Problem with Accel and Jerk settings for Marlin

[Jailcee 0]

Using Cura 2.5 and 2.6, my printer is set up as a custom printer using GCode flavor "RepRap (Marlin/Sprinter)".

In Cura I have these as my settings:

Print Accel: 800

Travel Accel: 850

Print Jerk: 10

Travel Jerk: 15

According to documentation (http://marlinfw.org/docs/gcode/M204.html and http://marlinfw.org/docs/gcode/M205.html) this should produce the following Gcode:

M204 P800 T850

M205 X10 Y10

What I find in the GCode instead is:

M204 S850

M205 X15

M204 S800

M205 X10

Using Cura I set both accelerations to 800 and both Jerk to 10.  Then I pulled the settings from the firmware using M503 and this is what I got:

Recv: echo:Acceleration (units/s2): P R T

Recv: echo:  M204 P800.00 R1000.00 T800.00

Recv: echo:Advanced: S T B X Z E

Recv: echo:  M205 S0.00 T0.00 B20000 X10.00 Y20.00 Z2.70 E5.00

So clearly Cura is not setting Y jerk and not interpreting the M205 reading correctly.  How could I fix this?

Edited June 26, 2017 by Guest

[gr5 2,167]

According to this:

http://reprap.org/wiki/G-code

You can't set the Y jerk differently than the X jerk with Marlin and Sprinter (other firmwares let you). So you will have to use the lower jerk value for the 2 axes.

There is a reason for this. Jerk is not the derivative of acceleration in Marlin. Instead it should be called "instant velocity change" or some other name. Jerk is a bad name. Marlin uses jerk only to determine the max velocity allowed at each junction. so if you are printing a circle and the cad made it a 10 sided circle and you are approaching the next line segment to print it looks at the two velocity vectors before and after that intersection. Then it subtract the 2 vel vectors to get the instantaneous velocity change at that corner. Then it gets the magnitude (speed change) of that vector. Then if that speed is > than the "jerk" speed it reduces the speed for that junction by the ratio you are over the jerk speed.

Real example: print speed is 100mm/sec. printing the corner of a cube (90 degree corner). The magnitude of speed change is 140mm/sec. If jerk is 10mm/sec then it will divide the speed by 14 and you get 7mm/sec max junction speed at the corner.

This is a crappy algorithm if the two speeds going into and out of the corner are different but if speeds are the same then it works most of the time. It does not allow a different jerk speed for Y versus X.

[burtoogle 513]

The current Marlin source does expect M205 to have separate X and Y values. It used to only take an X value which it then used for both X and Y as the reprap wiki describes. I can't see that adding the Y value will break old Marlin's so I have submitted a PR to make the M205 have both the X and Y values (which will be the same value) and it should work for both old and new Marlins. We will see...

[DaHai8 74]

Aside from the M205 X10 Y10 issue (which will only work with the most current version of Marlin firmware), I believe Cura is behaving properly:

This is setting the parameters before a Travel Move:

M204 S850

M205 X15

These are setting the parameters before Print Moves:

M204 S800

M205 X10

The above setting match your requests in Cura, it is just send these commands before each Travel or Print moves instead of what you are expecting with the M204 command. I believe Cura is adhering to the Lowest Common Denominator when generating gCode so that it can remain compatible with as many variations and versions of Firmware for 3D Printers as possible.

Also, the M503 only reports what is in the Firmware or EEPROM. Note that the above M204 and M205 do not alter EEPROM and Firmware is, well, firm...it cannot be changed without burning new firmware. So what is being currently used may not be what M503 reports.

If you want to have your Accel & Jerk settings set as the default, and EEPROM writing is enabled on your Firmware, you need to use the M500 command to save them in EEPROM and then DON'T change them during printing with the M204 and M205 commands (ie: disable Accel & Jerk settings in Cura)

Hope this helps.