Piezoelectric Materials - Saba Web Page：压电材料，该网页[精品].ppt

1 Seismic Transducers A seismic transducer consists of two basic components: Spring – Mass – Damper Element Displacement Transducer (MS Fig 4.77) (Note: xo = xi – xM) 2 Seismic Transducers – Acceleration Sensor Let’s explore the dynamic response of the spring-mass-damper element alone. Noting the sign conventions in Fig. 4.77, we have, from Newton’s second law for the motion of mass M, xM: (Where xM = xi – xo) (A classic 2nd Order System) We define (again) With the result 3 Seismic Transducer – Acceleration (cont). Let’s associate the following: Input is object acceleration Output is relative displacement of M and object Static Sensitivity (sec2) Classic 2nd Order System 4 Seismic Accelerometer – Freq. Responce 5 Seismic Accelerometer – Freq Response (cont) Question: Over what range of frequencies can we actually use a seismic accelerometer? Answer: To be most useful we desire a “flat” frequency response and a “linear” phase shift. In other words, we need SIG n  ~ 0.4 – 0.6 But recall that 6 Seismic Accelerator – “Readout” The previous discussion ignored the response of the displacement sensor used to measure xo!! We need to consider this! RECALL System 1 qi,1  System 2 qi,2   qo,1  qo,2 7 Seismic Accelerometer i. Resistive Potentiometer Readout 8 Seismic Accelerometer ii. Piezo Readout “Usable” range depends upon damping 9 Seismic Displacement How about a seismic displacement transducer? (We’ll let you do this one as homework). 10 Capacitance Transducers Consider a basic parallel plate capacitor, with C = Capacitance (pF) A = Plate Area (in2) x = Plate Separation (in) If either x or A are changed, then C will change!! 11 Basic Capacitance Transducer Geometries Linear Motion Rotational Motion Typical Capacitance Values 12 Capacitance Transducers – Signal Conversion Capacitance is not easy to measure directly. We need to convert “signal” to current or voltage. a. AC Voltage Approach We apply a constant amplitude AC voltage, Vex, at  = ex This will re