Capacitive Sensing

Capacitive sensing elements are incorporated into either electrical oscillator circuits or into a.c. deflection bridge circuits. An example of the latter is shown in Fig. 6.3117. In this the output voltage is ... 

Fig. 10.12 Electric field distribution in the capacitive sensing element utilizing time transients as...

Fig. 10.13 Transient signals obtained from the capacitive sensing element (Fig. 10.12), used for multivariate dynamic analysis (Kummer et al., 2006). Data for (a) pure alcohols and (b) ethanol/toluene mixtures of different ratios...

The use of capacitance measurement is based upon the principle that the concentration changes as particles settle through a suspension and will alter the effective dielectric constant between the sensing electrodes. A complete capacitance transducer consists of capacitance sensing electrodes together with capacitance sensing electronics, which is essentially a capacitance to voltage converter. 

The use of glass is also very common in other sensors than accelerometers using capacitive sensing technology. 

The testing concepts described so far make explicit use of the fact that additional drive electrodes, together with those used to capacitively sense a primary sensor input signal, may be used for built-in tests. Design for test concepts therefore appear to be limited to the class of sensors employing reversible transducer mechanisms (i.e., capacitance or piezoelectricity) and to their processing sequences. 

Comparisons of published results suggest that capacitive sensing is as good or better than alternative techniques, including tunnelling, piezoresistive, and piezoelectric interfaces [2], Other attractive features of capacitive interfaces include compatibility with many sensor fabrication processes, low temperature coefficient, and low power dissipation. 

Many different arrangements are used for capacitive sensing. Parallel-plate type interfaces sense variations of the plate spacing, z. Conversely, overlap type interfaces such as combs detect variations of the effective capacitor area. The two approaches and combinations thereof differ in sensitivity, displacement range, electrostatic force, and electrical spring constant generated. 

Fig. 6.1.8 Capacitive sensing with a) high, b) low impedance interfaces...

Capacitive readout circuits, such as that shown in Fig. 6.1.13 can be readily implemented with off-the-shelf components using operational amplifiers for single ended [25, 26] or differential designs [27, 28]. Special purpose chips for capacitive sensing [29, 30] are also available. 

Dielectric Sedimentation Method. The measurement relies upon the sensing of changes in the dielectric constant of medium between a pair of capacitance sensing plates placed around the sedimentation tube. 

Capacitance Method, Fig.1 Schematics of the capacitive sensing method. When a microparticle, like a cell, passes between a pair of electrodes, the capacitance of the system will be changed... 

Chen JZ, Darhuber AA, Troian SM, Wagner S (2004) Capacitive sensing of droplets for microfluidic devices based on thermocapillary actuation. Lab Chip 4 473 80... 

Integration of interdigital electrodes (IDEs) with microfluidic technology has been implemented for capacitance sensing in many bio-detection... 

Left) Illustration of the droplet-based inteifacial capacitive sensing principle and the equivalent electrical circuit... 

Murran reported a position sensor based on capacitance sensing methods [5]. The equivalent electrical circuit model of a pair of electrodes (ground electrode and control electrode) with an electrolyte droplet is shown in Fig. 7a. The equivalent capacitance is given by... 

Nie B et al (2012) Droplet-based intcrfacial capacitive sensing. Lab Chip 12 1110... 

After each iteration, to evaluate the test results, the test-outcome droplets are routed serially from pseudo-sinks to a capacitive-sensing circuit coimected to the electrode for the sink reservoir. The capacitive-sensing circuit can produce a pulse sequence corresponding to the detection of multiple test droplets. 

After the test-outcome droplets are read serially, the capacitive-sensing circuit generates a pulse sequence corresponding to the detection of these droplets. An additional evaluation step is required to analyze these pulse sequences to determine whether the microfluidic array under test has a defect. For example, if a row/column of an array is faulty, there is no generated pulse in... 

As described before, a lot of electrical activity is going on in the human body. Heart, muscle and brain activity are aU measurable because they cause a variation in electrical potential that is observable at the skin using electrodes. And that is also why the sensors that are developed to measure other physiological parameters are mainly based on electrical- and capacitive-sensing mechanisms. 

J. Wu, G.K. Fedder, L.R. Carley, A low-noise low-offset capacitive sensing amphfier for a 50-pg/VHz monolithic CMOS MEMS accelerometer, IEEE Journal of Solid-State Circuits 39 (2004) 722-730. 

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