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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.
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