FEEDRATE_MODE_SUPPORT. Adds G93 inverse time mode and G94 units per minute feedrate mode

[DerAndere1]

Description

Add option FEEDRATE_MODE_SUPPORT. When this option is enabled, the following G-codes are supported:

• G93 (inverse time mode): Feedrate (Value of the F word) is specified in strokes per minute.
• G94 (units per minute feedrate mode (default)

Background:

So far, Marlin only supported feedrate specified in units per minute (G94). Marlin uses the LinuxCNC definition of feedrate (

Marlin/Marlin/src/module/motion.cpp

Lines 1444 to 1465 in a506701

	/**
	* Calculate distance for feedrate interpretation in accordance with NIST RS274NGC interpreter - version 3) and its default CANON_XYZ feed reference mode.
	* Assume:
	* - X, Y, Z are the primary linear axes;
	* - U, V, W are secondary linear axes;
	* - A, B, C are rotational axes.
	*
	* Then:
	* - dX, dY, dZ are the displacements of the primary linear axes;
	* - dU, dV, dW are the displacements of linear axes;
	* - dA, dB, dC are the displacements of rotational axes.
	*
	* The time it takes to execute a move command with feedrate F is t = D/F,
	* plus any time for acceleration and deceleration.
	* Here, D is the total distance, calculated as follows:
	*
	* D^2 = dX^2 + dY^2 + dZ^2
	* if D^2 == 0 (none of XYZ move but any secondary linear axes move, whether other axes are moved or not):
	* D^2 = dU^2 + dV^2 + dW^2
	* if D^2 == 0 (only rotational axes are moved):
	* D^2 = dA^2 + dB^2 + dC^2
	*/

). Feedrate in units per minute is not very intuitive if more than 3 axes are involved. Thus, G93 inverse time mode is usually preferred for multi-axis machining.

Example:

G90 ; absolute positioning
G1 X0 Y0 ; move to X0 Y0
G93 ; inverse time mode
G1 X10 F60 ; this move takes 1/60 minutes, i.e. 1 second
G1 X5 F60 ; this move takes in 1/60 minutes, i.e. 1 second
G1 X10 F10 ; this move takes 1/10 minutes, i.e.  6 seconds

Caveats:

• Currently, fixed time motion (FT_MOTION) will probably not work correctly when G93 inverse time mode is active.
• Currently, in G93 mode, each move will be completed in ((1/F) minutes + time for acceleration + time for deceleration). It would be a further improvement if each move (including acceleration and deceleration) completes in (1/F) minutes.

Requirements

#define FEEDRATE_MODE_SUPPORT

Benefits

Support for G93 inverse time feedrate mode. This mode is preferred in multi-axis machining.

Configurations

Enable this new option in the default Configuration.h:
#define FEEDRATE_MODE_SUPPORT

Related Issues

• This feature was requested via Email
• [FR] G93 Inverse Time Mode #8290
• [1.1] G93/94/95 Path Control Modes (work-in-progress) #8311

[GMagician]

I think this will implement what required here #8290.
Just one doubt because AFAIK in standard CNC feedrate doesn't consider acc/dec, it is constant velocity target while interpolating or I am wrong?

[DerAndere1]

The following discussion indicates that my current implementation indeed behaves like LinuxCNC with regards to acceleration and deceleration: https://forum.linuxcnc.org/20-g-code/50870-inverse-time-feed-g93-ignored-with-4th-rotary-axis . Source code of Grbl planner.c also indicates that Grbl behaves the same.

[DerAndere1]

Extended to work with G2/G3. I tested various combinations of G93, G94, G0, G1 and G2 with FEEDRATE_MODE_SUPPORT, G0_FEEDRATE and ARC_SUPPORT enabled on real hardware.