Piezoelectric disks for Z contact detect and bed levelling
Posted by leadinglights
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Re: Piezoelectric disks for Z contact detect and bed levelling June 11, 2019 04:00PM |
Just reporting that I have been running some early tests in a "semi-real-world" situation that shows repeatability of well better than 0.5 gram at 2 grams of nozzle contact pressure.
10 microns of movement of the nozzle gives 0.60 Newtons ( say 62 grams or 2¼ ounce) of force in the middle of the bed.so 2 grams should equate to 0.067 micron. As my Z axis is 1mm/rev and ¼ step on a 1.8° stepper there is sledome even one step difference in trigger points.
Mike.
10 microns of movement of the nozzle gives 0.60 Newtons ( say 62 grams or 2¼ ounce) of force in the middle of the bed.so 2 grams should equate to 0.067 micron. As my Z axis is 1mm/rev and ¼ step on a 1.8° stepper there is sledome even one step difference in trigger points.
Mike.
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Re: Piezoelectric disks for Z contact detect and bed levelling June 12, 2019 05:12AM |
... are you interested in even finer resolutions and/or "non-contact" force measuring?
Developed some sensing methodes in my "active" time, when working in the micro/nanotech industry
Viktor
--------
Aufruf zum Projekt "Müll-freie Meere" - [reprap.org] -- Deutsche Facebook-Gruppe - [www.facebook.com]
Call for the project "garbage-free seas" - [reprap.org]
Developed some sensing methodes in my "active" time, when working in the micro/nanotech industry
Viktor
--------
Aufruf zum Projekt "Müll-freie Meere" - [reprap.org] -- Deutsche Facebook-Gruppe - [www.facebook.com]
Call for the project "garbage-free seas" - [reprap.org]
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Re: Piezoelectric disks for Z contact detect and bed levelling October 03, 2019 06:45AM |
I thought that I would just update you on the progress of my new piezoelectric controller. The controller is one that gives two signals to the controlling device (Arduino, Duet etc..) One signal is to show that a nozzle contact has been seen while the other signal shows that the signal is good - not noise or contamination on the nozzle.
The latest software and hardware working, although showing that more work is needed. The circuit and program are very sensitive to a good bed contact, responding in 4ms and about 15µm of travel after the nozzle first touches the bed. It also detects plastic on the nozzle which I simulated with a 0.25mm rubber gasket. Although it rejects some mechanical noise, such as most finger taps on the bed or dropping 8mm balls on the bed, it can be fooled with careful finger taps. Worse though is that detects noise through the workbench and frame. This was most unexpected as it 100% immune to rattling from the end of an X or Y movement or the beginning of the Z probe down move.
I am presently rewriting the software to modify the windows that a contact event must be within to improve this. Hopefully, I can beat the noise through the workbench problem as I can see what it is doing. It may be more difficult to beat the calibrated finger tap though.
I have put the hardware on [circuitmaker.com] but because circuitmaker is less common than say Eagle, I have the circuit diagram below. I will put Gerber and NC files on the same site once the latest changes have been done (improved EMC and test points)
Software is on [github.com]
Beware though, the software is written in Microchip assembler both for speed and because I prefer it to high-level languages. I am given to understand that assembly language causes terror in the unwary and may cause brain rot.
I will not be pursuing this beyond the point where it works to my satisfaction so anybody who wants to copy is most welcome. MIT license applies so even commercial as long as attribution is given.
Mike
The latest software and hardware working, although showing that more work is needed. The circuit and program are very sensitive to a good bed contact, responding in 4ms and about 15µm of travel after the nozzle first touches the bed. It also detects plastic on the nozzle which I simulated with a 0.25mm rubber gasket. Although it rejects some mechanical noise, such as most finger taps on the bed or dropping 8mm balls on the bed, it can be fooled with careful finger taps. Worse though is that detects noise through the workbench and frame. This was most unexpected as it 100% immune to rattling from the end of an X or Y movement or the beginning of the Z probe down move.
I am presently rewriting the software to modify the windows that a contact event must be within to improve this. Hopefully, I can beat the noise through the workbench problem as I can see what it is doing. It may be more difficult to beat the calibrated finger tap though.
I have put the hardware on [circuitmaker.com] but because circuitmaker is less common than say Eagle, I have the circuit diagram below. I will put Gerber and NC files on the same site once the latest changes have been done (improved EMC and test points)
Software is on [github.com]
Beware though, the software is written in Microchip assembler both for speed and because I prefer it to high-level languages. I am given to understand that assembly language causes terror in the unwary and may cause brain rot.
I will not be pursuing this beyond the point where it works to my satisfaction so anybody who wants to copy is most welcome. MIT license applies so even commercial as long as attribution is given.
Mike
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Re: Piezoelectric disks for Z contact detect and bed levelling November 16, 2019 12:37PM |
I received a flurry of PMs and emails a few days ago asking what progress there had been on the new piezo conditioner circuit so this is a report on the present status. There is some good news and some not so good news first.
The reliability and sensitivity remain good and new techniques have been introduced that help to reduce false "good nozzle contact" signals. Other good news is that it is looking more possible to discriminate particular reasons for reporting a bad contact event, in particular, the signal from a dirty nozzle should be detectable.
Principal bad news is that the goal of not needing to be tuned seems to be becoming more remote. A Delta printer with 3 underbed sensors mounted to a solid platform can likely be tuned by nothing more than selecting the Z probe speed. Mounting geometries for square beds on cartesian printers, particularly those with 3 point mounting are less tunable. 4 point mounting is ugly and prone to maladjustment, but may also not need tuning.
The problem here comes in two parts, The movement of the bed under the force of the nozzle is more than expected on the corner and, even at speeds as low as 2mm/sec, the plate bounces back giving rise to a hump in the contact response. The effect is shown in the attached diagram although this is not in the corner (even more messy) but about 2/3 of the way to the corner.
The upshot of all of this is that since the hump is dependent on the mass and the elasticity of the bed and other mechanical parts, controllers based on this will need tuning. If the bed is predictable then it should need no tuning and is almost a replacement for a microswitch - albeit with additional dirty nozzle detection.
Mike
The reliability and sensitivity remain good and new techniques have been introduced that help to reduce false "good nozzle contact" signals. Other good news is that it is looking more possible to discriminate particular reasons for reporting a bad contact event, in particular, the signal from a dirty nozzle should be detectable.
Principal bad news is that the goal of not needing to be tuned seems to be becoming more remote. A Delta printer with 3 underbed sensors mounted to a solid platform can likely be tuned by nothing more than selecting the Z probe speed. Mounting geometries for square beds on cartesian printers, particularly those with 3 point mounting are less tunable. 4 point mounting is ugly and prone to maladjustment, but may also not need tuning.
The problem here comes in two parts, The movement of the bed under the force of the nozzle is more than expected on the corner and, even at speeds as low as 2mm/sec, the plate bounces back giving rise to a hump in the contact response. The effect is shown in the attached diagram although this is not in the corner (even more messy) but about 2/3 of the way to the corner.
The upshot of all of this is that since the hump is dependent on the mass and the elasticity of the bed and other mechanical parts, controllers based on this will need tuning. If the bed is predictable then it should need no tuning and is almost a replacement for a microswitch - albeit with additional dirty nozzle detection.
Mike
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Re: Piezoelectric disks for Z contact detect and bed levelling February 11, 2021 06:25AM |
Short version for those for whom “Too long: did not read” is a mantra:
I am looking for suggestions for how to get software changes and any other comments or thoughts on my two-headed printer bed leveling method. Video at [www.youtube.com] and pictures below
Some years ago I became convinced that undebed piezoelectric sensors were the best technology for bed leveling in an FFF printer. In particular, the underbed piezos in my Delta printer were, and remain, completely reliable. Two other printers, both loosely based on the early Prusa Mendel design, had underbed piezos of the same design which were mostly reliable, though not bulletproof: I thought that the occasional less than sparkling result was down to the lack of rigidity of the unbraced sides of the design.
With my newest printer, I placed three underbed sensors well inboard from the edges. This was because the design of the bed gave only limited space for the placement of the sensors. It was only when I fully characterized the behavior of the sensors across the whole of the bed that I realized that there was a dynamic problem that gave rise to areas of insensitivity and that these areas were pretty much unpredictable – they also ended any hopes of getting the very accurate bed leveling that had been almost within my grasp. The effect was much like I imagine an early test pilot felt when he discovered that if he increased the angle of attack beyond a certain point that the aircraft would stall and he would plunge to his death.
In the intervening year, progress has been somewhat spasmodic until recently when I have started to get some very good results. The present approach is that there is only one underbed sensor plus one contact sensor mounted near the nozzles. The first nozzle is brought directly above the underbed sensor and a G30 probing operation called. The printing heads are then swapped and the G30 probing operation repeated and the Z distances for each is stored. Then both heads are withdrawn to a halfway position and the bed moved so that the contact sensor is directly above the same point that had just been checked with the nozzles. A third G30 operation is performed to obtain the offset at that point, followed by a G32, also using the contact sensor to obtain a leveling plane.
There is an added complexity in that the underbed sensor and the contact sensor have different outputs, presently wired to a RAMPS board with the underbed sensor being connected to the Z-Min limit switch pin and the contact sensor being connected to the X-Max pin.
Along with the added complexity though, there are some added benefits. The underbed sensor is able to determine if the nozzle has any plastic on the tip which would render a probing operation unreliable. Both of the probes can check that the baseline immediately before the probing operation is flat, not drifting with temperature changes, and is also free of mechanical or electrical noise. This last item can replace the normal necessity of having a delay immediately before the probing operation to allow the XY movement artifacts to die away. The outputs will require additional inputs to the host computer.
Where to next on this project depends on if I can get some features incorporated into one or more of the various firmwares that run on Arduino/RAMPS combinations. The main one is that two distinctly separate inputs to the host are needed for the probing with the nozzles and with the touch sensor. Also nice would be two more inputs, one to suppress probing until after the XY shaking has died away, ane one for the detection of plastic on the nozzle.
Mike
I am looking for suggestions for how to get software changes and any other comments or thoughts on my two-headed printer bed leveling method. Video at [www.youtube.com] and pictures below
Some years ago I became convinced that undebed piezoelectric sensors were the best technology for bed leveling in an FFF printer. In particular, the underbed piezos in my Delta printer were, and remain, completely reliable. Two other printers, both loosely based on the early Prusa Mendel design, had underbed piezos of the same design which were mostly reliable, though not bulletproof: I thought that the occasional less than sparkling result was down to the lack of rigidity of the unbraced sides of the design.
With my newest printer, I placed three underbed sensors well inboard from the edges. This was because the design of the bed gave only limited space for the placement of the sensors. It was only when I fully characterized the behavior of the sensors across the whole of the bed that I realized that there was a dynamic problem that gave rise to areas of insensitivity and that these areas were pretty much unpredictable – they also ended any hopes of getting the very accurate bed leveling that had been almost within my grasp. The effect was much like I imagine an early test pilot felt when he discovered that if he increased the angle of attack beyond a certain point that the aircraft would stall and he would plunge to his death.
In the intervening year, progress has been somewhat spasmodic until recently when I have started to get some very good results. The present approach is that there is only one underbed sensor plus one contact sensor mounted near the nozzles. The first nozzle is brought directly above the underbed sensor and a G30 probing operation called. The printing heads are then swapped and the G30 probing operation repeated and the Z distances for each is stored. Then both heads are withdrawn to a halfway position and the bed moved so that the contact sensor is directly above the same point that had just been checked with the nozzles. A third G30 operation is performed to obtain the offset at that point, followed by a G32, also using the contact sensor to obtain a leveling plane.
There is an added complexity in that the underbed sensor and the contact sensor have different outputs, presently wired to a RAMPS board with the underbed sensor being connected to the Z-Min limit switch pin and the contact sensor being connected to the X-Max pin.
Along with the added complexity though, there are some added benefits. The underbed sensor is able to determine if the nozzle has any plastic on the tip which would render a probing operation unreliable. Both of the probes can check that the baseline immediately before the probing operation is flat, not drifting with temperature changes, and is also free of mechanical or electrical noise. This last item can replace the normal necessity of having a delay immediately before the probing operation to allow the XY movement artifacts to die away. The outputs will require additional inputs to the host computer.
Where to next on this project depends on if I can get some features incorporated into one or more of the various firmwares that run on Arduino/RAMPS combinations. The main one is that two distinctly separate inputs to the host are needed for the probing with the nozzles and with the touch sensor. Also nice would be two more inputs, one to suppress probing until after the XY shaking has died away, ane one for the detection of plastic on the nozzle.
Mike
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Re: Piezoelectric disks for Z contact detect and bed levelling August 29, 2022 03:18PM |
... yes, RepRap is grown pretty "old" now (or better "matured") -- could be par with the oldest forums/communities in the web (or is it maybe?) - but still alife! 
I'm normally "swooping" several times a day through all of my forums (here mostly for "maintaining" like removing spam or approving posts or new users).
But my interests too are somehow "distracted" to more modern areas, like high power lasers, material processing and such ... something of them related to 3D-printing too
Edited 2 time(s). Last edit at 08/29/2022 03:20PM by VDX.
Viktor
--------
Aufruf zum Projekt "Müll-freie Meere" - [reprap.org] -- Deutsche Facebook-Gruppe - [www.facebook.com]
Call for the project "garbage-free seas" - [reprap.org]
I'm normally "swooping" several times a day through all of my forums (here mostly for "maintaining" like removing spam or approving posts or new users).
But my interests too are somehow "distracted" to more modern areas, like high power lasers, material processing and such ... something of them related to 3D-printing too
Edited 2 time(s). Last edit at 08/29/2022 03:20PM by VDX.
Viktor
--------
Aufruf zum Projekt "Müll-freie Meere" - [reprap.org] -- Deutsche Facebook-Gruppe - [www.facebook.com]
Call for the project "garbage-free seas" - [reprap.org]
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Re: Piezoelectric disks for Z contact detect and bed levelling August 30, 2022 05:56AM |
The problem is not repeatability. I can confirm repeatability for your sensor in the order of 3 microns or better. The problem is whether the reported contact is accurate.
I use a contact plate and dummy nozzle, both plated with silver, and cleaned every 5 or 6 contact operations to determine where contact occurs. A high impedance amplifier and a source voltage of only 1.5V reports contact very accurately indeed. Whatever sensor is being tested will report the contact a short time after the physical contact - I record the time between contact and the sensor responding with an oscilloscope. The distance traveled by the nozzle in the time is what I am reporting as the error
You do say that cleanliness does have an effect, but this will give rise to delays in the signal and hence a lower nozzle position when the contact is reported. The delays can be softness of contact (cleanliness) but also by:-
Very simply, comparing the plate with a guitar string, if the sensor is where it is shown on the drawing above and the string is plucked at position 'B', the sensor would see a reduced signal from the red line.
Mike
p.s., I knew I shouldn't have looked at the forum so soon after having earned my two-month abstention badge.
I use a contact plate and dummy nozzle, both plated with silver, and cleaned every 5 or 6 contact operations to determine where contact occurs. A high impedance amplifier and a source voltage of only 1.5V reports contact very accurately indeed. Whatever sensor is being tested will report the contact a short time after the physical contact - I record the time between contact and the sensor responding with an oscilloscope. The distance traveled by the nozzle in the time is what I am reporting as the error
You do say that cleanliness does have an effect, but this will give rise to delays in the signal and hence a lower nozzle position when the contact is reported. The delays can be softness of contact (cleanliness) but also by:-
- Attenuation is caused by XY distance between nozzle and sensor - that is, the speed of sound for a transverse wave in the bed.
- Attenuation is also caused by the sensor being closer to a clamping point than the nozzle. - equivalent to being a lever.
- Most importantly, the interplay between the harmonics on the bed - potentially this may cause a contact not to be reported at all - like the spot on a railway station where you can't hear the announcements.
Very simply, comparing the plate with a guitar string, if the sensor is where it is shown on the drawing above and the string is plucked at position 'B', the sensor would see a reduced signal from the red line.
Mike
p.s., I knew I shouldn't have looked at the forum so soon after having earned my two-month abstention badge.
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Re: Piezoelectric disks for Z contact detect and bed levelling October 26, 2022 01:53PM |
Hmm, I seem to have made it for nearly two months since my last posting - perhaps I am nearly cured of my RepRap addicition.
While archiving some old pictures that I had taken I found some that I had never published which shows that I did indeed "do the work" of investigating the response of the TAP sensors. In the picture below I have suspended a 4mm float glass bed over the existing bed of one of my printers. The glass bed is supported at four positions and there are three different positions where the TAP sensor can be connected.
The result of the tests was that the TAP sensor gave a signal output between 12 microns after the nozzle touched the plate when probed directly above the TAP sensor, and about 50 microns when the nozzle probed directly above one of the mountings. The probing across the whole surface did not give any consistent pattern but fell between the limits.
The best place for the sensor seemed to be on a line between the middle of the bed and about 1/3 of the way to one corner.
I admit that using a plain 4mm glass bed is a bit sucky, but that was the idea. If I had made the bed to the highest standards instead of using the assumptions that newbie builders may use then I would have proven very little.
So I stick with what I have said. While it is possible to use underbed sensors for bed leveling, the dynamic behavior is so unpredictable that it should be avoided.
Mike
While archiving some old pictures that I had taken I found some that I had never published which shows that I did indeed "do the work" of investigating the response of the TAP sensors. In the picture below I have suspended a 4mm float glass bed over the existing bed of one of my printers. The glass bed is supported at four positions and there are three different positions where the TAP sensor can be connected.
The result of the tests was that the TAP sensor gave a signal output between 12 microns after the nozzle touched the plate when probed directly above the TAP sensor, and about 50 microns when the nozzle probed directly above one of the mountings. The probing across the whole surface did not give any consistent pattern but fell between the limits.
The best place for the sensor seemed to be on a line between the middle of the bed and about 1/3 of the way to one corner.
I admit that using a plain 4mm glass bed is a bit sucky, but that was the idea. If I had made the bed to the highest standards instead of using the assumptions that newbie builders may use then I would have proven very little.
So I stick with what I have said. While it is possible to use underbed sensors for bed leveling, the dynamic behavior is so unpredictable that it should be avoided.
Mike
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