Home Z leaves nozzle 5mm above bed!

I have an Ormerod 1 (first edition from RS) which I have recently upgraded to use firmware version 1.23 and using the Mini Height Sensor from David Crocker's MiscSolutions (bought from E3D)
I followed the setting up instructions described in [miscsolutions.wordpress.com] which went well.
The results copied into congig.g were:
G31 Z1.805 P500
When I execute Homez.g (listing below), after psitioning the nozzle to X=100, Y=100, the nozzle moves down to about 2mm from the bed, then moves up by about 5mm and it is this position which is being set t Z=0 (i.e 5 - 7mm above the bed
The probe type is set to M558 P1.

; homez.g
G91               ; set movement to relative mode
G1 Z5 F200        ; move Z up 5mm
G90               ; set movement to absolute mode
G1 X100 Y100 F2000   ; move X and Y to probe point. ADJUST X and Y values to get Z probe over target.
;G30 X75 Y0 Z-99999
G30               ; home Z, using values from G31 in config.g
G92 Z0            ; set this position as Z=0

config.g
; Configuration file for RepRap Ormerod 1
; RepRapPro Ltd
;
; Copy this file to config.g if you have an Ormerod 1
; If you are updating a config.g that you already have you
; may wish to go through it and this file checking what you
; want to keep from your old file.
; 
; For G-code definitions, see [reprap.org]
; 
M111 S0                             ; Debug off
M550 P3D Printer                    ; Machine name (can be anything you like). With DHCP enabled connect to (example) [reprapproormerod1] (machine name with no spaces).
M551 Preprap                        ; Machine password (currently not used)
M540 P0xBE:0xEF:0xDE:0xAD:0xFE:0x14 ; MAC Address

;M552 P0.0.0.0                       ; Un-comment for DHCP
M552 P192.168.1.14                  ; IP address, comment for DHCP
M553 P255.255.255.0                 ; Netmask
M554 P192.168.1.1                   ; Gateway, comment for DHCP



M555 P2                             ; Set output to look like Marlin
G21                                 ; Work in millimetres
G90                                 ; Send absolute corrdinates...
M83                                 ; ...but relative extruder moves

; Axis and motor configuration
M569 P0 S1       ; Drive 0 goes forwards
M569 P1 S1       ; Drive 1 goes forwards
M569 P2 S1       ; Drive 2 goes forwards
M569 P3 S1       ; Drive 3 goes forwards
M569 P4 S1       ; Drive 4 goes forwards
M574 Y2 S1       ; set endstop for Y axis to high end Active High (PS 1/2/2020)
M574 X1 Z1 S2    ; Set endstop for X and Z axis to low end Z probe (PS 1/2/2020)

M208 X220 Y230 Z180 ; set axis maxima
M208 X-10 Y-10 Z-3 S1 ; set axis minima
M564 H0 ; Allow axes to move before they're homed

M906 X800 Y1000 Z800 E800           ; Set motor currents (mA)

M305 P0 T10000 B3988 R1000 H0 L0 ; Bed thermistor resistance (PS 1/2/2020)
M305 P1 T100000 B4138 C0 R1000 H0 L0 ;  Nozzle Thermistor series resistance (PS 1/2/2020) 
M307 H0 A96.8 C833.2 D3.7 S1.00 V0.0 B1 ; From auto calibration by PS after sending M303 H0 S60  13/04/2020
M307 H1 280.3 C54.0 D3.1 S1.00 V0.0 B0 ; From auto calibration by PS after sending M303 H1 S210 13/04/2020

M92 E408                            ; Set extruder steps per mm (PS 5/5/2020
M558 P1                             ; Use a modulated Z probe (change to P2 if you update to the modulated probe)
G31 Z1.805 P500                       ; Set the probe height and threshold (PS 12/5/2020)
M556 S75 X0 Y0 Z0                   ; Put your axis compensation here
M201 X500 Y500 Z15 E500             ; Accelerations (mm/s^2)
M203 X15000 Y15000 Z100 E3600       ; Maximum speeds (mm/min)
M566 X200 Y200 Z30 E20              ; Minimum speeds mm/minute
M563 P0 D0 H1                       ; Define tool 0
G10 P0 S-273 R-273                  ; Set tool 0 operating and standby temperatures
;M563 P1 D1 H2                          ; Define tool 1 Uncomment if you have a dual colour upgrade
;G10 P1 X19 S-273 R-273                ; Set tool 1 operating and standby temperatures  Uncomment if you have a dual colour upgrade

M84 S30 ;powers off motors after 30 seconds of inactivity
M143 H1 S230 ; (PS 19/02/2020)
M557 X50:200 Y0:200 S25  ;Define the mesh for mapping the bed
; M501 ; set active parameters to those stored in config-override.g

Can anyone help please!

Uk Folks. I think I've found the problem.
I misunderstood what exactly the G30 cmmand was doing. I had assumed that this command actually moved the head to Z=0 (i.e. touching the bed).
What it appaently actually does is to move the head down until the reading from the Z probe reaches P value (500 in this case) then records that position as the Z value (1.805mm in this case). The head is then moved up by 5mm to clear any obstacles on the bed.
The presence of the G92 Z0 in my homez.g file was wrong in that it was setting the position after leaving the G30 command (i.e. the trigger height + 5mm) as Z=0!
After removing the G90 Z0 command it now works as it should, and G1 Z0 moves the head to just touching the bed.

Problem solved.

Quote
PeterSimpson
The presence of the G92 Z0 in my homez.g file was wrong in that it was setting the position after leaving the G30 command (i.e. the trigger height + 5mm) as Z=0!
After removing the G90 Z0 command it now works as it should, and G1 Z0 moves the head to just touching the bed.

That's exactly right. I'm glad you sorted it.

---

Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].