The Research On Wide-measuring-range And Precise I/F Conversion Circuit

In the inertial navigation system?INS?,the quartz flexible accelerometer outputs a current signal,which requires an analog-to-digital conversion to perform navigation calculations.Due to the level restriction of domestic high-performance ADC,coupled with the embargo of the developed countries,the I/F conversion circuit is still the mainstream application in this field.As the name implies,the I/F conversion circuit converts the current signal into digital pulses.Because its working principles are current integration and charge balance,theoretically,it does not lose the accelerometer current signal,and the integration-accumulation principle of velocity and position in the INS is similar.This characteristic of the I/F conversion circuit has very important significance for improving the navigation accuracy,which is also one of the reasons why it has an important engineering application status in the medium and high precision INS.However,subject to the working method of traditional I/F conversion circuit,the three indicators of range,linearity,and resolution are mutually restricted.Under the circumstance that the performance of electronic components is unlikely to produce a qualitative leap at present,it is very difficult to further improve the performance of I/F board from pure hardware,and the room for improvement has gradually deceased.In this context,this paper makes two improvements to the traditional I/F conversion circuit:First,the switching control mode of the constant current source is changed from ternary constant width?or ternary metabolic width?to ternary extended width,which makes the range increased by 60%without reducing the linearity;Second,using FPGA,ADC,and other digital devices,the integral voltage is sampled by ADC,and the FPGA obtains data and logic flags in the mode of ternary extended width,then runs the pulses expansion algorithm and outputs the calculated expansion pulses.In the specific process of the subject,this paper has reorganized the working principle of the traditional I/F circuit,derived the definition and calculation method of circuit parameters of the ternary width-extended I/F conversion circuit.About the hardware circuit design,the analog circuit part focuses on the design of the integrator and the constant current source,some components are reselected and several details are improved;the digital circuit part focuses on the FPGA minimum system and the ADC acquisition circuit,and the control logic of the ternary extended width is integrated into the FPGA chip.The clocks required for the operation of the entire circuit system are synchronous clocks,ensuring the timing strictness.On the basis of the waveform and the analysis of the working characteristics of the ternary width-extended I/F conversion circuit,a pulses'number expansion algorithm is proposed,using the top-down modular program design method,the FPGA program is implemented using the ISE development platform.The test experiment of PCB board is performed according to the I/F board debugging specification,the experimental results are as follows:the ternary width-extended I/F conversion circuit?pulses are not extended?can measure over±40mA with the linearity reaching 5.35×10^-5(better than 1.05×10^-4),its scale factor is 788.2⁷90.5Hz/mA;however the scale factor of the extended pulses is 31.5³1.6kHz/mA,and the pulses'extension multiplier is 39.94⁴0.02,in addition,the full-scale frequency of extended pulses can reach 1.26MHz while the linearity can reach 2.69×10^-5(better than 7.60×10^-5);the experimental results show that the parameters meet the design requirements,and the work of this paper has achieved the established goal.