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Jan 09, 2016, 04:37 PM
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A lens revived


Canon EF 50mm mechanism (1 min 34 sec)


So after powering up the lens with a minimum of components, the motor turned briefly then stalled again. After manually turning it, it turned more consistently. It never stalled again.

The internet can't agree on what type of motor it is & might have confused it long ago with the stepper motor in the F1.8 or what they term as a "micro motor". The motor impedances are over 300k ohms. Steppers have much lower impedances. The motor looks like the other ultrasonic motor documented on the internet. Canon would get sued for printing USM on it if it was a stepper. So rest assured, it's a USM.

The only other teardown showing any detail of a USM motor was

http://www.lightcafe.net/forums/view...p?f=20&t=14241

Its wire bundle went to a soldered point which failed. The 50mm's wire bundle went straight in the motor with no soldering.

Based on the scarce documentation, USM motors work more like human muscles than what's marketed as muscle wire. They contain thousands of tiny ratchets which move the payload. In the lens, the tiny ratchets are on the end of a large piston shaped thing. Most of the volume of the motor is a pancake of ceramic & metal alloys dedicated to just generating the vibrations. Only the very end of the pancake converts the vibrations to motion.

Because the tiny ratchets are always engaged with the payload, the motor has very high torque. Turning the stiff lens gearbox wouldn't be a problem. There's no evidence turning the motor manually damages it.

For all the potential of piezo motors, they have yet to appear in any hobby store. There are no reverse engineerings of piezo motors, no oscilloscope plots of the voltages, no robots using them, no recycling of lens motors, no magnified views of the ratchets. The mane problem is they're slow. While they would solve the problem of torque, they would be too slow for a running robot.

As for the tendancy of 50mm F1.4's to break, there is a helical ring supporting the full weight of the glass elements. It's designed such that very little material is holding it in when fully extended. The issue is pulling on the glass rather than pushing on it. Repeatedly pulling a lens cap off, walking around with it fully extended by your side & side swiping a door, or a high vibration environment would do it. Didn't document the helical ring because there wasn't anything wrong with it. The video shows where the helical slot is bearing weight when fully extended.
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Jan 09, 2016, 04:43 PM
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RickC_RCAV8R's Avatar
Jack :

I was wondering as to the weight of the DSLR lenses as opposed to smaller ones used on the older vidicon tube based color CCTV cameras . I recently picked up some of these older cameras at a yard sale and am attempting a proper interface to drive them . The lighter SONY lenses are about half of the physical size of the DSLR lenses but may not have the necessary lens coatings needed for IR work . Dunno about this one ; yet .


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