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. Each output lead acts as a current source (due to photoelectric effect) either sourcing or sinking current based on the intensity of the light it is receiving and the distance from the centroid of the light spot from the terminal corresponding to the lead. One pair of leads corresponding to a direction pair (left-right or up-down) will be sourcing current while the other sinks current. For further details see the lateral-effect photodiode.
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. The target was a 1.5" diameter sphere covered with retroreflective tape, providing a decent brightness spot even at distances beyond one meter.
If light is directed at the sensor in a beam (such as from a flashlight or laser, or potentially the reflection from a distant target), the light available from the field of view can be overshadowed by the intensity of the light in the beam, resulting in uniform distribution of the light until the light source becomes occluded by the edge of the case or structure, resulting in partial shadow. In theory, if the beam were narrow with respect to the size of the sensing area, the placement of the beam on the sensing area should produce output currents which are representative of the proximity of the "dot" to each edge of the sensor. The sensing area is only 4mm square, and the "dot" produced by a laser pointer is 2mm in diameter with additional scatter, so the dot is anything but "small". To reduce this "large-dot" problem, an optical lens (salvaged from a laser pointer) and infrared filter (http://www.edmundoptics.com/onlinecatalog/displayproduct.cfm?productID=1918) were used.
Phase-Sensitive Detection Circuit
The difficulty in trying to identify a target which could be buried within background ambient light is differentiating between the ambient signal and the target signal. The method of achieving that separation that this system uses is called phase-sensitive-detection. Essentially, the circuit uses an active low-pass filter to compare the centroid of light with the illumination on to the centroid of light with the illumination off, by switching from one lead when the illumination is on to the other when it is off. The use of the other lead when the illumination is off allows the filter to compare