Using FSR at hotend level for an unoptrusive bed probing via the nozzle

Do you guys know of any actual direct nozzle to metallic bed current loop implementation? Am i unlucky or there is hard to find such a thing?

I admit that with such an implementation one would rely on the springs in the three point leveling to compensate for the extra time the nozzle presses into the bed before it registers the touch but I guess that is miliseconds. At low touching speeds there would be no issue.

Also how would one keep a Normally Closed loop with such a system?

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can't we just use something like this, wire it into the ramps,
[www.ebay.co.uk]
there are so many nicely machined items be a shame not to be able to use them,
but they are a little large if all that's really needed is an accurate micro switch.

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MechaBits
can't we just use something like this, wire it into the ramps,
[www.ebay.co.uk]
there are so many nicely machined items be a shame not to be able to use them,
but they are a little large if all that's really needed is an accurate micro switch.

Super interesting find ...:-o thats a really interesting product. It is powered by batteries so it surely outputs the necessary voltage to be able to directly send to arduino.

I've found during a brief search that "Accuracy 0.005MM" ...which is ...wow
And "There is a safety spring puller which puts The ball precisely back to the position when ball breaks away from ball seat"

I hope the better equipped guys around can tell more.

If edges can be found with this tool then a little programming and we can trace a perfectly milled piece and let the software auto-calibrate the whole printer, X,Y and Z ... Please tell me I'm not dreaming too much

Edit: even cheaper on aliexpress.

Edited 2 time(s). Last edit at 02/04/2016 08:01PM by realthor.

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Those center finders when they touch the metal object to find the edge, it completes an electrical circuit between the ball and spindle via the part mounted on the mill bed through the mill to the spindle. There is no switch in the center finder.

Edited 1 time(s). Last edit at 02/04/2016 08:47PM by stephenrc.

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realthor
Do you guys know of any actual direct nozzle to metallic bed current loop implementation? Am i unlucky or there is hard to find such a thing?

Did the schematic that I post a few posts earlier not sufficient ?
I'm not sure it is totaly correct but I expect so.

Some precisions :
- The switch represent the contact with hotend and bed.
- The 5V comes from the ramps
- The GND from the ramps to
- the last port is the entry of the ramps

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lyscanthrope

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realthor
Do you guys know of any actual direct nozzle to metallic bed current loop implementation? Am i unlucky or there is hard to find such a thing?

Did the schematic that I post a few posts earlier not sufficient ?
I'm not sure it is totaly correct but I expect so.

Some precisions :
- The switch represent the contact with hotend and bed.
- The 5V comes from the ramps
- The GND from the ramps to
- the last port is the entry of the ramps

Am I missing something here or my eyes condition just got worse :-/ .. I don't know what you are referring to...can you link that post? I only saw the magnetic snatching screw youtube video you linked ...

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stephenrc
Those center finders when they touch the metal object to find the edge, it completes an electrical circuit between the ball and spindle via the part mounted on the mill bed through the mill to the spindle. There is no switch in the center finder.

It seems totally doable to me. The electronic edgefinder has a led that goes on when touch is achieved. And it works to great tolerances. If the led can go on then it means a wire can be pulled off that and connected to the controller board. In normal operation it has to rotate at high velocities but the electronic version I don't see being excentric in any way so I assume static works as well for our purpose. I am really digging this. Hate to put it on the carriage (+weight) but totally awesome for the price.

Here's an expl:
https://youtu.be/f0od-cp_9dg?t=113

Edited 1 time(s). Last edit at 02/05/2016 05:06AM by realthor.

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I was refereing to this post [forums.reprap.org]
Well I haven't tested the circuit yet ...
It may look a bit "over specified" but I wanted to avoid any issue so I built an isolated circuit for the probing

Edited 1 time(s). Last edit at 02/05/2016 05:18AM by lyscanthrope.

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lyscanthrope
I was refereing to this post [forums.reprap.org]
Well I haven't tested the circuit yet ...
It may look a bit "over specified" but I wanted to avoid any issue so I built an isolated circuit for the probing

You'd have to walk me through the schematics as I know jack sh*t about electronics (dissapointed my father as he was stellar mechanical eng and he wanted me to be electronics eng ... I ended up in IT where lazyness is a featuer ).

So you want this: "it should be done using an isolated circuit for the hotend-bed in order to avoid shortcircuit !" and that circuit offes some protection. How? Please walk me through.

Regarding this method of directly making electrical contact between nozzle tip and bed, it is a normally closed circuit which doesn't offer crash protection in case the contact is missed (loose wires,etc). There has to be a backup solution available.

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realthor
It seems totally doable to me. The electronic edgefinder has a led that goes on when touch is achieved.

It's designed to approach a work piece from the side, it's not particularly useful for approaching straight down. It also takes you back to only working on a conductive surface. The idea of probing a reference block of known XY dimensions for auto-calibration is sort of interesting, except I never had any problems with XY calibration and it's a one time thing anyway, so probably not enough benefit to justify the effort. I may be missing something, but I think in this context the edge finder is a diversion.

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JamesK

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realthor
It seems totally doable to me. The electronic edgefinder has a led that goes on when touch is achieved.

It's designed to approach a work piece from the side, it's not particularly useful for approaching straight down. It also takes you back to only working on a conductive surface.
[...]I may be missing something, but I think in this context the edge finder is a diversion.

You might actually be right. Thanks for straightening us up

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JamesK
The idea of probing a reference block of known XY dimensions for auto-calibration is sort of interesting, except I never had any problems with XY calibration and it's a one time thing anyway, so probably not enough benefit to justify the effort.

For deltas the XY accuracy is difficult to achieve so they might benefit. For carthesians I've seen many problems getting circles right for example. Maybe this can be of use. But not for this sole purpose I guess. I would have liked it to be usable for up-down situations too. When we'll have 5 axis printers this well be possible to be flipped horizontally and used for both edges and surface ...

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@jamesK
I agree with you and have problem seeing how it will work coming from above.

@Realthor
I don't really see how you will manage to have a normaly closed circuit that goes open on contact AND on loose wire ...but I guess I just don't see it

oops, I misplaced U1, it should be between the resistor and the phototransistor, that would explain why you had trouble undertsing it Sorry

Just some details on it :
- B0506s (named T1) is a DC-DC isolated convertor (from 5V to 5V) so we feed it with 5V from ramps (endstop connector for exemple)
- S1 is the contact between the hotend tip and foil/bed/whatever
- PC817 is a optocoupler : when current pass trough the photodiode (the one on top) it will shine on the transistor will have its resistance going from very large to zero
- When the circuit (5V-R2-U1-PC817-ground) is open (when no contact of S1), U1 is at a high level
- When the same circuit is closed, U1 is at the ground

I hope I helped you a bit. I don't have the componant to test it yet, but I'll order them this week end. (testing out of the printer of course )


I could be wrong, but all this fuss is just to avoid the current (generated for this sensing method) to find the ground of the ramps by passing by other element.

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lyscanthrope
@Realthor
I don't really see how you will manage to have a normaly closed circuit that goes open on contact AND on loose wire .

Umm... Normally open was what I meant. Sorry for the messup.

Thanks for the explanation, I understand a bit better now. It's some common electronics concepts that I am lacking. So we put a lead on the hotend/nozzle and the other lead on the bed/probe/etc, there is no risk at all until contact is made? Is there a difference if we put 5V on the hotend side or ground on it? I would guess that ground should be safer if permanently fixed to the hotend side.

As far as I understand you are trying to avoid a short. Namely another "switch" that you could close by running current through the hotend right? (I'm so bad at this it's embarrassing )

Edited 1 time(s). Last edit at 02/05/2016 08:00AM by realthor.

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What we want to avoid is that the 5v is directly connected to the 0v of the ramps.
The problem is that we have
-hotend side :heat cartridge and thermistor
-bed :the same if heated bed

We expect them to already be isolated from the metallic structure (hot end and bed) for them work correctly.
The bad point is that we will put the bed(or hot end) to a given potential.... So if there is a problem with their isolation... We may have a short circuit.
I m not sure that there is a better solution between 5v on head or bed!

We sure aren't the first ones to think of it, there should already be such implementations at large ... we have just to find a couple and see their experience. Maybe that helps cause my knowledge kind of ends here.

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Cant the edge finder be used to probe the edge of the bed, next to the 3 point level screws, ie set Z above bed, probe moves to outside area of bed then moves in by say 30mm, if it doesnt trip its not low enough, so moves down by layer height...repeat at other points. As far as I can tell they do not rely on a electrical loop to work it's a self contained unit. I dont see that it needs to rotate to perform the task, it appears to only make the light easier to see, but that signal/on/off could be sent to arduino.
there are the non electronic ones, which appear to just slide the tip, giving a visual indication,
there are also some attachments for probing center of bed or other things. though I'm sure the attachments called wigglers & wobblers have to rotate.

Or is the goal for something more complex, to compensate for a warpped un-even bed?

Edited 1 time(s). Last edit at 02/05/2016 10:42AM by MechaBits.

I've been under the impression that they are a self contained unit too. They even come with their batteries. The problem is that it would need a clever (and new) algorithm for bed-level auto-compensation. It is perfectly doable I am sure. But a slanted bed is not all there is to bed imperfections. Warping, as you mention, is far more problematic than a slanted perfectly level bed. If any attempt to compensation should be implemented, it should work for both.

I have one question about this edge finder ... it has a ball at the tip. The ball is used to touch a side of a work-piece. It is supposed to rotate at high speeds and maybe this is helping find the edge. But a ball is round and thus any point in space touching a ball can be considered "edge". How does it discriminate between lateral touch at 90 degrees and 0 degrees? Does it have 100% precision when it touches a side and this precision decreases as it touches the edge lower and lower on the ball, that is when the edge is below the ball's horizontal axis? These questions I can not answer.

Edited 3 time(s). Last edit at 02/05/2016 05:31PM by realthor.

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Quote
lyscanthrope
What we want to avoid is that the 5v is directly connected to the 0v of the ramps.
The problem is that we have
-hotend side :heat cartridge and thermistor
-bed :the same if heated bed

We expect them to already be isolated from the metallic structure (hot end and bed) for them work correctly.
The bad point is that we will put the bed(or hot end) to a given potential.... So if there is a problem with their isolation... We may have a short circuit.
I m not sure that there is a better solution between 5v on head or bed!

I studied a bit these cartridges and there is very little probability that there is a direct contact between their leads and the bed or the heater block. Considering that similar attempts are already in the wild, albeit called experimental/prototype/etc.

I've looked into thermal pastes/glues/etc that can withstand 300C+ and are also non-conductive but wasn't too successful, not in the price range we've setup so we can avoid high-cost vitamins. And thermal grease or any other such compound is to be expensive.

One correction regarding one of my above statements: I suggested a backup solution for Z_Min because the possibility exist that thermoplastic buildup on the nozzle can prevent registering contact and the motors will push through the bed until such contact is made. So I believe there should be another couple of 10c mechanical switches that would register the bed going too far down if the hotend presses it and the nozzle-bed contact is not registered.

Edited 1 time(s). Last edit at 02/09/2016 06:43AM by realthor.

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