Z Probe Options
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Has anyone done some work on an ultrasonic sensor?
Perhaps something like this? It takes 5V and senses 2cm+ with an accuracy of 3mm. Now I understand this is not good enough for this application, but maybe there is another sensor that can work.
What kind of electrical interface should I be looking for if I do find something like this?
I used to sell a switchable modulated IR/ultrasonic sensor for Ormerod printers. The ultrasonic option worked reasonably well when the bed was cold. The problem was that the speed of sound in air is very temperature dependent. To mitigate this, I added a facility in RepRapFirmware to specify the variation of the trigger height with bed temperature - and that facility is still in RepRapFirmware. However, if you tried to probe with the ultrasonic sensor while the bed was heated, then air currents caused by convection tended to disturb the height reading. So I abandoned the ultrasonic technology in favour of differential IR.
I do like the idea of a non-contact solution, but do you think something like this will work with the Duet Wifi?
I'm thinking of using these on my next printer, because there are two x axis carriages, and to be relatively positioned precisely will require some very repeatable switches. But, could this also be used as a z-probe?
Edit: actually, they have a NC version of that switch or something similar, meant to link to that one. This link is for a NO switch.
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How do you propose to deploy the switch when probing, and retract it when printing? Or would you only use it to probe pillars at the side of the bed?
If you can make your dual carriage system stable enough that the relative heights of the nozzles remain constant to within 20um or so, then you can use an IR sensor or some other probe attached to one of the carriages, measure the relative heights of the two nozzles, and put that figure in the Z offset of the G10 command for the second carriage.
Another possibility would be to mount one or more tactile switches at one edge of the bed, opposite to the homing edge for that axis. Then you could use each nozzle to probe that switch, and measure the Z=0 position separately for each nozzle. But you could only use this for setting the Z=0 height, not for auto bed compensation.
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I would probably only install the probe when calibration was necessary, or use pillars around the bed.
I have only ever manually calibrated printers, so my mindset isn't used to auto compensation or anything. I envision simply using this to manually adjust the bed level and then setting z=0. My main question was to clarify if this specific type of switch would be directly compatible with the Duet Wifi. It seems so, but maybe there is some voltage requirement or the like I'm missing.
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Any kind of switch intended for low-current use will work. NC switches are preferred.
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This one looks like a neat solution
http://www.thingiverse.com/thing:1517363I especially like the use of the optical sensor and the design means that the probe is essentially dropped into place by the servo and not swung out on the servo, so its positional error must be very small.
Plus you could do away with the servo and just manually deploy/stow it when you wanted to do calibration.
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ive got an aluminium hotbed with a glass sheet on it would the ir sensor work with this solution
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ive got an aluminium hotbed with a glass sheet on it would the ir sensor work with this solution
Yes but you'll probably need to paint the top of the aluminium or the underside of the glass matt black, or use black paper between the glass and aluminium. Also, don't use 3dlac or hair spray as they can affect the reflectivity of the glass and give inconsistent readings.
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I'm going to say it, piezo is the way forward it's cheap, reliable and extremely accurate. Nozzle contact offers huge advantages. The best way to describe it, is that the tip of the nozzle is the tool, and it's the point in 3d space that the coordinate system describes not a position some way above and off to the side of the nozzle.
What better way to determine where the bed is than by touching it with as little as 15g of force? No x and y offsets.
My large delta calibrates to 0.006mm deviation across 300mm of bed!
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Well as we're all saying it, I'm going to chirp in and say that a Metrol positioning switch, attached to a hot end that slides in Z on steel dowels in bronze bushes is the way to go. All the advantages of using the nozzle to probe the bed but without the need for any signal conditioning that the piezo requires. Of course, designing the hot end that moves 0.3mm in Z but is solid in X and Y is a bit of a PITA…..........:)
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@npm1:
If you can mount it, go with the piezo system from Precision Piezo (DJDemonD).
Till recently I had a FSR system under the bed and it was good to probe with the nozzle. But the piezo is better.
