Any known issues with drifting inductive probes?

fho

I am a bit at loss here ... last week I ran my Duet 2 Wifi equipped Voron 2.1 basically non-stop on a production run. During the last few prints I noticed that during the bed leveling procedure the nozzle touched the bed once or twice. During the last bed leveling procedure I did, the inductive Z probe I used registered the bed even later/lower and the toolhead bent upwards to "compensate".

Now, I shrugged that off to a cheap faulty Z probe that just couldn't handle a week at 50°-100°C. I just replaced that with a fresh probe (same model) and that probe has the same problem. For me that rules out the probes themselves.

Apart from the probes there is only the Duet boards and the bed itself that may be a problem. The bed is a 1 cm lump of cold rolled aluminium... I think that should at most warp at 100°C, but I have no idea what that temperature does to its inductivity (?).

Just to rule out every possible problem: Are there any known problems with drifting inductive probes on the Duet 2 Wifi boards? (I should look up the version of the board itself ... but the printer is still running ).

o_lampe

@fho
not sure if there's a general issue with inductive probes, but I always heat up the bed to target temp, then switch it OFF before probing.
I guess the bed heater can influence the readings. Especially when in PWM mode

fho

@o_lampe Fair point ... I'll try that, although my bed does not run in PWM mode. Still that does not really explain why the trigger point apparently drifted gradually over time?

o_lampe

@fho said in Any known issues with drifting inductive probes?:

why the trigger point apparently drifted gradually over time?

..too close to the heater block? I've added a piece of cardboard covered with Kaptontape between hotend and probe as thermal shield.

fho

@o_lampe close? Like right next to it

Actually when I removed the probe after the printer was off for at least half an hour, the probe still was warm to the touch. The copper coil in there retains a lot of heat.

Although I do have a silicone sock over the heater block, that should at least prevent some of the heat escaping there.

fcwilt

@fho

Does your inductive sensor have a model number?

Any published specifications?

Inductive sensors can have varying sensing ranges, temperature ranges, switching frequencies, etc.

Perhaps you just need one with better specifications.

Frederick

dc42

@fho inductive sensors are generally temperature sensitive. That's why Prusa uses sensors with built in thermistors. You can add your own thermistor and configure temperature compensation in your G31 command if you want. You could also use bed temperature as a proxy for sensor temperature.

fho

@dc42 Thanks for the reply!

I am aware that inductive sensors are temperature sensitive. I am following the same warm-up procedure every time to mitigate that. That procedure involves letting the (enclosed) chamber warm up for ~20 minutes. I have a temperature sensor in the chamber (and two on the bed), so I am pretty sure that I have a fairly consistent routine there.

Thing is that even with the routine I seem to have lost several milimeters of "trigger height" over the last week. Paired with the fact that changing the sensor for a "fresh" one does not change that I am searching for other culprits.

(I am not really suspecting the duet board, just making sure that I covered all possibilities.)

fho

@fcwilt It's the one recommended by the Voron community: https://de.aliexpress.com/item/32803870919.html

So definitely no-name, but others use it successfully

fcwilt

@fho said in Any known issues with drifting inductive probes?:

@fcwilt It's the one recommended by the Voron community: https://de.aliexpress.com/item/32803870919.html

So definitely no-name, but others use it successfully

Thanks for the info.

If you are interested in what is available here is one of my sources:

Inductive Sensors

You can check the specs and perhaps get an idea about temperature sensitivity.

Frederick

fho

Well ... that was a false alarm ...

I checked the height of the sensor and checked the height of the nozzle in the toolhead ... nozzle was below sensor ... so I thought that was fine.

Turns out the round heat-sink has rattled itself lose over time and the heat-brake should have sat much higher in the cooler.

Sorry for the fuzz