2011

Using Optical Encoders to Improve Camera Phone Zoom Lens Accuracy

A wide range of commercial applications use cameras with a zooming mechanism. Perhaps the most ubiquitous is the camera phone. Camera phones may have an optical zoom, a digital zoom, or both. What’s the difference? An optical zoom actually changes the effective focal length of the camera lens such that the original image is magnified and could be captured by the image sensor (CCD or CMOS). With greater magnification, the light is spread across the entire image sensor and all of the pixels can be used. An optical zoom could be interpreted as a true zoom that will improve the quality of pictures captured.

altDigital zoom, on the other hand, is a bit different. In this case, a software algorithm is applied rather than the actual hardware movement (i.e., lenses positioning) to magnify the image. Such magnification involves only a certain portion of the captured image. This is known as the interpolation technique. Using such a technique, or algorithm, additional information needs to be added in order to enlarge the corresponding image portion. It may seem that the image captured is being magnified; however, there is only a certain portion of the real image information being utilized and the rest of the image is coming from the interpolation outputs.

altOne thing worth mentioning is that the higher the digital zoom, the smaller the portion of real information is taken. Therefore many of the originally captured pieces of information on the image sensor will be discarded and more interpolated image data will be incorporated into the resultant image.

Consequently, optical zooming is an important mechanism in determining the true zooming power of a camera phone that isn’t losing any image data. Having accurate lens positioning control in optical zoom is crucial to help ensure quality in an enlarged image. Figure 1 illustrates a typical example of the zooming mechanism in a camera module inside a camera phone. Lenses are aligned such that an image could be focused onto the image sensor (CMOS or CCD). The zooming mechanism involves synchronization movement between two or more lenses. By varying the distance between the lenses, the actual effective focal length of the camera lens changes accordingly. Hence, a magnified image would be captured by a CCD or CMOS image sensor.

To simplify the wiring process, encoders are mounted at the camera module shell and remain in a fixed position. The moving portion is the codestrip, which acts as the translator for the lens’ linear movement. Casting the window and bar image back to the encoder provides feedback on all the necessary information for prompt and accurate lens positioning. With a conventional zooming mechanism, a combination of mechanical cam and gearing is a common approach for lens position controlling. However, such an approach will suffer unavoidable wear and tear issues and the accuracy of lens positioning will degrade over time and directly impact the quality of zoomed images.

An AEDR-8400 encoder by Avago Technologies can help resolve these zooming issues. The feedback from the encoder provides necessary information for real-time calibration whenever there is any back-lashing from gears and mechanical cams. This can help ensure precise and accurate lens positioning. Furthermore, in some customized camera module designs, removing the mechanical cam is possible (Figure 2).

Incorporating the AEDR-8400 encoder into a piezo-actuator camera module, for example, can essentially eliminate the use of mechanical cams. And, because there is no mechanical cam involvement, there is no fixed zooming position and the new camera module system can now have a continuous zooming function (Figure 3).

In terms of power consumption, piezo-actuator systems tend to consume less power compared to voice coil and servo solutions. Also, a piezo-actuator solution could help keep the noise and vibration level to a minimum, which a stepper motor or voice coil solution cannot achieve.

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