Fads research based on Wireless SAW pressure senso

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Fads research based on Wireless SAW pressure sensor

1 introduction

fads uses pressure sensor arrays distributed at different positions on the front perimeter of the aircraft (or both sides of the wing) to measure the pressure, and indirectly obtains the dynamic and static pressure through calculation, so as to obtain atmospheric data such as vacuum velocity, Mach number, air pressure altitude, etc. NASA's Dryden Flight Research Center began research on embedded atmospheric data systems in the 1860s. This kind of sensor system is used in many aircrafts such as F-14, f/a-18, X-31, X-33, X-34 and X-38, but it uses traditional pressure sensors and requires lengthy cables, which is not conducive to use in smaller weapons and ammunition

wireless surface acoustic wave pressure sensor has the advantages of small volume and wireless measurement. Therefore, using wireless surface acoustic wave pressure sensor, the embedded atmospheric data system can apply the embedded atmospheric data system to smaller weapons and ammunition, and combine it with small and cheap strapdown inertial navigation system to form a cheap but high-precision integrated navigation system, which can be conveniently used to improve the hit accuracy of small ammunition

2 basic principle

fads is generally installed in the front of aircraft. In order not to affect the installation of radar and fire control devices, fads is also installed in the front of the wing. The fads of F-14 is composed of 23 pressure sensors, which are installed at the front of the fuselage. The fads system of X-33 is composed of six pressure sensors, which are installed at the front of the fuselage. There is no fixed regulation on the number of pressure sensors. On the F-14 aircraft, the layout of its fads

pressure sensor is shown in Figure 1

the more the number of pressure sensors in fads system, the better the fault tolerance performance, but the more complex the calculation of the system, the higher the system performance requirements. However, due to the corresponding pressure difference required to measure the angle of attack and side slip angle, there must be a relatively symmetrical pressure measurement point around the center point

the aerodynamic model of fads system combines the potential flow model and the modified Newtonian flow model (the former is mainly applicable to subsonic conditions after the end of the experiment, and the latter is mainly applicable to supersonic conditions) with a correction coefficient ε Combined, the aerodynamic model with compensation under different Mach conditions is formed. ε The value of is selected by comprehensively considering the compression effect, aerodynamic shape, system influence and other factors. In flight, it can be regarded as a function of angle of attack, sideslip angle and Mach number, and its functional relationship can be determined before flight

the derivation process of aerodynamics is omitted here, and a complete aerodynamic model of fads system is given.

in the formula: Pi is the pressure measured by the I (i=1, 2... 23) pressure sensor (referred to as point I); QC is dynamic pressure; P ∞ is static pressure; M ∞ is Mach number; ε Is the forming pressure coefficient; α Is the angle of attack; β Is the sideslip angle; φ I is the circumferential angle of point I; λ I is the cone angle of point I; θ I is the incident angle of point I (the angle between the surface normal direction of this point and the incoming velocity vector); G function is a definite monotone function. Through the measurement of pressure at the pressure point and the corresponding algorithm, the values of dynamic and static pressure, Mach number, angle of attack and slip angle can be obtained. Through these values, atmospheric parameters such as air pressure height and vacuum velocity can be calculated

3 Wireless SAW pressure sensor

fads is installed in the front end of the aircraft. Electronic force measurement requires a small pressure sensor for point measurement. The wireless SAW pressure sensors reported in literature [4], [5] are all realized through delay lines. The insertion loss and propagation loss of delay lines are large, which affect the telemetry distance of SAW pressure sensors. Moreover, the shape of SAW delay lines is flat and long, which is not suitable for consideration before adjusting various control conditions. 1. It is suitable to be installed in aircraft tensile testing machine, pressure testing machine, steel bar bending testing machine There are thousands of kinds of shear tear testing machines, and the front end measures the pressure at a point. The structure of SAW pressure sensor shown in Figure 2 can change the shape of SAW pressure sensor and increase the wireless measurement distance of SAW pressure sensor

unlike saw pressure sensor with SAW delay line structure, the structure shown in Figure 2 adopts two single ended resonators in parallel. When the SAW resonator resonates, the Rayleigh wave is superimposed many times through the reflection grid, and its energy is superimposed many times, so the propagation loss and insertion loss are relatively reduced, and it has a high Q value. At the same time, SAW resonator has high sensitivity, high accuracy, and can remain stable for a long time. Using this structure design to measure pressure wirelessly can reduce the propagation loss and insertion loss of SAW pressure sensor, so this principle structure has good application potential

when there is pressure on the surface of the sensor, the piezoelectric film of the wireless SAW pressure sensor will deform, and the strain of the film material will change the propagation speed of the surface acoustic wave, thus changing the central resonant frequency of the surface acoustic wave. By wirelessly detecting the change of central resonant frequency of SAW pressure sensor, the data of pressure change can be obtained. Assuming that temperature has little effect on the central resonant frequency of the two single ended resonators, the relationship between the change of the central frequency difference of the two resonators and the measured pressure can be expressed as

where S1 and S2 represent the pressure sensitivity coefficients of single ended resonator 1 and single ended resonator 2 respectively, which is related to the parameters of single ended resonator and diaphragm. The pressure sensitivity coefficient can be expressed as

, where: R is the radius of the diaphragm; H is the thickness of diaphragm; E is young's modulus; μ S is Poisson's ratio; R1 and R2 are the linear coefficients of the substrate material of two single ended resonators to the mechanical disturbance

the pressure value can be obtained by wirelessly measuring the change of the central frequency difference between the two resonators of saw, which greatly improves the application flexibility of the embedded atmospheric data system

4 wireless measurement structure

in the embedded atmospheric data system, the advantages of wireless measurement of SAW pressure sensor can improve the flexibility of its application. Figure 3 shows the wireless measurement structure of SAW pressure sensor

the whole wireless measurement system is composed of signal interrogation and signal receiving circuits. The signal transmitting device is composed of a reference oscillator, an RF pulse generator and a receive/transmit (t/r) transfer switch; The signal receiving device is composed of integral down converter and two-way a/d converter. The signal processor module processes the data obtained after a/d conversion, and obtains the phase orthogonal I-channel signal and Q-channel signal. By processing the I-channel signal and Q-channel signal, the frequency and phase change information generated by the SAW sensor due to physical changes can be obtained. At the same time, the signal processor module generates pulse control signals and receive/send conversion signals to coordinate the conversion of reception and transmission

since it takes time for the pulse signal to be transmitted, processed in the SAW sensor and returned after the signal receiving and transmitting circuit transmits the pulse interrogation signal, it is necessary to define mutually independent transmission and reception intervals and convert them through the receive/send switch. The reference oscillator generates an interrogation signal when the pulse signal is sent, and provides a local oscillator reference signal for the integral down conversion when the signal is received. After down conversion, it is digitized by dual a/d converter, and the reception and transmission of signals are controlled by programmable logic devices. After signal processing, it is connected with the inertial navigation system to form a small integrated navigation system

5 simulation results

the overall performance of SAW pressure sensor can be analyzed through the equivalent circuit of saw single ended resonator. Figure 4 shows the equivalent circuit of saw single ended resonator

in the figure: C1 represents the dynamic capacitance caused by the elasticity of the substrate; L1 is the dynamic inductance caused by substrate inertia; R1 is the dynamic resistance caused by damping; C0 is the static capacitance of the interdigital transducer. The saw pressure sensor is obtained by two single ended resonators in parallel. Through the simulation of HP eesoft software, the relationship between the echo admittance amplitude and frequency of the two resonators can be obtained. The simulation results are shown in Figure 5

it can be seen in Figure 5 that the two SAW resonators have two central resonant frequencies, 434 MHz and 434.4 MHz respectively. These two frequencies are within the measurement range of ISM standard wireless frequency, which can effectively carry out wireless measurement. (end)

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