I would not agree they are "bad". Piezo is what it is. IMHO the sensors created by precisionpiezo Andromeda and Orion are pretty good (much better IMHO than stand-alone disks where the puny force on the wires will detach them from the disk along with a piece of disk coating)
Most issues I have (and you have as well from what I see) are the way we are using the technology. When you use the way they designed it, it works out of the box
No, the sensors themselves are really good. Any sensors measuring strain or deformation mounted beneath the bed have their disadvantages, simply due to their type of placement. They are simply in a place where forces are not... well, "focused", to put it simple (unsure which is the correct english wording here...). If you place a sensor into the hotend correctly there is no way for the contact force but to go through the sensor (in case of a piezo I count the deformation element it is glued onto as part of the sensor), and the lever arm always more or less stays the same.
In case of the bed however, there are at least two other ways - and lever arms change, depending on where you push. In theory, you always have the same result - in practice, there are various effects causing problems. It is the same as with several force transducers being mounted below let's say a crane or one mounted directly into the hook... Thus, even an ideal bed sensor array will not work as good as a hotend sensor. The question is always: does it work well enough anyway?
In my case, I tried to use them as close as possible to the way they were intended to be, but as I work within the restraints of my printer, that only works as well as it allows...
I disagree with you here and I have to say, I disagree without ANY proof but I am somehow sure that there is a way to incorporate a piezo-like sensor in this type of head mount without losing more than 5mm of Z space. [...]
but since I actually gave up as even the cheap aliexpress bltouch knockoff give results that are consistent and repeatable, do not require thorough nozzle clean before probing and work "every time" ...
You dont' know how that thing looks inside - there is not that much material there even if it looks massive If you could get custom made piezo sensors that are not round for a decent price, it would be rather easy (but would need a complete redesign anyway) and could probably be done without losing z height. But a 20mm disc or even something larger would be really hard, even with redesigning the whole thing. As mentioned in the other thread we talked, there is exactly one place where I could drill a 2...3mm hole through the whole assembly for a - much elongated - BLTouch pin without interfering with something somewhere and letting the cable chain still move as it needs. And all these things need to stay in place...
Hm, in case I need something SLS printed, I use Rapidobject. For Germany, they have good prices, and no fuss with customs. And the Alumide they offer is simply great
That is weird. I had a similar idea myself at the beginning, luckily I have 4ch scope (both digital and analog storage) and I looked at the signal from 3 and 4 andromeda's directly without amp and in my case they don't cancel out. The issue is just in how "sharp" the "touch" is in my case.
Yes, I had hoped for this, but even with just two channels on my old Tek, I can see the signal peaks with the overlayed vibration being something near λ/2 apart... I guess the resonance frequency of my heatbed fits much too good to its size...
For e.g. this is the test that works for me [...]
The test looks very much the same than the one I did, except that I used a Ø6mm plastic rod I had lying around (length around 30cm). To get a signal rising above the level of the vibration (i.e. something not canceling itself out) I needed to rise that one some five to 6 cm. No way to get such an impulse with my z axis. As soon as I split the sensors, a gentle tip with the finger was enough...
Anyway, I have the Precision Piezo v2.85 amp (i.e. the latest variant they sell) in use - and I got feedback from them.
So here is the solution for anyone who might need it in the future: the digital output of v2.85 does indeed pull the signal line to ground - and that's it. Thus, you can simply wire two of these amplifiers (I would like to support them and not buy the chinese ripoffs) in parallel and that's it
Regarding the trimmer: yes, I understand why you don't like that thing lasting just 20 full turns. But on the other hand: the lighter (i.e. smaller) the part, the less problems with mechanical disturbances. These trimmers you suggest are in comparison relatively heavy and in my old company I learnt there were problems with those when being built stiffly into force transducers - they got killed or worn-out by the mechanical impulses over time in some cases. Thus, even if I had the space I would not mount that one directly benath my print bed facing downwards as I do with the amplifier - even if I don't know if the vibrations come anwhere near the critical level
That's why I gave up on Piezo's.
But now imagine you had a [barometric sensor]...
I like the idea - but apart from the sealing problem, you would have to cope with the air getting hot and thus expanding as soon as someone turns on the heat bed - And this effect will continue for quite some time after the bed has reached its temperature, so I guess you need an amplifier that checks for pulses and discards any change benath a certain value per second - meaning you end up with something similar to a piezo (albeit requiring less speed). I would rather try some silicone oil or so which expands much less and minimise the volume of that to the absolute minimum.
I guess it might be easier to use the principle of piezo force transducers instead - these do not monitor the voltage peak of the piezo disc as "our" piezo amplifiers do but rather the electric charge created by the piezo. By doing so, you can even measure semi-static forces up to a certain point.