An integrated circuit (IC) developed specifically for vehicle systems, frequently opens the door for a number of non-automotive applications. This occurred with accelerometers developed for crash detection that now provide input for steadying circuitry in cell phone cameras and impact detection in computers. An electric field imaging device, or e-field sensor, developed by Freescale Semiconductor for occupant detection systems in vehicles, could have similar wide-spread usage outside of cars.
The e-field device is an analog IC that uses a sine wave generator to create a low-level electric field and detects changes in that electric field by measuring capacitance. The MC34940 integrates support for up to seven simple electrodes, which can be used independently to provide information for performing multiple functions. Based on the equation for a capacitor (C=k?¸0A/d), the strength of the electric field is:
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Directly proportional to the area (A) of the electrode,
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Directly proportional to the dielectric constant (k) of the material between the electrodes, and
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Inversely proportional to the distance (d) between objects.
Simply changing the variables allows one e-field imaging device to sense proximity, touch, size, shape and differentiate the material inside the field. An example of an appliance application for e-field sensing is in washing machines.
Four electrode pads aligned in an octagon pattern underneath the washing machine's drum can provide out-of-balance detection. To obtain consistent results, the drum must be grounded and made of a conductive material. The drum serves as the grounding plate of the capacitors formed with the four electrodes. The four e-field values E1, E2, E3 and E4 provide information regarding the status of the spinning drum. With a balanced drum, the distances between the drum and the four pads are identical and all four capacitance measurements are the same. If the load is unbalanced, the drum will lean toward one direction and cause closer spacing (lower capacitance) to one of the pads.
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 Load Sensing Comes Clean. A four element e-field sensor detacts an unbalanced load in a washing machine when the drum leans in one direction, causing closer spacing to one of the pads and greater spacing for the others. |
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In the washing machine application, in addition to the out-of-balance control, one e-field sensor can perform multiple functions such as sensing water level, soap content and water quality. This multi-function approach can control cost and open up new features in several other appliance applications including refrigerators, and in industrial situations that need multiple measurements.
Contact: Philip Sieh, applications engineer, Freescale Semiconductor (philip.sieh@freescale.com).
For more information about Freescale's e-field sensor, go to
http://rbi.ims/ca/4930-517.
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