Many tracking applications require that the system identify the location of information within a visual field. This could be seeking a color, or a shape, or just the centroid of light intensity. This page discusses the use of a duo-lateral photodiode to sense the centroid of light within its field of view. The documented method uses a passive retro-reflective target and optics.
The system is designed to obtain the location of a reflective target within its visual field and track it at high speeds. Figure 1 is a system diagram showing the functional blocks of the system. The system needs to be able to obtain the position of the target relative to the current direction the "eye" is facing, which requires that it be able to distinguish between the target signal and potential ambient light, and then produce a signal which the microcontroller can utillize to drive motors which orient the eye toward the target.
The system contains the following elements:
- Eye structure on a 2-DoF gimbal
- Position-sensing detector
- Target illumination circuit
- Phase-sensitive detection circuit
The position sensing detector used in this system is a DLS-4, a two-dimensional lateral-effect position-sensing device from OSI Optoelectronics. This sensor (as shown in the image at right) has a 4mm x 4mm sensing area and has four output leads, one pair for each axis of measurement.
Illuminating the target was achieved via eight TSHF 5210 infrared-emitting diodes (IRED's)(http://www.vishay.com/docs/81303/tshf5410.pdf). These were arranged into two parallel groups of four which were each soldered together as part of the "eye"structure. The IRED illumination circuit was pulsed synchronously with the phase-sensitive detector switch at 20 kHz.
Phase-Sensitive Detection Circuit