FengHongling Li
