Twotrees SK1 Z-probe failure

General Discussion
1

Basic Information:

Printer Model: Twotrees SK1
MCU / Printerboard: Twotrees proprietary (very similar to Makerbase SKR PI), STM32F401 on the main board, and RP2040 on the toolhead
Host / SBC: Twotrees proprietary (very similar to Makerbase SKI PI)
klippy.log attached
klippy.log (141.9 KB)

Describe your issue:

When homing, Z-probe does not trigger, and toolhead hits the plate. Even though sensor itself clearly triggers (a LED lights up when sensor is close to a metal), the QUERY_PROBE command still reports z probe is open.

Everything was working fine for the recent few weeks. Printer suddenly stopped printing, and after reboot it no longer can home on Z axis. There were no recent configuration changes (most of them are even factory).

The print head features a quite rare sensor, named PuXey LT-04N. I tested the sensor separately, and it seems to work - the signal line gets low when metal is near the sensor (though it looks a bit like a low duty cycle PWM - it is low most of the time, but there are 1ms pulses every 100ms)

It is not really clear whether this is a HW or SW issue. Can someone suggest a test? This particular toolhead had issues previously, but after a patch it worked normally for about a month.

2

Well, if:

  • it worked before
  • you did not change config
  • you did not update the software components

then it is quite likely that it is a failing hardware or issue with the wiring.

AFAIK, an inductive sensor should stay triggered as long as it detects something.

3

I am pretty sure it is. But I cannot understand where exactly.

I tried to flash Micropython to the toolhead’s microcontroller to see whether it sees the value change on the pin, and it seems to work fine. Now I would like to understand why Klipper does not see it.

Is there a way to log communication with the toolhead and see what the toolhead’s reply on probe query command?

4

No, you would need to hook up an oscilloscope or just a digital multimeter.

Basically these are just switches. Depending on the type, either:

  • Signal line is pulled up to high (3.3V)
  • Signal line is pulled to GND (0V)

During the sensor is detecting something the switch state needs to be stable.

5

This indeed was a hardware problem - the toolhead contains a MCU board with 24V->5V DC-DC. It is a known problem that main board and toolhead board use poor quality solder that tend to break on a little force or vibration. Perhaps this was a reason why 5V became 4V, and the sensor simply did not work as expected. I soldered everything around the DC-DC and the issue has gone.

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