The biggest challenge when designing the autofocus system was finding the right motor to drive the lens unit back and forth. Having decided on a new autofocus mechanism using a motor and lead screw, I needed an appropriate stepping motor. My search lead me to the smallest stepper motor in the product range of Faulhaber. The screenshot above is of the 3D model provided by the company of the motor.
At less than 6mm in diameter it could fit into the deadspace around the outside perimeter of the lens side of the camera. The height however, of 9mm, posed a greater problem. To leave enough space for a lead screw which can push the lens unit a good distance away from the sensor (this is what happens when the camera focuses on a close subject), I needed to place the motor as low as possible in the 17mm of depth inside the camera body.
The motor was 9.6mm in length according to the website, but as I examined their CAD model I noticed this didn't allow for the height of the thread at the top. Including this it was pushing 11.5mm, leaving me not a lot of room to play with. I decided therefore to push the bottom of the motor into the unibody by 1.5mm, with a 3mm high wall build up around it to hold it firmly in place. The motor reaches from the edge of the unibody up through both circuit boards and sprouts a lead screw. A dog legged joint then connects a nut on the lead screw to the lens unit, allowing for the motor which placed in the far corner away from obsticals such as the rear shutter button and the sensor housing, to reach the lens unit without hitting the front of the body when being driven upwards.
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| Infinity focus with the lens at it's default position. The sensor shade has been made white so we can see the parts easier. |
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| Macro focus with the lens pushed away from the sensor. |
Below we can see a view of the mechanism from the back. The lead screw and the supporting pins on which the lens unit slides along all secure into the inside of the front face of the camera to avoid any skewing which may harm the function. Placing the motor where it is is just about okay, however if a lens is specified which demands greater distance between the lens and the sensor when macro focusing, the stepper may need to be reconfigured and placed on it's side, and cog drive a vertical lead screw in order for the system to fit within the limited space inside the body.
