[moved] How to control an CD drive with a dc motor instead of a stepper motor?

[dan2zep]

First of all hello there, this is my first thread.

So I started working on a mini cnc machine made from old CD drives. I plan to use the mechanisms to build a plotter like structure. When I tore them down I was expecting a stepper motor at the beginning of the lead screw, but what I got was a standard dc motor with no encoders or any means for a feedback signal.

How is the control achieved? Can't be open loop since there's a DC motor with no linear or rotatory encoders. There's also some looseness in the transmission mechanism which definitely discards the open loop option because of the poorly precision it would result in.

I kindly appreciate any suggestions or just someone pointing me the right way.

Regards, Daniel

 

[FlapJack]

Can you post a picture looking at the end of the motor shaft. The plastic disc may be for an optical encoder.

 

[ads-ee]

Read this explanation of how CDs work. Part of the encoding is on the CD itself. Also the tracking of the laser is done using extra tracking beams that keep the main beam on the data see this.

A CD tracking mechanism probably isn't going to be a good fit for what you are trying to do.

 

[dan2zep]

Damn, guess I'd have some sort of linear encoder (have lots-of from salvaged printers) or find a CD drive with a stepper.

(Hope to post in the future less trivial threads)

Thanks!

 

[KlausST]

Hi,

The lens positioning is made with two systems:
* one is the DC motor with the lead thread (slow, but high range)
* the other is a magnet to move the lense. (Fast, but with low range)

The feedback is made with the laser and multiple sensors. The sensors give the information if the beam is on the track, or left, or right of the track (while maintaining track data). It can only work when the disc is rotating.

Klaus