BLTouch alternative for hotbed with magnets

[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.