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

I am not sure about pricing, have seen prices from $7 to about $20 for a single sensor, which puts it on about the same level with capacitive probes, so I am wondering why th FSRs have only been used (to my knowledge) for 3 point bed leveling, under the glass - so no heated bed -, and not at printer head level.

There is one out the box instructable for a mini kitchen scale load cell used as the sensor for weight reduction when the nozzle touches the bed. I am thinking at using a FSR instead, placed somewhere between the hotend mount and the X carriage.

What are the obstacles with FSRs that they are not used in printer head setups? I get it that for a bed leveling they are expensive (you need 3) and that a heated bed is a no-no in most cases, but why not in the hotend mount itself? The hotend, in most cases, is clamped and it just "hangs" there from a mechanical perspective, so there would be little to no force pressing on the FSR unless one is printing, when there would be little force and when the nozzle touches the bed, when it should be calibrated to register.

It is an almost invisible solution that should work.

Edit: they are said to have a 0.02mm repeatability, which might not be enough (phidgets.com sais that "They are more appropriate for qualitative rather than precision measurements.").

But how do they compare to the very inexpensive Inductive sensors and Capacitive sensors we buy from ebay/aliexpress? Are those better for the same pricerage? I get it that infrared ( a la dc42, laser, capacitive displacement sensors and eddy-current sensors are the best but I am talking about this low pricerange of $5-$15). I would be also very curious how do these inexpensive probes we try to adopt measure up against the classical mechanical switches' repeatability.

Edited 2 time(s). Last edit at 02/01/2016 06:20AM by realthor.

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I believe it has been done, but I think there are some issues:

1. The mechanical complexity of fitting an FSR into the print head, while keeping the head securely mounted. Some people have used hinged hot end mounts, but these cause the nozzle to scrape along the bed as it is pushed upwards.

2. If you probe with the nozzle cold, then any filament stuck to the bottom of the nozzle will affect the trigger height. If you probe with the nozzle hot, this may damage some bed surfaces e.g. BuildTak and probably PEI.

3. You need to pre-tension the FSR such that a small upwards force on the nozzle will trigger it. This pre-tensioning adjustment may not be stable. With FSRs under the bed instead, you don't need to do this.

Edited 1 time(s). Last edit at 02/01/2016 07:14AM by dc42.

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I've seen the hinged hotends mainly used by deltas but they use microswitches and they tilt quite a bit the hotend, something I didn't quite like. This FSR could be almost hidden solution and the amount of move after hitting the bed would be minimal should the FSR register the force right away.

I understand the concern for heat on different build surfaces... that is a very valid point.

Still this is the most unobtrusive solution out there. I haven't seen small inductive or capacitive probes. Those are bulky and in the way.

I am thinking about sandwitching the FSR+Neoprene Rubber (or whatever material) between one of the Hotend's upper shoulders and the mount that clamps it to the X carriage. Something like this:



Pre-load solutions are not complicated and the nozzle touching the bed would send an upward force that will try to push the hotends through their mount, thus activating the FSR at low forces. I hope I am understanding/thinking this right and that the FSR can sense the very small amount of force squishing it between the plate and the plastic mount.

Edit: I believe FSR work in an NO state so I believe we can actually preload it to a certain degree so it stays NC, then when the nozzle presses into the bed it lift sme weight off the FSR and the calibration shoudl be done in such a way that at very little force the logic (controlboard) registers an endstop trigger (registering a breaking of the circuit in a NC switch) and that would protect also from random head crashes into the bed:



I'm sorry if I am talking rubbish here but this is my understanding and some solutions I am thinking of because I can't use a permanent Inductive/Capacitive probe due to space restrictions and also don't want to spend too much on an IR probe (which I suppose is north of $30). Also less knowledgeable people can easier think out of the box cause their box is so small

Edited 4 time(s). Last edit at 02/01/2016 01:26PM by realthor.

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If the sensor used foil strain gauges then there would be no movement - or undetectably small anyway.
Something like these

The trickiest bit would be designing a strain element to bond a set of the gauges onto.

Mike

To make a system like the hinged microswitch in the hotend mount, but using a bridge of 4 strain gauges I think that the foil gauges could be mounted on a simple steel beam and a quad op-amp could be wired as a simple instrumentation amp and comparitor - should be achievable for about $10. I have tended to use a ring element but that was for measurement while this is more a presence/absence sensor.

Mike

You are already loosing me here but if you are patient enough with electronics noobs I might follow .

Sote what I am thinking of is more the latter solution than the hinged one: Have an already preloaded element that registers a closed circuit and when the tension on it decreases by a minimum amount, it would register an open circuit. Is this still achievable with what you suggest? (I am now watching explanations on the terms above)

The FSRs used in Kossel implementations could be calibrated so they don't register under the bed's weight but they will register if an additional force to the existing weight was present. I am thinking the same way here but in reverse. so we always have an NC circuit that would Open when a certain amount of force is taken away from the preload.



Edited 2 time(s). Last edit at 02/01/2016 12:29PM by realthor.

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Foil strain gauges are resistive elements typically of 350 ohms each, they are often about 15mm long by 5mm wide and a small fraction of a mm thick. They are usually used in sets of 4 glued (epoxy will do) to a chunk of metal that can be slightly deformed by a load - the chunk of metal is the strain element and the 4 strain gauges are mounted and wired in what is called a bridge - this makes for great sensitivity; they can be sensitive enough that the bend caused by a feather on the end of a 1 foot long 1 inch diameter bar could be detected - and that was back in the 1960s. They seem to be quite cheap on ebay although in the past they could be had by the thousands for next to nothing in surplus stores whenever any aircraft development project came to an end

Mike

That is very interesting. I watched a few videos and read a few pages of explanations. They seem to be a heck of a solution for both weight and space. The only problems are the need for skills in electronics which people don't usually have.

The FSR seems to be a more friendlier solution but one which is not very suitable for accurate measurements. My diagram above would work only for FSRs. The beauty with FSRs is that they don't seem to need any extra electronics.

The challenge with using strain gauges for nozzle touch is what is the least amount of work/materials/skills to transform the resistance sampled at different intervals (maybe 100 times/sec) into voltage and have the arduino do the sampling/calculation. I believe some amplification is also needed, which means extra electronics. Basically a digital switch needs to be programmed so that the end-stop is triggered when the calibrated difference is achieved. Having gotten to this point I believe the Instructables video I linked in the first post was about the exact thing we are discussing here (a load cell) but there was extra electronics needed for that project, totaling about $20 with parts sourced from ebay. I admit it, DC42's IR probe has a great appeal considering all the above.

With all this added complexity I believe the FSR is still the way to go for an easier and still cheap approach.

Edited 6 time(s). Last edit at 02/01/2016 01:52PM by realthor.

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Moaaa ... look what I've found:

Strain Gauge Module: $7.20


Things are getting more and more interesting. Now I find these modules all over, on Aliexpress (they free-ship to where i am that's why I am mostly interested).

Can these modules be used with a 3D printer? At $7-$10 they are really a true contender for the precision measurements (something that FSRs are not able to provide past a certain level).

Edit: There is even an Arduino Shield for load cells.

Now all that is needed is a Brain to put together some useful instructable.

Edited 1 time(s). Last edit at 02/01/2016 07:40PM by realthor.

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Here is one example
[www.thingiverse.com]

On www.seemecnc.com forums check for mhackney's solution, he has a nice setup with good repeatability.

Quote
WZ9V
Here is one example
[www.thingiverse.com]

On www.seemecnc.com forums check for mhackney's solution, he has a nice setup with good repeatability.

Still on the thingiverse link: good resource I wonder how on earth did I miss it :-/ .

From the comments I see that:
1) they suggest another controller board whereas the Kossel Wiki claims that direct connection can be done to the controller board of the 3d printer;
2) "Where I expected the whole mechanism to travel significantly, in fact it has no visible movement before the FSRs detect a change." ...this is great news as it is exactly what I expect/design for
3) "noticed that the FSR would be decompressed when the nozzle touched the build surface vs. most other designs where it is compressed." ... again, exactly what I want in my design

Thanks for sharing. Do you use a FSR solution or you are too interested?

Edit: from the seemecnc forums I also get confirmation that it can be attached straight to the board.

I am closer to feeling comfortable enough to start designing around it and ordering a couple FSRs


But i have to be honest, I am more and more intrigued about the Strain Gauge thing if the module I linked above can be the only interconnect between the Strain Gauge stamp and the arduino ...

Edited 1 time(s). Last edit at 02/01/2016 09:02PM by realthor.

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My current FSR solution is not working well but that's due to some other problems I need to fix. I use a bed mounted solution and intend to upgrade it to the one used by mhackney on seemecnc. [forum.seemecnc.com]

When doing a bed mounted solution watch out for the pressure tilting the bed. That is one reason I like mhackney's approach but it needs something like the "star" found under the Onyx bed they sell. The "star" is essentially 1/4 in melamine support under the bed that makes a spot to mount the plungers. I like his system because it allows the bed to float without rotating and isolates the FSR from the heater.

With a cartesian style printer usually the bed is moving so I wouldn't put the FSR but in the printer head, although because all of the wires there the noise could be substantial, affecting the readings maybe. But sure offers an invisible and lightweight solution for bed probing.

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I used a print head FSR based levelling system on my kossel mini and it worked very well with a couple of caveats;

Using the FSR connected directly to my smoothieboard did not work well for me, I used a very simple op-amp comparator to convert from an analogue to digital signal. I could also adjust the trigger point on this circuit.

I used three FSRs because, at least with a delta, when you probe near the edges of the bed the effector can tilt and the probe will trigger at different heights depending on where the probe is placed relative to the hot end.



In the image the FSRs are yellow and actuation relies of flexing of the red hot end mount relative to the purple effector frame.

Moriquendi

With a Cartesian printer one could use less than 3 FSRs. For 2 Hotends I plan to place it in between them.

All I am counting on is that while relieving very little pressure of the FSR I'd be able to get a trigger. Can I do that without an intermediary op-amp or whatever board? What is the behavior of the FSR if you connect it straight to the controller board of the printer?

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realthor
All I am counting on is that while relieving very little pressure of the FSR I'd be able to get a trigger. Can I do that without an intermediary op-amp or whatever board? What is the behavior of the FSR if you connect it straight to the controller board of the printer?

That would work if you use a board and associated firmware that supports an analog Z probe, e.g. Duet/RepRapFirmware. You would still need a resistor so that that FSR and resistor form a potential divider between +3.3V and ground.

If you use more than one FSR, then using John't trinket board or similar would still be advisable.

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Quote
dc42

Quote
realthor
All I am counting on is that while relieving very little pressure of the FSR I'd be able to get a trigger. Can I do that without an intermediary op-amp or whatever board? What is the behavior of the FSR if you connect it straight to the controller board of the printer?

That would work if you use a board and associated firmware that supports an analog Z probe, e.g. Duet/RepRapFirmware. You would still need a resistor so that that FSR and resistor form a potential divider between +3.3V and ground.

If you use more than one FSR, then using John't trinket board or similar would still be advisable.

Having to use a board will likely make the FSR thing loose its price advantage. At that level I'd rather use your IR sensor

But I insist , the RepRap Wiki for the Kossel FSR states that: "Johann's first prototype used the heated bed thermistor (analog) input pin, but that's actually not necessary. The endstop (digital) input pin works just fine, without any additional parts.". I have a glass bed and I can't use Inductive Probes as a consequence; moreover I can't find cheap Capacitive probes that are both reasonable in price and have small diameter (I'd use Inductive Probes 6mm and 8mm OD, but Capacitive Probes seem to start at 12mmOD - this one is 12mm and I haven't seen thinner). So I'm betting big on FSR without additional costs implied by boards/etc.

What am I understanding wrong?

Edited 2 time(s). Last edit at 02/02/2016 06:00AM by realthor.

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But I insist , the RepRap Wiki for the Kossel FSR states that: "Johann's first prototype used the heated bed thermistor (analog) input pin, but that's actually not necessary. The endstop (digital) input pin works just fine, without any additional parts.".

I would never rely on a digital pin responding correctly to an analog signal. At the very least, the pressure needed to trigger it is likely to vary with the temperature of the MCU.

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I found that using an op amp comparator board made tuning much much easier. I initially ran without it and it does work if everything is tuned perfectly. However I found that the FSRs would drift over time.

The FSRs are made of two conductive layers spaced apart by a very small distance. When the FSR is pressed the two layers flex and touch, with a low force the contact between the layers is poor and the resistance is high, as you add more force the pressure improves the contact and the resistance drops until it reaches a minimum. When you start removing force the layers spring back towards their flat shape reducing the contact and increasing resistance.

Crucially there is nothing other than the flex of the conductive layers to return them to their flat shape. The conductive layers are printed onto thin, transparent, plastic (Polycarbonate possibly?). If you leave the FSR under pressure or use a concentrated force (a bolt head in my case) you can permanently bend these plastic sheets. I don't think that FSRs can be used in a situation where pressure is removed to activate the signal because after a while (not very long in my experience) the plastic bends permanently and your sensor will no-longer activate when you remove the force.

The other issue I had and the reason I used the op-amp is that in order to make the FSRs work with the digital input on the smoothieboard the off state had to have very high resistance while the on state needed quite low resistance meaning the the probe needed quite a lot of force to activate. With the op amp I could choose my own trigger point so that on the off state the FSR was already triggered to some degree(medium resistance) and only a very little force would drop the resistance enough to switch the op-amp and activate the probe. The op-amp comparator could be as simple as two components and cost almost nothing, I put mine together dead-bug style in about five minuets.

Moriquendi

Quote
Moriquendi
I found that using an op amp comparator board made tuning much much easier. I initially ran without it and it does work if everything is tuned perfectly. However I found that the FSRs would drift over time.

The FSRs are made of two conductive layers spaced apart by a very small distance. When the FSR is pressed the two layers flex and touch, with a low force the contact between the layers is poor and the resistance is high, as you add more force the pressure improves the contact and the resistance drops until it reaches a minimum. When you start removing force the layers spring back towards their flat shape reducing the contact and increasing resistance.

Crucially there is nothing other than the flex of the conductive layers to return them to their flat shape. The conductive layers are printed onto thin, transparent, plastic (Polycarbonate possibly?). If you leave the FSR under pressure or use a concentrated force (a bolt head in my case) you can permanently bend these plastic sheets. I don't think that FSRs can be used in a situation where pressure is removed to activate the signal because after a while (not very long in my experience) the plastic bends permanently and your sensor will no-longer activate when you remove the force.

The other issue I had and the reason I used the op-amp is that in order to make the FSRs work with the digital input on the smoothieboard the off state had to have very high resistance while the on state needed quite low resistance meaning the the probe needed quite a lot of force to activate. With the op amp I could choose my own trigger point so that on the off state the FSR was already triggered to some degree(medium resistance) and only a very little force would drop the resistance enough to switch the op-amp and activate the probe. The op-amp comparator could be as simple as two components and cost almost nothing, I put mine together dead-bug style in about five minuets.

Moriquendi

It's a bummer that the depressing to activate wouldn't work well... but it's only a design choice so pressing is just the same. BTW, isn't any design going to place a certain amount of constant force (wight of the bed, which can be substantial) which will mean that the FSR will be easier and easier to activate in time, conducting to potentially false triggers?

Is there a video tutorial about building such a device? You have to consider that many of us have zero experience in electronics and even if we are willing to put our sweat into it we have no clue how to do that. Maybe this board is something that is open source so we can replicate it at home? I'd like to dive my teeth into some basic electronics but even if parts sourcing and soldering wouldn't pose as much of a problem, the board making (small, unobtrusive) is.

What do you think of Strain gauges then? There are boards we can buy (I've linked them above) with the strain gauge already attached. Wouldn't it be about the same? The board would raise a digital trigger to the controller board. The only thing that would be needed is a metallic (most of the times) beam that will provide a surface to stretch.

@DC42: would a strain gauge signal be able to be picked up by the Duet without an amplifier circuit?

Edited 1 time(s). Last edit at 02/02/2016 07:07AM by realthor.

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@DC42: would a strain gauge signal be able to be picked up by the Duet without an amplifier circuit?

No, a strain gauge needs an instrumentation amplifier and up to 3 precision resistors depending on the configuration of the strain gauge.

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I have been looking at a guide for FSRs at [www.sparkfun.com] and they look very interesting. Having said that, I have been disappointed with FSRs in the past. While their poor repeatability and reliability are not too important if used for a presence/absence detection, I found that the need for a compliant elastic mounting almost always gave problems whenever moderate force was applied - such as in doing maintenance on the device. Strain gauge load cells are much more forgiving.

Although I won't say that it is impossible, interfacing a foil strain gauge directly to an Arduino in probably pretty impractical - a heroically accurate ADC along with sophisticated multi sampling, a full bridge strain gauge and thin load element may do it but an amplifier is a better bet. A true instrumentation amplifier would obviously be desirable but for presence/absence sensing a simple 2 op-amp circuit would probably hack it - maybe with a third wired as a comparator.

Just a passing thought: If the load cell was between the extruder and nozzle then it would not only be able to detect nozzle touching the build stage for leveling and Z zero setting, but it would also give an indication of back pressure - warning of nozzle blockage.

Mike

Having looked back at realthors original submission, I found that the kitchen scale load cell is almost cirtainly a strain gauge based device - possibly using a silicon sensor but otherwise not much different to an etched foil type. In addition, the HX-711 is little more than a "heroically accurate ADC" as I said earlier - O.K., it has got cleaver zeroing etc..

Although the strain element is far from the ideal shape for using on an extruder, I haven't come up with anything better so I may get a kitchen scale load cell and HX-711 ADC board to try out.

Mike

Guys, please look up my post about the ready to buy strain gauges and the additional board they come with. Is that enough of a circuitry to be able to send an on/off signal to the Arduino ADC?
[http://forums.reprap.org/read.php?1,619119,619713#msg-619713]

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Although it would work I should make two points

• The output is analogue. You can put an analogue output into a digital input but it is unreliable, kludgy and you will be shunned by most engineers.
• As shown it has only a single leg of the bridge. To get the best sensitivity and immunity to temperature effects you will need a full bridge (O.K. a half bridge - but some engineers may shun you)

If you go for a full bridge, the HX-711 that you quoted can be used and is cheaper. It will however need some programming as it doesn't give an on/off output but a two wire serial output.

That board will certainly interface with an arduino, the question is whether it will do what you want repeatable and reliably which I have my doubts about.

Strain gauges fundamentally measure the amount of stretch of the active element, in this case a minute etched trace on a plastic backing. In a load cell a calibrated strain gauge is carefully bonded to a calibrated metal bar such that any stretch in the upper or lower surface of the bar (such as applying a weight to the end) is also transferred to the strain gauge. That stretch changes the physical size of that tiny etched track making it thinner thereby increasing the resistance. So far so good, but, as dc42 has alluded to strain gauges are not easy to use and even harder to use well. The change in resistance of the gauge is so small that you need a powerful amplifier to produce a measurable signal, a powerful amp that isn't a high precision amp introduces a lot of noise into the signal that can easily swamp your reading (the board you linked to uses a LM358, about as basic an amp as you can get, categorically not a precision amp). Furthermore, temperature changes affect the resistance of the strain gauge, more than the strain readings do infact, so you have to take great care to eliminate thermal effects from your readings. Normally you would have an identical strain gauge next to your measuring gauge that doesn't get flexed and subtract the signal. Long story short, I would be quite surprised if that strain gauge module gave you anything but headaches.

I've been through many iterations looking for a cheap, reliable, accurate (enough) and repeatable Z-probe. What I've currently settled on is a version of the touch probes used in industrial CNC machines.



This is one corner of my delta effector. The red part clamps to the hot end and is bolted to the effector frame (purple) by a sprung arm (at the back). In the frame is a pair of 90deg PCB pins, the brass bolt in the red clamp bridges the two pins making a circuit. When the hot end is pushed into the bed during probing it lifts one of the brass bolts fractionally, enough to break the circuit and trigger the probe. The three sets of pins are in series so breaking any connection triggers the probe.



It's not perfect and has required some tuning and fiddling but so far it's been the best system I've tried and it has the advantage of being dirt cheap. If I had space under my effector I might go for dc42s IR sensor but I want my ball joints as close to the nozzle as possible and I don't like the idea of having an offset between the sensor and the nozzle.

Moriquendi

That is a very neat idea, much like the Renishaw touch probe.

If I can make a suggestion, the presumably gold plated brass contacts may have a bit too much springiness. Replacing them with firmly mounted stainless steel balls should reduce this a bit.
I made a touch probe using stainless steel balls with sections of stainless steel tube as the shorting contacts and I used two magnets set to repel each other as the spring to keep everything together. The repeatability was certainly in the low 10s of microns.

Mike

Quote
leadinglights
That is a very neat idea, much like the Renishaw touch probe.

If I can make a suggestion, the presumably gold plated brass contacts may have a bit too much springiness. Replacing them with firmly mounted stainless steel balls should reduce this a bit.
I made a touch probe using stainless steel balls with sections of stainless steel tube as the shorting contacts and I used two magnets set to repel each other as the spring to keep everything together. The repeatability was certainly in the low 10s of microns.

Mike

I am too looking for a minimal move system, that's why I was researching FSRs before I learned about Strain Gauges. If we can have a even simpler solution, especially one that is zero (or close to zero) movement I am in. I started with the thought of having a current loop ran through nozzle and bed and when contact is made it registers but that requires again a steel or conductive bed. So my research led me to printer head-only solution... which yours and Moriquendi's seems to satisfy.

Can you post a sketch so we can get some inspiration?

@Moriquendi: I find it difficult to imagine how that works in real life , can you make a video of the probing? I also see that you clamp your hotend by its lower side, possibly screwing the stainless heatbreak into the effector. What is the red part made of? Aluminum? Sorry for being insistent but could you post a video or some pictures?

Edited 1 time(s). Last edit at 02/02/2016 02:37PM by realthor.

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The touch probe that I made was for a milling machine. Sadly I have no photos of it but you can get a good idea of how it works by Googling for "diy renishaw touch probe"
It is such a good idea that I will try to get it into my project bucket - for me the main/only problem with the strain gauge was to come up with a load cell form that didn't introduce constructional or operational probems.
Having said that, I think that I would only use a nozzle touch system to establish a datum; my problem with them is that all nozzles suffer from some degree of incontinence with blobs left all over the bed. For the actual bed leveling I would use an optical system such as DC42s differential IR sensor

@Moriquendi: I re-read your explanation and I've understood this time. Sorry for being stupid sometimes.

I've searched the net for CNC probing DIYs and found this one, which looks awfully similar to what @leadinglights is suggesting: [www.thingiverse.com]



I really like this ballbearing or some other round on round solution because of the minimal contact that breaks at even the slightest move. Does this mean that having more such round-on-round contacts would be better? Another question: if the move is very small, isn't the current still able to transfer through air via minuscule electric arcs?

Quote
Moriquendi
It's not perfect and has required some tuning and fiddling but so far it's been the best system I've tried and it has the advantage of being dirt cheap. If I had space under my effector I might go for dc42s IR sensor but I want my ball joints as close to the nozzle as possible and I don't like the idea of having an offset between the sensor and the nozzle.

Now I rephrase my request: I'd like to know more about the connection to the board, calibration, tuning... Would it be too much to ask for a tutorial? In the meantime I'll look around maybe such thing already exists.

I was almost ready to settle for a spring loaded CNC Zero Touch Plate that has some UHDPE pads and can be dragged around by the nozzle and then pressed into for touch ...


Now this is some milion points probing [youtu.be] ... not sure how it works though:


Edited 3 time(s). Last edit at 02/02/2016 04:51PM by realthor.

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