SENSING WITH CHARGE

A new type of sensor based on a capacitance sensing principle promises to solve some intriguing and diverse design problems.

Claimed to the the ‘world’s first’ charge transfer (‘QT’) sensor, the Qprox can turn almost any object into a proximity sensor, so it is readily embedded into or behind the surfaces of existing products. Simply clipping a sense lead to anything metallic imparts a proximity effect. That means that a metal sheet or wire can be placed under a carpet to make a people sensor, replacing passive IR systems or mechanical switch based mats. Other applications include hidden mass detectors, harsh environment sensing, switch replacement and level sensing.

The QT sensor is essentially an exercise in switching circuitry. By charging a sense electrode (which can be anything electrically conductive) to a fixed potential, then transferring that charge to a charge detector comprising another known capacitor, the capacitance of the sense electrode can be readily ascertained. The charge and transfer operations are conducted by switches; while electromechanical switches would work well, in actual practise, MOSFET transistors are used for the purpose. The control of these MOSFETs is performed by digital logic. The conversion of the real world signal from analog form to digital form can be accomplished by any number of commercially available ADC chips. A more detailed technical description can be found at the company’s Web site on www.interquant.com.

Several products that incorporate the technology are currently available from Quantum Research in the US and its UK distributor Sensor Consultants. The QProx-E2S, shown in Figure 1, is an evaluation board sensor that can discern femtofarads of capacitance change with a background level of hundreds of picofarads. In addition to the natural noise immunity of the sensor, it also contains both linear and nonlinear filters, plus a post detection event filter. The board suppresses background capacitance caused by large sensing surfaces and the use of shielded cable.

The QTM-1001 module shown in Figure 2 implements the charge-transfer sensing method of capacitance sensing that closely duplicates the E2S evaluation board, in a 3.8cm x 8.7cm x 1cm format. Critical circuitry is encapsulated for environmental protection. All components are surface mount, and connection is via a 12-pin header strip. The module, which is CE certified, is almost identical to the E2, but comes without an LCD display audio indicator and RS-232 serial link.

An integrated circuit which also contains much of the functionality of the module is also available. Dubbed the QT9701, the device provides a low cost solution for engineers with substantial production volumes.

The sensor was originally developed to control an automated water tap, and is currently in test at several plumbing companies in the US. However, the sensor is also finding uses in other more esoteric applications.

One project underway at the University of Florida is using the device to determine the ripeness of fruit, while another has used an array of sensor elements for the development of a non-tactile computer ‘mouse’ shown in Figure 3.

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