BLTouch alternative for hotbed with magnets

[jensus11]

Hi everyone, I bought a new hotbed with magnets on the bottom. Unfortunately, the BLTouch doesn't measure properly. When it's over a magnet, it doesn't have enough power to pull the pin back and gives a message that the probe is in use.

What is a perfect alternative that works perfectly with magnets?

[oliof]

@jensus11 so you are using an older model BLTouch with a metal pin? Sinve version2.1 or so BLTouch comes with nylon pins. Or are you saying you nagnets are so strong it overpowers the BLTouch mechanism further up?

In the former case, you may be able to get a replacement nylon pin and it may work (or get a newer BLTouch with nylon pin). In the latter case you will need to consider another probe like klicky/euclid if you still want to probe the actual surface; or an inductive probe or scanning probe if you want to go touchless.

[jensus11]

Hi oliof,

I have the BLTouch in V3.1, so with a nylon pin. I think the mechanism doesn't work with the strong magnets.

Can you tell me a good inductive/scanning probe?

[oliof]

@jensus11 inductive probes, the Pepperl+Fuchs superpinda style ones are good since they are not floating with temperature (as much) and their higher frequency allows for somewhat faster probing. You can get equivalent ones in various places, for example CR3D or RatRig.

Scanning probes usually require CAN connections, you can either go with the Roto Toolboard , Duet3D SZP, or Mellow SB2040v3 Max or SHT36v3 Max versions. Note that temperature compensation for these has not been finished yet and use as a Z probe is somewhat limited (no beacon contact style nozzle probing)

[dc42]

@jensus11 inductive sensors can often detect magnets too. It's not usually the magnetic field that affects the sensor, it's the change in magnetic permeability and/or electrical conductivity that affects the inductive sensor. Flexible magnetic beds are usually thin enough that the inductive sensor sees what's underneath them too.

[jensus11]

For me, it is definitely the magnet. The BLTouch measures 4 positions perfectly. Then the magnet comes and the pin is not retracted and flashes blue and red. There are also neodymium magnets which are embedded in the plate from below.

[dc42]

@jensus11 yes BLTouch is affected by magnetic fields. My comment was about inductive sensors.

[jensus11]

ok. I'll order an inductive one and try it out and then report back.

Unfortunately, I have a problem with my Keenovo heating pad. I've just installed it. When I want to heat the bed to 60 degrees, everything is quiet up to 55 degrees. From 55 degrees onwards the heating pad makes noise. A humming noise that only stops when it's turned off.

Do you have any ideas? The heating pad is new.

[dc42]

@jensus11 the humming sound is most likely caused by the PWM being reduced as the temperature nears the target. It's unlikely that you will be able to get rid of it completely (other than by using a different type of bed heater) but you may be able to shift it to a less annoying pitch using the M950 F parameter.

[jensus11]

That sounds good, what I have to change it?

M307 H0 R0.475 K0.931:0.000 D7.84 E1.35 S1.00 B0

M308 S0 P"e0temp" Y"thermistor" T100000 B3950 C7.06e-8           	; configure sensor 0 as thermistor on pin bedtemp
M950 H0 C"bed" T0                                        			; create bed heater output on bedheat and map it to sensor 0
M140 H0                                                  			; map heated bed to heater 0
M143 H0 S100								             			; set temperature limit for heater 0 to 100C 

[dc42]

@jensus11 add the F Q parameter to the M950 command. Assuming it's a DC heater, try e.g. Q10, Q30, Q100.

[jensus11]

I want to test the F25 and became this message.

Fehler in Startdatei macro Zeile 69: exactly one of EFHJPSR must be given

M950 H0 C"bed" T0 F25

[droftarts]

@jensus11 Use the Q parameter for PWM frequency. See https://docs.duet3d.com/en/User_manual/Reference/Gcodes#m950-create-heater-fan-spindle-led-strip-or-gpioservo-pin

Ian

[jensus11]

I'm using this external mosfet. If I use a different one, will the noise go away?
With Q50 the annoying sound is almost silent. Can I use it or will something break?

[dc42]

@jensus11 Q50 is fine. You will probably hear a slight buzz.

[jensus11]

@dc42
I haven't bought an inductive sensor yet. I can't find one, they're sold out everywhere.

But an idea is, of it is possible to define measuring points for the BLTouch so that I can find the position of the magnet and measure there and use this for the heightmap.

[droftarts]

@jensus11 you can define points in the mesh for the BLTouch to avoid, see the notes on G29 S4 here: https://docs.duet3d.com/User_manual/Reference/Gcodes#notes-8

Ian

[jensus11]

I have defined the mesh point in the probePoint.csv file.
But the result looks not good. I think the magnets are so strong and interfere with the BLTouch when measuring.
So on, I need an inductive sensor.

[jensus11]

I have now installed an inductive sensor. Unfortunately it doesn't work that way either.
The measurement stops above a magnet and this message appears.

Error: M280: Probe already triggered before probing move started.

What can I do?

[troydeming]

I have been using a bltouch with a magnetic bed for years with no issues. did you get it figured out?