Sensor Chair
The sensor chair is a device that measures the body (hand and foot) positions and motions of a seated occupant. It was developed for a performance of music (written expressly for the Chair system by Tod Machover) and magic by the magicians Penn and Teller at our Digital Expression Conference in October of 1994. It is now used as a one of the performance instruments in the Brain Opera. Click here for some demo videos and a few more bits of info.
As labeled on the chair layout diagram, above, the copper plate (A) affixed to the top of the chair cushion is a transmitting antenna being driven at roughly 70 kHz. When a person is seated in the chair, they effectively become an extension of this antenna; their body acts as a conductor which is capacitively coupled into the transmitter plate.
Four receiving antennas (B) are mounted at the verticies of a square, on poles placed in front of the chair. These pickups receive the transmitted signal with a strength that is determined by the capacitance between the performer's body and the sensor antenna. As the seated performer moves his hand forward, the intensities of these signals are thus a function of the distances between the hand and corresponding pickups. The pickup signal strengths are digitized and sent to a Macintosh computer, which estimates the hand position. A pair of pickup antennas are also mounted on the floor of the chair platform, and are used to similarly measure the proximity of left and right feet, providing a set of pedal controllers. In order for a performer to use these sensors, he must be seated in the chair, and thus coupled to the transmitting antenna. Other performers may also inject signal into the pickup antennas if they are touching the skin of the seated individual, thus becoming part of the extended antenna system. The sensor antennas are synchronously demodulated by the transmitted signal; this produces a receiver tuned precisely to the waveform broadcast through the performer's body and rejects background from other sources. Before a show, the performer moves his hand around the perimeter of the sensor range, after which a linear least-squares fit produces a calibration that gives good tracking coordinates.
A pair of footswitches (D) are incorporated in this system to provide sensor-independent triggers. These are used for changing parameters when the foot pedals are dedicated to generating musical sounds, or for instigating t riggers when the performer is not seated, hence is unable to use the sensors.
The hand sensor antennas (B) are composed of a copper mesh encased inside a translucent plastic bottle. A halogen bulb is mounted inside this mesh which is illuminated with a voltage proportional to the detected sensor sign al (thus is a function of the proximity of the performer's hand to the sensor), or driven directly by the Macintosh computer as a MIDI light-instrument. Four lights are mounted below the platform (F); these are correspondingly driven by the foot-sensor signals or directly through MIDI. A digital display (E) is also mounted on one of the sensor posts; this is similarly defined as a MIDI device, and is driven by the Macintosh to provide performance cues (i.e. amount of time or triggers remaining in a particular musical mode, etc.). The sensors are used to trigger and shape sonic events in several different ways, depending on the portion of the composition that is being performed. The simplest modes use the proximity of the performer's hand (or head in the case of Teller's closing bit) to the plane of the hand sensors (z) to trigger a sound and adjust its volume, while using the position of the hand in the sensor plane (x,y) to change the timbral characteristics. Other modes divide the x,y plane into many zones, which contain sounds triggered when the hand moves into their boundary (i.e. the percussion mode). Several modes produce audio events that are also sensitive to the velocity of the hands and feet.
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