Extendable Z-probe with Shape Memory Alloy actuator

This is a Z-probe touch sensor with an SMA (Shape Memory Alloy) actuator to extend the probe tip. In order to see the fine detail of a contact event, I have used a piezo disk as I am familiar with both the vices and virtues of these devices. I have put this on this forum in the hope that it may be of use to others; but also in the hope of some useful input from others.

A proof of concept device has been tried and shows that there are a number of problems and that a re-design is needed. Nonetheless, there seems to be sufficient promise to make it worthwhile to investigate further. I have put a video of this on YouTube. [www.youtube.com]

As far as why this exists: I am going away from using three underbed sensors for bed leveling for reasons which have been discussed elsewhere. [reprap.org] Instead, I am using a single underbed sensor at a good datum position with a second sensor of the touch type on the effector. While a BLTouch would be satisfactory, it would not allow me to explore what is happening dynamically on the bed. I have now made several piezo touch sensors and they are ably to resolve the contact in the sub-micron, sub-millisecond, and milliNewton range. [reprap.org] & [reprap.org]

The SMA actuator consists of a length of Nitinol wire 0.1mm in diameter by 227mm long which contracts by about 4% when it is heated to above 60C and is able to exert a pull of 143 grams (about 5 ounces). Incidentally, this length of wire weighs only 0.011grams (about 0.0004 of an ounce). The wire is doubled over twice to fit into the available space and to magnify the pull by 4 times. The quad set of wires pulls on a spring lever which then moves the actuator rod - the maximum final force is a little over 100 grams and maximum travel about 9mm. Below is a photograph of the top end of the device along with a CAD representation showing the route of the Nitinol wire along with the spring lever and pulleys used for load sharing between the lengths.



As far as what is wrong with this prototype, the following is a partial list:-

• On this device, it is difficult to get the right preload on the actuator rod at full extension (100 grams) and a livable retract time at the same time. Lots more work needs to be done on the springs.
• The sliding of the actuator rod does not seem to be acceptably smooth. Similar touch sensors had smooth follow-through but they used leaf springs - possibly the top adjustor is contributing a sideways force. This sideways force seems to also introduce a bit of unwanted sideways movement of the tip.
• Fitting the Nitinol wire is a nightmare. I don't think the pulleys will be necessary if the lengths of all four quarters are the same and omitting these will make the fitting much easier.
• The device is far too heavy as it weighs 26 grams. It is also far too complicated as it was designed at the vice instead of on the computer - bits were fitted as the need became apparent and even the piezo disk had to be trimmed to fit.

So any constructive comments or suggestions gratefully received.

Mike

Edited 2 time(s). Last edit at 06/26/2021 02:23PM by leadinglights.

... I've used SMA's for tweezers - my fastest recovery time was at roughly 0,7s, but with a special design, with lower masses/forces and "acting" against strong springs.

For faster recovery you'll need two "counteracting" SMA's to reduce the retract time, or thermoelectric cooling.

Or you'll change to NiMnGa-alloys, which acts like PZT's, but with magnetic field instead of voltage -- there are some designs/prototypes (actor 20mm long, 1mm moving range, 1kN force), acting and releasing with up to 1kHz! ... but pretty much money needed for (my two 20x5x5mm blocks - only one left - were 400€ each!!)

Edited 1 time(s). Last edit at 06/26/2021 10:09AM by VDX.

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Viktor
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Aufruf zum Projekt "Müll-freie Meere" - [reprap.org] -- Deutsche Facebook-Gruppe - [www.facebook.com]

Call for the project "garbage-free seas" - [reprap.org]

Thanks Viktor, I will look into NiMnGa alloys which I haven't come across before. Hopefully I will be able to find a less stratospheric price - for example, Nitinol can be had from Sigma Aldrich for £320 for 1 meter [www.sigmaaldrich.com] or through Amazon for £1.39 per meter [www.amazon.co.uk] from Formgedächtnistechnik.

The "counteracting" SMAs method looks good at the moment, possibly having PCBs with the SMA actuators and driver circuits along with Piezo sensor and conditioning circuit. FR4 pcbs from some of the cheap PCB manufacturers like JLCPcb have the mechanical accuracy and strengths well as all the onboard circuitry.

Mike

From an admittedly short search for info on Magnetic Shape Memory Alloys like Ni₂MnGa, it looks like the magnetic driver will have to be too big to be usable - I will retire that for some future use.

A quick test with a bistable Nitinol actuator powered with short pulses of high current to switch states looks promising. It looks like I can get everything I need as long as I can allow > 2 seconds between switching from extended to retracted - I don't think this will ever be a problem. First calculations make it look likely that I can even keep the weight of the complete device with piezo and supporting electronics to a little over 10 grams - As long as I can avoid adding complexity.

Mike

... you could add a jet of cold air to reduce the recovery times

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Viktor
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Aufruf zum Projekt "Müll-freie Meere" - [reprap.org] -- Deutsche Facebook-Gruppe - [www.facebook.com]

Call for the project "garbage-free seas" - [reprap.org]

Quote
VDX
... you could add a jet of cold air to reduce the recovery times

The cold air jet could be switched on and off by a valve with an SMA actuator. The slow recovery time of the valve could be improved by a jet of cold air which.......... Must stop adding complexity.

The latest development removes the push-rod entirely and replaces it with a swing-out lever reduces the weight by about 1 gram and does reduce complexity. I was also looking at fitting a half-bridge foil strain gauge to the lever - on balance though, I am happy with the piezo circuit and software.

Mike