Smart Effector Strain Gauge Magnetic Immunity

[louvanna]

The strain gauge in the Duet3D smart effector is susceptible to changes in magnetic flux, dB/dt. Specifically, the bed heater under thermostatic control, and I suspect to a lesser extent the local hotend heater. ( This is not the magnetic ball susceptibility due to Hall-effect on various electronic devices. )

The lack of magnetic immunity causes programming of the bed levelling and compensation routines to halt thermostatic control of the both a heated bed, as well as the hot-end. While the bed is probed, these devices are in the off-state. This then causes the temperature to drift, cooling. I have not quantified the dimensional shift imparted by this thermal transient. I have however wished to overcome the delay required to re-heat the devices in the overall multi-step routine.

I have analysed and simulated the necessary update(s) to the SE, which were pulled from the Github. I have generated a report/presentation on the needed, minor updates to achieve a greatly improved magnetic immunity. This forum is possibly not the correct place for such a document.

I would like to deliver to the designers of the SE this report such that its findings might be incorporated in the next revision of the SE. I have applied best practices for strain gauge as well as my knowledge of magnetic susceptibility to achieve a much improved device.

A little of my background. My professional career is currently focused on the simulation of electro-magnetics. Currently, I am employed as an engineer in MRI industry, developing next generation medical devices and the MRI system itself, which must be capable of functioning in the highly stressful MRI environment. I have at my disposal very large compute GPU clusters, which were employed in the simulation of a virtual SE.

[dc42]

Hi @louvanna, we would welcome your suggestions for improving magnetic immunity of the strain gauge circuitry on the Smart Effector. Please email them to info at duet3d dot com marking the email for my attention.

I am aware that some types of print cooling fan generate enough varying magnetic field to trigger the effector, as this has been reported on the forum. This is not normally an issue as there is no need to run the print cooling fan while probing. However, I am surprised that you are unable to probe with the bed heater running. I use a 240VAC 350W bed header and I leave it running during probing. I guess your bed heater must generate more magnetic field than most. Does it perhaps use a spiral trace pattern instead of the more common zigzag pattern?

[louvanna]

@dc42 You understand correctly, yes my bed heater is spiral and DC-12V
The thermostat switching causes a change in current, on/off, which is dB/dt. This then induces a voltage in the strain conductor. That spiral heater has also been modelled, and analysed for the stimulus in my simulations. A labyrinth heater conductor would have less magnetic flux, but still potentially enough to trip the strain threshold. Gladly, amend this work to a main-line bed heater, etc.

I will email the document from my other workstation, when I log-in next day.