Electric field proximity sensing

Electric Field Proximity Sensing or EFPS is a sensory system that relies on the fact that an electric field can be perturbed by the existence of a nearby object, provided it is at least slightly conductive. One type of EFPS is The People Detector. The People Detector is a micro-electronic based device that can detect the presence of both moving and stationary objects through solid materials. Its ability to operate through any non-conductive material permits complete invisibility. The sensor functions by detecting small changes in an ultra-low-power electromagnetic field generated between two remotely located antenna electrodes. Its range is adjustable from a few centimetres [inches] to 4 m [over 12 feet]. Electric field proximity detectors can detect partially conducting or conducting objects and does not depend on impedance to ground.
Advantages.
Because EF sensors provide comparatively low amounts of data, they have the potential to provide several advantages over optical systems. Electric field systems can be made smaller, lighter, and power efficient (valuable qualities in an age of ever shrinking electronics such as palm and watch based computers). Since EF sensors penetrate non-conducting materials, they can remain hidden allowing them to remain protected from weather. EFPS systems today are used in a wide variety of industrial and commercial applications, including automobile airbag systems (to determine where passengers are seated), in advanced robotic manipulators (to determine the properties of an object in the manipulator's grasp), and in home automation, to determine which rooms are occupied.
Electric fish.
This is the same sensing system that is used by the electric catfish to navigate through muddy waters. These fish make use of spectral changes and amplitude modulation to determine factors such shape, size, distance, velocity, and conductivity. The abilities of the electric fish to communicate and identify sex, age, and hierarchy within the species are also made possible through electric fields. EF gradients as low as 5nV/cm can be found in some saltwater weakly electric fish.
MIT prototype.
Researchers at MIT have created a prototype of a 3-dimensional EFPS called the "Fish". It can monitor objects in a 3-dimensional space. The People Detector operates in only 2-dimensions and does not have a microprocessor.
Technology review.
"Technology Review", a science and technology magazine published by MIT, featured EFPS in their September 2007 computing section. Intel had created a robotic arm that could distinguish between plastic bottles that had water present or not. The robotic arm did not have to touch the bottles to determine which ones had water inside. Researchers applied an oscillating voltage to an electrode, in the thumb of the robot for example, which created an electric field. As the field interacted with the water in the bottle, special sensors could determine the change in the electric field and then cause the robot to behave in certain ways.