– utilize capacitance change
• due to changes in the position of the dielectric material between the two pieces
• due to the shifting position of one of the pieces and the area of the face-to-face
• due to the addition of distance between the two keeping
Figure 3.31 Capacitive position sensor: (a) horizontal media shift, (b) rotary shift, (c) shifting plate distance
– the capacitance value is directly proportional to the area and is proportional to the distance
C = 0.0885 d A k
– quite sensitive but bad linearity
– bridge circuit as in inductive sensor can be used with capacitor connected parallel with high resistance to give DC path for opamp input
– the second alternative changes the capacitance change to the oscillator frequency change
• center frequency 1 – 10 MHz
• frequency change for capacitance change is quite small compared to Co capacitance
Figure 3.32 Position sensor use in electronic circuit:
(a) the capacitance becomes frequency, (b) the capacitance becomes pulse
– Inexpensive circuit solution with dual inverter CMOS relaxation oscillator