Probe Temperature Compensation

Introduction

Temperature can significantly affect bed probing and as a consequence first layer quality. To address this, probes like the P.I.N.D.A V2 include a thermistor so they can compensate for measurements taken at different temperatures. However, in order to find these compensation values a calibration process is required (G76).

Since the probe temperature is not the only factor affecting first layer quality, the current implementation can compensate for the bed, probe, and extruder, but only the first two can be calibrated automatically. The measured values are used during G29 mesh bed leveling to adjust the probe measurements at different temperature readings.

During the calibration process it’s important to keep other parts at a constant temperature to prevent them from affecting measurement. For the bed this is fairly easy since Marlin controls its temperature. And for the probe, Marlin can control its proximity to the bed. On some printers (e.g., Průša MK3) it may be necessary to shield the probe from active fans or it won’t heat up enough. Setting the extruder to something like 140°C can also help.

The probe calibration table starts at 30°C, the bed at 60°C, and the extruder at 180°C. In reality we might not reach maximum temperatures while calibrating, so linear regression and extrapolation are used to fill in the gaps. While this is hardly exact, it’s still better than applying the last value for higher temperatures. The more measurements taken, the better the extrapolated values will be.

The calibration process simply does some probing at a lower temperature (e.g., probe at 30°C with bed constant) and uses that measurement as the base value. After heating up the probe or bed by an incremental value (+5°C) another probe reading is taken and the measured offset is stored in the appropriate table. This process is repeated multiple times.

After calibration print, verify sanity, and eventually modify single values (outliers) with the M871 command.

G76 bed calibration process

During bed calibration the probe temperature is held constant (e.g., 30°C).

• Move the probe to the cooldown point.
• Heat up the bed to 60°C.
• Move the probe to the probing point (0.5mm above heatbed).
• Wait until the probe heats up to the target (30°C).
• Probe the bed to get a base value and increase bed temperature by 5°C.
• To get the rest of the calibration values the following steps are repeated until the maximum bed temperature is reached or a timeout occurs:
  • Increase the bed temperature by 5°C.
  • Move the probe to the cooldown point.
  • Wait until the probe temperature is below 30°C and the bed has reached the new target temperature.
  • Move the probe to the probing point and wait until the probe reaches the target temperature (e.g., 30°C).
  • Probe the bed to get a delta value.
• In the case of a timeout, compensation values for higher temperatures will be extrapolated from the existing values.

Note that some beds distort when heated and this will invalidate results. If your bed distorts you may need manual calibration for the bed.

G76 probe calibration process

While probe calibration is active bed temperature is held constant (e.g., 110°C).

• Move the probe to the cooldown point.
• Heat up the bed to maximum temperature (e.g., 110°C).
• Move the probe to the probing point and lower to just 0.5mm above the bed.
• Wait until the probe heats up to the target (30°C).
• Probe the bed to get a base value.
• To get the rest of the calibration values the following steps are repeated until the maximum probe temperature is reached or a timeout occurs (i.e., the probe doesn’t get any hotter):
  • Increase the target temperature for the probe by 5°C.
  • Wait until the probe reaches the new target temperature.
  • Probe the bed to get a delta value.
• In the case of a timeout, compensation values for higher temperatures will be extrapolated from the existing values.

Manual calibration process

The extruder (and perhaps the bed) must be calibrated manually. Here is a potential process:

• Record the Probe Z Offset (call it V0).
• Set the extruder (or bed) temperature and wait for it to stabilize.
• Use the Z Probe Wizard to get a new value for the Probe Z Offset (call it V1).
• Use M871 to set the value V0 - V1.
• Repeat with the next temperature…

Configuration

The feature can be enabled and configured in Configuration_adv.h. Most of the numbers quoted above are defaults and can be modified.

1. Make sure you have a probe and at least one temperature sensor: probe, bed and/or extruder.
2. Enable the option for each temperature sensor you wish to use:
   • PTC_PROBE, PTC_BED or PTC_HOTEND
3. Set up the temperature range to use when calibrating each sensor:
   • PTC_*_START, PTC_*_RES and PTC_*_COUNT
4. If you already have Z offset measurements, populate them:
   • PTC_*_ZOFFS
5. Set the behavior for temperatures outside the calibrated range, extrapolation or (default) clamping to the min/max value:
   • PTC_LINEAR_EXTRAPOLATION
6. Set BED_MAXTEMP in Configuration.h.
7. Set the park position for G76 to wait for probe to cool down:
   • PTC_PARK_POS
8. Set the probe position for G76 to wait for probe to heat up and perform Z-probings:
   • PTC_PROBE_POS
9. Set the probe temperature for G76 to target while calibrating the bed.
   • PTC_PROBE_TEMP
10. Set the height above Z=0.0 for heating the probe:
    • PTC_PROBE_HEATING_OFFSET
11. Calibrate the Z value for NOZZLE_TO_PROBE_OFFSET (the Probe Z Offset) at the temperatures given by PTC_*_START.
12. Run G76 command to start calibration process.
13. Use M871 command to check/adjust values in tables.
14. Use M500 command to store values in EEPROM.

Saving and Loading

The G76 or M871 commands only store their results in SRAM, so you must save the data to EEPROM with M500 to preserve the data across reboots. If you’ve used G76 or M871 and don’t want to use the results, you can send M501 to load the last-saved values or M502 to reset them to zero.

Example values

These values were calibrated using a genuine MK52 and P.I.N.D.A V2 probe. The values for the probe above 50°C are extrapolated. Note that you can always tidy up your compensation curve manually using the M871 command.

Probe

°C	30°C	35°C	40°C	45°C	50°C	55°C	60°C	65°C	70°C	75°C	80°C
µm	0	-5	-27	-46	-57	-63	-80	-98	-115	-133	-150

Bed

°C	60°C	65°C	70°C	75°C	80°C	85°C	90°C	95°C	100°C	105°C	110°C
µm	0	3	11	27	30	35	37	37	39	50	55