Digital micrometer for positioning

Has anyone tried attaching a digital caliper to the X and Y axis?
I recently picked up a cheap one that has a port for connecting it to a computer. I haven't had time to investigate it but it seems to me that this could solve some of the problems with accurately positioning the print head.

I think the issue with those is the frequency of measurements is a bit slow.

Perhaps it could be used to detect when there is a missed step.

A more usual way to do this is to add linear or rotary-encoders
to the axis. The result is stepper-motors operated as servo-motors
to reach abolute positions while detecting and countering missed stepps.

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Digital Calipers are often used as linear encoders on machine tools see the following image.

Linear encoder milling

The data I think is sent 3 times per second from the port which may be sufficient. Here is another link to some info.

Interface

martinprice2004 Wrote:
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> Digital Calipers are often used as linear encoders
> on machine tools see the following image
....
Well. that's all manual machines where an operator can read
the value with his eyes.
Last time I checked a RepRap didn't have eyes.


3x a second means that even at slow 32mm/s you
have already moved 10.67mm between 2 meassurements.
Not enough for a closed feedback loop that tells a
stepper of 400 steps per revolvolution and <5mm
movement per revolution how far it has moved and
how to do minute adjustments to it's speed.

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Thanks!
Once I get my reprap working I will start looking into this.
Even if it only checks the position immediately before starting the extruder, I imagine that print quality will improve.

Yes If you are using stepper motors 3 times per second would be enough as you are only verifying position.

We are confusing this with a closed loop system where the signal feedback is used to control the motor position in real time. This is how servo motors are controlled in CNC and you would need faster feedback for this.

One use I could see for what you are proposing is to set the initial datum of the print head before you start to print. Accurate postioning of the Z axis is vital when putting down your first layer to get good adhesion and avoid warping the part. It would be great if you could get positional feedback, particularly if you are doing multiple unattended builds. Perhaps you could spring load it to one side of the build platform to verify Z position.

the z axis doesn't move very fast so 3/second wouldn't be too bad.

Here are some resources for the output signal:

[www.compendiumarcana.com]
[mdmetric.com] (at the bottom of the page)