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This document describes Klipper's stepper phase adjusted endstop system. This functionality can improve the accuracy of traditional endstop switches. It is most useful when using a Trinamic stepper motor driver that has run-time configuration.

A typical endstop switch has an accuracy of around 100 microns. (Each time an axis is homed the switch may trigger slightly earlier or slightly later.) Although this is a relatively small error, it can result in unwanted artifacts. In particular, this positional deviation may be noticeable when printing the first layer of an object. In contrast, typical stepper motors can obtain significantly higher precision.

The stepper phase adjusted endstop mechanism can use the precision of the stepper motors to improve the precision of the endstop switches. When a stepper motor moves it cycles through a series of phases until in completes four "full steps". So, a stepper motor using 16 micro-steps would have 64 phases and when moving in a positive direction it would cycle through phases: 0, 1, 2, ... 61, 62, 63, 0, 1, 2, etc. Crucially, when the stepper motor is at a particular position on a linear rail it should always be at the same stepper phase. Thus, when a carriage triggers the endstop switch the stepper controlling that carriage should always be at the same stepper motor phase. Klipper's endstop phase system combines the stepper phase with the endstop trigger to improve the accuracy of the endstop.

In order to use this functionality it is necessary to be able to identify the phase of the stepper motor. If one is using Trinamic TMC2130, TMC2208, TMC2224 or TMC2660 drivers in run-time configuration mode (ie, not stand-alone mode) then Klipper can query the stepper phase from the driver. (It is also possible to use this system on traditional stepper drivers if one can reliably reset the stepper drivers - see below for details.)

Calibrating endstop phases

If using Trinamic stepper motor drivers with run-time configuration then one can calibrate the endstop phases using the ENDSTOP_PHASE_CALIBRATE command. Start by adding the following to the config file:

[endstop_phase]

Then RESTART the printer and run a G28 command followed by an ENDSTOP_PHASE_CALIBRATE command. Then move the toolhead to a new location and run G28 again. Try moving the toolhead to several different locations and rerun G28 from each position. Run at least five G28 commands.

After performing the above, the ENDSTOP_PHASE_CALIBRATE command will often report the same (or nearly the same) phase for the stepper. This phase can be saved in the config file so that all future G28 commands use that phase. (So, in future homing operations, Klipper will obtain the same position even if the endstop triggers a little earlier or a little later.)

To save the endstop phase for a particular stepper motor, run something like the following:

ENDSTOP_PHASE_CALIBRATE STEPPER=stepper_z

Run the above for all the steppers one wishes to save. Typically, one would use this on stepper_z for cartesian and corexy printers, and for stepper_a, stepper_b, and stepper_c on delta printers. Finally, run the following to update the configuration file with the data:

SAVE_CONFIG

Additional notes

  • This feature is most useful on delta printers and on the Z endstop of cartesian/corexy printers. It is possible to use this feature on the XY endstops of cartesian printers, but that isn't particularly useful as a minor error in X/Y endstop position is unlikely to impact print quality. It is not valid to use this feature on the XY endstops of corexy printers (as the XY position is not determined by a single stepper on corexy kinematics). It is not valid to use this feature on a printer using a "probe:z_virtual_endstop" Z endstop (as the stepper phase is only stable if the endstop is at a static location on a rail).

  • After calibrating the endstop phase, if the endstop is later moved or adjusted then it will be necessary to recalibrate the endstop. Remove the calibration data from the config file and rerun the steps above.

  • In order to use this system the endstop must be accurate enough to identify the stepper position within two "full steps". So, for example, if a stepper is using 16 micro-steps with a step distance of 0.005mm then the endstop must have an accuracy of at least 0.160mm. If one gets "Endstop stepper_z incorrect phase" type error messages than in may be due to an endstop that is not sufficiently accurate. If recalibration does not help then disable endstop phase adjustments by removing them from the config file.

  • If one is using a traditional stepper controlled Z axis (as on a cartesian or corexy printer) along with traditional bed leveling screws then it is also possible to use this system to arrange for each print layer to be performed on a "full step" boundary. To enable this feature be sure the G-Code slicer is configured with a layer height that is a multiple of a "full step", manually enable the endstop_align_zero option in the endstop_phase config section (see config/example-extras.cfg for further details), and then re-level the bed screws.

  • It is possible to use this system with traditional (non-Trinamic) stepper motor drivers. However, doing this requires making sure that the stepper motor drivers are reset every time the micro-controller is reset. (If the two are always reset together then Klipper can determine the stepper phase by tracking the total number of steps it has commanded the stepper to move.) Currently, the only way to do this reliably is if both the micro-controller and stepper motor drivers are powered solely from USB and that USB power is provided from a host running on a Raspberry Pi. In this situation one can specify an mcu config with "restart_method: rpi_usb" - that option will arrange for the micro-controller to always be reset via a USB power reset, which would arrange for both the micro-controller and stepper motor drivers to be reset together. If using this mechanism, one would then need to manually configure the "endstop_phase" config sections (see config/example-extras.cfg for the details).

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