Research On Multi-Channel Quadrature Lock-in Amplification Detection System Of Wind Tunnel Balance And Its Characteristics

The balance signal detection system is the main way to get test data in wind tunnel test.The strain gage balance can convert the aerodynamic load into the corresponding voltage difference signal output,which can be detected,processed and uploaded by the balance signal detection system,which can accurately reflect the wind tunnel aerodynamic test results.The accuracy of wind tunnel balance signal detection directly determines the credibility and analysis value of wind tunnel force test data.From the level of the testing object,the strain gage balance has a rigid structure,the deformation caused by the aerodynamic load is small,and the force signal reflected is weak;from the level of the test environment,the noise is everywhere and the useful signal is submerged.It is difficult to detect accurately.With the diversified development of wind tunnel test technique,the wind tunnel test environment is becoming more and more complex.The interference factors such as aerodynamic noise,mechanical noise and electromagnetic noise in the wind tunnel bring challenges to the detection accuracy and stability of the balance signal detection system.For the design of the balance signal detection system,this dissertation takes the six-component strain gage balance as the research object from two aspects: automatic gain control loop and quadrature lock-in amplifier detection technique.The main work of this dissertation is summarized as follows:1.A wind tunnel balance multi-channel signal detection system based on quadrature lock-in amplifier structure is designed.it mainly includes six-component balance bridges driving signal generation module,orthogonal reference signal generation module,quadrature lock-in amplifier module,data acquisition module and so on.The system has1-channel drive signal output,6-channel balance signal input and 6-channel 18-bit high-precision analog to digital converter.The frequency of both driving signal and reference signal is 10 k Hz and the frequency is adjustable.The common source driving mode is adopted to drive the six-channel strain bridges at the same time,which has a good consistency in the circuit.2.An automatic gain control loop with digital feedback structure is designed.The structure is applied to the driving signal generation module of the six-component strain balance bridge and the quadrature lock-in reference signal generation module.The gain closed-loop control is realized by using the processor and variable gain amplifier.The amplitude stability of the bridge driving signal and reference signal is improved.The test results show that the amplitude fluctuation of driving signal and reference signal is less than0.01 V.3.In the quadrature lock-in amplifier circuit,the influence of the phase difference between the reference signal and the driving signal on the signal-to-noise ratio of the two quadrature detection channels is studied,based on which a signal phase closed-loop control structure with digital phase shift function is designed.The structure uses the processor timer interrupt delay to complete the phase control,which makes the signal-to-noise ratio of the two quadrature detection channels consistent or equal,improves the response speed of the system and ensures the detection stability of the system.Under the laboratory test conditions,the bridge driving signal 1V/10 k Hz and the reference signal 1V/10 k Hz are set.The strain resistance changes with a fixed resistance value of 1Ω.Six quadrature lock-in detection channels of the system are tested.The test results show that the resolution of the system for detecting the balance bridge signal is 39μV,the maximum fluctuation of the detection data is about 60 μV,and the maximum signal detection error is about 10%.It has good detection stability and accuracy,and realizes the function of detecting weak bridge signal in noisy environment.