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Report on under bed sensor problems

Posted by leadinglights 
Report on under bed sensor problems
January 11, 2020 10:52AM
I have been looking at some of the problems experienced by some users of piezoelectric discs used for under-bed leveling in 3D printers. Problems have been reported by experienced builders of 3D printers with knowledge of both the mechanical and electronic design so it seemed likely that the cause of the problems would be obvious.

I think I have run the causes of the problems that people have been experiencing to ground – though it has been quite a detective story.

I present below exhibit A: a series of plots set against the bed of my printer. Each plot shows the memory of the values stored in the buffer of the conditioning circuit for 128 readings over 32 milliseconds. The marker with the label “Contact” shows the point at which the nozzle met a contact plate directly below it. The time between the first contact and the detection of the event by the conditioner circuit is about 27 milliseconds on the front left corner but about 8 milliseconds at the bed centre. As the nozzle speed was 2 millimetres per second this means that the travel after initial contact spanned from 16 microns to 54 microns. A difference of 38 microns would be unacceptable with layers of less than 0.1mm – since this is a printer made with fanatical attention to detail other printers might just fare even worse.



To explain what is going on here, may I direct your attention to exhibit B: A reconstruction of the print bed as seen from the side. There are only two sensors shown in the interest of simplicity.



When the nozzle strikes the bed the piezo disc below the sensor has the pressure on it reduced – this was the design intent and it works as planned. Unfortunately, the bed acts as a lever with the centre of mass of the bed as a fulcrum and the piezo disk on the opposite side experiences an increase of pressure

Exhibit 3 shows the output from the three piezo sensors and a sum of the three outputs. This diagram needs a bit of explanation as it contains a bit of fiction and artistic license. The three piezo traces are from the buffer memory of three conditioner circuits and a record of 128 samples immediately before data collection stopped. The blue trace is from the right hand piezo and the conditioner responds when a certain condition has been met i.e. when it would normally respond when a contact event has occurred. The signal from the right-hand conditioner is used after a 5 millisecond delay as a signal to the back and left-hand conditioner to stop also and the data from all three are harvested. Since the back and left signals are delayed they are plotted as shifted along by 5 milliseconds relative to the first trace and a sum of all three has also been plotted.



The fiction mentioned above is in that an additional 8 readings (2ms) have been tacked onto the end of the blue (left piezo) trace and represent a continuation of the trajectory. This is approximately what does happen and helps to illustrate the summed waveform. The timing and general shape of the waveform is very close to what is seen in the first diagram.

A diagram of the interconnections on the test rig are shown below along with a photograph of the same.





Although not conclusive or even complete, perhaps this will give anybody else using piezos or other underbed sensors some idea of where to start in working around the problems which arise if a good accuracy is to be hoped for. I would point out that in the first diagram, with the contact pressure set very high, that triggering spanned about 36 microns – bad but not unusable. If the noise band is small enough then a spread of less than 5 microns is quite achievable.

Mike

Edited 2 time(s). Last edit at 01/11/2020 11:11AM by leadinglights.
Re: Report on under bed sensor problems
June 24, 2020 06:18PM
Hello Mike

Found your post interesting, what is the z axis calibration in your chart? II assume that the printer is a delta from the photo that was included. I did a number of repeatabiliy tests using the Precision Piiezo production electronics with the piezos nested in the center+/- of a rectangular bed, the piezos ranged from the smallest-10mm to the largest 40mm all mounted under bed. the tests were conducted on each size piezo diameter. The published resonant frequency for each size was (from memory) 10mm- 10kHz to abt 800 Hz for the 40mm one. My conclusion was to use the smaller disks in order to minimize low frequency pick-up. I was not party to any discussions at the time, several years [email protected]di received all of my data at the time and working with his delta , using multiple piezos. I found in my tests the underbed mounting ideal. The contact point underbead was through an insulating blanket, promptin Idris to claim that my setup was using the Piezo as a "microphone" I left the scene following a stroke, and am back now with some limitations- and in the process of building a delta Rostock Max V3.2 , featuring the Duet 32 bit board with the effector based on the smart design

So in your chart the traces represent a pickup by the others in the printer?

Regards,
Stef
Re: Report on under bed sensor problems
June 25, 2020 02:23AM
Dear Mike,

I found your analysis very intesting. I did not understand every detail yet and will have to reread. But I have some ideas already, which I want to share with you:

Besides mass lever effect, I see other effects also, where I don't know how it contributes to the result:
- the natural frequency (eigenfrequency) of the aluminium plate could play a role, especially the transient response
- the springs and their oscillations could play a role. The left/right upper/down piezo elements curves are different, maybe due to differences of the springs/screws/...
- the piezo elements may be different

I wondered whether the analysis of all piezos together when a contact is done would get better results. Adding (or the emphasis of single elements) of all piezo signals e.g. may sum up the first big peak above the threshold and shortens the time.
If the curve at the position is known, it would be possible to correct the time delay and recalculate the measurement.
Another aspect would be analyzing the effect of diffferent springs, different speeds and materials.

Best regards, Joerg

Edited 2 time(s). Last edit at 06/25/2020 02:36AM by JoergS5.
Re: Report on under bed sensor problems
June 26, 2020 08:30AM
A true luxury of questions. I will do my best to answer all of them.

@Chowa,
  • The Z-axis is not shown on the chart and the Y-axis is in ADC counts. Having said that, with a Z speed of 2mm/second the 32ms X duration covers a Z travel of 64µm. Z drive is from a 0.9° stepper driving a 1mm pitch lead screw with anti-backlash nuts.
  • The printer is not a Delta but a slightly unusual Cartesian type where the bed moves in both X and Y-axes. This is to have the heaviest bits like multiple extruders where they are only required to move slowly. views of this printer in [www.youtube.com] and used as an engraver in [www.youtube.com]
  • The resonant frequency of a piezo disk is quoted with it in open air and retained only at the periphery. When it is clamped into an assembly it has less significance than the tuned frequency of a guitar string if the palm of a hand is rested against it.
  • In the first series of plots, all three underbed piezos are wired in parallel while the contact record is approximately in the position for each graph shown on the bed. the second set of plots, marked as Combined.png, shows the output from all 3 piezos along with a calculated sum. The calculated sum of the voltages and the measured values are tolerably close.

BTW, I have also fairly recently had a stroke but have the good fortune of having no residual deficit.

@JoergS5
  • The interaction of the mass/leverage/elasticity of a plate held in 3 places and struck by a nozzle in a number of others is understandably complex and probably best done with a Finite Element Analysis package. The last time I used one of these was back when computers had green screens, an experience I would rather not repeat.
  • The natural frequency of a plate such as a printer bed is in the hundreds of hertz, say somewhere in the middle of a piano keyboard. That this is a cycle time some milliseconds should have warned me that getting things down to the hundreds of millisecondsEDIT: microseconds would be difficult - particularly when the sensors are inboard of the corners.
  • As the springs are held in compression and forces move through the piezo disk and mounting mechanism, only parasitic oscillations are expected from them and these only at a low level. I have tried to show this in the diagram below.
  • Piezo elements were closely matched on a jig made for this purpose.
  • The piezo element outputs are summed simply by paralleling them. The only purpose of looking at the outputs separately was to find where the error was coming from.



EDIT: I got a message that the text in the picture made no sense so I have replaced it - hopefully with a sensible one

The present status of this project, at least for this printer, is that I have abandoned multiple underbed sensors. I am using a single underbed sensor to detect nozzle contact and measure Z height at a single point. Bed leveling/compensation is now done with a sort of piezo/microswitch/semi_BL-Touch sort of thingy. [reprap.org]
For others though, I can only offer the following:-
Keep the positions of underbed sensors, Piezo or otherwise, at or beyond the edge of the bed
and remember that object lessons can be found even in the distant past.

Quote
The Rime of the Ancient Mariner
He went like one that hath been stunned,
And is of sense forlorn:
A sadder and a wiser man,
He rose the morrow morn.

Mike

Edited 3 time(s). Last edit at 06/27/2020 02:40PM by leadinglights.
Re: Report on under bed sensor problems
March 23, 2021 11:11AM
I have been looking at this again as I am doing some tests on my newest printer and have been annoyed that I couldn't really get to grips with the problem (Spoiler alert, I still can't, but results are intriguing)

Just to summarize what has gone before in this project: Underbed piezos are "Best in Class" as bed sensors if done correctly, but there are many ways in which they can be used incorrectly - in which case they really really really suck.

My current printer uses them incorrectly, with three sensors mounted in a triangle somewhat inboard from the edge of the printing area. The photo below shows the bottom view of the print bed and carriage



Recent progress on my printer has involved using a touch sensor mounted on the extruder carriage to map the bed. Think BLTouch, but with the printer heads being retracted instead of the sensor being extended. Once I had this running, I decided that it would be possible to map the relative sensitivity of the bed across its surface. To this end, I put new, selected piezo disks in each of the three underbed sensors and did a comparison using the Repetier bed height map tool first with the touch sensor and then with the three underbed sensors.

Results were that the map was similar to that achieved in the run mentioned earlier in this thread except that I was checking over many more points - 4 runs each of 64 points (8 X 8 grid), each run being slightly displaced to give 264 data points. The errors relative to the touch probe were that near the sensors were pretty good at 3 to 5 microns, but far from the sensors, the errors rose to about 50 microns although even this was pretty chaotic, with 10µm errors just 5mm away from the 50µm errors.

The test that made it really suck was when I deliberately replaced one of the sensors with one that was 25% more sensitive. Despite the greater sensitivity, there were points on the bed where it was not possible to detect a nozzle contact. Changing the nozzle speed from 1mm per second to 5mm per second only moved the areas of insensitivity, it did not eliminate them.

There are two takeaways from this: It can be done properly. The Delta printer below detects a contact repeatably within 5µm of actual contact. The sensors are mounted on the periphery of the print bed.



The other takeaway is that no underbed sensor is immune to the problems I have experienced here and that positioning of the sensors is critical

Mike
rq3
Re: Report on under bed sensor problems
March 29, 2021 12:14PM
Mike, I have an under bed piezo sensor on my delta, of a completely different design. I sure wish I could attach my notes or pictures, but the forum keeps complaining that they are too large (although they are not). I'd even be happy to send you a sensor if you're interested.

Rip
Re: Report on under bed sensor problems
March 29, 2021 02:17PM
Hi rq3,
I am not sure of the limits that the forums software has on size, but I think that it is 800 kilobytes, not 8 megabytes as advertised when you try to attach it.
For pictures, I typically use old-fashioned Windows Paint to reduce them. Paint is now found in the apps tray under Windows Accessories. Just open the (.jpg, .png etc.) anduse the size button to change it to a reasonable size - even 640 X 480 is usually quite acceptable on a forum posting, then re-save the file under a new name.
For Word files, I think that saving them as .docx instead of .doc will dramatically reduce the size. Other ways are to edit photographs in the files to reduce them to an acceptable minimum size..

I am always interested in seeing other bed leveling sensors - but beware as I am not above pilfering others' ideas.

Mike
rq3
Re: Report on under bed sensor problems
March 29, 2021 07:55PM
Mike, I managed to post the docx file in its entirety on thingiverse:
[www.thingiverse.com]

The basic idea is not new, and in fact is how automobile engine knock sensors work. Feel free to leverage as you see fit.
I'm surprised that apparently this hasn't been stumbled across in the printer community, and I'm shocked at how well, and easily, it works.
Again, I'm happy to send one along if of any use to you.

Rip

Edited 2 time(s). Last edit at 03/29/2021 08:02PM by rq3.
Re: Report on under bed sensor problems
March 30, 2021 05:33AM
Hi rq3,
I read your document on Thingiverse with great interest. It is obvious that you have put a great deal of thought into the subject and I can't find any significant problems. In particular, the idea of filtering out high-frequency vibrations and most of the first order acceleration component is new, as is the use of a mass attached to the piezo sensor to affect some of this filtering.

Having said that, I can see some possible problems allied to ones that I have experienced. The following applies to underbed sensors only: I know of no equivalent for in-probe sensors - although it does not mean that some boojum does not exist somewhere. Also, there is nothing miraculous about piezos, or any other type of sensor that will render them immune to these problems.

Primary amongst these is the fact that a build stage is dynamic and the movement that a sensor will be looking for can be different in different areas and can be chaotically unpredictable in some areas. This can be most clearly seen on something called a Chladni plate or sand plate and trivial differences of frequency can have major changes in areas of insensitivity.

A second effect is based on the fact that piezo disks can act as pyroelectric generators, responding to changes in temperature in an equally surprising sensitivity. This can affect the offset of the sensor and cause unexpected changes in response.

You can make an underbed sensor and check it for response every square millimeter of the surface and find no faults, but when you change the dynamic only slightly, a new build surface for example, you can find the nozzle grinding into the bed without triggering the sensor. Even worse, you can publish your design and have it copied by many readers who follow your plans almost to the letter, only to find some of them getting angry because they kept damaging nozzles and build stages. Worse still when you have an argument with a correspondent who you had been on very good terms only to end by calling them a blithering idiot - I have been there.

Out of simple interest, I would like to see the source code for your Picaxe MCU Edit: Woops!! I didn't see the code at the end of the document, just assumed it was a bill of materials.
When my own latest source code is good enough to be seen in polite company I will publish it on Github.

Mike

Edited 1 time(s). Last edit at 03/30/2021 09:37AM by leadinglights.
rq3
Re: Report on under bed sensor problems
April 02, 2021 06:13PM
Mike, I hear you. My company manufactures aircraft avionics. 99.9% of my customers are terrific. The remainder have done things that make my jaw drop. And these are folks that are licensed by the Federal Aviation Administration (FAA) to install my products, and other products like them, on aircraft.

During phone calls my first question is always, "Did you read and understand the installation manual?". The only legally correct answer is, "Yes". We're talking aviation here. My next question is always, "What can I change in the manual to make it clearer?" Almost always I get crickets for a response, especially after I have walked through the manual with the customer over the phone. Once in a great while I get a good suggestion, and it gets into the latest revision of the manual, but that hasn't happened in many years. Which is a good thing, because the manual is approved by the FAA, and they have to bless any changes. It's time consuming.

The latest fun one was an installer who phoned to ask how to bench check the device. I ran through the simple process with him. At the end he remarked that the device was already installed in the aircraft, but he wanted to bench check it. When I suggested that perhaps he had is priorities a bit askew, he said that he couldn't remove it because, "I thought the connectors looked a little flimsy, so I hot melt glued them in place. I can't take it out." He even e-mailed me photos to prove it.

These are very expensive mil-spec connectors we're talking about here. Nowhere is there any mention of hot melt glue anywhere in my manual, or that of the manufacturer of the connector. Sometimes you just have to fire a customer. I suggested that he rip the thing out, and send it to me for a full refund, which didn't fly well with him because HIS customer (the aircraft owner) would NOT be happy. So I suggested that he have his customer call me so that I could tell him he should really, really, really find someone else to work on his aircraft. You can imagine how that went over, and left me in the position of trying to decide whether to report this yahoo to the FAA. I probably should have, but its a grey area and his actions weren't life threatening. This time. I haven't heard a squeak since then.

The joys of trying to make the world a better place.

Edited 1 time(s). Last edit at 04/02/2021 06:14PM by rq3.
Re: Report on under bed sensor problems
April 03, 2021 06:34AM
Hi rq3,
It may not have been apparent in my earlier posting, but I was the Yahoo. Let me explain, and perhaps clarify why I seem to be regarding it almost as a religious mission to tell everybody to take care with underbed sensors.

I fitted all of my own printers with variations of underbed piezo sensors. In every case the sensors were bulletproof and all reports I heard from others were the same. After a year or two I had heard of only a very few reports of failure, and most of these from people who misunderstood the operation of the sensor - typically floating the bed loosely on springs without any force transmission to the disk.

One email correspondent was not amongst the Yahoos but seemed to have problems of varying sensitivity. Both he and I had sufficient mechanical nous to be able to model the forces on the piezos and the mechanical and electrical response or the piezos. Despite this, I couldn't duplicate his results even though I tried to copy his layout - including the inward mounting of the sensors. In the end, we ended the correspondence on bad terms as I implied that he was just plain wrong.

It is only now that I know that the dynamic component can be more important than the static balance of forces and that if I had moved one of the sensors by a few mm then I would have seen the same failure to trigger.

My opinion of underbed sensors of any kind is now that you should mount multiple sensors at or beyond the periphery of the bed unless you are using only a single sensor to be probed at only one position on the surface.

Mike

Edited 1 time(s). Last edit at 04/03/2021 06:34AM by leadinglights.
Re: Report on under bed sensor problems
April 04, 2021 06:44AM
A further comment on the dangers of underbed probing.



Mike
Re: Report on under bed sensor problems
July 08, 2021 06:58AM
Just a quick report on the status of this investigation - for both remaining readers of this forum. I have now used a touch sensor on my old delta printer to demonstrate this problem even on a printer that has given very reliable bed leveling for many years with underbed sensors. On the diagram below, I leveled the bed first using the three underbed piezo sensors with probes at S1, S2 and S3, and using a 5 X 5 grid at the Red points on the diagram, errors were within 40 microns of flat. Changing to a touch sensor [reprap.org] I probed as close to possible the same positions. The points within the triangle agreed within 10 microns but on those outside the triangle, there could be up to 50 microns difference to the previous points as at point P1. Worse still, with some points on the bed outside the square there was a difference of up to 80 microns between measurements with each method.



To recap on previous findings.
  • There are two sources of the problem in underbed sensors: Dynamic problems caused by the difference in time between the pressure seen at each sensor and dynamic problems caused by different sensitivities of each sensor.
  • This problem is not restricted to piezo sensors although different types of sensors with different time characteristics will give different results, they will all suffer from some version of the above errors.
  • If the sensors are mounted substantially inboard of the periphery of the build stage the problems will be quite severe. Conversely, if the triangle of sensors encloses the whole of the bed, as in the blue triangle, then the errors become very small. A manageable solution can probably be found with sensors just beyond the edge of the bed.

Mike
rq3
Re: Report on under bed sensor problems
July 08, 2021 05:31PM
Mike, I don't know what printer firmware you're using, but if it's Marlin, there is an "interrupt enable" function that is disabled by default. If your control board can handle it (most 32 bit boards can), enabling the interrupt function eliminates, for all practical purposes, the delay between receipt of a signal and the controller's response to the signal. In other words, it does away with the delay and motor "coasting" effect.

On my under-bed piezo design, just enabling the interrupt function made a drastic (as in orders of magnitude) difference in the standard deviation of repetitive nozzle contacts wiith the bed. The absolute worst case SD I get is 3 microns, anywhere on the bed, and that is if I do a repeatability test while the bed is heating from cold to 60C. On the other hand, I'm not completely sure I trust Marlin's implementation of the test and its maths. I'm by no means a coder, but I mean to look into the base C++ one of these days.

As I've noted before, my sensor is a mass loaded piezo disc, and the entire assembly is glued under the bed. There just one, it responds to dropping a BB from one inch anywhere above the bed, and is insensitive to temperature changes and stepper noise and vibration. There are no springs, levers, or other extraneous parts, and no adjustments other than a one time software "set and forget" for sensitivity.

Just a thought.

Edited 1 time(s). Last edit at 07/08/2021 05:32PM by rq3.
Re: Report on under bed sensor problems
July 09, 2021 05:19AM
@rq3. Your mass-loaded piezo sensor may well avoid the problems that I have seen but it is not impossible that it will be troublesome on other beds - like areas that are completely dead. I was happy with my tests on my early printers until I drilled down in the elusive search to get triggering within 1 micron/1 millisecond/1 gram when I started seeing problems - then my newest printer gave me some clues that the underbed sensor that I used method had feet of clay.

One thought in passing is that there is a potential problem but also offers a potential solution. Since your sensor is quite dynamic it may fail to register a contact event if the nozzle is contaminated with soft plastic residues and you are probing far from the sensor. The solution is to first probe directly over the sensor. The piezo output wave shape will be much less sharp if there is plastic on the nozzle and software to detect this softness and stop the leveling should be fairly simple.

On the firmware front; although I only use Repetier firmware on a Mega2560/RAMPS combination, this seems to accept signals from my controller boards at somewhere in the few tens of microseconds. In all of the research, the controller board and firmware are only there to edit: lower the nozzle and then stop it in a reasonable time. The actual measurements are taken by entirely separate instrumentation - you can see some of this earlier in this thread, particularly [reprap.org]

Mike

Edited 1 time(s). Last edit at 07/10/2021 02:28PM by leadinglights.
Re: Report on under bed sensor problems
February 12, 2022 11:37AM
An update on the progress (or otherwise) of this project, but first a recap. I have used underbed piezoelectric sensors for several years now and have had no problems. Despite this, I heard occasional complaints that others had experienced areas on their print beds where they got no response at all and the nozzle kept pushing on the bed until the motor(s) stalled.

To investigate this I mapped the bed using the three underbed probes with an analog output to see what the real-world pressure response was. What I found was quite alarming: There are places even on a rigid and well-constructed bed where the sum of forces due to dynamic movement cancels out and the detected point of contact can be in error, or even not be found at all. I did in fact find that there were points of non-detection even on my best printer that I had not found as they were between points on any mesh probing pattern I had used.

The way I am trying to address this is to use only one underbed probe which finds the Z nozzle contact point for only a single XY position, that position being where the nozzle is directly over the underbed probe. A touch probe mounted on the effector would then take the measurements for bed leveling and/or mesh compensation.

I have made several versions of a touch probe using Nitinol shape memory alloy wire for an actuator and a piezo disk as a sensor [reprap.org]. The most recent of these weighed less than 10 grams and was very sensitive but it has proven very difficult to make is simple or robust. The SMA wire path for example is folded to keep the size down, but this needs no fewer than 18 tiny PEEK pulleys.

Having abandoned the Nitinol actuator, I have decided to try a method dating to the first RepRap bed leveling experiments - the Allen key probe method. There is a good demonstration of this on Youtube at [www.youtube.com] where "Design Prototype Test" uses a photoelectric detector instead of a microswitch. I will use a somewhat similar setup but will use my trusty piezo sensors to detect the touch as this will allow the best trigger pressure to be found.

I have also been testing two prototype piezo sensors provided by rq3. I should stress that I have found no problem with these sensors on my test setup despite mapping across the whole of the bed surface at 5mm intervals. Despite this, I am unable to predict the dynamics of possible beds with differing material properties, geometries and suspensions.

For the future, my main thrust will be to explore high sensitivity touch probes used in conjunction with a single underbed piezo pressure transducer. This method should be able to check for nozzle cleanliness as well as obtaining the nozzle contact datum and data for bed leveling and compensation.

Mike

Edited 1 time(s). Last edit at 02/12/2022 02:34PM by leadinglights.
rq3
Re: Report on under bed sensor problems
February 14, 2022 12:42PM
Mike, after using my under-bed piezo for over a year, my experience has been that IF the nozzle is not scrupulously clean, or there is a cat hair on the bed, the M48 repeatability test in Marlin will either fail, or show an excessive standard deviation (over 3 microns).

If I DO get a repeatability of 3 microns or more, it's pretty much a definitive indication that the nozzle isn't clean or there's contamination on the bed (or the bed is heating up or cooling down).

Once addressed, I have no issues with repeatability, delta calibration, or bed leveling over the entire bed area.

I've been using a diamond tipped nozzle since last November, and it's amazing at shedding plastic. Nothing sticks to the diamond, and the diamond printing tip area is optically polished. It makes a very satisfying contact with the bed, and surprisingly has caused absolutely no discernible damage to the bed. Unlike a brass or steel nozzle, it just skates on the polished diamond area if something goes badly wrong.
Re: Report on under bed sensor problems
February 14, 2022 02:19PM
Hi rq3, I will have to modify my hotend to accept a diamond nozzle as all of my nozzles have M4 threaded unions, but it sounds like a modification worth using.

The failure to trigger on dirty nozzles was part of why I started on this long investigation. Having two trigger points, one for a good contact and one for any contact allowed the detection of plastic on the nozzle. It was only after doing some quite thorough data gathering at various speeds and pressures that I came to the conclusion that it would be difficult or impossible to rely on underbed sensing even for the "any contact" detection. Even having a bed that is apparently without problems does not mean that somebody else using a quite similar bed won't have disastrous inaccuracies or failures to trigger.

I have a thought about a touch sensor that is very simple to make as well as being light and robust. Initially, it will use a piezo sensor to investigate accuracy. This will as stated in other posts be used with a single underbed sensor to get what is called "Z offset" and address effector tilt problems.

Mike
rq3
Re: Report on under bed sensor problems
February 14, 2022 03:51PM
Quote
leadinglights
Hi rq3, I will have to modify my hotend to accept a diamond nozzle as all of my nozzles have M4 threaded unions, but it sounds like a modification worth using.

The failure to trigger on dirty nozzles was part of why I started on this long investigation. Having two trigger points, one for a good contact and one for any contact allowed the detection of plastic on the nozzle. It was only after doing some quite thorough data gathering at various speeds and pressures that I came to the conclusion that it would be difficult or impossible to rely on underbed sensing even for the "any contact" detection. Even having a bed that is apparently without problems does not mean that somebody else using a quite similar bed won't have disastrous inaccuracies or failures to trigger.

I have a thought about a touch sensor that is very simple to make as well as being light and robust. Initially, it will use a piezo sensor to investigate accuracy. This will as stated in other posts be used with a single underbed sensor to get what is called "Z offset" and address effector tilt problems.

Mike

Mike, your post prompted another bout of curiosity. An ohmmeter says the sintered diamond nozzle has a resistance of about 10 ohms between the polished tip and the brass body. Interesting.
Re: Report on under bed sensor problems
April 10, 2022 12:10PM
A new item I am exploring in this quest: A printer that uses 4 (yes four) sensors, each on the extreme corners of the build plate. A video showing this printer is on YouTube at [www.youtube.com] Although the printer shown uses expensive and heavy strain gauges - and will need quite complicated support electronics, the placement of the sensors at the corners should reduce dynamic and sensor miss-match problems to a lower area than three sensors.

Although I still have to build mechanical and mathematical models to check this out, I think (hope) that a version using piezo sensors will get around the combination of dynamic behavior and sensor miss-match which gives unexpected - and occasionally damagingly incorrect readings.

Mike
rq3
Re: Report on under bed sensor problems
April 12, 2022 05:38PM
Quote
leadinglights
A new item I am exploring in this quest: A printer that uses 4 (yes four) sensors, each on the extreme corners of the build plate. A video showing this printer is on YouTube at [www.youtube.com] Although the printer shown uses expensive and heavy strain gauges - and will need quite complicated support electronics, the placement of the sensors at the corners should reduce dynamic and sensor miss-match problems to a lower area than three sensors.

Although I still have to build mechanical and mathematical models to check this out, I think (hope) that a version using piezo sensors will get around the combination of dynamic behavior and sensor miss-match which gives unexpected - and occasionally damagingly incorrect readings.

Mike

Mike, I recently got a report of someone using one of the TAP-100 sensors I invented on an Anet A8. He reported that it was the best sensor he's ever used, and this is on a bed slinger. Why pursue complexity (other than that it's fun)? Four strain gauges!?
Re: Report on under bed sensor problems
April 13, 2022 02:03PM
Quote
rq3
.......................... I recently got a report of someone using one of the TAP-100 sensors I invented on an Anet A8. He reported that it was the best sensor he's ever used, and this is on a bed slinger. Why pursue complexity (other than that it's fun)? Four strain gauges!?

Hi rq3. I still have not seen any problem with the TAP100 probe so it certainly seems to be a candidate for top-dog in the sensors. Having said that, I thought that the three underbed sensor design that I used was bulletproof until a background of makers kept talking about blind spots where the piezo didn't trigger. These problems do seem to be real and affect ALL underbed sensors to a greater or lesser extent, giving rise to different sensitivities across the bed surface: Sometimes this sensitivity may give rise to unacceptable errors - the errors in FSR underbed sensors can be awful and even simple underbed microswitches can exhibit really poor sensitivities in some parts of the bed.

Hopefully, the TAP100 will have errors in the few micron region so small enough to disregard on most beds. On the other hand, we won't know until it has been tried on quite a large number of designs, with each builder taking different attitudes on how to implement your design.

As far as the four sensors scheme goes, I am trying to combine four piezo sensors with a three-point bed adjustment. This will need two sensors to be mounted on one adjuster and this has all sorts of mass and compliance implications.

The main project at the moment is a piezo touch probe for bed leveling and mapping, combined with a single underbed sensor for setting Z zero when the nozzle touches the bed. I will try to get a video on YouTube in the next week or so.

Mike
rq3
Re: Report on under bed sensor problems
April 13, 2022 06:32PM
Quote
leadinglights
The main project at the moment is a piezo touch probe for bed leveling and mapping, combined with a single underbed sensor for setting Z zero when the nozzle touches the bed. I will try to get a video on YouTube in the next week or so.
Mike

Again, why the complexity? If a single sensor (of whatever type), can repeatably and reliably detect contact between the nozzle and the bed, what more is there? It will deal with Z zero AND bed leveling. After all, Z zero is just a single point case of bed leveling.

As a thought experiment, a pure gold bed being probed by an atomic force microscope, even in atmosphere, is capable of sub-nanometer resolution. Only one probe, and only one sensor. I'm just not following you on the need for different measurement approaches for what is basically the same thing.

Since the distance from the printing nozzle to the printing bed is really the only parameter of interest, only reliably determining that distance anywhere on any bed is of any interest. The simplest method of doing that is of most interest.

I'm looking forward to the YouTube video!
Re: Report on under bed sensor problems
April 14, 2022 01:04PM
What I have been able to do with a single underbed sensor is to determine not only the point of contact but also the quality of contact. The quality of contact is poor if there is any plastic on the nozzle or if the printing surface has a bubble under it etc..
A single underbed sensor works with no X, Y, or Z offset however I have not found any underbed sensor that can reliably map the bed without significant errors. I maintain that this is true with ANY underbed sensor - consider you own earlier post:-

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rq3 February 14th 2022
after using my under-bed piezo for over a year, my experience has been that IF the nozzle is not scrupulously clean, or there is a cat hair on the bed, the M48 repeatability test in Marlin will either fail, or show an excessive standard deviation (over 3 microns).

If the nozzle is making a soft contact due to either plastic on the nozzle or, for example, bubbles under a Kapton film or a cat hair on the surface, the deviation over the surface may look like an uneven bed in need of compensation. Even worse, it may fail to register a contact at all.

Since I can only reliably read a single point with high accuracy, it is necessary to use a different probe for mapping the bed. Although a probe such as a BLTouch would do it, with a piezo touch probe I can now see the variation between probe operations to better than 1 micron.

There is a video with the dual-sensor setup, [www.youtube.com] but this is on my development printer. The dual sensor setup I am working on at the moment is a retrofit to my Delta printer.

Mike
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