Auto Tool Height Measurement

[Terry]

H3 appears to do the same as H4, but also changes the min travel extent value.

I think this maybe a bug as that isn't the expected behaviour.

[fcwilt]

@Terry said in Auto Tool Height Measurement:

H3 appears to do the same as H4, but also changes the min travel extent value.

I think this maybe a bug as that isn't the expected behaviour.

If it does that it would seem to be a bug - nothing in the docs suggests it should do that.

What firmware version are you using?

I have 3.2.2 and can test that.

Frederick

[Terry]

Thanks. I am on 3.2.0 currently and this axis and switches are on a Duet 3 Expansion 3HC board.

[Terry]

On a typical CNC platform, I would expect at least 3 triggers per axis.

Endstops would be min and max travel indicators and would be continuously active unless explicitly disabled so they could stop motion before a physical collision (the machine may also include emergency stop cutouts at the physical limits to cut power in a runaway controller scenario). It should be possible to detect unexpected over-travel as a result of lost steps and recover by re-homing.

One or more reference triggers should also exist, for accurate and efficient homing. These should be inboard of the travel limits to avoid harm to the machine during homing operations and may sit in the middle of the travel range to enable efficient re-calibration during long operations. Typically they would only be active during reference operations.

Endstops need to be active during homing in case the assumed start position or travel direction is wrong.

Single active endstop-as-home-reference is a cost-saving hack that is acceptable for a $500 3D printer but can result in injury or physical damage to the machine in larger and more powerful setups, so it would be nice if we could support best practice in the future?

[fcwilt]

@Terry said in Auto Tool Height Measurement:

Thanks. I am on 3.2.0 currently and this axis and switches are on a Duet 3 Expansion 3HC board.

Could you use G60 along with G1 moves with the R parameter to obtain the desired result?

Frederick

[fcwilt]

@Terry said in Auto Tool Height Measurement:

Endstops would be min and max travel indicators and would be continuously active unless explicitly disabled so they could stop motion before a physical collision

You can do that with triggers. From one of my printers:

; ****************************************************************************************************
; external buttons (P = pin #, T = trigger #, R = when to trigger (0 = always, 1 = printing), S = what edge (I->A = 1, A->I = 0, -1 = ignored)
; --- T0 does emergency stop
; --- T1 does pause (see M25)
; --- T# runs macro trigger#.g
; ****************************************************************************************************

M950 J2 C"!^duex.e6stop"
M581 P2 T0 R0 S0			; does emergency stop

M950 J1 C"^e0stop"
M581 P1 T2 R0 S0			; runs trigger2 which does M999 (board reset)

[fcwilt]

@Terry said in Auto Tool Height Measurement:

Ok, setting and resetting the endstops in the macro allows me to sense the button press but unfortunately G1 H4 doesn't do what I expected. It seems that what it does is travel until the switch is triggered and then sets the user position to the value of the move that was commanded.

I think I did not understand what you were saying.

With G1 H4 and G1 H3 the axis is at some position when the end stop is triggered - this position should be what appears in the DWC.

Does that not happen?

Frederick

[Terry]

With both G1 H3 and G1 H4, the machine and user positions are overwritten as soon as the switch is triggered. In absolute mode, the axis value is set equal to the destination value that was commanded in the move. In relative mode, the axis position value is set to 1.

So, for example, starting at machine coordinate V=50 and commanding G1 H4 V2, the axis will move towards V=2 but if the switch is pressed before it gets there, motion will stop and the current position of V will be declared to be V=2, changing the calibration.

[Terry]

@fcwilt G60 wouldn't work because calibration is lost.

[Terry]

@fcwilt Triggers can't stop motion in a controlled manner, so at best you can call an emergency stop and lose system calibration. This is why endstop circuitry needs to exist inside the motion control loop - the endstop triggers need to be interrupts that can cancel motion at very low latency without losing position.

[fcwilt]

@Terry said in Auto Tool Height Measurement:

@fcwilt G60 wouldn't work because calibration is lost.

How so?

If you save the current position and then return to it how does that affect calibration?

Frederick

[Terry]

G60 would store a position relative to the calibration referenced against the fixed endstop. Any move detecting the mobile endstop will cause the axis to re-reference against that position. Any subsequent G2 R would return to a saved position relative to the new calibration, so not the original absolute position in the machine framework.

[fcwilt]

@Terry said in Auto Tool Height Measurement:

G60 would store a position relative to the calibration referenced against the fixed endstop. Any move detecting the mobile endstop will cause the axis to re-reference against that position. Any subsequent G2 R would return to a saved position relative to the new calibration, so not the original absolute position in the machine framework.

Under firmware 3.2.2 I did a H3 move.

When the end stop was triggered the movement stopped and the M208 setting for Z min was updated using the Z position at the moment the end stop was triggered.

How does that cause the reference for the axis to change?

Frederick

[Terry]

But did your machine and user position for that axis also change?

[Terry]

This will only be visible if you are testing against an endstop that is different to the one you referenced from in your homing move...

[Terry]

For clarity, I home in the positive direction (but physically downwards) for my axis and reference this as the highest axis distance. I then swap out the high end endstop for the low end endstop and probe in the negative direction (physically upwards) until the switch hits the tool.

This recalibrates the axis position to the target value that was in the move command, losing the information about how far I have actually moved in the original coordinate system.

[Terry]

Ideally, what is needed is a generic probing command that supports motion on any axis. This would be necessary for a 3 degrees of freedom probe mounted in a spindle if you wanted to touch off on a workpiece in X, Y and Z, in any case.

[fcwilt]

@Terry said in Auto Tool Height Measurement:

This will only be visible if you are testing against an endstop that is different to the one you referenced from in your homing move...

Different in what way?

I don't think it is possible to have more than one end stop configured at the same time for a given axis - aside from the situation where you have multiple Z steppers with an end stop for each.

---

I did this:

I moved Z down to 100 and issued G91 G1 H3 Z-150.

I waited a bit and triggered the end stop - it was around 47.

M208 now reported min of 47. I reset Z min with M208 Z0:240 which is correct for my printer.

Moving to 0 and the end stop was just triggered as it should be.

---

This I did this:

I moved Z down to 100 and issued G91 G1 H4 Z-150.

I waited a bit and triggered the end stop - it was around 53.

Moving to 0 and the end stop was just triggered as it should be.

Nothing else appears to have changed.

---

Frederick

[Terry]

Just found G38.2, which appears to be more suitable.

After testing with

M558 K1 P8 C"!1.io2.in" F1000
G38.2 V1 P1

I get the same problem! There appears to be a bug where the machine and user positions are always updated to be equal to the requested destination move, even though the move was interrupted by an endstop or probe trigger.

[fcwilt]

@Terry said in Auto Tool Height Measurement:

Just found G38.2, which appears to be more suitable.

After testing with

M558 K1 P8 C"!1.io2.in" F1000
G38.2 V1 P1

I get the same problem! There appears to be a bug where the machine and user positions are always updated to be equal to the requested destination move, even though the move was interrupted by an endstop or probe trigger.

Perhaps so - because it did not happen in my testing - and the "home" position (which for my testing was 0) remained unchanged.

I know that 3.2.0 had a bug which prevented my 3 Z stepper printer (each stepper had its own end stop) from homing.

Frederick