Multi-frequency capacitance sensor
 The ratio C.sub.1 /C.sub.2 between the capacitances of two capacitors is obtained by connecting them into the frequency-determining circuit of an oscillator in at least three different combinations, namely C.sub.1 along, C.sub.2 along and both or neither of C.sub.1 and C.sub.2 ; the resultant oscillation frequencies are measured, and a signal representative of changes in C.sub.1 /C.sub.2 is derived from the oscillator frequencies, preferably by means of digital circuitry. In one preferred mode C.sub.1 /C.sub.2 is obtained by forming signals proportional to ##EQU1## and in another preferred mode proportional to ##EQU2## In still another preferred mode C.sub.1 /C.sub.2 -1 is obtained from the expression T.sub.1 -T.sub.3 /T.sub.2 -T.sub.3 and used as the indication of changes in C.sub.1 /C.sub.2. In the foregoing, f.sub.1, f.sub.2, f.sub.3 and f.sub.4 are the oscillator frequencies produced respectively by connecting C.sub.1 alone, C.sub.2 along, neither C.sub.1 nor C.sub.2, and both C.sub.1 and C.sub.2 into the frequency-determining circuit, and T.sub.1, T.sub.2, T.sub.3 and T.sub.4 are the periods of oscillations at f.sub.1, f.sub.2, f.sub.3 and f.sub.4 respectively. 
Moore Products Co.
G
G01R
inventional
Chih C.
Chang