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A High-Precision Capacitive Sensor System For D...

The Accumeasure product line features capacitive sensor products for high resolution gap and displacement measurements that require a high level of accuracy that is both stable and repeatable. A capacitive sensing system consists of a probe and amplifier. Capacitive measurements can be performed in a multitude of environments using non-contact passive capacitance probes that are not affected by magnet fields, temperature, humidity, nuclear radiation, and pressure. This is largely due to the nature of the rugged and passive capacitance probes.

A High-Precision Capacitive Sensor System for D...

MTI Instruments offers high speed, high resolution and highly accurate 1D laser sensors (non-contact linear position sensor) for measuring displacement and position. Our laser measurement systems allow easy position setup and alignment with its visible laser position spot (check out laser safety standards before handling laser sensors). Quality and process control applications that require accurate warpage, alignment, lead position on integrated circuits, sheet and web thickness measurements can be precise and repeatable as our 1D laser displacement measurement sensors are equipped with an auto gain feature making it unaffected by surface (i.e. energy reflections) texture, color or stray light. Our large line up of non-contact displacement sensors including laser triangulation sensors provide operating distances to 300mm, measurement ranges to 200mm and measurement resolution to less than 1 micron.

MTI Instruments offers large measurement range and standoff distance fiber-optic measurement sensors and probes that provide ultra-sensitive linear output response. Our modular (interchangeable) non-contact fiber optic sensors systems have dual channel that enables the user to make simultaneous measurements. Our fiber optic probes are designed to automatically compensate large changes in reflectivity making it possible to monitor dynamic reflectance of up to 100:1 making it ideal for measuring vibration of ultrasonic horns, modal analysis of disk drive suspension, displacement and timing of fuel injectors and lateral motion measurements.

There are two general types of capacitive displacement sensing systems. One type is used to measure thicknesses of conductive materials. The other type measures thicknesses of non conductive materials or the level of a fluid.

A capacitive sensing system for conductive materials uses a model similar to the one described above, but in place of one of the conductive plates, is the sensor, and in place of the other, is the conductive target to be measured. Since the area of the probe and target remain constant, and the dielectric of the material in the gap (usually air) also remains constant, "any change in capacitance is a result of a change in the distance between the probe and the target."[4] Therefore, the equation above can be simplified to:

Where α indicates a proportional relationship.Due to this proportional relationship, a capacitive sensing system is able to measure changes in capacitance and translate these changes in distance measurements.

The operation of the sensor for measuring thickness of non-conductive materials can be thought of as two capacitors in series, with each having a different dielectric (and dielectric constant). The sum of the thicknesses of the two dielectric materials remains constant but the thickness of each can vary. The thickness of the material to be measured displaces the other dielectric. The gap is often an air gap, (dielectric constant = 1) and the material has a higher dielectric. As the material gets thicker, the capacitance increases and is sensed by the system.

One of the more common applications of capacitive sensors is for precision positioning. Capacitive displacement sensors can be used to measure the position of objects down to the nanometer level. This type of precise positioning is used in the semiconductor industry where silicon wafers need to be positioned for exposure. Capacitive sensors are also used to pre-focus the electron microscopes used in testing and examining the wafers.

In the disc drive industry, capacitive displacement sensors are used to measure the runout (a measure of how much the axis of rotation deviates from an ideal fixed line) of disc drive spindles. By knowing the exact runout of these spindles, disc drive manufacturers are able to determine the maximum amount of data that can be placed onto the drives. Capacitive sensors are also used to ensure that disc drive platters are orthogonal to the spindle before data is written to them.

Capacitive displacement sensors can be used to make very precise thickness measurements. Capacitive displacement sensors operate by measuring changes in position. If the position of a reference part of known thickness is measured, other parts can be subsequently measured and the differences in position can be used to determine the thickness of these parts.[5] In order for this to be effective using a single probe, the parts must be completely flat and measured on a perfectly flat surface. If the part to be measured has any curvature or deformity, or simply does not rest firmly against the flat surface, the distance between the part to be measured and the surface it is placed upon will be erroneously included in the thickness measurement. This error can be eliminated by using two capacitive sensors to measure a single part. Capacitive sensors are placed on either side of the part to be measured. By measuring the parts from both sides, curvature and deformities are taken into account in the measurement and their effects are not included in the thickness readings.

While capacitive displacement sensors are most often used to sense changes in position of conductive targets, they can also be used to sense the thickness and/or density of non-conductive targets as well.[4] A non-conductive object placed in between the probe and conductive target will have a different dielectric constant than the air in the gap and will therefore change the Capacitance between probe and target. (See the first equation above) By analyzing this change in capacitance, the thickness and density of the non-conductor can be determined.

Capacitive displacement sensors are often used in assembly line testing. Sometimes they are used to test assembled parts for uniformity, thickness or other design features. At other times, they are used to simply look for the presence or absence of a certain component, such as glue.[9] Using capacitive sensors to test assembly line parts can help to prevent quality concerns further along in the production process.

Capacitive displacement sensors share many similarities to eddy current (or inductive) displacement sensors; however capacitive sensors use an electric field as opposed to the magnetic field used by eddy current sensors [10][11] This leads to a variety of differences between the two sensing technologies, with the most notable differences being that capacitive sensors are generally capable of higher resolution measurements, and eddy current sensors work in dirty environments while capacitive sensors do not.[10]

Functionally complete, they integrate a multiplexer, an excitation source, switched-capacitor DACs for the capacitive inputs, a temperature sensor, a voltage reference, a clock generator, control and calibration logic, an I2C-compatible serial interface, and a high-precision converter core, which includes a second-order Σ-Δ charge-balancing modulator and a third-order digital filter. The converter works as a CDC for capacitive inputs and as an ADC for voltage inputs.

A simple technique for monitoring liquid levels is to immerse a parallel-plate capacitor in the liquid, as shown in Figure 3. As the liquid level changes, the amount of dielectric material between the plates changes, which causes the capacitance to change as well. A second pair of capacitive sensors (shown as C2) is used as a reference.

With its two capacitance measurement channels, the 24-bit AD7746 is ideally suited for level-sensing applications. Figure 4 shows the system block diagram. The sensor and reference capacitances are converted to digital and the data is transmitted via the I2C port to the host PC or microcontroller.

From this equation, the measured capacitive is proportional to the length submerged into water, as the approximate capacitance per length of track for a coplanar sensor remains constant. Performing system calibration using LabVIEW software can help achieve higher accuracy.

Micro-Epsilon offers the largest range of high precision displacement sensors, infrared temperature sensors, color sensors as well as dimensional measurement devices and systems for industrial applications. Whether your field is research and development, manufacturing automation or machine building, we offer solutions that meet the specific requirements of your individual measurement task.

All the parasitics on either side of the sensor capacitance Cs can be simplified to one parallel capacitor by omitting the ground connection, reducing the total parasitic capacitance. The equivalent circuit in this case is included in Fig. 5e. However, in this case, the parasitic capacitance will be in parallel to the sense capacitance. This effect is undesired, because charge redistribution from the measurement signal over the parallel capacitor occurs. This would result in a worse system performance compared with the parasitic capacitance being connected to ground, as shown in Fig. 5d. The best way of connecting the sensor is summarised by Fig. 5f, thus including the connected ground.

The silicon die is therefore packaged on both sides using glass dies fabricated from a Pyrex wafer, as illustrated in Fig. 6g. Because the sensor is bulk micromachined, the moving parts have the same thickness as the rest of the die, in this case the frame. To allow the mass to move freely within the glass-silicon-glass stack, silicon spacers are used to have a controlled and known separation between the mass/beam system and the outer glass dies as shown in Fig. 6h. The stack of glass outer dies, spacers and MEMS-chip is glued together by ultraviolet curable Norland Optical Adhesive. 041b061a72


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