Z Probe Options



  • I have a bed consisting of aluminum with some PEI bonded to it with 486MP. Right now I use a limit switch for Z, but I am wondering what the options are for Z probes.


  • administrators

    The IR sensor can be made to work very well with thin PEI sheet if you paint the underside black before bonding it to the bed. So you would need to do some work and buy a new sheet of 468MP to get good results. See https://miscsolutions.wordpress.com/2015/11/30/print-bed-surface-roundup/ for more details.

    If the PEI sheet is thin enough, an inductive sensor would be able to detect the aluminium, assuming you have enough room to mount one on your hot end. It would measure the height above the aluminium rather than the height above the PEI, but that wouldn't matter if the thickness of the PEI+adhesive is uniform.



  • Are there any mechanical switch style options? I have seen people put a limit switch on a z probe. Something like the BLTouch would be nice for mechanical feedback - but I understand the servo on it wouldn't work with RRF yet.

    I would ideally have something like it, even if it is just in place without moving.


  • administrators

    Yes you can use a mechanical switch. I used to have one on my Duet-controlled Mini Kossel. The switch was deployed and retracted by moving the head in certain ways outside the normal printable area.


  • administrators

    zzing

    The switch design David mentions is Johann Rocholl's design for the mini kossel. You can see more information about it from our (old) mini kossel documentation:
    https://docs.google.com/document/d/1G-wlL1J4Soj5tfgQbkFJF60nPH1amiRDJ-fVzMi-_JA/pub

    It works reasonably well if the components are well chosen and is cheap to source, but I think David's IR sensor is a better solution.



  • I am not going to do anything to my current bed for an IR sensor unless IR works with aluminum.



  • 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?


  • administrators

    @zzing:

    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.



  • @dc42:

    @zzing:

    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?

    http://uk.misumi-ec.com/vona2/detail/110300151320/?HissuCode=N-MSTKD-ASD&PNSearch=N-MSTKD-ASD&searchFlow=results2products

    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.


  • administrators

    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.



  • 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.


  • administrators

    Any kind of switch intended for low-current use will work. NC switches are preferred.



  • This one looks like a neat solution
    http://www.thingiverse.com/thing:1517363

    I 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.



  • ive got an aluminium hotbed with a glass sheet on it would the ir sensor work with this solution



  • @npm1:

    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.



  • 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!



  • 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…..........:)



  • @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.


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