Capacitive Imaging Research Articles

Theoretical model and optimal design of silicon micromachined ultrasonic imaging transducers

A theoretical model and mathematical description for silicon micromachined electrostatic or capacitive ultrasonic imaging transducers have been developed. According to the model the basic performance parameters of such a transducer, such as natural frequencies, eigenfunctions, resonance and anti-resonance frequencies, and the mechanical impedance of the diaphragm can be predicted from the geometry of the transducer and property parameters of materials used. The paper reveals that this type of transducers has two basic operation modes, corresponding to the resonance of a mass-spring oscillator comprised of the diaphragm and the air cushion, and the first-order bending mode of the diaphragm itself respectively, and presents an optimal method for extending the bandwidth by making the two modes coupled, and thereby provides a theoretical basis for the optimal design.

Capacitive Resistivity Imaging with Towed Arrays

The capacitive resistivity (CR) technique is a generalization of the conventional DC resistivity method that facilitates measurements of electrical resistivity on engineered surfaces and highly resistive ground. The CR methodology allows the use of towed sensor arrays, thus enabling the rapid collection of high-resolution resistivity data. Under quasi-static conditions, CR data are equivalent to galvanic DC measurements so that CR datasets can be interpreted with conventional DC inversion algorithms. In this study, we demonstrate that the methodology and fundamental parameters of the CR technique facilitate spatial sampling at the centimeter scale for towing speeds of the order of [Formula: see text]. We argue that the dipole-dipole array is the most suitable geometry for dynamic CR measurements and present example data acquired with a prototype instrument using plate-wire combinations arranged in an equatorial geometry. The information content of raw CR data is found to be dominant over towing-induced noise and a direct comparison with DC profile data shows good agreement between both techniques. Based on these findings, we show that tomographic imaging is possible using datasets acquired with moving arrays. Closer investigation of the practical aspects of towed-array capacitive resistivity imaging (CRI) highlights the similarities with galvanic multi-electrode surveys, but the different geometric constraints and sampling regime of CRI give rise to advantages (high lateral resolution) as well as disadvantages (limitations in vertical resolution). We conclude our study with recommendations for practical CRI survey procedures and present a field example where we have successfully imaged a subsurface target in 3D. Towed-array CRI is found to provide equivalent (and in some ways superior) information about the shallow subsurface when compared to DC ERT (electrical resistivity tomography); it therefore widens the scope of electrical imaging surveys to environments where the conventional methodology would be impractical.

Single-sided capacitive imaging for NDT

A capacitive imaging technique has been developed to characterise the internal features and properties of a wide range of materials and structures. The new technique is very simple in operation, using two coplanar electrodes on one surface of the sample. Images have been obtained using this technique, and are presented in this paper. These include the imaging of changes in metals, insulators, fibre-reinforced polymer composites, and concrete. Underwater operation is also demonstrated.

Inductive and capacitive array imaging of buried objects

Despite ongoing research and development efforts, landmine and unexploded ordnance infestation continues to be a problem. Remediation efforts typically utilise inexpensive hand-held metal detectors that rely on the principle of electromagnetic induction. While these devices often detect all relevant metal objects, they generally cannot discriminate between harmful objects and harmless clutter. False indications continue to far outnumber verified detections. One promising method for improving the discrimination between harmful and harmless objects is the use of sensor arrays that provide images of hidden objects and complementary information about the buried object from a variety of angles. This paper will review the progress of high-resolution inductive and capacitive sensing array development for buried object detection. These sensor arrays use unique designs incorporating a single drive with multiple sense elements. The drive creates a shaped magnetic or electric field pattern that concentrates the field energy into longer wavelength spatial modes with 'focused' penetration into the ground. Images obtained from scans over buried objects provide a basis for spatial filtering and signal processing while multiple sensor arrays placed at different positions within the drive provide different 'views' of buried objects and clutter. Model-based grid measurement methods will also be reviewed as a real-time method for multiple property measurements, and real-time data analysis/image generation.

An integrated charge amplifier for a smart tactile sensor

A novel circuit for read-out of a sensor capacitance is presented. This circuit is generally referred to as a charge amplifier, and is implemented in a bipolar full-custom design for the accumulation of tactile data from a tactile imaging sensor. A capacitance is measured with two local feedback Miller capacitances in a trans-impedance amplifier, whereby the output signal is a direct measure of the deflecting capacitance. The performance of this circuit is estimated theoretically and measurements prove its capability to detect capacitance changes of 15–20 fF from a nominal capacitance of 1.25–2.5 pF. This new design features a reduced number of circuit components, provided that they are properly selected to minimize their inherent effect on the transfer and linearity of the amplifier. It is shown that the charge amplifier can be included in a capacitive pressure imaging array with high spatial resolution. An additional circuit element to select the tactile data from a prototype pressure-sensitive touch array is reported as a preliminary experiment for the construction of a fully-integrated intelligent capacitive tactile sensor.

Integrated tactile imager with an intrinsic contour detection option

This paper presents a 9 row by 9 column capacitive tactile imaging sensor. As it can act like a differential operator, it is intrinsically suitable for edge detection. The sensor consists of a conductive rubber layer placed on top of an isolating rubber layer, which is in turn attached to a silicon wafer. The 28 × 28 mm integrated circuit is covered by 81 square-shaped aluminium electrodes and also contains the addressing logic and the signal-conditioning circuits. Each electrode establishes a tactile element (taxel). The operation involves the capacitive measurement of the indentation pattern of the non-conductive natural rubber layer, caused by the pressure distribution acting on the sensor surface, when scanning the taxels. The local deformation of the isolating rubber between a selected square and the common conductive rubber electrode causes a change in the capacitance as low as 1 fF, which can be detected using a specially designed driving method and phase read-out.