Z sometimes homes in wrong direction

[p8blr]

Sometimes when I turn on my printer z starts moving down instead of up towards the nozzle and I have no idea why.

Duet 6XD
(6) 3HC
(2) 1XD
Sammy
Raspberry Pi 5 in SBC mode

Running 3.5.4 (although this error has persisted on other fw versions)

(7) Teknic Clearpath servos

; homez.g
; called to home the Z axis

if sensors.analog[0].lastReading < 155 || sensors.analog[1].lastReading < 95 ; if bed and chamber aren't hot enough
	M291 P"Probing when bed and chamber are below target temps may result in innacurate Z offset. Press OK to continue anyway." R"Low Temperature Warning" S3 T0

M18 Z	; disable in case teknic motors try to recover
G4 S1
M17 Z	; droftarts suggestion to enable then wait 1 second
G4 S1

M98 P"prepare.g"	; does everything necessary to enable motion and duet reset
if {global.errorFlag}
	M99 ; exit current file

M98 P"soundNotifyShort.g"
G4 S1	; not sure if necessary but used to work with this wait

M402						; retract probe
M98 P"uvHomeCond.g"			; home U and V if they aren't homed
G1 U0 V0 F999999			; retract tools

M98 P"condHomeXY.g"			; home X and Y if they aren't homed
if {global.errorFlag}
	M99 ; exit current file

if move.axes[2].machinePosition <= 10 	; ensure nozzle is at least 10mm from bed
	G91                					; relative positioning
	G1 H2 Z10 F400     					; lower Z relative to current position
	G90									; absolute posistioning
	
; test for servo error
M17 Z						; enable servos - this wakes up the servos so they are ready to accept commands
G4 S1						; wait - sending commands too early can cause missed steps and a servo error
M98 P"servoErrorZ.g"		; check for servo error
if {global.errorFlag}		; bail if errors
	M99 ; exit current file
G91							; relative positioning
G1 H1 Z-2 F500				; try moving z just a little bit upwards
M98 P"soundNotifyShort.g"	; beep
G4 S1						; wait - sending commands too early can cause missed steps and a servo error
M98 P"servoErrorZ.g"		; check for servo error
if {global.errorFlag}		; bail if errors
	M99 ; exit current file
G1 H2 Z2 F500				; move in positive z now that we know it's safe
M98 P"soundNotifyShort.g"	; beep
G4 S1						; wait - sending commands too early can cause missed steps and a servo error
M98 P"servoErrorZ.g"		; check for servo error
if {global.errorFlag}		; bail if errors
	M99 ; exit current file

; probe z
G90							; absolute posistioning
G1 X525.9 Y507.8 F20000		; go to middle of bed (was X625.9 Y607.8)
M106 P2 S255 				; turn on fans to clean bed
M106 P3 S255
G4 S0.1						; wait
G30							; probe z
M98 P"blinkWhiteFast.g"
M98 P"servoErrorZ.g"		; check for servo error
M201 Z1000					; default accel
M106 P2 S0 					; turn off fans
M106 P3 S0

M98 P"soundAlertShort.g"
G4 S1	; not sure if necessary but used to work with this wait
set global.errorFlag = false

config.g
driveSetup.g
variables.g

homez.g

homez.g calls the following:

airEstopObserve.g
blinkWhiteFast.g
condHomeXY.g

doorLock.g
duetResetEnable.g
homex.g
homey.g
prepare.g
servoErrorAll.g
servoErrorZ.g
soundAlertShort.g
soundNotifyShort.g
uvhome.g
uvHomeCond.g
vacVerify.g

[dc42]

How far does it move down? You have this in homez.g:

if move.axes[2].machinePosition <= 10 	; ensure nozzle is at least 10mm from bed
	G91                					; relative positioning
	G1 H2 Z10 F400     					; lower Z relative to current position
	G90									; absolute posistioning

so it will move down if it thinks the bed is too close to the nozzle.

[p8blr]

@dc42 I notice the issue tends to happen when it's at Z max, and I have some overtravel switches there that will estop the printer if there's an overtravel so I never get to see how far it moves. If the axis isn't homed, what is the value of that variable? I think this is a good point, I will look into it. Thank you.

[dc42]

@p8blr if the axis is flagged as not homed then you can't assume anything about the value of that variable. If the machine has just been powered up or rebooted then it's quite likely to have the value zero. If the axis became un-homed because you disabled the Z motor then it could be anything.

One option is to reduce the motor current while you make that move, so that if it tries to go drop the bed lower than the minimum stop, it won't do any damage. Or you could configure that overtravel switch as the axis endstop temporarily, then use H1 instead of H2 for that move.