principle of laser tracking system

We have already seen about laser tracking system in my previous post, now let us see the working principle of laser tracking system. Here we are going to track the tip of the finger as shown in the figure below. The laser beam from the source is made to incident over the steering mirror. The steering mirror is tilted in such a way that the reflected beam falls on the finger tip. The laser beam will form a circular saccade on the tip of the finger.

Fig.Laser Tracking System

The photodetector will detect the backscattered laser signal from the finger(see the diagram below). The saccade is supposed to remain fully inside the tip of the finger while tracking [fig (a)]. The backscattered signal will be high when the saccade remains fully inside the finger tip. As the object moves, a small portion of the saccade may fall outsid the finger tip, and the backscattered signal will momentarily drop. Due to the synchronous operation of the beam-steering mirrors and the photdetection, an accurate recentering vector(blue arrow) is computed [fig (b)]. The center of the saccade is updated accordingly [fig (c)]. By repeating these three steps continously we can track the movement of the finger tip.

Fig. Updating center of saccade

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about laser tracking system

why laser based tracking became famous?

The tracking system at present, scans the entire background along with the object to be tracked and by using intense image processing it separates the object from its background. After separating the object, the motion of the object is detected. This method is complex and expensive. But in laser tracking system instead of scanning the entire background along with the object, we will scan only the object. So no image processing is required at all. We can directly detect the motion of the object. This method requires only few components, hence very simple and inexpensive.

Components used in laser tracking system

The laser tracking system requires three main components. Performance of these components decides the accuracy and efficiency of tracking system. They are

• Laser source - a focusable laser diode delivering required optical power. The required optical power depends on the distance between the object and the tracking system. When the object is at greater distance, high optical power is required and vice versa. since the target is likely to represent a small portion of the total field of view, the illuminating energy can be efficiently concentrated, and therefore the system will be extremely power-efficient and work equally well with distant targets. based on safety the laser source is classified into six types. they are class 1, class 2, class 2a, class 3a, class 3b and class 4.
• Steering mirrors - can be tilted in both x and y axis, which is used for focusing the laser beam towards the object which is being tracked. please see the post regarding steering mirrors.
• Photodetector - a transducer capable of accepting an optical signal and producing an electrical signal containing the same information as in the optical signal. As the requirements for applications vary a lot concerning wavelength, maximum optical power, dynamic range, linearity, quantum efficiency, bandwidth, size, robustness and cost, there are many types of photodetectors. For laser tracking application we require very narrow bandwidth, wide angle photodetectors.