it seems that a lot of cheaper CDROM drives position their laser by using a standard, imprecise DC-motor and using the laser tracking as feedback.
Besides cost, the track on an optical disc runs in a spiral, and thus the continuous motion of a DC motor is advantageous because the feedback loop is adjusting the linear velocity of the head instead of its absolute position, whereas a stepper motor would have to make discontinuous jumps. This helps tracking performance and decreases noise (both electrical and acoustical.)
Except that a CDROM wants to be able to seek quickly (say 100 milliseconds) while also read a disc at 1x speed (taking 75 minutes).
Having a DC motor which can run smoothly across 5 orders of magnitude is pretty much impossible. Stiction will always be more than 0.05% of the motors power output.
You therefore have to use a "go-stop" approach based on laser feedback instead.
> On startup, it'll run the carriage into that switch so it knows exactly where it is. With my setup missing that switch, how does it know where the carriage is? Basically by doing the same thing: it'll run the carriage into the end stop for a good while. If the carriage reaches its limit, it'll slip back and stay there. This gives quite some wear-and-tear on the mechanics and isn't the most elegant solution by a long shot, but for a device that isn't used much and with me not having enough GPIOs left to hook up switches, it had to do.
As I understand it, this is basically the same solution the Apple II disk drives used to return the head to track zero. Owners of the Disk ][ will remember the loud repetitive clicking sound it made at startup. That's the sound of the drive controller moving trying to move the head outward 40 times and hitting the end stop most of the time.
Then the process is "move towards the endstop", then "switch off the motor current, if we are pushing against the endstop spring, then we'll be pushed back a step". Repeat this process 40 times.
Based on the science described in mark robers jelly pool (where he makes an actual proper size pool of properly set jello/jelly), probably not possible to print proper gelatin. At least not without it being a bunch of tiny particles rather than a congealed clean solid:
https://www.youtube.com/watch?v=DPZzrlFCD_I
It would have been a cool project, even if he used 8020 extrusion like most similar projects.
Instead, he used salvaged gear for this: He pulled old stepper motors from hard disk drives, and even used an old laptop battery as a power supply. Impressive!
His whole website is an absolute goldmine of awesome projects and hacks. I thought this was really cool: Optical Mouse Cam: https://spritesmods.com/?art=mouseeye
The non-obvious way to greatly speed this method up is to use an array of nozzles and dip them all at the same time. Draw them upward while squirting your resin at the appropriate times and you have a 3D printer that prints as fast as you can move something up and down...
Imagine a sewing machine, but every time it moves up a 3D part is left behind in a binder. A new container is whisked in to replace it and the process repeats. Patent #10,870,239 issued, and more are pending :)
This is the principle behind the rapid liquid prototyping process. Positive displacement of a UV cured crosslinking polymer into a self healing substrate allows building at m/sec instead of cm/min.
If we have grant money left over at end of year, I was planning to work on a bench scale unit.
Besides cost, the track on an optical disc runs in a spiral, and thus the continuous motion of a DC motor is advantageous because the feedback loop is adjusting the linear velocity of the head instead of its absolute position, whereas a stepper motor would have to make discontinuous jumps. This helps tracking performance and decreases noise (both electrical and acoustical.)