Electric Field Sensing is well-suited to measuring conductive objects, as well as objects whose dielectric constant is substantially different than the surroundings. In practice, metals, the human body, fruits and vegetables, and water-based liquids all work well because of their high conductivity and/or dielectric contrast. Thin plastic cases, fabric, thin sheets of paper and thin glass cannot be sensed well (thin means relative to the electrode spacing). On the positive side, this means that Electric Field Sensing can operate through thin layers of plastic, fabric, paper, or glass if desired. In this paper, we make use of this last property by sensing through plastic fingertips.
Electric Field Sensing: Schematic representation of a single electric field sensing channel. If the target object is within the sensor electrode pair’s field of view, then as the sensor-to-object spacing changes (or more generally, as the geometry changes), the signal at the receive electrode changes.In Electric Field Sensing, illustrated in image to the left, a low frequency (100KHz) AC voltage is applied to a transmit electrode (labeled TX), which via capacitance C0 induces a displacement current in the receive electrode (labeled RCV).[2] The position of the target object relative to TX and RCV modifies the capacitance C0 between TX and RCV, and thus the amplitude of the signal received at RCV. If the target object is well-coupled to ground, the displacement current received at RCV decreases as the electrodes approach the object.
