Nozzle contact probe reliable and safe?

schpongo

Hey I'd like to have some input on my idea of using the nozzle as a z probe.

Originally I had planned on using a inductive sensor for my z probe on my delta printer.
But I seem to be getting unreliable results from that setup and was no able to setup my printer using the auto calibrate function.
So I moved away from the inductive sensor and manually calibrated my delta with the help of this guide (german http://forums.reprap.org/read.php?245,590895,612438).

The guide requires you to check the distance of the bed and the nozzle several times. For that reason I want to start using a z probe.

Here's my idea. I have a delta printer whose frame is mostly made out of metal. If I connect the GND pin from the E0 endstop to the frame, set the end stop to active low, electrically isolate the effector from the rest of the frame and connect the STP pin from the E0 end stop to the effector I can use my nozzle to probe for my bed.

I just did a quick test and was successful.
After cleaning the tip of the nozzle I was able to use the tip of the nozzle as a z probe. While doing so I measured the current through the STP pin (1,4mA).

I would now like to know if anyone else has any experience with such a setup and if I maybe neglecting something:

Is this setup electrically safe for the duet?
Is it electrically safe, as the frame is connected to protective earth?
How reliable do you guys think this will be?

Thanks for the help.
Greetings Daniel

dc42

That's fine in theory, but here are some possible issues:

1. If you forget to clean the nozzle, it may have plastic stuck to the end and not make electrical contact with the bed. Then the effector will try to drive into the bed. You could reduce the motor current during probing using M913 to try to reduce the damage when this happens.

2. Shorts between the element of a cartridge heater and its case are not uncommon. If you get such a short, it will feed 12V or 24V into the endstop input.

Have you seen the Duet3D Smart Effector?

schpongo

Yes this is clearly a problem. But for calibration one could either clean the the tip of the nozzle manually or let the nozzle heat up and have it run over a cleaning pad.
That pad could be made up of an abrasive or a solvant which removes the filament which is being used.
Ah that is good to know. Sadly my current set up does not allow me to electricity isolate the hot end from the effector.

I've heard of the smart effector but haven't looked at it closely.

Are there any other concerns or experiences?

Greetings Daniel.

DjDemonD

We recommend to probe at 130 deg C, this is below ooze temperature for all filaments, clip the excess off the nozzle. Then do G30 just off bed centre, if there is any residual filament it gets squashed down by this initial probe. Now run your normal probing routine and then heat up to first layer temp and print. The thermal expansion from 130 to 250 deg C for a v6 heater block is about 0.02mm if you want to factor it into any Z offset.

As for electrical contact as a probe, its the most accurate you can get, as long as you take care with shorts etc. as David pointed out. However unless you have some electrically conductive PEI lying around that did not cost £1000/sheet its usefulness is limited as who wants to print onto bare aluminium?

schpongo

@DjDemonD:

We recommend to probe at 130 deg C, this is below ooze temperature for all filaments, clip the excess off the nozzle. Then do G30 just off bed centre, if there is any residual filament it gets squashed down by this initial probe. Now run your normal probing routine and then heat up to first layer temp and print. The thermal expansion from 130 to 250 deg C for a v6 heater block is about 0.02mm if you want to factor it into any Z offset.

As for electrical contact as a probe, its the most accurate you can get, as long as you take care with shorts etc. as David pointed out. However unless you have some electrically conductive PEI lying around that did not cost £1000/sheet its usefulness is limited as who wants to print onto bare aluminium?

Thanks for the answer.
The information you provided me with ist exactly what I was looking for.

How commen are electrical short in the heater cartridge and are they detectable in any way?

Well at the moment I'm still printing on bare aluminum.
But if I were to use a sheet of carbon for better adhesion I would maintain the electrical conductivity.

Greetings Daniel

DjDemonD

Hi Daniel, I have never seen a shorted heater cartridge, but it's not impossible and they are relatively high current, so the effect would be fairly unpleasant and possibly expensive (new duet board).

So it is worth avoiding, use some resistors in your sensor circuit or some other more sophisticated electronic protection (not really my area of expertise).

If you can crack the challenge of an affordable, conductive build plate then you have a winning product on your hands. Imagine a kit, in it there is a conductive printing surface which connects to a small PCB providing some protection, a trigger output to go to your controller and nice LED's, there is also a wire to connect to your heater block. Neat yes?

Conductive PEI is available but scandalously expensive. However other conductive sheets that you can print onto such as polycarbonate (with appropriate sanding) might be worth pursuing. Indeed most of these things have quite high resistance so the risk from a short is probably lower. One such conductive PEI can also be used as a bed heater, now we are talking convenient, but very expensive at this point in time.