The human movement laboratory

CODA

The active markers used by the CODA MPX30 consist of an infrared light emitting diode (figure 1) powered and controlled by a separate small driver box which houses the drive circuitry and a rechargeable battery (figure 2).

The driver boxes receive trigger information from the sensor units (figure 3) causing them to flash in rapid succession. This triggering process allows the signal received from each marker to be identified. As a result the system is able to identify each individual marker, even when markers are in close physical proximity.

The sensor units each contain three photo-electric detector arrays mounted on a rigid frame. In front of each sensor is a mask of light opaque lines. As infrared light enters the sensor unit through three flat windows, the light passes through a mask before reaching each of the photo-electric detector arrays. As a result shadows are cast on the detector arrays. As the light emitting diodes move, the location of the shadows on the detector arrays also move. The new pattern of shadows is then cross-correlated with the original pattern in real time by a digital signal processing card. The masks of the outside two detector arrays produce vertical shadows, while that of the middle sensor produces horizontal shadows. As a result the outside two sensors are used to assess lateral movement, while the middle sensor is used to assess vertical movement.

Technical

The following data was provided by Charnwood Dynamics Limited.

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