3D printer - Piezo Z probing

[tekkydave]

15 minutes ago, Gina said:

Thanks Dave, I wondered if I might do something like that.  Hadn't thought of using the bigger sort with a hole to go over the motor shaft but between extruder and motor seems a good place.  Certainly food for thought

Yes, I've seen that part on Thingiverse.

The main issue is getting the motor/extruder to 'pivot' around a line joining the bottom 2 screws. This translates the upward movement of the nozzle into the horizontal movement which puts pressure on the piezo. The piezo itself needs to be deformed to generate sufficient volts to trigger the circuit. Having it supported around its edges and pressure applied to the centre is best. If it is just pressed flat between 2 surfaces it is acting more like a microphone and will generate millivolts rather than volts.

[Gina]

5 minutes ago, tekkydave said:

The main issue is getting the motor/extruder to 'pivot' around a line joining the bottom 2 screws. This translates the upward movement of the nozzle into the horizontal movement which puts pressure on the piezo.

Yes, I'm aware of that and it's what I've tried to achieve.

6 minutes ago, tekkydave said:

The piezo itself needs to be deformed to generate sufficient volts to trigger the circuit. Having it supported around its edges and pressure applied to the centre is best. If it is just pressed flat between 2 surfaces it is acting more like a microphone and will generate millivolts rather than volts.

That's an interesting point that I wasn't aware of - I thought it was just compression, though I have seen bending mentioned.  As it stands, I'm getting triggering all the time the Z motors are running, even with the trigger level turned well up though below maximum.

I really want to get this working and "throwing my toys out of my pram" was just sheer frustration!

[tekkydave]

1 minute ago, Gina said:

Yes, I'm aware of that and it's what I've tried to achieve.

That's an interesting point that I wasn't aware of - I thought it was just compression, though I have seen bending mentioned.  As it stands, I'm getting triggering all the time the Z motors are running, even with the trigger level turned well up though below maximum.

I really want to get this working and "throwing my toys out of my pram" was just sheer frustration!

Sounds like it's just acting as a microphone then. Might be interesting to hook the piezo sensor up to a scope and see what the relative amplitude you are getting from normal z movement and a 'tap on the nozzle'. If you can boost the deformation of the sensor you might be able to turn the sensitivity down on the pcb to make the z movement noise insignificant. Try putting a fibre washer or similar device behind the piezo and something smaller in the centre in front so it gets deformed more

 

[tekkydave]

Loads of info here if you are not aware of it (Reprap forum). May take some wading through though

 

[Gina]

Since virtually all my parts are 3D printed, I can make them any shape I like.  Pressing in the middle and supporting around the edge may be what I have in my Mini printer that works so well.  The sensor as delivered had a significant burr on the substrate side from drilling and the support is tubular so that I could screw it to the already made part.  I thought it best not to try to remove the burr as I didn't want to damage the sensor.

I have been wading through the RepRap forum posts but by no means read them all yet.

[Gina]

I see what you mean about these piezo sensors acting as a microphone.  Took the one out of the Concorde and set it up on the table - then shouted at it - it flickered blue!! ?

OK - so I've got a loud voice ?

Edited March 9, 2019 by Gina

[tekkydave]

I bet you felt better for it afterwards, though ? 

 

[Gina]

I feel like screaming at things sometimes!!

[Gina]

Amazing!  My "Pink string and sealing wax" Mini printer is sitting there happily printing parts for my ASC precisely and virtually inaudibly, though with a 0.25mm nozzle and 0.2mm layers, taking rather a long time, while my flagship printer, Concorde, stands somewhat aloof grumbling and groaning and point blank refusing to use a piezoelectric Z probe.

I thought that by concentrating on build quality my Concorde printer would be precise and fast using standard tried and tested 3D printer construction techniques.  Well, it is precise once going and XY motion is reasonably quiet but Z motion using two single start threaded rods and NEMA23 motors is noisy and slow.  Z is precise once the dreaded Z height is corrected for the inaccurate inductive Z probe and the bed is pretty level thanks to my beefy design of the Z carriages.  Surprisingly, changing from all metal to "plum" type shaft couplers made no noticeable difference to either noise or transmitted vibration.

Of course, Concorde is very much bigger than the Mini and probably bigger than many other 3D printers so the print bed will be heavier than most, the main weight being the 500mm x 500mm x 4mm glass plate.  Maybe I'll do a test without the glass plate - it's separate from the 2mm aluminium plate and bed heater.

A bit later...  Tried Z drive without glass plate and it does make less noise.

Edited March 10, 2019 by Gina

[Gina]

Worked out the weight of the glass plate - 2.5Kg.  The aluminium sheet is just over half the weight of the glass plate then there's the frame and the Z carriages.  I guess the total comes to something like 4-5Kg.  Is it possible that these 1mm pitch trapezoidal shape screws are not up to this much loading. viz. 2.5Kg each?  Also, are the small 57mm NEMA23 stepper motors powerful enough?  Am I expecting too much of this drive system with the loading I have?

[Gina]

I've dragged the printer out of its place and have it turned on its back looking st the NEMA23 motors with a box of grommets to my side.  Thinking of trying grommets as anti-vibration mounts which is something I've done in the past to quieten motors.  OTOH I'm wondering if I should buy the proper mountings from Makers Hut at £16 the pair :- Nema 23 Stepper Motor Damper Pad Anti Vibration CNC 3D Printer Mill  

[Gina]

Here's a very rough diagram of how I'm using grommets as vibration isolators.  M5 bolts through the motor holes, grommet, 8mm hole in box bottom, grommet, washer and nut.

[Gina]

Motors attached with two bolts each so far (may be enough).  Job done ready for testing...

A bit later... Well, the noise is less, particular from the motors - I can now hear a chugging noise from the threaded rods.  Also, there seems to be slightly less vibration of the box.  Might have achieved sufficient improvement that I can get the piezoelectric probing to work.

Edited March 10, 2019 by Gina

[Gina]

Found where the noise was coming from - motor slightly out of alignment so redid the mounting with more accuracy but now the motor won't turn - investigating.

[Gina]

The motor had moved while I was marking out - now fixed.  Now a lot quieter and just faintly felt vibration on the box - very much better.  New X carriage parts will have to be printed on the Mini printer unless I can get piezo sensing bodged up to work as in all the moving about the bracket that holds the inductive probe go broken.  This printer is rather too big for this room except when in its cubby-hole.

[Gina]

I need to get the piezo sensitivity down more somehow.  When I have the parts cooling fan on at more than around 40% it triggers.  Not that I shall have that fan on when Z homing or bed probing.  Fortunately the hotend fan doesn't trigger the sensor. 

The problem is that the sensor still triggers almost continuously when the bed is moving.  I think the problem may be that it's between extruder motor and rear X carriage plate ATM and I reckon vibrations are being transmitted along the X rail to the X carriage and the motor is staying still while the back plate moves.  I need to implement your idea @tekkydave and put the sensor between extruder and motor and hope the motor dampens out the vibrations coming into the X carriage.

[tekkydave]

I think if you can get some decent deformation of the piezo going on you should be able to turn the sensitivity down

 

[Gina]

I already have the sensitivity at minimum but I reckon I can reduce the movement at the sensor.

Here are some photos of the current X carriage partly dismantled.  The slot at the top took the umbilical cable bundle and can be changed.  This strikes me as a good place for the sensor.  The extruder can be fastened to the motor at the bottom only (or maybe just lightly screwed together).

 

This is where Ive had the sensor for testing.

Edited March 10, 2019 by Gina

[Dr_Ju_ju]

If its still too sensitive, try reversing the connection between piezo & controller....  it's one of the recommendations to be found on the site.

[Gina]

After a break outdoors this afternoon burning up some rubbish after a bit of a clear-out, I managed to get my brain into gear and design parts for the Concorde piezo sensor. 

Removed clamp for umbilical from the front plate of the X carriage and used that place for the piezo sensor (see 1st CAD model).  This is a hollow of 21mm OD for the 20mm sensor and a central hole so that it's only supported around the edge.  Between the extruder and this X carriage plate will be a spacer 1mm thick increased to 2mm along the bottom edge and with a lump for the middle of the sensor (see 2nd model).  This arrangement flexes the sensor rather than just pressing it.

Edited March 11, 2019 by Gina

[Gina]

Spacer plate sitting on back of extruder body.

[Gina]

Testing time today...

[Dr_Ju_ju]

Hoping it works ok, for you..

[Gina]

X carriage assembled sufficiently to test piezo sensor.  Piezo triggers with a tap on the nozzle with a small spanner but didn't trigger raising bed from around 200mm but one motor stopped after about 50mm!    The dreaded Z drive is still causing problems!!  If it's not one thing is's another! ?

[Gina]

With the bed manually moved to within 10mm of touching the nozzle I was able to move the LHS of the bed up to the nozzle.  Needed to increase the sensitivity and reverse the sensor plug to get the sensor amp to trigger.  The sensor didn't trigger from the Z motion alone.  So I think I may have the piezo probing working at last. 

Now I have to find out why the RH Z drive is jamming.  I tried to stop the Z drive by hand but couldn't - the NEMA23 is a mighty powerful stepper motor.  In view of that I can't understand why the drive doesn't work - I can't see anywhere it could be binding other than the threaded nut on the threaded shaft.  I guess I shall just have to take it all apart again (tomorrow).