Bed auto Leveling change: Raise Z before homing

- Added "Z_RAISE_BEFORE_HOMING" for raising Z the defined distance
before homing. This is useful to avoid Z-Probe collision when hotend is
near bed.

- Fixed the issue of Z not going bellow Z_PROBE_OFFSET when
"min_software_endstops" is true.
Now the Z_PROBE_OFFSET is not set in Z_MIN_POS, it is added after
homing.

Marlin/Configuration.h

@@ -309,7 +309,10 @@ const bool Z_MAX_ENDSTOP_INVERTING = true; // set to true to invert the logic of
   #define X_PROBE_OFFSET_FROM_EXTRUDER -25
   #define Y_PROBE_OFFSET_FROM_EXTRUDER -29
   #define Z_PROBE_OFFSET_FROM_EXTRUDER -12.35
-  
+
+  #define Z_RAISE_BEFORE_HOMING 4       // (in mm) Raise Z before homing (G28) for Probe Clearance.
+                                        // Be sure you have this distance over your Z_MAX_POS in case
+    
   #define XY_TRAVEL_SPEED 8000         // X and Y axis travel speed between probes, in mm/min
   
   #define Z_RAISE_BEFORE_PROBING 15    //How much the extruder will be raised before traveling to the first probing point.
@@ -330,12 +333,7 @@ const bool Z_MAX_ENDSTOP_INVERTING = true; // set to true to invert the logic of
 #define Y_MAX_POS 205
 #define Y_MIN_POS 0
 #define Z_MAX_POS 200
-
-#ifndef ENABLE_AUTO_BED_LEVELING
 #define Z_MIN_POS 0
-#else
-#define Z_MIN_POS (-1*Z_PROBE_OFFSET_FROM_EXTRUDER)  //With Auto Bed Leveling, the Z_MIN MUST have the same distance as Z_PROBE
-#endif
 
 #define X_MAX_LENGTH (X_MAX_POS - X_MIN_POS)
 #define Y_MAX_LENGTH (Y_MAX_POS - Y_MIN_POS)

Marlin/Marlin_main.cpp

@@ -920,19 +920,28 @@ static void homeaxis(int axis) {
       axis_home_dir = x_home_dir(active_extruder);
 #endif
 
+    current_position[axis] = 0;
+    plan_set_position(current_position[X_AXIS], current_position[Y_AXIS], current_position[Z_AXIS], current_position[E_AXIS]);
+	
     // Engage Servo endstop if enabled
     #ifdef SERVO_ENDSTOPS
-#if defined (ENABLE_AUTO_BED_LEVELING) && (PROBE_SERVO_DEACTIVATION_DELAY > 0)
-    if (axis==Z_AXIS) engage_z_probe();
-	else
-#endif
+      #if defined (ENABLE_AUTO_BED_LEVELING) && (PROBE_SERVO_DEACTIVATION_DELAY > 0)
+        if (axis==Z_AXIS) {
+          #if defined (Z_RAISE_BEFORE_HOMING) && (Z_RAISE_BEFORE_HOMING > 0)
+            destination[axis] = Z_RAISE_BEFORE_HOMING * axis_home_dir * (-1);    // Set destination away from bed
+            feedrate = max_feedrate[axis];
+            plan_buffer_line(destination[X_AXIS], destination[Y_AXIS], destination[Z_AXIS], destination[E_AXIS], feedrate, active_extruder);
+            st_synchronize();
+          #endif
+          engage_z_probe();
+        }
+	    else
+      #endif
       if (servo_endstops[axis] > -1) {
         servos[servo_endstops[axis]].write(servo_endstop_angles[axis * 2]);
       }
     #endif
 
-    current_position[axis] = 0;
-    plan_set_position(current_position[X_AXIS], current_position[Y_AXIS], current_position[Z_AXIS], current_position[E_AXIS]);
     destination[axis] = 1.5 * max_length(axis) * axis_home_dir;
     feedrate = homing_feedrate[axis];
     plan_buffer_line(destination[X_AXIS], destination[Y_AXIS], destination[Z_AXIS], destination[E_AXIS], feedrate/60, active_extruder);
@@ -1198,6 +1207,9 @@ void process_commands()
           current_position[Z_AXIS]=code_value()+add_homeing[2];
         }
       }
+      #ifdef ENABLE_AUTO_BED_LEVELING
+         current_position[Z_AXIS] -= Z_PROBE_OFFSET_FROM_EXTRUDER;  //Add Z_Probe offset (the distance is negative)
+      #endif
       plan_set_position(current_position[X_AXIS], current_position[Y_AXIS], current_position[Z_AXIS], current_position[E_AXIS]);
 #endif // else DELTA