Field Imaging

We are investigating the use of near-field, quasi-electrostatic measurements ("electric field sensing") for measuring the configuration of human bodies and parts, particularly hands. This sensing technique is useful in close proximity to an "activated object" such as a desk or chair.

One of the nice things about electric field sensing is that it can be used to make as many or as few measurements as are required by the problem. A video camera always returns many numbers, even if only a few are needed. By using only a few electric field sensors, one can quickly and inexpensively get an "average" measurement of the conductivity distribution.

Inference

We are exploring the continuum from a single channel proximity detector, through three-d mouse, gloveless dataglove, and all the way up to full three-dimensional imaging. Each task requires more measurements. In all cases, the challenge is to infer as much as possible about the world from the measurements.

Instrumentation

The first EF sensing hardware developed by the Physics and Media group is known as "the fish," because weakly electric fish use similar mechanisms to sense their environments, and because, like a fish, our sensor can navigate in a 3d environment, whereas a mouse spends its life in a 2d field. The fish has one transmit channel and four receive channels.

The SmartFish is a new, all digital successor to the original fish. The smart fish supports 9 transceiver electrodes, for a total of 9^2 = 81 possible measurements. Unlike the original Fish, the SmartFish has no manual adjustments: all the parameters can be adjusted in software, hence the name SmartFish.

The ScanFish is a 16 electrode multiplexing array designed by Joe Paradiso for research on imaging. It can make 16^2 or 256 possible measurements.

The School of Fish can be thought of as a distributed implementation of the ScanFish. Each member of the school (Guppy) is an autonomous transceiver, consisting of an electrode, EF sensing cicuitry, and communications circuitry. The guppies work together to make many measurements quickly (much more quickly than the ScanFish can, since the guppies can work in parallel). The number of measurements that can be made can be scaled up indefinitely by increasing the size of the school. In the first implementation, the guppies will be networked on RS-485; later implementations will use wireless communications.

History and Relation to "Capacitive Sensing"

Electric field sensing is a descendant of the Theremin, one of the first electronic musical instruments, and the first early example of electric field sensing. Other relatives are Max Matthews' Radio Batton, and the sensors traditionally known as "capacitive sensors," such as those found in elevator buttons.

Though we measure the physical quantity capacitance, our sensing technique is somewhat different than what is normally known as capacitive sensing. In capacitive sensing, the amount of current dumped out of a single electrode is measured. As a hand approaches the electrode, the electrode becomes coupled more strongly to ground (through the person). In the shunt mode version of our technique, there are distinct transmit and receive electrodes. As a hand approaches, the signal decreases. But since each measurement depends on two electrodes, we can make n(n-1)/2 distinct measurements using n electrodes, rather than just n, the number we can make using "capacitive sensing".