docs: Update Probe_Calibrate.md with information on common probe errors
Add a section describing PROBE_ACCURACY, describing probe location bias, and describing probe temperature bias. Signed-off-by: Kevin O'Connor <kevin@koconnor.net>
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Kevin O'Connor
KevinOConnor
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@ -43,13 +43,17 @@ the new values take effect.
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# Calibrating probe Z offset
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Providing an accurate probe Z offset is critical to obtaining high
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quality prints.
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Providing an accurate probe z_offset is critical to obtaining high
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quality prints. The z_offset is the distance between the nozzle and
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bed when the probe triggers. The Klipper `PROBE_CALIBRATE` tool can be
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used to obtain this value - it will run an automatic probe to measure
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the probe's Z trigger position and then start a manual probe to obtain
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the nozzle Z height. The probe z_offset will then be calculated from
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these measurements.
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Start by homing the printer and then move the head to a position near
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the center of the bed. Navigate to the OctoPrint terminal tab and run
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the `PROBE_CALIBRATE` command to start Klipper's probe calibration
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tool.
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the `PROBE_CALIBRATE` command to start the tool.
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This tool will perform an automatic probe, then lift the head, move
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the nozzle over the location of the probe point, and start the manual
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@ -65,3 +69,116 @@ results to the config file with:
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```
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SAVE_CONFIG
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```
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# Repeatability check
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After calibrating the probe X, Y, and Z offsets it is a good idea to
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verify that the probe provides repeatable results. Start by homing the
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printer and then move the head to a position near the center of the
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bed. Navigate to the OctoPrint terminal tab and run the
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`PROBE_ACCURACY` command.
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This command will run the probe ten times and produce output similar
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to the following:
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```
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Recv: // probe accuracy: at X:0.000 Y:0.000 Z:10.000
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Recv: // and read 10 times with speed of 5 mm/s
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Recv: // probe at -0.003,0.005 is z=2.506948
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Recv: // probe at -0.003,0.005 is z=2.519448
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Recv: // probe at -0.003,0.005 is z=2.519448
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Recv: // probe at -0.003,0.005 is z=2.506948
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Recv: // probe at -0.003,0.005 is z=2.519448
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Recv: // probe at -0.003,0.005 is z=2.519448
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Recv: // probe at -0.003,0.005 is z=2.506948
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Recv: // probe at -0.003,0.005 is z=2.506948
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Recv: // probe at -0.003,0.005 is z=2.519448
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Recv: // probe at -0.003,0.005 is z=2.506948
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Recv: // probe accuracy results: maximum 2.506948, minimum 2.519448, average 2.513198, median 2.513198, standard deviation 0.006250
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```
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Ideally the tool will report an identical maximum and minimum value.
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(That is, ideally the probe obtains an identical result on all ten
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probes.) However, it's normal for the minimum/maximum distance to
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differ by one Z step_distance or up to 5 microns (.005mm). So, in the
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above example, since the printer uses a Z step_distance of .0125, the
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results would be considered normal.
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If the results of the test show a minimum and maximum distance that
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differs by more than 25 microns (.025mm) then the probe does not have
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sufficient accuracy for typical bed leveling procedures. It may be
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possible to tune the probe speed and/or probe start height to improve
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the repeatability of the probe. The `PROBE_ACCURACY` command allows
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one to run tests with different parameters to see their impact - see
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the [G-Codes document](G-Codes.md) for further details. If the probe
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generally obtains repeatable results but has an occasional outlier,
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then it may be possible to account for that by using multiple samples
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on each probe - read the description of the probe `samples` config
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parameters in the
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[example-extras.cfg](https://github.com/KevinOConnor/klipper/tree/master/config/example-extras.cfg)
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file for more details.
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If new probe speed, samples count, or other settings are needed, then
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update the printer.cfg file and issue a `RESTART` command. If so, it
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is a good idea to
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[calibrate the z_offset](#calibrating-probe-z-offset) again. If
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repeatable results can not be obtained then don't use the probe for
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bed leveling. Klipper has several manual probing tools that can be
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used instead - see the [Bed Level document](Bed_Level.md) for further
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details.
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# Location Bias Check
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Some probes can have a systemic bias that corrupts the results of the
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probe at certain toolhead locations. For example, if the probe mount
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tilts slightly when moving along the Y axis then it could result in
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the probe reporting biased results at different Y positions.
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This is a common issue with probes on delta printers, however it can
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occur on all printers.
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One can check for a location bias by using the `PROBE_CALIBRATE`
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command to measuring the probe z_offset at various X and Y
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locations. Ideally, the probe z_offset would be a constant value at
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every printer location.
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For delta printers, try measuring the z_offset at a position near the
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A tower, at a position near the B tower, and at a position near the C
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tower. For cartesian, corexy, and similar printers, try measuring the
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z_offset at positions near the four corners of the bed.
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Before starting this test, first calibrate the probe X, Y, and Z
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offsets as described at the beginning of this document. Then home the
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printer and navigate to the first XY position. Follow the steps at
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[calibrating probe Z offset](#calibrating-probe-z-offset) to run the
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`PROBE_CALIBRATE` command, `TESTZ` commands, and `ACCEPT` command, but
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do not run `SAVE_CONFIG`. Note the reported z_offset found. Then
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navigate to the other XY positions, repeat these `PROBE_CALIBRATE`
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steps, and note the reported z_offset.
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If the difference between the minimum reported z_offset and the
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maximum reported z_offset is greater than 25 microns (.025mm) then the
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probe is not suitable for typical bed leveling procedures. See the
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[Bed Level document](Bed_Level.md) for manual probe alternatives.
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# Temperature Bias
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Many probes have a systemic bias when probing at different
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temperatures. For example, the probe may consistently trigger at a
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lower height when the probe is at a higher temperature.
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It is recommended to run the bed leveling tools at a consistent
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temperature to account for this bias. For example, either always run
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the tools when the printer is at room temperature, or always run the
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tools after the printer has obtained a consistent print temperature.
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In either case, it is a good idea to wait several minutes after the
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desired temperature is reached, so that the printer apparatus is
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consistently at the desired temperature.
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To check for a temperature bias, start with the printer at room
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temperature and then home the printer, move the head to a position
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near the center of the bed, and run the `PROBE_ACCURACY` command. Note
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the results. Then, without homing or disabling the stepper motors,
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heat the printer nozzle and bed to printing temperature, and run the
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`PROBE_ACCURACY` command again. Ideally, the command will report
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identical results. As above, if the probe does have a temperature bias
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then be careful to always use the probe at a consistent temperature.
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