hi help me out guys

Unfortunately, it's not as simple as that. The lens is actually moved across the disc by two different means. The rack and pinion part that you mention, is only responsible for the coarse movement of the optical block. The fine movement of the lens, required to track the very fine data pits on the disc, is accomplished using a variable current supplied to a coil which is attached to the lens mount. As the current to this coil is ramped up by the tracking servo, the lens mount bends progressively to one side, following the data track. When the amount of displacement is approaching the mechanical limit of movement, the coarse servo's output to the sled motor, will have reached a point where there is enough drive current to cause the motor to ' roll over ' by one pole ( 120 deg, normally ). This has the effect of moving the sled on by an (admittedly tiny )amount, but still enough to easily see with the naked eye. The lens is bent back the opposite way by reversing the current through the tracking coil, and so the sequence begins again. The lens thus moves across the disc as a sort of ' wave ' motion.

I don't think that the mechanism employed in a CD player would be of sufficient quality to satisfy your needs on this project, but you may just get success if you tried using a DVD mechanism instead. In general, because of the much tighter tolerances required to play a DVD, the gears and rack and slides, are much better made, and of a generally higher precision. Most DVD lasers come as a complete deck assembly, so if you have a repair shop anywhere nearby you, they will normally have scrap ones coming out of their ears, that they will be happy to let you have some.

Arfa

Yep, there's a rather complex feedback system to keep the beam on-track in all optical drives, whehter it be within 1.6 um for a CD or 0.5 um for a DVD.

No mechanical open loop system of rack and pinoun gears and stepper motors will come anywhere close.

The usual way of doing this is with some type of feedback. In a harddrive, it's used to be s special optical grating or similar sensor. In newer ones, it's embedded servo information on the disks themselves. For the CD and DVD, the optical pickup actually has to follow the track becuase the runout may be order of 0.5 millimeter - 100s of times the acceptable tract deviation.

Perhaps if we knew more about your intended application, there could be other suggestions as to a solution.

--- sam | Sci.Electronics.Repair FAQ:

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Suggestion: It would be more precise to employ a 5 phase stepper motor with 0.36 or even 0.18 deg/step. That is 1000 / 2000 pulses per rotation, it would turn 0.00036 / 0.00018 deg per control pulse. Use Google / Ebay to find it.

Its a two stage control loop ,for coarse movement it is motor driven,for the accurate alignment something like a loudspeaker coil and a photodetector in a feedback loop are used in sideways and focus control, so the motor part is not very accurate. The motor part just tries to keep the coil more or less in the centre position,while the coil does the tracking and stepping.

Check out sources such as:

You might get some good ideas.

i need to make a linear movement of 1 micron or something close to that.this is used in crystal growth,microscopic stage movement etc.whenever i microstep the stepper motor i can make it move by 0.014 degrees.now need to convert this 0.014 degree or higher value into a micron linear movement...

can the rack pinion arrangement of a dvd player do the trick without any additional component..ie will that satisfy my need to convert angular movement of 0.014 degree to linear motion of 1 micron or maybe even 10 micron.or should i have any other movement attached to it...reply as soon as possible please

You may well be able to do it using a DVD mech, depending on the mechanical configuration of your stepper motor. There are two types of mechanism configuration. The first uses a conventional motor with a diameter of about

The other type of drive uses a smaller motor, either conventionally mounted, and driving a worm gear via a normal gear, and then via another gear, driving a coarse or fine pitched worm screw, which then drives the sled via a friction pawl, or mounted horizontally, and driving the worm screw via a couple of conventional gears. I think that this setup would probably give the best precision, but the other type would be easier to interface your stepper motor to.

As I said, if you have a repair shop you could go to, pop in and ask. They will probably have a box full of scrap decks that you can look through to see if anything is appropriate to your needs. The only reason that we all keep them, is for the occasional motor or gear that we can cull from them to complete an urgent repair.

Arfa

No way no how are you going to get repeatable movement of 1 micron with the motor driven mechanism from an optical pickup. Period. Sorry, 1 micron is an incredibly small distance. The thickness of the grease film on the gears is probably much greater. The backlash is probably 100 times greater. The motor itself can't be controlled to within orders of magnitude of this precision.

Even 10 microns is not likely. The mechanisms in a CD or DVD player optical pickup don't need to be this precise since the fine tracking feedback takes care of fine movement.

The fine tracking coil movement could be controlled to 1 micron but it will require some sort of feedback. For example, it would be theoretically possible to reflect the beam from the pickup off of a grating and back to the photodiode array in the optical pickup and then combine the coarse and fine tracking as done in a CD or DVD player to move the lens in increments of 1 micron or even less over a distance of 2 or 3 cem. But this won't help you easily move anything much more massive than the lens because the bulk of what moves does so in coarse steps.

There are all kinds of mechanisms for doing this as you are probably aware. Some of the most precise use piezo electric actuators and are capable of repeatable linear movement measured in nanometers. Of course, they are not cheap.

Somewhat less precise but perhaps adequate would be micrometer driven linear slides used in optical systems and elsewhere. You can probably pick up something on eBay for very little cost that would be much much better than anything derived from the CD or DVD player.

--- sam | Sci.Electronics.Repair FAQ:

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You're probably right Sam, but he's not trying to use the deck in its ' original ' form. If you look at the top of the post, he's already got a motor microstepping, so all the feedback and servo stuff of a deck in ' normal ' use is not relevant to his question, except to explain, as I did in the first place, that the mechanics of a CD player do not handle the micromovement of the lens, on their own. All he is trying to do is to convert the fine microstepping of his motor to a linear motion. I'm pretty sure that all you say about play in the mechanism is right, and he probably won't get the order of precision that he's looking for. Certainly, he wouldn't from a CD transport, but the tolerances of the mechanics are much better with DVDs, and I would have thought that as it should be a fairly simple job to interface his ( already tightly controlled ) motor to the deck, it's got to be worth a try, if only to prove that it's not going to be accurate enough.

Arfa

Part of the question (and I may have missed it) is "What triggers the stepper motor?" If the OP pushes a button to manually initiate a single step, then repeatability, accuracy, backlash, etc don't matter much, because he can manually re-adjust until he gets what he wants. If it steps 1 micron the first time, and 3 microns the second time, then he can just back up. It will be annoying, and it will be inaccrate, but it won't be impossible to use.

The second part of the question is "What are the REQUIRED (as opposed to the DESIRED) results when a step is initiated?" If the mechanism overshoots, as on the example above, can he just back up, or does it destroy a delicate crystal by ramming a probe into it, for example? If the latter, then repeatability and accuracy are very important.

No one has yet mentioned a tiny pantograph mechanism as a reducer for a rack-and-pinion mechanism. I have no idea where to get one or how to fabricate one, but then I am not a machinist.

Or how about a high-quality machinist's micrometer? I have a relatively inexpensive Brown & Sharp micrometer for which 40 turns = 1 inch (one turn equals 25 mils = 0.025 inches). Check my math on this, but

Anyway, until the OP answers these questions, our answers are pure speculation. But Sam is probably right - the distances here are so tiny that a backlash-laden, grease-sticky nylon mechanism may be completely unusable.

Bill

I just stumbled across an interesting tutorial on micrometers as used in an intro ME course at University of Toronto. It suggests that you can get an accuracy of +/- 3 micrometers (3 microns) with an ordinary good-quality mechanical micrometer.

Bill Jeffrey ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~`

http://www.upscale.utor> Part of the question (and I may have missed it) is "What triggers the

I believe you'll need to consider a feedscrew and precision linear slide combination. You can pretty much forget about finding something useable in consumer goods that would be worthwhile for you to invest your time in.

Making predictable motion in the resolution you require isn't a trivial matter. I've used digital readout measuring instruments that were capable of 10 microns resolution, and any motion in microns is subject to numerous environmental conditions. Even moderate temperature changes will have a dramatic effect on the materials of precision instruments, and vibration will cause unpredictable changes.

For precision linear movement you should consider a precision ball screw to be driven by your stepper. An attempt to utilize rack and pinion will include many more problems with regard to precision movement. Ball screws utilize a "nut" that has a continuous loop of circulating balls, and this design provides a very low frictional load, compared to a standard solid nut.. also, the ball-to-thread motion provides finer resolution.

Additionally, a ball screw can be driven by a rotational reduction method, such as anti-backlash gears or timing belt arrangements. It doesn't sound as though your application would involve forces that could actually apply reverse force to a ballscrew, but if it did, you would want to include a braking mechanism to prevent the load from driving the ballscrew. The holding torque of the stepper motor may be sufficient. Ballscrews have such low friction that a load is capable of becoming a driving force (if a significant weight is raised and lowered vertically, for example).

If you intend to look for sources of useable parts (avoiding bying suitable new individual components), you might find acceptable parts in laboatory or manufacturing quality control inspection equipment.

Cheers WB .............

suchi snipped-for-privacy@yahoo.com wrote:

Use a spline gear with shaft connected to an anvil.

See link below for a possible