33/51 4.3 Relative Displacement:Translational and Rotational VIll.Capacitance Pickups Capacitance of two parallel plates with air in the gap: C==0.259 (4.41) 4πkX where ccapacitance,pF A≌plate area,in2 xAplate separation,in 1 in =2.54cm) k会Coulomb constant(静电力常量) Use current source or voltage source to measure C? 34/51 4.3 Relative Displacement:Translational and Rotational VIll.Capacitance Pickups Gold-coated surfaces Glass disks (Stationary plates of capacitor) Stretched stainless steel diophragm (Movable plate of capacitor) Pressure 2.500 Hz Pressure b Figure 4.36 Differential-Capacitor Pressure Pickups4.3 Relative Displacement: Translational and Rotational VIII.Capacitance Pickups ‹ Capacitance of two parallel plates with air in the gap: ఌబఢೝ ସగ௞ ୅ ୶ ୅ ୶ (4.41) where c≜capacitance, pF A≜plate area, inଶ x≜plate separation, in ( 1 in =2.54cm) k≜Coulomb constant (静电力常量) ‹ Use current source or voltage source to measure C ? 33/51 4.3 Relative Displacement: Translational and Rotational VIII.Capacitance Pickups Figure 4.36 Differential-Capacitor Pressure Pickups 34/51