Capacitive proximity sensors of this proposed type would be based on the capaciflector concept, with an extension of hardware and software designs to incorporate capabilities for scanning in frequency and analyzing the resulting capacitance-vs.-frequency data. The proposed frequency-scanning sensors would perform all the functions and offer all of the advantages of capaciflectors; in addition, they would provide information on materials within their capacitive-sensing ranges.
Capaciflectors and related topics have been described in a number of prior articles in NASA Tech Briefs during the past several years. A typical capaciflector includes a sensing electrode and a driven shielding electrode, both excited at the same voltage, frequency, and phase via voltage followers. The voltage follower that drives the sensing electrode also includes an operational-amplifier circuit for measuring the sensing-electrode current, which includes a component proportional to the excitation voltage and to the capacitance between the sensing electrode and any objects in the vicinity.
The figure illustrates the basic measurement principle of a frequency-scanning capaciflector. In the case of a nearby dielectric object, the sensed capacitance can be regarded as two capacitances in series: an airgap capacitance between the sensing electrode and the facing surface of the object, and a capacitance between the facing surface of the object and electrical ground. The dc values of these capacitances depend partly on the geometry of the electrode and the object. The ac capacitance through the object is proportional to its dc capacitance and varies with frequency according to the frequency dependence of the permittivity of the object material. Thus, it is possible to use the shape of the measured capacitance-vs.-frequency curve to differentiate between the effect of geometry and the effect of the object material, and to distinguish among materials with different permittivity-vs.-frequency curves.
A computer could store data on capaciflector responses as functions of frequency for sensed objects made of a variety of known materials. Then the computer could compare data from a capaciflector frequency scan with the stored data by use of a mean-square estimator with the amplitude of the frequency response as a free variable:
mina,m ∑(arf,m - cf)2,
where a is the unknown amplitude, rf,m is the stored frequency response of the mth material at frequency f, and cfis the measured capaciflector response (sensing-electrode current or proportional signal) at frequency f. That is, the computer could choose a combination of amplitude and material to minimize the mean squared error between the measured response cf and the stored response rf,m.
Some applications for this sensor include rapid determination of rock types, determining the type of snow and ice accumulation on aircraft wings, and determining if passengers are carrying weapons or explosives.
This work was done by Charles E. Campbell, Jr., of Goddard Space Flight Center.
This invention is owned by NASA, and a patent application has been filed. Inquiries concerning nonexclusive or exclusive license for its commercial development should be addressed to
the Patent Counsel
Goddard Space Flight Center; (301) 286-7351
Refer to GSC-13618