2011

Using Optical Encoders to Improve Camera Phone Zoom Lens Accuracy

Encoder Operating Principle

altThe miniature incremental encoder, AEDR-8400, comes in a surface-mount leadless package, measuring 3.00mm x 3.28mm x 1.26mm, making it the smallest optical encoder with digital outputs. It incorporates both an LED light source and photo detector IC in a single SO-6 (Small Outline, 6 Pin) package and employs reflective technology to sense rotary or linear motions. The small size and reflective technology allows the encoder to be used in a wide range of commercial applications, particularly where space and weight are primary concerns, such as the zooming mechanism in a camera phone. The encoder offers 254 lines per inch (LPI) resolution, which is equivalent to 10 lines per mm (LPmm) with two channel digital outputs. The encoder can operate in a temperature range of -20°C to 85°C. One of the critical criteria in the camera module of a camera phone is being able to operate at a lower voltage level. With the typical operating voltage of 2.8V, the AEDR-8400 encoder will comfortably suit the needs of this application.

altFigure 4 shows the optical arrangement of an AEDR-8400 encoder used with a reflective codestrip where the lens focuses the light from the LED onto the window and bar of the codestrip. The reflected images of the window and bar are focused on the photodiodes. As the codestrip moves, an alternating pattern of light and shadow cast by the window and bar, respectively, falls upon the photodiodes. The detector IC converts this pattern into digital TTL-compatible outputs representing the codestrip linear motion and hence the lens’ movements. An important parameter is resolution, which is defined as the density of window/bar in a unit distance and is typically defined as lines per inch (LPI) or lines per mm (LPmm). Higher resolution means “finer” control of the linear motion.

altThe AEDR-8400 encoder is designed so that the LED and detector IC of the encoder should be placed parallel to the window/bar orientation. As such, the encoder is robust against radial play. This concept is illustrated in Figure 5.

altThe overall camera module design can be shrunk compared to a stepper motor solution or a voice coil solution. The motor size is comparable to the piezo-actuator; however, the removal of mechanical cams and gearing enables the overall camera module dimension to be decreased further to meet existing market demands. The AEDR-8400 encoder helps to provide precise positioning control between both lenses and results in a better quality image. In addition, the synchronization of the lens movement can be performed fast and accurately.

altThe encoder outputs, namely Channel A and Channel B, are characterized by their quadrature relationship. As shown in Figure 6, there is a phase shift of 90 electrical degrees between the channels. In addition, the channels are also characterized by their four states (i.e., State 1 to State 4), each spanning a nominal 90 electrical degrees. Information about linear motion, such as movement speed and distance traveled, can be derived from the parameters of the output such as pulse period and number of pulses. Meanwhile, the direction of linear movement is determined by the phase relationship between the two outputs.

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