🚸 Fix extra Z raises

[thinkyhead]

Before homing we don't know where Z is, but on most machines we know it's above 0. And once Z has been raised we can be pretty certain it's at or above current_position.z.

This PR adds a flag to indicate that Z has been powered on and its Z position should be trusted to be at or above current_position.z. If "Z" is disabled on a machine that has a falling toolhead then the flag is cleared when the Z stepper is powered off.

[thijstriemstra]

after this change (I think although my last marlin compile was few weeks ago) I had to define Z_AFTER_HOMING to 0 or otherwise the MPCNC would move to z of 5 after homing.

In Configuration.h the default comment is:

//#define Z_AFTER_HOMING         10   // (mm) Height to move to after homing (if Z was homed)

not sure why my machine moved to 5 and not 10 which seems to be the default value?

[thisiskeithb]

not sure why my machine moved to 5 and not 10 which seems to be the default value?

@thijstriemstra: The reason your Z height after homing was 5 instead of 10 is due to how Marlin handles Z clearance settings:

1. Z_AFTER_HOMING is not defined by default:

   Marlin/Marlin/Configuration.h

   Line 1844 in 9192ae5

   //#define Z_AFTER_HOMING 10 // (mm) Height to move to after homing (if Z was homed)

2. When Z_AFTER_HOMING is not defined, Marlin falls back to Z_CLEARANCE_FOR_HOMING:

   Marlin/Marlin/src/module/motion.h

   Lines 420 to 424 in 9192ae5

   	#ifdef Z_AFTER_HOMING
   	#define Z_POST_CLEARANCE Z_AFTER_HOMING
   	#else
   	#define Z_POST_CLEARANCE Z_CLEARANCE_FOR_HOMING
   	#endif

3. Z_CLEARANCE_FOR_HOMING is also not defined by default:

   Marlin/Marlin/Configuration.h

   Line 1841 in 9192ae5

   //#define Z_CLEARANCE_FOR_HOMING 4 // (mm) Minimal Z height before homing (G28) for Z clearance above the bed, clamps, ...

4. ...so Z_CLEARANCE_FOR_HOMING is defined as Z_CLEARANCE_BETWEEN_PROBES:

   Marlin/Marlin/src/inc/Conditionals-3-etc.h

   Lines 557 to 561 in 9192ae5

   	#ifdef Z_CLEARANCE_BETWEEN_PROBES
   	#define Z_CLEARANCE_FOR_HOMING Z_CLEARANCE_BETWEEN_PROBES
   	#else
   	#define Z_CLEARANCE_FOR_HOMING 5
   	#endif

5. Z_CLEARANCE_BETWEEN_PROBES is defined as 5 by default:

   Marlin/Marlin/Configuration.h

   Line 1721 in 9192ae5

   #define Z_CLEARANCE_BETWEEN_PROBES 5 // (mm) Z Clearance between probe points

Enable & set your Z_AFTER_HOMING value (like you did), otherwise the "safe" default of 5 is used.

The conditionals are a bit spread out, but hopefully that clears things up.

[thijstriemstra]

thanks for your excellent explanation @thisiskeithb. I previously had this defined:

# Z clearance between probe points (mm).
# Set to 0 since we're only using a single probe point,
# and consider the homed Z position to be Z0.
# Without this change, Marlin will use a default clearance
# of 5, and move the Z up by 5 after homing is done, which
# we don't want for this machine.
opt_set Z_CLEARANCE_BETWEEN_PROBES "0"

Which "stopped working" and the Z_AFTER_HOMING setting makes more sense when I look at the machine's behaviour, so looks like a bug was definitely fixed (here).