Thermal-based Flexible Micro Sensor Array For Shear Stress Measurement

Shear stress is an important physical parameter in the process of fluid control and it is of great application value in drag reduction of vehicles,diagnosis of medical diseases and military equipment monitoring,etc.Micro array composed of multi-channel shear stress sensors may be qualified for temporal and spatial distribution measurement,which is the foundation of active closed-loop flow controlling.Shear stress sensor on the basis of heat transferring and flexible polymer film can be of great flexibility and it is suitable for shear stress measurement on irregular walls.Micro sensor array in this dissertation has structural characteristics of non-suspended hot-wires,polyimide-platinum-polyimide sandwiched,concentrated and dispersed leading-wires combined,backside electrical contracts,which can be manufactured by microfabrication process with multi-layer flexible polymers.Output errors induced by fluid temperature disturbance in air and water can be compensated respectively by static and dynamic temperature compensation methods,which are also useful in other fluid occasions.After calibration,the micro sensor array and its testing system can be used with application features of miniaturization,array type,suitable for irregular walls and water/air dual uses.Main accomplishments of this dissertation can be summarized as follows:1.Principle analysis and structure optimization design of the thermal-based flexible shear stress sensor.There is a significant linear correlation between hot-wire's power and cube root of fluid's shear stress in the effective working range,which can be used to calculate the value of shear stress by measuring electrical properties of the hot-wire.Polyimide and platinum are selected as structural materials of the flexible substrate and the hot-wire respectively,which can offer great flexibility and reduce heat consumption in the film.Compensation units are adjacent and parallel to the hot-wires so that resistance variation ratios induced by fluid temperature disturbance or film stretching are almost the same,which can be compensated by temperature compensation methods.Combination of concentrated and dispersed leading-wires can reduce the amount of leading-wires and welding-pads greatly,which is beneficial for connecting the sensor array and the testing circuit conveniently.Welding-pads are arranged on the opposite side of the hot-wires so that film installation and electrical contracts are easy to implement.By this method,extra protections for welding-pads and connection wires are not necessary and connection wires will not be attacked by the flowing fluid,which ensures the reliability of the system.Planar and sectional temperature analyses are carried out qualitatively and quantitatively,based on which relations between power,fluid heat flux,shear stress and fluid temperature are studied.The designed dimension of the hot-wire is 1000×5×0.2?m³ and its theoretical resistance is 106?,which are determined by the optimization analysis and design.The overall thickness of the film is only 50?m.2.Manufacturing of the micro sensor array based on flexible microfabrication technology.Detailed fabrication process of the micro sensor array is introduced,including spin coating and thermal curing of polyimide,metal sputtering of hot-wire layer,electroplating and patterning of copper mask layer,nickel leading layer and nickel welding layer,etching of hot-wire layer with mask layer patterned,sputtering of waterproof layer.Analyses and selections are carried out for key procedures,including fabrication and release of the flexible film,shaping of the miniature hot-wires and bonding of several structural layers.Flexible micro sensor array with 20 channels is eventually fabricated,which can be used in air or water.3.The constant-temperature mode and its corresponding temperature compensation circuit.Equivalent models for thermal-electric and electric-thermal transforming are established based on the working principle of the thermal shear stress sensor.System simulation model is built with the combination of sensor and constant-tempearture circuit.In the constant-tempearture mode,the temperature of the hot-wire remains unchanged.Output voltage will increase with the fluid shear stress increasing,while it will decrease with the fluid temperature increasing.With the static temperature compensation loop working,output errors induced by fluid temperature disturbance decrease dramatically by adjusting the magnification of the compensation loop.With the static compensation circuit working,the temperature of the hot-wire increases synchronously with fluid temperature increasing and output voltage is independent of the fluid temperature.The error analysis shows that smallest fluctuation and minimum deviation between output voltage and initial value can be achieved by adjusting the compensation region.The shear stress sensitivity in some fluids are relevant to the fluid temperature.Thus,temperature function should be established and dynamic compensation module is connected to the output of the static temperature compensation circuit to realize temperature compensaiton in different fluids.4.Measurement and calibration of the flexible thermal-based shear stress micro sensor array.Preliminary characterizations reveal that thermal coefficient of the resistances are 2137 ppm/°C in average and the linearity is better than 99.96%.The resistance of the hot-wire is almost constant when the film is bent in parallel mode,while it is slightly increased due to streching when the film is bent in vertical mode.Waterproof feasibility is verified and the film can operate in water for a long time.Measured start-up power and power-temperature coefficient of the hot-wire are 2.5 mW and 7.66°C/mW respecvively.Maximal working temperature of the hot-wire is higher than550°C.In addition,correction coefficient should be introduced in the magnification of the compensation loop between the theoretical values and the measured ones.Correction coefficient is set to 0.83 in this dissertation.Temperature distributions in different channels are measured in the same time and the results show that the hot-wires in different channels do not interfere with each other and the compensation units hardly heat up.The output stability and the start-up repeatability are satisfactory.Overheat ratio of the resistance is proportional to the fluid shear stress response.Sensitivity in air and water are measured 0.092 V²/Pa^1/3 and 0.0123 V²/Pa^1/3 respectively through wind tunnel and circulating water channel,which verifies the feasibility of the micro sensor array working in different fluids.