Firstly, I must apologize for coming a month late to this party but I only just stumbled across this thread on a Google search for something else.

It sounds like the problem is something I came across and did a bit of research on. The problem is not with piezo sensors, but with the dynamic behavior of the bed being unpredictable in much the manner of the sand plates (Chladni plates) beloved of high school science demonstrations.

I found two ways to combat this:-

1. The underbed sensors are much more predictable if they are placed beyond the periphery of the probing area - having three closely matched piezos helps a lot as well.

2. Use a single underbed sensor to get the accurate nozzle contact and only probe directly above this sensor. A second sensor of the contact type such as a BLTouch is used got get other data for bed leveling/mapping. This does require that there is some way to select the sensor to tbe used for each purpose.

I have had a great deal of success with the second method and am able to get results with an accuracy of better than 3 microns at all points on the bed.

More info on this at:-
https://reprap.org/forum/read.php?424,865620
https://reprap.org/forum/read.php?424,874556
https://reprap.org/forum/read.php?1,635075,881434#msg-881434

Mike

There are several possibilities, even including connecting the disk directly to an analog input of your controller (protective diodes may be advisable). The output of a piezo disk is actually very high, see the first entry in https://reprap.org/forum/read.php?1,635075 where an output of 8V was obtained from a 27mm piezo disk. The conditioning circuits are more to clean up the signal to get a reliable and consistent detection of a contact event.
It sounds like you want to use a puck-type detector which can be used to get the Z probe offset from the BLTouch device. There is a puck probe called the BIQU marketed by Aliexpress which they used to call a piezo probe - I have heard nothing either good or bad about it though.
I hope to put the 4th generation of my piezo conditioning board on GitHub in the near future. The older one was left in a pretty incomplete state but my latest software (the hardware is unchanged) seems to be pretty bulletproof.

Mike

A flat piezo element used in the same manner as a foil strain gauge element certainly sounds lie it is worth trying. I would think that most places that you could put a single element on a PCB effector would have differing sensitivities in the X and Y planes but that would not be likely to have any effect on a straightforward Z probing operation. Thermal effects may be more of a problem but they can probably be worked around as they are with piezo diaphragms.

Another one that may be worth looking at are thickness mode piezo rings such as this one from Steiner and Martins https://www.steminc.com/en/piezo-ring-18x12x12mm-18-mhz-2-wl At $5 per ring they are a lot more than diaphragms but still not very pricey.

Mike.

p.s., I am the Mike that DjDemonD mentioned a few postings ago. I haven't yet got to 1 gram having hit the wall at 5 grams but will try again over the next few days.

I would like to be able to contribute some data for under-bed piezo sensors but unfortunately the cost of the Duet3D prevents me from doing so at the moment. I suspect from oscilloscope and logic analyzer traces that it is at least in the 1 micron region but Arduino/Ramps/Repetier firmware only reports to 0.01mm. Of course, if anybody knows how I can coax Repetier firmware to report G30 either to 0.001mm or in raw Z stepper steps I may be able to give you the data

Mike

@James:

…...........
There are many use cases here: Missing motors, miswired motors, the tip hitting the bed, the tip hitting anything (e.g., the part being built when it gets dislodged from the bed), a wire or something getting caught in the works, any mechanical part on the printer breaking... Is there a reasonable way of addressing such issues, or will the solution have to be "Be careful"?

In another place, nebbian came up with an idea to use a piezo disc connected to a stepper motor mounting to detect the load from nozzle contact http://forums.reprap.org/read.php?1,635075,756496#msg-756496 While I can think of several reasons why that may not work, it may answer precisely the question you have put .

Mike

There is now a wiki page addressing the underbed piezo sensors at http://reprap.org/wiki/Underbed_Piezo-electric_sensors

Mike

As most firmware does not look at the sensor other than when probing this does not normally create a problem. However, the LED on the piezo sensor boards on my 3D printers flash a lot during printing so there is signal to work with. If the amplifier put out an analogue signal instead of a logic one then perhaps the firmware could be set to look for error indicating values.

Mike

The point made by James in the first posting on this thread is not quite true for piezo disks - although perhaps I spread this about a bit with an early posting on the RepRap forum http://forums.reprap.org/read.php?1,635075,635164#msg-635164 which is a load of doggy-doo.

There is very little variability at different positions on the bed as long as the piezos are fairly closely matched - within batches of no-name piezos there has been less than 3% variability of output or of capacitance. The reason that this is true is that the charge generated on each disk is fairly proportional to applied force. Simply paralleling three discs will give the correct output even if one of them gets a negative output - think of a three legged table with the legs bolted to the floor, the sum the compressive forces on the three legs equals the weight put on the table even if you sit on an unsupported corner causing the opposite leg to be under tension

Mike

I purchased to batches of 10 no-name 27mm discs on ebay from two different suppliers and tested about 17 of them. I also got 10 Murata discs from Farnell and tested only 5 of them as they had to have leads soldered on and this was a destructive learning curve.

The Murata did score on giving a decent output at temperatures at just over 100C while the no-name were really poor - see http://forums.reprap.org/read.php?1,635075,655510#msg-655510

Mike

Just a quick observation on consistency: I found that the branded ones (Murata) were polarised in the opposite direction to the unbranded ones and had a wider spread of sensitivity, the most sensitive being about twice as good as the least. By comparison the unbranded ones fell into less than a 10% spread.

Mike