Improvement - G29 Option for Not Retracting Servo

This change introduces an improvement to G29 command on Marlin.

Auto bed leveling operation's reliability is based on the repeatability of the Z-probe switch and the servo. This change introduces an option to G29 command. When the G29 command is sent with an "e" option, during auto bed levelling the servo is not retracted between probes which decreases the bias on auto bed levelling resulting from servo.

G29 command does the auto bed probing as it is.

G29 E command engages the servo on first probing point, probes all points and retracts the servo after probing the last point.

Please comment your opinions, test on your printer and check the code on a programmer's perspective. (I am not a good programmer.)

Marlin/Marlin_main.cpp

 }
 
 /// Probe bed height at position (x,y), returns the measured z value
-static float probe_pt(float x, float y, float z_before) {
+static float probe_pt(float x, float y, float z_before, int retract_action=0) {
   // move to right place
   do_blocking_move_to(current_position[X_AXIS], current_position[Y_AXIS], z_before);
   do_blocking_move_to(x - X_PROBE_OFFSET_FROM_EXTRUDER, y - Y_PROBE_OFFSET_FROM_EXTRUDER, current_position[Z_AXIS]);
 
 #ifndef Z_PROBE_SLED
-  engage_z_probe();   // Engage Z Servo endstop if available
+   if ((retract_action==0) || (retract_action==1)) 
+     engage_z_probe();   // Engage Z Servo endstop if available
 #endif // Z_PROBE_SLED
   run_z_probe();
   float measured_z = current_position[Z_AXIS];
 #ifndef Z_PROBE_SLED
-  retract_z_probe();
+  if ((retract_action==0) || (retract_action==3)) 
+     retract_z_probe();
 #endif // Z_PROBE_SLED
 
   SERIAL_PROTOCOLPGM(MSG_BED);
                   z_before = current_position[Z_AXIS] + Z_RAISE_BETWEEN_PROBINGS;
                 }
 
-                float measured_z = probe_pt(xProbe, yProbe, z_before);
+                float measured_z;
+                //Enhanced G29 - Do not retract servo between probes
+                if (code_seen('E') || code_seen('e') )
+                   {
+                   if ((yProbe==FRONT_PROBE_BED_POSITION) && (xCount==0))
+                       {
+                        measured_z = probe_pt(xProbe, yProbe, z_before,1);
+                       } else if ((yProbe==BACK_PROBE_BED_POSITION) && (xCount == AUTO_BED_LEVELING_GRID_POINTS-1))
+                         {
+                         measured_z = probe_pt(xProbe, yProbe, z_before,3);
+                         } else {
+                           measured_z = probe_pt(xProbe, yProbe, z_before,2);
+                         }
+                    } else {
+                    measured_z = probe_pt(xProbe, yProbe, z_before);
+                    }
 
                 eqnBVector[probePointCounter] = measured_z;
 
 #else // AUTO_BED_LEVELING_GRID not defined
 
             // Probe at 3 arbitrary points
-            // probe 1
-            float z_at_pt_1 = probe_pt(ABL_PROBE_PT_1_X, ABL_PROBE_PT_1_Y, Z_RAISE_BEFORE_PROBING);
-
-            // probe 2
-            float z_at_pt_2 = probe_pt(ABL_PROBE_PT_2_X, ABL_PROBE_PT_2_Y, current_position[Z_AXIS] + Z_RAISE_BETWEEN_PROBINGS);
-
-            // probe 3
-            float z_at_pt_3 = probe_pt(ABL_PROBE_PT_3_X, ABL_PROBE_PT_3_Y, current_position[Z_AXIS] + Z_RAISE_BETWEEN_PROBINGS);
-
+            // Enhanced G29
+            
+            float z_at_pt_1,z_at_pt_2,z_at_pt_3;
+            
+            if (code_seen('E') || code_seen('e') )
+               {
+               // probe 1               
+                z_at_pt_1 = probe_pt(ABL_PROBE_PT_1_X, ABL_PROBE_PT_1_Y, Z_RAISE_BEFORE_PROBING,1);
+               // probe 2
+                z_at_pt_2 = probe_pt(ABL_PROBE_PT_2_X, ABL_PROBE_PT_2_Y, current_position[Z_AXIS] + Z_RAISE_BETWEEN_PROBINGS,2);
+               // probe 3
+                z_at_pt_3 = probe_pt(ABL_PROBE_PT_3_X, ABL_PROBE_PT_3_Y, current_position[Z_AXIS] + Z_RAISE_BETWEEN_PROBINGS,3); 
+               }
+               else 
+               {
+	        // probe 1
+	        float z_at_pt_1 = probe_pt(ABL_PROBE_PT_1_X, ABL_PROBE_PT_1_Y, Z_RAISE_BEFORE_PROBING);
+
+                // probe 2
+                float z_at_pt_2 = probe_pt(ABL_PROBE_PT_2_X, ABL_PROBE_PT_2_Y, current_position[Z_AXIS] + Z_RAISE_BETWEEN_PROBINGS);
+
+                // probe 3
+                float z_at_pt_3 = probe_pt(ABL_PROBE_PT_3_X, ABL_PROBE_PT_3_Y, current_position[Z_AXIS] + Z_RAISE_BETWEEN_PROBINGS);
+               }
             clean_up_after_endstop_move();
 
             set_bed_level_equation_3pts(z_at_pt_1, z_at_pt_2, z_at_pt_3);