double bump probing as a feature

Why double touch probing is not a good thing.

It's widely believed we can get better __probing__ results when using a double touch when probing.

Let's compare to double touch __homing__.
Or better let's begin with single touch __homing__.
We home to find out out position, so our position is unknown.
To find the endstop we have to move into the direction of the endstop.
The maximum way we have to move is a bit longer than the axis length.
When we arrive at the endstop - when it triggers, the stepper pulses are stopped immediately.
It's a sudden stop. No smooth deacceleration is possible.
Depending on the speed and the moving mass we lose steps here.
Only if we approached slow enough (below jerk speed?) we will not lose steps.

Moving a complete axis length, that slow, takes for ever.
To speed up homing, we now make the first approach faster, get a guess about our position,
back up a bit and make a second slower approach to get a exact result without losing steps.

What we do in double touch probing is the same. But the difference here is:
a. we already know where we are
b. if the first approach is to fast we will lose steps here to.
But this time there is no second approach to set the position to 0. We are measuring only.
The lost steps are permanent until we home the next time.

So if you experienced permanently rising values in M48 you now know why. (Too fast, suddenly stopped, first approach)

What can we do to improve probing?
We can use the information about our current position.
We can make a really fast, but deaccelerated, move to a place we know it is a bit before the trigger point.
And then move the rest of the way really slow.

Marlin/Configuration.h

 
 // X and Y axis travel speed (mm/m) between probes
 #define XY_PROBE_SPEED 8000
-// Speed for the first approach when probing
+// Speed for the first approach when double-probing (with PROBE_DOUBLE_TOUCH)
 #define Z_PROBE_SPEED_FAST HOMING_FEEDRATE_Z
-// Speed for the second approach when probing
+// Speed for the "accurate" probe of each point
 #define Z_PROBE_SPEED_SLOW (Z_PROBE_SPEED_FAST / 2)
+// Use double touch for probing
+//#define PROBE_DOUBLE_TOUCH
 
 //
 // Allen Key Probe is defined in the Delta example configurations.

Marlin/Marlin_main.cpp

       planner.bed_level_matrix.set_to_identity();
     #endif
 
-    do_blocking_move_to_z(-(Z_MAX_LENGTH + 10), Z_PROBE_SPEED_FAST);
-    endstops.hit_on_purpose();
-    set_current_from_steppers_for_axis(Z_AXIS);
-    SYNC_PLAN_POSITION_KINEMATIC();
+    #if ENABLED(PROBE_DOUBLE_TOUCH)
+      do_blocking_move_to_z(-(Z_MAX_LENGTH + 10), Z_PROBE_SPEED_FAST);
+      endstops.hit_on_purpose();
+      set_current_from_steppers_for_axis(Z_AXIS);
+      SYNC_PLAN_POSITION_KINEMATIC();
 
-    // move up the retract distance
-    do_blocking_move_to_z(current_position[Z_AXIS] + home_bump_mm(Z_AXIS), Z_PROBE_SPEED_FAST);
+      // move up the retract distance
+      do_blocking_move_to_z(current_position[Z_AXIS] + home_bump_mm(Z_AXIS), Z_PROBE_SPEED_FAST);
+    #else
+      // move fast, close to the bed
+      do_blocking_move_to_z(home_bump_mm(Z_AXIS), Z_PROBE_SPEED_FAST);
+    #endif
 
-    // move back down slowly to find bed
-    do_blocking_move_to_z(current_position[Z_AXIS] - home_bump_mm(Z_AXIS) * 2, Z_PROBE_SPEED_SLOW);
+    // move down slowly to find bed
+    do_blocking_move_to_z(current_position[Z_AXIS] -2.0*home_bump_mm(Z_AXIS), Z_PROBE_SPEED_SLOW);
     endstops.hit_on_purpose();
     set_current_from_steppers_for_axis(Z_AXIS);
     SYNC_PLAN_POSITION_KINEMATIC();

Marlin/example_configurations/Cartesio/Configuration.h

 
 // X and Y axis travel speed (mm/m) between probes
 #define XY_PROBE_SPEED 8000
-// Speed for the first approach when probing
+// Speed for the first approach when double-probing (with PROBE_DOUBLE_TOUCH)
 #define Z_PROBE_SPEED_FAST HOMING_FEEDRATE_Z
-// Speed for the second approach when probing
+// Speed for the "accurate" probe of each point
 #define Z_PROBE_SPEED_SLOW (Z_PROBE_SPEED_FAST / 2)
+// Use double touch for probing
+//#define PROBE_DOUBLE_TOUCH
 
 //
 // Allen Key Probe is defined in the Delta example configurations.

Marlin/example_configurations/Felix/Configuration.h

 
 // X and Y axis travel speed (mm/m) between probes
 #define XY_PROBE_SPEED 8000
-// Speed for the first approach when probing
+// Speed for the first approach when double-probing (with PROBE_DOUBLE_TOUCH)
 #define Z_PROBE_SPEED_FAST HOMING_FEEDRATE_Z
-// Speed for the second approach when probing
+// Speed for the "accurate" probe of each point
 #define Z_PROBE_SPEED_SLOW (Z_PROBE_SPEED_FAST / 2)
+// Use double touch for probing
+//#define PROBE_DOUBLE_TOUCH
 
 //
 // Allen Key Probe is defined in the Delta example configurations.

Marlin/example_configurations/Felix/DUAL/Configuration.h

 
 // X and Y axis travel speed (mm/m) between probes
 #define XY_PROBE_SPEED 8000
-// Speed for the first approach when probing
+// Speed for the first approach when double-probing (with PROBE_DOUBLE_TOUCH)
 #define Z_PROBE_SPEED_FAST HOMING_FEEDRATE_Z
-// Speed for the second approach when probing
+// Speed for the "accurate" probe of each point
 #define Z_PROBE_SPEED_SLOW (Z_PROBE_SPEED_FAST / 2)
+// Use double touch for probing
+//#define PROBE_DOUBLE_TOUCH
 
 //
 // Allen Key Probe is defined in the Delta example configurations.

Marlin/example_configurations/Hephestos/Configuration.h

 
 // X and Y axis travel speed (mm/m) between probes
 #define XY_PROBE_SPEED 8000
-// Speed for the first approach when probing
+// Speed for the first approach when double-probing (with PROBE_DOUBLE_TOUCH)
 #define Z_PROBE_SPEED_FAST HOMING_FEEDRATE_Z
-// Speed for the second approach when probing
+// Speed for the "accurate" probe of each point
 #define Z_PROBE_SPEED_SLOW (Z_PROBE_SPEED_FAST / 2)
+// Use double touch for probing
+//#define PROBE_DOUBLE_TOUCH
 
 //
 // Allen Key Probe is defined in the Delta example configurations.

Marlin/example_configurations/Hephestos_2/Configuration.h

 
 // X and Y axis travel speed (mm/m) between probes
 #define XY_PROBE_SPEED 8000
-// Speed for the first approach when probing
+// Speed for the first approach when double-probing (with PROBE_DOUBLE_TOUCH)
 #define Z_PROBE_SPEED_FAST HOMING_FEEDRATE_Z
-// Speed for the second approach when probing
+// Speed for the "accurate" probe of each point
 #define Z_PROBE_SPEED_SLOW (Z_PROBE_SPEED_FAST / 2)
+// Use double touch for probing
+//#define PROBE_DOUBLE_TOUCH
 
 //
 // Allen Key Probe is defined in the Delta example configurations.

Marlin/example_configurations/K8200/Configuration.h

 
 // X and Y axis travel speed (mm/m) between probes
 #define XY_PROBE_SPEED 8000
-// Speed for the first approach when probing
+// Speed for the first approach when double-probing (with PROBE_DOUBLE_TOUCH)
 #define Z_PROBE_SPEED_FAST HOMING_FEEDRATE_Z
-// Speed for the second approach when probing
+// Speed for the "accurate" probe of each point
 #define Z_PROBE_SPEED_SLOW (Z_PROBE_SPEED_FAST / 2)
+// Use double touch for probing
+//#define PROBE_DOUBLE_TOUCH
 
 //
 // Allen Key Probe is defined in the Delta example configurations.

Marlin/example_configurations/K8400/Configuration.h

 
 // X and Y axis travel speed (mm/m) between probes
 #define XY_PROBE_SPEED 8000
-// Speed for the first approach when probing
+// Speed for the first approach when double-probing (with PROBE_DOUBLE_TOUCH)
 #define Z_PROBE_SPEED_FAST HOMING_FEEDRATE_Z
-// Speed for the second approach when probing
+// Speed for the "accurate" probe of each point
 #define Z_PROBE_SPEED_SLOW (Z_PROBE_SPEED_FAST / 2)
+// Use double touch for probing
+//#define PROBE_DOUBLE_TOUCH
 
 //
 // Allen Key Probe is defined in the Delta example configurations.

Marlin/example_configurations/K8400/Dual-head/Configuration.h

 
 // X and Y axis travel speed (mm/m) between probes
 #define XY_PROBE_SPEED 8000
-// Speed for the first approach when probing
+// Speed for the first approach when double-probing (with PROBE_DOUBLE_TOUCH)
 #define Z_PROBE_SPEED_FAST HOMING_FEEDRATE_Z
-// Speed for the second approach when probing
+// Speed for the "accurate" probe of each point
 #define Z_PROBE_SPEED_SLOW (Z_PROBE_SPEED_FAST / 2)
+// Use double touch for probing
+//#define PROBE_DOUBLE_TOUCH
 
 //
 // Allen Key Probe is defined in the Delta example configurations.

Marlin/example_configurations/RepRapWorld/Megatronics/Configuration.h

 
 // X and Y axis travel speed (mm/m) between probes
 #define XY_PROBE_SPEED 8000
-// Speed for the first approach when probing
+// Speed for the first approach when double-probing (with PROBE_DOUBLE_TOUCH)
 #define Z_PROBE_SPEED_FAST HOMING_FEEDRATE_Z
-// Speed for the second approach when probing
+// Speed for the "accurate" probe of each point
 #define Z_PROBE_SPEED_SLOW (Z_PROBE_SPEED_FAST / 2)
+// Use double touch for probing
+//#define PROBE_DOUBLE_TOUCH
 
 //
 // Allen Key Probe is defined in the Delta example configurations.

Marlin/example_configurations/RigidBot/Configuration.h

 
 // X and Y axis travel speed (mm/m) between probes
 #define XY_PROBE_SPEED 8000
-// Speed for the first approach when probing
+// Speed for the first approach when double-probing (with PROBE_DOUBLE_TOUCH)
 #define Z_PROBE_SPEED_FAST HOMING_FEEDRATE_Z
-// Speed for the second approach when probing
+// Speed for the "accurate" probe of each point
 #define Z_PROBE_SPEED_SLOW (Z_PROBE_SPEED_FAST / 2)
+// Use double touch for probing
+//#define PROBE_DOUBLE_TOUCH
 
 //
 // Allen Key Probe is defined in the Delta example configurations.

Marlin/example_configurations/SCARA/Configuration.h

 
 // X and Y axis travel speed (mm/m) between probes
 #define XY_PROBE_SPEED 8000
-// Speed for the first approach when probing
+// Speed for the first approach when double-probing (with PROBE_DOUBLE_TOUCH)
 #define Z_PROBE_SPEED_FAST HOMING_FEEDRATE_Z
-// Speed for the second approach when probing
+// Speed for the "accurate" probe of each point
 #define Z_PROBE_SPEED_SLOW (Z_PROBE_SPEED_FAST / 2)
+// Use double touch for probing
+//#define PROBE_DOUBLE_TOUCH
 
 //
 // Allen Key Probe is defined in the Delta example configurations.

Marlin/example_configurations/TAZ4/Configuration.h

 
 // X and Y axis travel speed (mm/m) between probes
 #define XY_PROBE_SPEED 8000
-// Speed for the first approach when probing
+// Speed for the first approach when double-probing (with PROBE_DOUBLE_TOUCH)
 #define Z_PROBE_SPEED_FAST HOMING_FEEDRATE_Z
-// Speed for the second approach when probing
+// Speed for the "accurate" probe of each point
 #define Z_PROBE_SPEED_SLOW (Z_PROBE_SPEED_FAST / 2)
+// Use double touch for probing
+//#define PROBE_DOUBLE_TOUCH
 
 //
 // Allen Key Probe is defined in the Delta example configurations.

Marlin/example_configurations/WITBOX/Configuration.h

 
 // X and Y axis travel speed (mm/m) between probes
 #define XY_PROBE_SPEED 8000
-// Speed for the first approach when probing
+// Speed for the first approach when double-probing (with PROBE_DOUBLE_TOUCH)
 #define Z_PROBE_SPEED_FAST HOMING_FEEDRATE_Z
-// Speed for the second approach when probing
+// Speed for the "accurate" probe of each point
 #define Z_PROBE_SPEED_SLOW (Z_PROBE_SPEED_FAST / 2)
+// Use double touch for probing
+//#define PROBE_DOUBLE_TOUCH
 
 //
 // Allen Key Probe is defined in the Delta example configurations.

Marlin/example_configurations/adafruit/ST7565/Configuration.h

 
 // X and Y axis travel speed (mm/m) between probes
 #define XY_PROBE_SPEED 8000
-// Speed for the first approach when probing
+// Speed for the first approach when double-probing (with PROBE_DOUBLE_TOUCH)
 #define Z_PROBE_SPEED_FAST HOMING_FEEDRATE_Z
-// Speed for the second approach when probing
+// Speed for the "accurate" probe of each point
 #define Z_PROBE_SPEED_SLOW (Z_PROBE_SPEED_FAST / 2)
+// Use double touch for probing
+//#define PROBE_DOUBLE_TOUCH
 
 //
 // Allen Key Probe is defined in the Delta example configurations.

Marlin/example_configurations/delta/biv2.5/Configuration.h

 
 // X and Y axis travel speed (mm/m) between probes
 #define XY_PROBE_SPEED 4000
-// Speed for the first approach when probing
+// Speed for the first approach when double-probing (with PROBE_DOUBLE_TOUCH)
 #define Z_PROBE_SPEED_FAST HOMING_FEEDRATE_Z
-// Speed for the second approach when probing
+// Speed for the "accurate" probe of each point
 #define Z_PROBE_SPEED_SLOW (Z_PROBE_SPEED_FAST / 2)
+// Use double touch for probing
+//#define PROBE_DOUBLE_TOUCH
 
 // Allen key retractable z-probe as seen on many Kossel delta printers - http://reprap.org/wiki/Kossel#Automatic_bed_leveling_probe
 // Deploys by touching z-axis belt. Retracts by pushing the probe down. Uses Z_MIN_PIN.

Marlin/example_configurations/delta/generic/Configuration.h

 
 // X and Y axis travel speed (mm/m) between probes
 #define XY_PROBE_SPEED 4000
-// Speed for the first approach when probing
+// Speed for the first approach when double-probing (with PROBE_DOUBLE_TOUCH)
 #define Z_PROBE_SPEED_FAST HOMING_FEEDRATE_Z
-// Speed for the second approach when probing
+// Speed for the "accurate" probe of each point
 #define Z_PROBE_SPEED_SLOW (Z_PROBE_SPEED_FAST / 2)
+// Use double touch for probing
+//#define PROBE_DOUBLE_TOUCH
 
 // Allen key retractable z-probe as seen on many Kossel delta printers - http://reprap.org/wiki/Kossel#Automatic_bed_leveling_probe
 // Deploys by touching z-axis belt. Retracts by pushing the probe down. Uses Z_MIN_PIN.

Marlin/example_configurations/delta/kossel_mini/Configuration.h

 
 // X and Y axis travel speed (mm/m) between probes
 #define XY_PROBE_SPEED 4000
-// Speed for the first approach when probing
+// Speed for the first approach when double-probing (with PROBE_DOUBLE_TOUCH)
 #define Z_PROBE_SPEED_FAST HOMING_FEEDRATE_Z
-// Speed for the second approach when probing
+// Speed for the "accurate" probe of each point
 #define Z_PROBE_SPEED_SLOW (Z_PROBE_SPEED_FAST / 2)
+// Use double touch for probing
+//#define PROBE_DOUBLE_TOUCH
 
 // Allen key retractable z-probe as seen on many Kossel delta printers - http://reprap.org/wiki/Kossel#Automatic_bed_leveling_probe
 // Deploys by touching z-axis belt. Retracts by pushing the probe down. Uses Z_MIN_PIN.

Marlin/example_configurations/delta/kossel_pro/Configuration.h

 
 // X and Y axis travel speed (mm/m) between probes
 #define XY_PROBE_SPEED 8000
-// Speed for the first approach when probing
+// Speed for the first approach when double-probing (with PROBE_DOUBLE_TOUCH)
 #define Z_PROBE_SPEED_FAST HOMING_FEEDRATE_Z
-// Speed for the second approach when probing
+// Speed for the "accurate" probe of each point
 #define Z_PROBE_SPEED_SLOW (Z_PROBE_SPEED_FAST / 2)
+// Use double touch for probing
+//#define PROBE_DOUBLE_TOUCH
 
 // Allen key retractable z-probe as seen on many Kossel delta printers - http://reprap.org/wiki/Kossel#Automatic_bed_leveling_probe
 // Deploys by touching z-axis belt. Retracts by pushing the probe down. Uses Z_MIN_PIN.

Marlin/example_configurations/delta/kossel_xl/Configuration.h

 
 // X and Y axis travel speed (mm/m) between probes
 #define XY_PROBE_SPEED 8000
-// Speed for the first approach when probing
+// Speed for the first approach when double-probing (with PROBE_DOUBLE_TOUCH)
 #define Z_PROBE_SPEED_FAST HOMING_FEEDRATE_Z
-// Speed for the second approach when probing
+// Speed for the "accurate" probe of each point
 #define Z_PROBE_SPEED_SLOW (Z_PROBE_SPEED_FAST / 2)
+// Use double touch for probing
+//#define PROBE_DOUBLE_TOUCH
 
 // Allen key retractable z-probe as seen on many Kossel delta printers - http://reprap.org/wiki/Kossel#Automatic_bed_leveling_probe
 // Deploys by touching z-axis belt. Retracts by pushing the probe down. Uses Z_MIN_PIN.

Marlin/example_configurations/makibox/Configuration.h

 
 // X and Y axis travel speed (mm/m) between probes
 #define XY_PROBE_SPEED 8000
-// Speed for the first approach when probing
+// Speed for the first approach when double-probing (with PROBE_DOUBLE_TOUCH)
 #define Z_PROBE_SPEED_FAST HOMING_FEEDRATE_Z
-// Speed for the second approach when probing
+// Speed for the "accurate" probe of each point
 #define Z_PROBE_SPEED_SLOW (Z_PROBE_SPEED_FAST / 2)
+// Use double touch for probing
+//#define PROBE_DOUBLE_TOUCH
 
 //
 // Allen Key Probe is defined in the Delta example configurations.

Marlin/example_configurations/tvrrug/Round2/Configuration.h

 
 // X and Y axis travel speed (mm/m) between probes
 #define XY_PROBE_SPEED 8000
-// Speed for the first approach when probing
+// Speed for the first approach when double-probing (with PROBE_DOUBLE_TOUCH)
 #define Z_PROBE_SPEED_FAST HOMING_FEEDRATE_Z
-// Speed for the second approach when probing
+// Speed for the "accurate" probe of each point
 #define Z_PROBE_SPEED_SLOW (Z_PROBE_SPEED_FAST / 2)
+// Use double touch for probing
+//#define PROBE_DOUBLE_TOUCH
 
 //
 // Allen Key Probe is defined in the Delta example configurations.