以壓電材阻抗為依據之荷重量測元件之數值模擬研究

Abstract

結構桿件荷重值是結構安全監測的重要數據之一，其可由荷重元(Load Cell)量測得知。傳統荷重元係利用應變計和橋式電路組合而成，當其受到了拉力或壓力時，將產生與作用力成正比的電壓輸出。壓電材料的阻抗反應能代表其受激勵下的動態特性。本研究的目的係利用有限元素法(ANSYS)模擬以壓電材阻抗值為依據之荷重量測元件。此荷重量測元件係由壓電材料黏貼於鋁塊(母材)上組合而成，欲量測之荷重(壓力)系加諸於鋁塊上。模擬過程係於母材施加不同荷重，再分別對壓電材料施加激勵電壓後分析得到各別相對的阻抗值。並探討不同外力作用於荷重量測元下，其外力與阻抗變化的關係，並探討外力與動態勁度 k_str 之關係。模擬結果顯示，壓電阻抗為主的荷重量測元件之外力與其阻抗變化之關係近乎呈線性變化。外力與動態勁度 k_str 亦是。表示利用壓電材料的阻抗反應能夠有效的反應鋁塊所受的外力多寡，此結果將有利於未來壓電式荷重元之發展。

External Load of structure members, which can be measured by load cells, is one of the important data for structural health monitoring. Traditional load cells are based on a combination of strain gages and the bridge circuit. When under tension or compression, they produce a voltage proportional to the force output. Impedance of piezoelectric materials represents its dynamic characteristics under external excitation. This study presents a numerical simulation of a piezoelectric-impedance based load measurement unit using finite element method (ANSYS). This load measuring unit is a combined form, consists of an adhered piezoelectric material and an aluminum block (base metal), to measure external load (compression) is imposed on the aluminum block. Simulation of different load applied on base metal, the respective relative impedances of piezoelectric material are analyzed based on the different excitation voltages. Under the different external loads to the measuring element, the relationship between the piezoelectric-impedances with external load and the piezoelectric-impedances with dynamic stiffness of k_str are investigated. Simulation results indicated that the measured external loads to the impedance changes is almost linearly changes of the load measuring unit. External load and dynamic stiffness of k_str is also in linear. The simulation results also reveal that the piezoelectric impedance response is able to express the amount of external load applied on the aluminum block. This consequence will be conducive to the future development of piezoelectric-based load cells.