The stepping motor rotates the spindle into either direction in small steps, so that the scan head moves at a certain distance. This motor resides in any flatbed scanner. It rotates the reducer (those cog-wheels you see in the snapshot) and moves the carriage in which the optical unit, lamp and matrix are. A special controller chip is responsible for changing the direction and rotation speed of the motor. The precision with which the carriage moves is referred to as mechanical resolution on Y-direction.
Optical resolution of a scanner is X-direction,
and mechanical resolution is Y-direction
Overall, optical resolution is determined by the number of elements in a matrix line divided by the width of the operational area. Mechanical resolution is the number of steps the scanning carriage can make in Y-direction. You can often come across a notation like “600x1200” in a scanner’s specification. The first number stands for optical and the second – for mechanical resolution of the scanner. There is also the so-called interpolated resolution, which may be a few times higher than optical resolution, but doesn’t depend on the physical capabilities of the device. I would call it “scaling resolution”. Interpolation (enlargement of the original image) is performed by the scanner software. You can disregard this parameter altogether as it’s quite possible to do the same interpolation in an image-editing software like Photoshop.
Insides of the motor
The external side of the motor core is connected with a gearing which is a simplest reducer. Its large cog-wheel pulls in the belt with the connected scan head.