Mini height sensor

@dc42. Just ordered one of these for a corexy that I'm planning. I'm thinking of using it for homing all 3 axes. The hot end will move in the X and Y directions only, the bed will move in the Z direction. Question1, I'm assuming that the X and Y tab surfaces can be simply printed PLA, is this correct? Question 2, how accurately would the tabs need to be positioned in the Z direction, relative to each other and relative to the probe? Question 3, leading on from question 2, what would the optimum position (height wise) of the tabs be in relation to the probe and what is the tolerance?

Many thanks in advance.

Yes, you can use printed PLA for the homing tabs - that's what I have on my Ormerod. Any colour except possibly black will probably work.

I suggest you aim to position the tops of the homing tabs about 1.75mm +/- 0.5mm below the bottom edge of the sensor board. If you get them too low, you can always glue a piece of card in top to raise them a little.

You will need to design the homing sequence to ensure that you know which tab has triggered the sensor. Here is one possibility:

1. Start the homing sequence by increasing Z by a few mm so that the bed will not trigger the sensor.

2. Do a course homing of X and Y simultaneously, as in my sample CoreXY homing file.

3. Move back by about 5mm in both X and Y directions, so that you can be sure that the head is not over either tab.

4. Home X, then move to x=5.

5. Home Y.

6. Move XY to bed centre, and home Z.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

Thank you. I have an original RRP Mendel tricolour which uses the RRP probe for Z and X and a micro switch for Y - it sounds like the Ormerod is the same. RRP recommended painting the x tab white but of course theirs was a different probe but that's why I asked the question. If I locate the X tab on the end of the X axis (a la Mendel), and the Y tab somewhere in the middle of one of the font or rear to frame rails, can't I simply lift the bed, home X, move X to the position where it will find the Y tab, home Y, move to the centre of the bed and home Z? Why do I need to do the course homing of X and Y simultaneously first?

Edit, just thought it through and of course if the hot end was at the extreme Y position, then the Y tab could trigger the X homing. Ignore the above.

Edited 1 time(s). Last edit at 05/17/2016 03:05PM by deckingman.

I know exactly what you mean as I have an RRP tricolour Mendel too, however its soon going to be stripped down and have a P3 frame added

The problem is you need XY to be homed before you can home Z...... and until you home Z you don,t know how far off the bed you are , so you will be unable to get an accurate trigger for Y home

Edited 1 time(s). Last edit at 05/18/2016 05:46AM by bgkdavis.

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RepRapPro Mendel 3 Tricolour
RepRapPro Fisher
-Carbon Arms
-Easy adjust Carriage+effector
-axis stiffness mods
HE3D -600 delta
-Duet 0.8.5
-PanelDue
-DC42 Height probe
-RobotDigg metal components
Simplyfy3D
RS Design Spark CAD

Quote
bgkdavis
The problem is you need XY to be homed before you can home Z...... and until you home Z you don,t know how far off the bed you are , so you will be unable to get an accurate trigger for Y home

That would be true for a Cartesian printer with a bed moving in the Y direction. However, deckingman is planning a CoreXY printer, in which the bed moves in the Z direction.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

Sorry didn't realize it was a gantry design

Just about any homing system will be screwed if you start homing from a poor position but this problem may be fixable by first moving 10mm in Y before homing X, in much the same way the RRP mendel moves 10 mm in Z before homing X, this way you can be guaranteed that the tab it sees isn't the Y tab.... of course, if you start homing at Y max then that 10mm will crash the limits.... but again, just about any homing system can be screwed if you start homing from a poor position

Edited 1 time(s). Last edit at 05/18/2016 07:10AM by bgkdavis.

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RepRapPro Mendel 3 Tricolour
RepRapPro Fisher
-Carbon Arms
-Easy adjust Carriage+effector
-axis stiffness mods
HE3D -600 delta
-Duet 0.8.5
-PanelDue
-DC42 Height probe
-RobotDigg metal components
Simplyfy3D
RS Design Spark CAD

Quote
bgkdavis
Sorry didn't realize it was a gantry design

.... of course, if you start homing at Y max then that 10mm will crash the limits....

Yup, done that !

@dc42. I'm not sure using the probe for homing all 3 axes is going to be viable in this particular instance. The reason is that I'm design the printer around a Diamond hot end which, because of the 3 heat sinks angled out at 27 degrees and set in a circle 120 degrees apart, makes it quite wide in every direction. This is coupled with the fact that I want to hang the entire thing between 2 horizontal V slot extrusions which combined adds another 40mm to the width of the X carriage. I've managed to design a mount for the sensor which will get it in line with the nozzle tip in the Y direction but about 30mm away from the nozzle tip in the X direction - not ideal but that's as good as it's going too get (and at least it'll stop it getting too hot). Also, I want to keep the motors inboard of the frame which further restricts Y axis travel. So, the culmination of all this rambling means that the nozzle tip is about 120mm away from the frame at Y=0, so my Y tab is going to be about 120mm long. It may work but it looks pug ugly and I may end up making it out of metal, but even so it could get bent. I may end up resorting to micro switches or some such for the Y homing.

However, while thinking about this and designing tabs for the sensor, it struck me that the tabs need to be in pretty much the same Z position as the bed. And then I got to thinking that the actual X and Y homing positions need to be the edges of the glass bed (I'll be using glass on top of an aluminium spreader of some sort). As you know, the usual method is to home to some sort of sensor, then adjust the position by whatever the offset is to the print plate. But why not use the glass as the homing tabs? I'm thinking along the lines of homing Z, then go up a tiny bit (say 1 or 2 mm), enough to clear the bed but still within range of the sensor. Then track left until the sensor reading falls off a cliff, which should be the left hand edge of the glass and that will be the X homing. One could also then track right to find the other edge of the glass which would be the X max position. Then do X+something and track forwards to find the front edge of the glass which will be Y home. Then maybe track backwards to find the rear edge which would be Y max. Of course, the bed would have to be reasonably flat to prevent the sensor from crashing into it but if the machine is built properly, it should be reasonably flat and level to start with. Also, one would still need to apply small offsets for the sensor position relative to the nozzle in the X and Y directions.

I guess it depends how the sensor works. Does the reading "fall off cliff" if the surface drops from(say 1 or 2 mm) to (say 5mm plus)? If it would work, it's quite an elegant solution - no tabs needed and you get X and Y max as well as X and Y min.

What do think? Crazy ramblings of an old man or something viable?

Edited 1 time(s). Last edit at 05/22/2016 01:53PM by deckingman.

What you suggests sounds ok and needs no homing tabs. The only risk I see is that the nozzle might be off the edge of the bed to start with. So how about this:

1. Make sure you have good mechanical stops at the upper ends of the X and Y axes, to prevent the nozzle going very far off the edges of the bed in those directions.

2. At the start of the homing sequence, reduce the X and Y motor currents, then move both X and Y by quite slowly by about +10mm. Chances are that the head won't be near max X or Y already, but if it is then the reduced speed and current will avoid damage.

3. Move X and Y by -5mm and restore the motor currents.

At this point, the head is +5mm from where it was originally in both the X and Y directions, and at least 5mm short of the upper limits. So if the probe can't move more than 5mm off the edge of the bed, it is now guaranteed to be over the bed.

4. Home Z, then X and Y as you described. Then move the head to the centre of the bed, and home Z again.

However, this is getting rather complicated, so adding a Y microswitch is probably a simpler option.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

Yes, you are probably right. As this is still a design exercise I'm making provision for X and Y tabs for the probe. The X tab is adjustable in both the X and Z directions to allow for build tolerances and variations. The Y tab is adjustable in all 3 axes. (Using V slot or T slot extrusion for the frame makes these things easy to do). I've also incorporated provision for a Y microswitch so I have a contingency. We'll see which works best if and when I ever get around to building the thing. Thanks for your help - much appreciated. If every you think about building a garden deck, I'll return the favour.