Piezo20 probe and piezo kit now available
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Sounds like a good idea. I find the sunon maglev 30mm fans are quiet, reliable and powerful you can get them from Farnell for around £13 each.
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Here's another possible configuration:
1. Drill and tap the top end of an E3D heatsink with 3 small holes, probably M2.5 or possible M3. I hate groove mounts.
2. Take a piezo element with the active element about 20mm diameter. Perhaps this one http://www.digikey.co.uk/product-detail/en/murata-electronics-north-america/7BB-27-4L0/490-7714-ND/4358154. Drill a central hole for the Bowden tube and collet, and 3 smaller holes around it.
3. Attach the disk to the top of heatsink using 3 nylon screws, with a piece of insulation between the piezo and the heatsink. The purpose of the insulation is so that if the heater cartridge develops a short and makes the heatsink live, we don't get a short between the heater and the piezo.
4. Clamp the brass surround of the piezo disc between the effector (which has a hole in it for the top of the heatsink to pass through) and a printed ring.
For the electronics, I would take one of my IR sensors, remove the optical components, and feed the piezo into the analog input. Then program it to produce a nice clean pulse when it detects a shock. Piezos produce a good voltage from a very small mechanical shock if the load resistance is very high.
I'd have a go at this myself, but I don't have a drill press for drilling the top of the E3D heatsink.
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That's a good approach and neatens the design. As long as the area of contact between the heatsink and the piezo is fairly small it will flex enough. Maybe a small washer with 4mm ID and 6-8mm OD between the heatsink and the piezo transducer. Good idea with the ir board it seems logical it would work. The board Moriquendi came up with is probably very similar to it but has two pots to adjust the sensitivity to suit various piezo discs. But as a finished product using a known and tested piezo this would be unnecessary. I'm fairly sure these boards can only be supplied in fairly small numbers at present.
My only concern would be the whole hotend is suspended from the piezo unless I'm not visualising what you mean correctly. This might reintroduce the wobbly nozzle issue. My rough design doesn't look nice but it does work and the nozzle is quite firm I think I'll work on making this smaller and neater for now.
Agreed re. groove mounts they are useless. The 90 deg tee I based mine on clamps the hot end hard and effectively turns a groove mount into a flat mount with m4 bolts.
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That David sounds like a plan although don't quite get how you would fasten the Hotend to the Effector with this arrangement any chance of a quick Sketch.
I am sure I could drill and tap 3 holes in the top of a heatsink at M3 and maybe use a piece of Bare PCB G10 board as the insulator.
Doug
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That David sounds like a plan although don't quite get how you would fasten the Hotend to the Effector with this arrangement any chance of a quick Sketch.
I am sure I could drill and tap 3 holes in the top of a heatsink at M3 and maybe use a piece of Bare PCB G10 board as the insulator.
Doug
See https://dl.dropboxusercontent.com/u/19369680/Scan029.pdf (sorry about the poor drawing, I flunked Art at school).
A is the top of the hot end heatsink
B is the insulator
C is the piezo transducer. The thicker part is where the element is, the thinner part is where the brass substrate extends beyond the element all round.
D is the effector
E is the printed ring. It will need a small tunnel in it to make room for the wires attached to the piezo element.
F are screws. The two (actually 3) inner ones at least are Nylon to avoid shorting the piezo element out or connecting the piezo to the hot end metalwork.If the 27mm O.D element I found proves too insensitive, we could use a 35mm one instead.
PS - RS also sells a 27mm OD piezo element, see http://uk.rs-online.com/web/p/piezo-buzzer-components/7243162/. They also sell M2.6x6 nylon screws.
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OK I get that the bit that worries me is if 3 Nylon screws will be substantial enough to hold the hotend especially should it contact anything.
How about using Mica washers as in the type used to attach 7405 type things to heatsinks and use std Stainless screws would need to drill the Piezo disc a little larger tho and I wonder if there is enough space to get it all in.
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It's worth trying but I wonder if the Piezo will flex enough, and whether the hotend will wobble but I'm happy to be proved wrong.
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I think it's a question of getting the balance right. If the piezo disc is large (i.e. inner diameter of F is much larger than outer diameter of A), it will produce a large output but the head may wobble. If it is small (inner diameter of F not much larger than outer diameter of A), it will be much more rigid but produce a smaller output. I think the same will apply to any design, except for designs that use some other means to constrain the relative movement of the hot end and the effector to be in the Z direction only.
The diameter A is about 16mm on my E3Dv6 and I expect the arrangement to be very sensitive because of the direct contact between the hot end and the piezo. That's why I think a piezo for which the outer diameter of the element (which should be slightly smaller than the inner diameter of F) is 20mm may be large enough. But I could be completely wrong.
There is also the risk that a high-speed head crash could break the piezo element. I don't think this is very likely because of the brass backing. However, the ring F could be made to come over the top of the inner 3 screws so that it acts as a stop if the hot end gets pushed too high.
I guess another option that avoids tapping the E3D and drilling more than 1 hole in the piezo is to epoxy the brass substrate of the piezo to the top of the insulator, and epoxy the bottom of the insulator to the top of the hot end heatsink. Bonding epoxy to aluminium can be tricky, but I have read that sanding followed by wiping with IPA helps, so does sanding followed by etching in citric acid.
PS - thinking about it some more, it may be acceptable to bond or screw the piezo element directly to the top of the hot end heatsink, and design the electronics to survive a short between the piezo and +24V.
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I think in the long term we will want to go with screw mounting, rather than bonding, as it is easier to assemble and easier toreplace a malfunctioning part
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This is very encouraging, I am sure various efforts will probably coalesce into a single design we can all benefit from. There's a good bit of long term testing to be done, Leadinglights and Moriquendi have got a good few months if not 6 or more on their under-bed piezo system which places more demand on the piezo transducers than hotend mounting (under normal conditions anyway).
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Hi all,
I've followed DjDemonDs link over from reprap. I have a couple of boards if anyone wants them.
I think it's worth mentioning with respect to mounting hot ends that the piezo effect is based on a change in pressure rather than the pressure itself. The signal you get changing from 10N to 20N of force is the same as you would get going from 100N to 110N. To put it another way, a steady pressure on the element produces no output only a change in pressure. I see no reason why you couldn't clamp the piezo element rigidly between the hot end and the carriage/effector, the element should respond to the additional force of a z-probe event just as well.
Moriquendi
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Hey Moriquendi, welcome to the forum and thanks for the clarification. Are your boards documented anywhere?
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The eagle files with a BOM and basic tuning instructions can be found herehttps://www.dropbox.com/sh/3q38ew98kt7f8sq/AAB_uA6tQL2ETlGcnfdijm2Ra?dl=0 the original circuit design was LeadingLights, I tuned it to my needs and put together the PCB design
I got 12 boards made up and offered to post the spares to anyone in the UK in exchange for a £5 donation to http://www.great-dane-care.org/index.html a charity of which I'm a trustee. That offer stands for the remaining boards that I have (four I think at this point). Please let me know if I'm breaking forum rules offering these, to be clear, I'm not making anything personally from these boards.
If there is demand I can make more though I have a new PCB design that is easier to make (components on one side only) and easier to tune (trimmers actually visible to the naked eye)
Moriquendi
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This is how my effort works I clamp the piezo element fairly tight in between the two halves of the assembly eliminating the wobbly nozzle, which is what you cannot do with fsr's or microswitches.
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Just a thought guys. I've seen people using magnets to hold the hot end to the effector. I think the idea is that in the event of crash, the hot and just breaks away before any damage occurs but it also allows for rapid changing of the hot end. Could you use a similar arrangement but with a Piezo sandwiched between the magnets? So the magnets exert a constant force which is broken (changes) when the nozzle touches the bed? I don't have a Delta so have no idea how you guys mount the hot end. Also I don't know if a negative change in pressure works the same way as a positive change, hence "just a thought".
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Hi all,
I've followed DjDemonDs link over from reprap. I have a couple of boards if anyone wants them.
I think it's worth mentioning with respect to mounting hot ends that the piezo effect is based on a change in pressure rather than the pressure itself. The signal you get changing from 10N to 20N of force is the same as you would get going from 100N to 110N. To put it another way, a steady pressure on the element produces no output only a change in pressure. I see no reason why you couldn't clamp the piezo element rigidly between the hot end and the carriage/effector, the element should respond to the additional force of a z-probe event just as well.
Moriquendi
Hi Moriquendi, thanks for joining in this discussion.
What you say is true of cylindrical piezo elements, but not of bimorphs, which are the type of element I believe we are talking about. Bimorphs are made from two oppositely-polarised sheets stuck together. They respond to being flexed. When you flex them, one sheet has to expand, the other has to contract, so you get a voltage between the two faces. They are far more sensitive to small movements than piezo cylinders, but of course not nearly as rigid. Conversely, they produce far more movement for a give voltage than a cylinder does, which is why they are used in buzzer elements. When I was working with CO2 lasers, my colleagues were using piezo cylinders about 50mm long for mirror positioning and needed 2Kv of drive to get 10um movement. I was building miniature lasers, so I used bimorphs and I was able to use a much smaller element and reduce the voltage to 250V.
I don;t think you would get nearly enough sensitivity from a cylindrical piezo element. If you look at the ones listed at https://www.kistler.com/gr/en/products/components/force-sensors/#1__component__force__sensor__fz_up_to_2_5_k_n_562_lbf_9131_b, even the smallest one senses "up to 2.5kN". We want to sense a small fraction of 1N.
So this is why the mounting arrangement I proposed is designed to let the piezo flex a little when the nozzle contacts the bed.
- David
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You may well be right, it would make sense for the buzzer elements I'm using to seek to operate at the lowest voltage for the maximum volume, nobody wants 2Kv in their alarm clock. I'm using piezo elements in the bending mode under the bed of my delta but they seemed to respond well to being tapped while flat on the desk as well. I think the circuit could be tuned to work with a smaller signal without much difficulty, perhaps someone should try it.
Moriquendi
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You might like to try the following experiment - I just did, using a 27mm piezo disc:
1. Support the disk on a ring around its periphery. Connect the piezo to an oscilloscope. Tap the centre and watch the voltage. I get a pulse of 20V.
2. Put the disc (or as much of it as you can without the wires getting in the way) between two flat surfaces, with the top one being an insulator (e.g. glass). Push down on the top surface, but not directly over the piezo, to clamp it flat, Use your other hand to tap on the top surface directly over the piezo. I get a pulse of 100mV, so 200x weaker.
Given the size of the 20V pulse, I am confident that a 27mm diameter piezo (20mm active element) will be sufficient, despite the fact that only a ring near the periphery of the active element will be free to flex, because the central section will be clamped or glued to the hot end heatsink. The main question for me is whether the amount of flex that the piezo allows will permit too much lateral movement of the nozzle. Attaching the piezo at the bottom of the hot end heatsink would be better from a vibration perspective, but shielding it adequately form the heater block would be difficult.
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David
just seen your post on the Google forum re Haydens balls and ends and can say they are superb and the Delrin ends will work fine with the balls at 90 degrees but not out the side of the robotdigg carriages.
Your idea of a PCB Effector makes me wonder if strain gauges could be used as a sensor setup in place of the Piezo disc?
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You might like to try the following experiment - I just did, using a 27mm piezo disc:
1. Support the disk on a ring around its periphery. Connect the piezo to an oscilloscope. Tap the centre and watch the voltage. I get a pulse of 20V.
2. Put the disc (or as much of it as you can without the wires getting in the way) between two flat surfaces, with the top one being an insulator (e.g. glass). Push down on the top surface, but not directly over the piezo, to clamp it flat, Use your other hand to tap on the top surface directly over the piezo. I get a pulse of 100mV, so 200x weaker.
…...........I suspect there is something wrong with your setup as I find using the piezoelectric disks in bending gives about 3 times the sensitivity as using them in compression, not the 200 times that you have found. This time 3 gain is not cost-free however as there is a considerable penalty in terms of compliance although such is the sensitivity of piezo disks that even this penalty is unnoticeable.
A big plus for using piezo disk in bending is that it is easy to engineer out the mechanical cross-talk that has plagued the piezo bed leveling in my Cartesian printer. If the three sensors under the bed can be replaced with one on the hot-end that is even better.
Mike
