I am looking for different types of encoders for closed-loop control of motors for use in miniature autofocus cameras. The encoder can be either rotary or linear. An ideal would be some sort of resolver or absolute encoder.

Thanks.

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The older ones used a linear potentiometer. The newer ones eliminated the encoder and used a stepping motor. Coarse focus was achieved by automatically calibrating the focus lense position to the zoom setting, zoom also done with a stepping motor. Both stepping motors initialized by moving to a home location when power is first applied to the camera and then keeping track of the step count. Fine focus is achieved by moving the focus lense back and forth slightly and analyzing the frequencies present in the video. The idea being that the maximum amplitudes of high frequencies will be greatest at the best focus.
The automatic calibration routine was built into the lense control microprocessor. When enabled, the routine steps through many zoom positions with the camera viewing a special chart. The micro remembers the best focus position for each zoom setting and has an interprolation routine for positions in between the zoom steps utilized in the procedure.
Jerome -

Thanks for the overview. It helped.

Mike

Jerome Silverman said:
The older ones used a linear potentiometer. The newer ones eliminated the encoder and used a stepping motor. Coarse focus was achieved by automatically calibrating the focus lense position to the zoom setting, zoom also done with a stepping motor. Both stepping motors initialized by moving to a home location when power is first applied to the camera and then keeping track of the step count. Fine focus is achieved by moving the focus lense back and forth slightly and analyzing the frequencies present in the video. The idea being that the maximum amplitudes of high frequencies will be greatest at the best focus.
The automatic calibration routine was built into the lense control microprocessor. When enabled, the routine steps through many zoom positions with the camera viewing a special chart. The micro remembers the best focus position for each zoom setting and has an interprolation routine for positions in between the zoom steps utilized in the procedure.
Going back through my notes on lenses for video cameras, I see that the linear feedback potentiometer was mechanically related to the zoom lense position. The focus lense was mechanically linked to the zoom lense through spiral grooves in the rotating lense barrel. The ability to maintain focus through the various zoom positions was dependent upon the precision of the lense assembly construction. The early autofocus systems added a focus lense servomotor and used a reflective infra red triangulation system mechanically linked to the zoom lense position and the focus lense position and a separate autofocus optical system.
The next advance was to use the camera lense as the autofocus optical system. There was a system that used ultrasonic reflection rather than infra red but it did not work very well. As these systems were refined it became possible to have zoom ratios improve from 6:1 to about 12:1. When the scheme was developed that calibrated the focus lense to the zoom position and stored the data in an eprom linked to a lense micro, optical zoom of 20:1 became possible with inexpensive lenses. Since the coarse focus was linked to the zoom lense position and the fine focus was determined by analyzing the frequencies present in the image, there was no need for a focus lense position encoder and I dont recall ever seeing one used in the cameras and camcorders that I was familiar with.

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