docs: add initial temperature_probe documentation

Signed-off-by:  Eric Callahan <arksine.code@gmail.com>

docs/Eddy_Probe.md

@@ -54,3 +54,91 @@ result in changes in reported Z height. Changes in either the bed
 surface temperature or sensor hardware temperature can skew the
 results. It is important that calibration and probing is only done
 when the printer is at a stable temperature.
+
+## Thermal Drift Calibration
+
+As with all inductive probes, eddy current probes are subject to
+significant thermal drift.  If the eddy probe has a temperature
+sensor on the coil it is possible to configure a `[temperature_probe]`
+to report coil temperature and enable software drift compensation. To
+link a temperature probe to an eddy current probe the
+`[temperature_probe]` section must share a name with the
+`[probe_eddy_current]` section.  For example:
+
+```
+[probe_eddy_current my_probe]
+# eddy probe configuration...
+
+[temperature_probe my_probe]
+# temperature probe configuration...
+```
+
+See the [configuration reference](Config_Reference.md#temperature_probe)
+for further details on how to configure a `temperature_probe`.  It is
+advised to configure the `calibration_position`,
+`calibration_extruder_temp`, `extruder_heating_z`, and
+`calibration_bed_temp` options, as doing so will automate some of the
+steps outlined below.
+
+Eddy probe manufacturers may offer a stock drift calibration that can be
+manually added to `drift_calibration` option of the `[probe_eddy_current]`
+section. If they do not, or if the stock calibration does not perform well on
+your system, the `temperature_probe` module offers a manual calibration
+procedure via the `TEMPERATURE_PROBE_CALIBRATE` gcode command.
+
+Prior to performing calibration the user should have an idea of what the
+maximum attainable temperature probe coil temperature is.  This temperature
+should be used to set the `TARGET` parameter of the
+`TEMPERATURE_PROBE_CALIBRATE` command.  The goal is to calibrate across the
+widest temperature range possible, thus its desirable to start with the printer
+cold and finish with the coil at the maximum temperature it can reach.
+
+Once a `[temperature_probe]` is configured, the following steps may be taken
+to perform thermal drift calibration:
+
+- The probe must be calibrated using `PROBE_EDDY_CURRENT_CALIBRATE`
+  when a `[temperature_probe]` is configured and linked.  This captures
+  the temperature during calibration which is necessary to perform
+  thermal drift compensation.
+- Make sure the nozzle is free of debris and filament.
+- The bed, nozzle, and probe coil should be cold prior to calibration.
+- The following steps are required if the `calibration_position`,
+  `calibration_extruder_temp`, and `extruder_heating_z` options in
+  `[temperature_probe]` are **NOT** configured:
+  - Move the tool to the center of the bed.  Z should be 30mm+ above the bed.
+  - Heat the extruder to a temperature above the maximum safe bed temperature.
+    150-170C should be sufficient for most configurations.  The purpose of
+    heating the extruder is to avoid nozzle expansion during calibration.
+  - When the extruder temperature has settled, move the Z axis down to about 1mm
+    above the bed.
+- Start drift calibration.  If the probe's name is `my_probe` and the maximum
+  probe temperature we can achieve is 80C, the appropriate gcode command is
+  `TEMPERATURE_PROBE_CALIBRATE PROBE=my_probe TARGET=80`.  If configured, the
+  tool will move to the X,Y coordinate specified by the `calibration_position`
+  and the Z value specified by `extruder_heating_z`.  After heating the extruder
+  to the specified temperature the tool will move to the Z value specified
+  by the`calibration_position`.
+- The procedure will request a manual probe.  Perform the manual probe with
+  the paper test and `ACCEPT`.  The calibration procedure will take the first
+  set of samples with the probe then park the probe in the heating position.
+- If the `calibration_bed_temp` is **NOT** configured turn on the bed heat
+  to the maximum safe temperature.  Otherwise this step will be performed
+  automatically.
+- By default the calibration procedure will request a manual probe every
+  2C between samples until the `TARGET` is reached.  The temperature delta
+  between samples can be customized by setting the `STEP` parameter in
+  `TEMPERATURE_PROBE_CALIBRATE`.  Care should be taken when setting a custom
+  `STEP` value, a value too high may request too few samples resulting in
+  a poor calibration.
+- The following additional gcode commands are available during drift
+  calibration:
+  - `TEMPERATURE_PROBE_NEXT` may be used to force a new sample before the step
+    delta has been reached.
+  - `TEMPERATURE_PROBE_COMPLETE` may be used to complete calibration before the
+    `TARGET` has been reached.
+  - `ABORT` may be used to end calibration and discard results.
+- When calibration is finished use `SAVE_CONFIG` to store the drift
+  calibration.
+
+As one may conclude, the calibration process outlined above is more challenging
+and time consuming than most other procedures.  It may require practice and several attempts to achieve an optimal calibration.