Added servo actuated enstop coding to allow G28 command to engage and retract a servo to specified angles.

Marlin/Configuration.h

 // leaving it undefined or defining as 0 will disable the servo subsystem
 // If unsure, leave commented / disabled
 //
-// #define NUM_SERVOS 3
+//#define NUM_SERVOS 3 // Servo index starts with 0
+
+// Servo Endstops
+// 
+// This allows for servo actuated endstops, primary usage is for the Z Axis to eliminate calibration or bed height changes.
+// Use M206 command to correct for switch height offset to actual nozzle height. Store that setting with M500.
+// 
+//#define SERVO_ENDSTOPS {-1, -1, 0} // Servo index for X, Y, Z. Disable with -1
+//#define SERVO_ENDSTOP_ANGLES {0,0, 0,0, 70,0} // X,Y,Z Axis Extend and Retract angles
 
 #include "Configuration_adv.h"
 #include "thermistortables.h"

Marlin/Marlin_main.cpp

   #if (NUM_SERVOS >= 5)
     #error "TODO: enter initalisation code for more servos"
   #endif
+
+  // Set position of Servo Endstops that are defined
+  #ifdef SERVO_ENDSTOPS
+  for(int8_t i = 0; i < 3; i++)
+  {
+    if(servo_endstops[i] > -1) {
+      servos[servo_endstops[i]].write(servo_endstop_angles[i * 2 + 1]);
+    }
+  }
+  #endif
 }
 
 void setup()
 static void homeaxis(int axis) {
 #define HOMEAXIS_DO(LETTER) \
   ((LETTER##_MIN_PIN > -1 && LETTER##_HOME_DIR==-1) || (LETTER##_MAX_PIN > -1 && LETTER##_HOME_DIR==1))
-
   if (axis==X_AXIS ? HOMEAXIS_DO(X) :
       axis==Y_AXIS ? HOMEAXIS_DO(Y) :
       axis==Z_AXIS ? HOMEAXIS_DO(Z) :
       0) {
+
+    // Engage Servo endstop if enabled
+    #ifdef 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) * home_dir(axis);
     destination[axis] = current_position[axis];
     feedrate = 0.0;
     endstops_hit_on_purpose();
+
+    // Retract Servo endstop if enabled
+    #ifdef SERVO_ENDSTOPS[axis] > -1
+      servos[servo_endstops[axis]].write(servo_endstop_angles[axis * 2 + 1]);
+    #endif
   }
 }
 #define HOMEAXIS(LETTER) homeaxis(LETTER##_AXIS)
     }
   }
   return false;
-}
+}