Optical Pressure Sensor Based On Laser Displacement Measurement

As is an important pressure signal sensing and energy conversion device, in the previous twohundred years, pressure sensors have been gained the great improvement from the progress ofmechanical measurement, the non electrical quantity measurement, Microelectromechanicalsystems (MEMS) to optical pressure sensor so its performance were swiftly promoted tominiaturization with volume and structure, function of intelligence, diversification ofmanufacturing process and materials development. Compared with the MEMS pressuresensors, optical pressure sensor based on laser displacement measurement (LDM), whichconstructed with the pressure chamber, pressure strain diapgragm and laser probe macrostructure, have many better properties of minimal differences, precise mathematical model,and combined with the advantages of high precision and high reliability from the laserdisplacement measuring system. On the other hand, LDM optical pressure sensors wereindependent with special process equipments, and easy to complete the experimental platformfor calibration and real time measurement. In this paper, LDM optical pressure sensor’spressure harmonic model and displacement model, general scheme, signal analysis, signalprocessing, pressure calibration method of harmonic model, set up the pressure, have beenmade the overall implementation. The main research results are as follows:(1) The harmonic model of LDM pressure sensor and the search algorithm for optimalharmonic parameter with variable step are proposed and tested. Strain element analysis ofcircular diaphragm with clamped edge under uniformly distributed load, using thedisplacement function and its boundary conditions of circular diaphragm to set up stressfunction and deflection function. The power function with parameter of lambda, exp [lambda(1-r/a)], was selected to modulate the nonlinear deflection of circular diaphragm. ApplyingBubnov Galerkin variational method, the maximum deflection function and deflectionfunction of circular diapgragm was deduced. The effect on harmonic model by harmonicparameter was examined. Using the harmonic parameters on the modulation characteristics ofharmonic model, the deflection function of S.Timoshenko was regarded as the referencemodel, and deflection error interval with least squares solution for the reference model and theharmonic model reached under the normalization condition to obtain the harmonic parameterrange and harmonic margin under the corresponding relative radius. The search algorithm ofoptimal harmonic parameter with variable steps was poposed with the integral of normalized deflection error which its relative radius was selected in the range of circular membrane, andthe integral result is judged by integral threshold termination condition. S-shaped searchacceleration mode was designed to modify the step size factor, and update the search step.When meet the integral threshold of the termination condition, the search progress stop socurrent harmonic parameter was the optimal one.(2)The reflecting-type measurement model of laser triangulation for LDM optical pressuresensor is proposed and formulated. The structure design for the general scheme are carried outand built the experimental platform. The displacement measurement range for maximumdeflection of circular diaphragm is magnified4.7669times with reflecing-type opticalstructure. Semiconductor laser diode with power of7mW and with wave length of650nm, thelinear array CCD image sensor with pixel pitch of8um and effective pixel number of3648, afocusing lens and a reflector mirror are layouted with optimal placement. The driver circuitfor laser diode, the timing driver circuit for CCD sensor, and analog-front-end are designed.With the composition by different materials which have the thickness of50um and10um,130GPa and205GPa of the elastic modulus, four kinds of pressure strain circular diaphragmare designed. And, the pressure chamber is finished. As a result, the measurement range ofmaximum deflection for laser probe has0~6.12mm. The lookup-table are used to compensatethe pixel imaging offset of linear array CCD image sensor.(3) Signal characteristics of LDM optical pressure sensor are researed and the laserdisplacement model is derived. The deconvolution algorithm with three order maximumcorrelation kurtosis is applied for blind extraction of sub-pixel peak position. Analysing theeffect of surface roughness on the laser intensity signal, skewness and kurtosis of laser signalintensity in the observation sequence are test. Four groups of the sample data with laserintensity signal which are regarded as Weibull probability distribution are used for parameterestimation with maximum likelihood estimation method. The impact of laser intensity signalto subpixel peak position is discussed with the Gauss distribution, sub-Gauss distribution andsuper-Gauss distribution characteristics. Combining the pressure deflection harmonic equationwith deflection displacement equation, the pressure-pixel analysis model is derived. Theinfluence on the characteristics of the input and output by the structural parameters, theharmonic parameters, material parameters are induced. The three order correlation kurtosismaximum criterion, iterative solution of deconvolution inverse filter, convergence andstability conditions, is derived and verified.(4) Pressure calibration and test. Servo motor, ball screw, calibration block and laser probeare established an experiment platform to calibrate laser probe’s displacement measurement. The laser probe obtained the micro displacement of0.25mm per step, and calibration curveand the precision0.5218percentage in measurement range of6mm. And, a pressure chamberand a laser probe are used for pressure calibration platform for four kinds of pressure circulardiaphragm. Pressure calibration input loaded with10KPa per step in the range of0-100KPa,100KPa per step in the range of0-600KPa, and obtain the pressure calibration curve and fullrange calibration precision of2.7055perentage. Finally, LDM optical pressure sensor is carryout the pressure test with four kinds of pressure circular diaphragm which its resolutionreached109.723Pa/pixel,21.9466Pa/pixel,173.0247Pa/pixel and34.6049Pa/pixel,respectively.