probe_eddy_current: Rework tap_threshold to use hz/mm

Rework the tap_threshold to use units of hz/mm (instead of hz).  This
will make it easier to implement tap calibration based on results of
PROBE_EDDY_CURRENT_CALIBRATE and previous tap attempts.  It can also
make the tap_threshold more stable with different probe speeds and
sensor rates.

As a result of this change, existing user configurations will need to
be updated.  It is hoped that existing configurations will have a
value that is sufficiently low that they will reliably get an error
until they can recalibrate.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

docs/Config_Changes.md

@@ -8,6 +8,11 @@ All dates in this document are approximate.
 
 ## Changes
 
+20260501: The handling of the `[probe_eddy_current]` `tap_threshold`
+config option and associated `TAP_THRESHOLD` G-Code parameter has
+changed. It will be necessary to recalibrate the value. See the
+[eddy probe documentation](Eddy_Probe.md) for calibration directions.
+
 20260408: The script `lib/canboot/flash_can.py` has been updated to
 the most current version from
 [Katapult](https://github.com/Arksine/katapult) and as such renamed to

docs/Config_Reference.md

@@ -2350,15 +2350,13 @@ sensor_type: ldc1612
 #   settings do not have an effect if using a probe "METHOD" of
 #   "scan", "rapid_scan", or "tap".
 #tap_threshold:
-#   Noise cutoff/stop trigger threshold (in Hz). Specify this value to
-#   enable support for "METHOD=tap" probe commands. See Eddy_Probe.md
-#   for more information. Larger values make the tap detection less
-#   sensitive. That is, larger values make it less likely the toolhead
-#   will incorrectly stop early due to noise, while increasing the
-#   risk of the toolhead not correctly stopping when it first contacts
-#   the bed. If this value is specified then one may override its
-#   value at run-time using the "TAP_THRESHOLD" parameter on probe
-#   commands. The default is to not enable support for "tap" probing.
+#   Descent stop threshold (in Hz/mm) for "tap" probing. Larger values
+#   reduce the chance of the toolhead incorrectly stopping early due
+#   to noise, while increasing the risk of the toolhead not correctly
+#   stopping when it first contacts the bed. See Eddy_Probe.md for
+#   more information. This value may be overridden at run-time using
+#   the "TAP_THRESHOLD" parameter on probe commands.  The default is
+#   to not enable "tap" probing.
 #tap_z_offset: 0.0
 #   The Z height (in mm) of the nozzle relative to the bed at the
 #   contact point detected during "tap" probing. Nominally this would

docs/Eddy_Probe.md

@@ -137,8 +137,8 @@ PROBE METHOD=tap
 
 Here is an example sequence of threshold values to test:
 ```
-1 -> 5 -> 10 -> 20 -> 40 -> 80 -> 160
-160 -> 120 -> 100
+50 -> 250 -> 500 -> 1000 -> 2000 -> 4000 -> 8000
+8000 -> 6000 -> 5000
 ```
 Your value will normally be between those.
 - Too high a value leaves a less safe margin for early collision -
@@ -156,8 +156,8 @@ z: 1.010 # noise 0.000400mm, MAD_Hz=14.000
 ```
 The estimation will be:
 ```
-MAD_Hz * 2
-11.314 * 2 = 22.628
+MAD_Hz * 100
+11.314 * 100 = 113.14
 ```
 
 To further fine tune threshold, one can use `PROBE_ACCURACY METHOD=tap`.
@@ -173,8 +173,7 @@ the speed of the descent.
 If you take 24 photos per second of the moving train, you can only estimate
 where the train was between photos.
 
-It is possible to reduce the descending speed. It may require decrease of
-absolute `tap_threshold` value.
+It is possible to reduce the descending speed.
 
 It is possible to tap over non-conductive surfaces as long as there is metal
 behind it within the sensor's sensitivity range.

klippy/extras/probe_eddy_current.py

@@ -529,6 +529,7 @@ class EddyTap:
                                        self._tap_threshold, above=0.)
         if not tap_threshold:
             raise self._printer.command_error("Tap not configured")
+        params = self._param_helper.get_probe_params(gcmd)
         # Setup mcu filter (scale internal values to milli-hz)
         sos_filter = self._trigger_analog.get_sos_filter()
         sos_filter.set_filter_design(self._filter_design)
@@ -537,7 +538,9 @@ class EddyTap:
         sos_filter.set_offset_scale(0, s, auto_offset=True)
         self._trigger_analog.set_raw_range(0, MAX_VALID_RAW_VALUE)
         # Set mcu trigger to tap_threshold
-        samp_thresh = int(FRAC_HZ * tap_threshold + 0.5)
+        sps = self._sensor_helper.get_samples_per_second()
+        adj_thresh = tap_threshold * params['probe_speed'] / sps
+        samp_thresh = int(FRAC_HZ * adj_thresh + 0.5)
         self._trigger_analog.set_trigger('diff_peak_gt', samp_thresh)
         self._current_tap_threshold = tap_threshold
     # Measurement analysis to determine "tap" position
@@ -703,7 +706,7 @@ class EddyTap:
         return final_coeffs
     def _error_detect(self, msg):
         raise self._printer.command_error("Unable to detect tap: %s" % (msg,))
-    def _analyze_pullback(self, measures, start_time, end_time, speed):
+    def _analyze_pullback(self, measures, start_time, end_time):
         reactor = self._printer.get_reactor()
         self._validate_samples_time(measures, start_time, end_time)
         # Correlate measurements to toolhead position at time of measurement
@@ -720,10 +723,10 @@ class EddyTap:
         z_contact, freq_contact, depress_slope, slope, slope2 = coeffs
         reactor.pause(0.)
         sps = self._sensor_helper.get_samples_per_second()
-        contact_slope_delta_per_sample = (depress_slope - slope) * speed / sps
-        if contact_slope_delta_per_sample < self._current_tap_threshold:
+        contact_slope_delta = depress_slope - slope
+        if contact_slope_delta < self._current_tap_threshold:
             self._error_detect("insufficient slope delta (%.6f vs %.6f)"
-                               % (contact_slope_delta_per_sample,
+                               % (contact_slope_delta,
                                   self._current_tap_threshold))
         if slope >= 0. or slope2 < 0.:
             self._error_detect("invalid free air slope (s=%.6f s2=%.6f)"
@@ -764,7 +767,7 @@ class EddyTap:
         start_time = retract_start_time - 0.010
         end_time = retract_start_time + 0.150
         self._gather.add_probe_request(self._analyze_pullback, start_time,
-                                       end_time, start_time, end_time, speed)
+                                       end_time, start_time, end_time)
     def pull_probed_results(self):
         return self._gather.pull_probed()
     def end_probe_session(self):

test/klippy/eddy.cfg

@@ -63,7 +63,7 @@ y_offset: -4
 sensor_type: ldc1612
 speed: 10.0
 intb_pin: PK7
-tap_threshold: 30
+tap_threshold: 1500
 
 [bed_mesh]
 mesh_min: 10,10