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Eddy Current Inductive probe

This document describes how to use an eddy current inductive probe in Klipper.

Currently, an eddy current probe can not precisely home Z (i.e., G28 Z). The sensor can precisely do Z probing (i.e., PROBE ...). Look at the homing correction for further details.

Start by declaring a probe_eddy_current config section in the printer.cfg file. It is recommended to set descend_z to 0.5mm. It is typical for the sensor to require an x_offset and y_offset. If these values are not known, one should estimate the values during initial calibration.

The first step in calibration is to determine the appropriate DRIVE_CURRENT for the sensor. Home the printer and navigate the toolhead so that the sensor is near the center of the bed and is about 20mm above the bed. Then issue an LDC_CALIBRATE_DRIVE_CURRENT CHIP=<config_name> command. For example, if the config section was named [probe_eddy_current my_eddy_probe] then one would run LDC_CALIBRATE_DRIVE_CURRENT CHIP=my_eddy_probe. This command should complete in a few seconds. After it completes, issue a SAVE_CONFIG command to save the results to the printer.cfg and restart.

Eddy current is used as a proximity/distance sensor (similar to a laser ruler). The second step in calibration is to correlate the sensor readings to the corresponding Z heights. Home the printer and navigate the toolhead so that the nozzle is near the center of the bed. Then run a PROBE_EDDY_CURRENT_CALIBRATE CHIP=my_eddy_probe command. Once the tool starts, follow the steps described at "the paper test" to determine the actual distance between the nozzle and bed at the given location. Once those steps are complete one can ACCEPT the position. The tool will then move the toolhead so that the sensor is above the point where the nozzle used to be and run a series of movements to correlate the sensor to Z positions. This will take a couple of minutes. After the tool completes it will output the sensor performance data:

probe_eddy_current: noise 0.000642mm, MAD_Hz=11.314 in 2525 queries
Total frequency range: 45000.012 Hz
z: 0.250 # noise 0.000200mm, MAD_Hz=11.000
z: 0.530 # noise 0.000300mm, MAD_Hz=12.000
z: 1.010 # noise 0.000400mm, MAD_Hz=14.000
z: 2.010 # noise 0.000600mm, MAD_Hz=12.000
z: 3.010 # noise 0.000700mm, MAD_Hz=9.000

issue a SAVE_CONFIG command to save the results to the printer.cfg and restart.

After initial calibration it is a good idea to verify that the x_offset and y_offset are accurate. Follow the steps to calibrate probe x and y offsets. If either the x_offset or y_offset is modified then be sure to run the PROBE_EDDY_CURRENT_CALIBRATE command (as described above) after making the change.

Once calibration is complete, one may use all the standard Klipper tools that use a Z probe.

Note that eddy current sensors (and inductive probes in general) are susceptible to "thermal drift". That is, changes in temperature can 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.

Homing correction macros

Because of current limitations, homing and probing are implemented differently for the eddy sensors. As a result, homing suffers from an offset error, while probing handles this correctly.

To correct the homing offset. One can use the suggested macro inside the homing override or inside the starting G-Code.

Force move section have to be defined in the config.

[gcode_macro _RELOAD_Z_OFFSET_FROM_PROBE]
gcode:
  {% set Z = printer.toolhead.position.z %}
  SET_KINEMATIC_POSITION Z={Z - printer.probe.last_probe_position.z}

[gcode_macro SET_Z_FROM_PROBE]
gcode:
  {% set METHOD = params.METHOD | default("automatic") %}
  PROBE METHOD={METHOD}
  _RELOAD_Z_OFFSET_FROM_PROBE
  G0 Z5

Tap calibration

Eddy current probes support a special kind of probing referred to as "tap" probing. This mechanism directs the toolhead to descend until the nozzle makes contact with the bed. That bed contact may cause a change in sensor measurements which can be detected and used to halt further downward movement. The nozzle is then lifted away from the bed and the sensor measurements during the lifting movement are analyzed to determine the location where the nozzle breaks contact with the bed.

In order to utilize "tap" probing it is necessary to configure a tap_threshold parameter. This parameter determines when downward toolhead movement during a "tap" probe should be halted. A value too large could result in a nozzle/bed contact not detected, which could result in the nozzle crashing uncontrollably into the bed. A value too small could result in a "tap" probe attempt halting before making contact with the bed, which could result in wildly inaccurate probe results.

The PROBE_EDDY_CURRENT_TAP_CALIBRATE command can be used to configure an appropriate tap_threshold value. This tool may be run after completing the main PROBE_EDDY_CURRENT_CALIBRATE calibration. Follow these steps to calibrate tap_threshold:

  1. Verify that both the nozzle and bed are clean. Enable the printer, home the printer, move the toolhead to a position near the center of the bed, and make sure the nozzle is between 3 - 10 millimeters from the bed.

  2. The next step involves commanding the nozzle to make contact with the bed. This process always has some risks, so be prepared to issue an emergency halt (M112) if the probing descent does not stop after contacting the bed. When ready issue the following command: PROBE_EDDY_CURRENT_TAP_CALIBRATE TAP=guess This command analyzes the data found during the main probe calibration to make an initial coarse guess for the tap_threshold value and it then performs the corresponding "tap" probe. Ideally the above command will cause the probe to descend until it hits the bed, lift away from the bed, and then report a valid probe result. If not, see the paragraphs at the end of this section to troubleshoot. If the attempt was successful then continue to the next step.

  3. The next step is to run another tap probe with a "refined" threshold setting. The tool utilizes information gathered during a previous successful tap probe to determine this improved threshold. Make sure that the nozzle is near the center of the bed, that it is between 3 - 10mm above the bed, be ready to issue an emergency halt, and then run the following command: PROBE_EDDY_CURRENT_TAP_CALIBRATE TAP=refine Ideally this command will also succeed; if not, see the paragraphs at the end of this section to troubleshoot. If the attempt was successful then continue to the next step.

  4. If probing with the refined threshold is successful then the next test is to verify that it is stable over multiple probe attempts. Make sure that the nozzle is near the center of the bed, that it is between 3 - 10mm above the bed, be ready to issue an emergency halt, and then run the following command: PROBE_EDDY_CURRENT_TAP_CALIBRATE TAP=verify This command will probe the bed five times in a row. Ideally the above command will also succeed; if not, see the paragraphs at the end of this section to troubleshoot. If the attempt was successful then continue to the next step.

  5. If all of the above steps are successful then one can issue a SAVE_CONFIG command to save the "tap_threshold" parameter to the printer.cfg file. Calibration should now be complete.

If any of the steps above did not succeed then it may be necessary to troubleshoot and manually determine an appropriate tap_threshold. This is done by running commands of the form: PROBE METHOD=tap TAP_THRESHOLD=<value> Where <value> is a threshold to test.

In general, if a probe attempt halts before making contact with the bed, then this indicates that the provided TAP_THRESHOLD parameter is too low. Try increasing it by about 10% and retry. Similarly, if a probe attempt does not halt after making contact with the bed then it indicates that TAP_THRESHOLD is too high. Consider decreasing the attempted value in half.

If the automated calibration tool failed during the initial "guess" stage, then one can use the tap_threshold value reported by the tool as a starting point for manual attempts. Once a successful probe attempt is completed then one can return to the main steps described above starting at the "refine" stage.

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 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. If the printer to be calibrated is enclosed, it is strongly recommended to set the max_validation_temp option to a value between 100 and 120.

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 thecalibration_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.

Errors description

Possible homing errors and actionables:

  • Sensor error
    • Check logs for detailed error
  • Eddy I2C STATUS/DATA error.
    • Check loose wiring.
    • Try software I2C/decrease I2C rate
  • Invalid read data
    • Same as I2C

Possible sensor errors and actionables:

  • Frequency over valid hard range
    • Check frequency configuration
    • Hardware fault
  • Frequency over valid soft range
    • Check frequency configuration
  • Conversion Watchdog timeout
    • Hardware fault

Amplitude Low/High warning messages can mean:

  • Sensor close to the bed
  • Sensor far from the bed
  • Higher temperature than was at the current calibration
  • Capacitor missing

On some sensors, it is not possible to completely avoid amplitude warning indicator.

You can try to redo the LDC_CALIBRATE_DRIVE_CURRENT calibration at work temperature or increase reg_drive_current by 1-2 from the calibrated value.

Generally, it is like an engine check light. It may indicate an issue.

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