G28 with a delayed G29 FAILURE?

[Hankus]

When testing ABL on my Prusa 10" i3v with Ramps 1.4, I encountered a problem. I was using Proterface to manually send G28 and G29 commands to my printer and found if I delayed sending a G29 command following the sending of a G28 command I would get an error message "echo: Home X/Y before Z" and the ABL would run. The minimum interval that I measured that would produce this failure was exactly 60 seconds from the time the Z sensor retracts upon finishing the G28 command and the instant G29 is submitted. A minute is an abnormal amount of time between these two commands but it made me wonder if others had gotten caught by this issue while testing, and for my own curiosity what causes it?

[jasay]

I believe this comes from "Z safe homing" in the ABL section of the firmware. You're steppers probably time out and turn off at which point the machine is no longer absolutely sure about the position (what if someone bumped the axis while the stepper was off) and if you homed the Z axis you may try to drop the axis with the z-probe off the bed, never trigger the probe, and smash the nozzle(s) into the bed. Not a huge deal for i3v since the nuttraps will just lift off the z-axis 5mm nuts, but now you've lost a level x-axis, so still annoying at the least.

relevant section of configuration.h:

 #define Z_SAFE_HOMING   // This feature is meant to avoid Z homing with probe outside the bed area.                          // When defined, it will:
                          // - Allow Z homing only after X and Y homing AND stepper drivers still enabled
                          // - If stepper drivers timeout, it will need X and Y homing again before Z homing
                          // - Position the probe in a defined XY point before Z Homing when homing all axis (G28)
                          // - Block Z homing only when the probe is outside bed area.

[kablahz]

There is a setting in one of the configuration files, I believe the configuration_adv.h that has a timeout value for the steppers. Once the steppers timeout the x and y location is no longer valid thus causing the error when running g29. I believe it is called stepper_deactivate_timer or something along those lines, I will update later with the exact location if no one chimes in.
edit:

Here is the value you need to change. I believe the default is 50? Either way I changed mine to about 250. Printbus has a writeup on this where he calculates the max Z time for a g28(if the extruder had finished a very tall print) and set that value.

//default stepper release if idle
#define DEFAULT_STEPPER_DEACTIVE_TIME 200

[printbus]

I don't have ABL, but the MakerFarm default for the z-axis value in configuration.h HOMING_FEEDRATE is only 50mm/minute. So, if you have a 12-inch (300mm) printer, a worst-case z-axis travel to home can take 300/50 or 6 minutes.

[Hankus]

Thanks all for the responses. Kablahz you nailed it with the DEFAULT_STEPPER_DEACTIVE_TIME statement. I checked my Configuration_Adv.H file and it was set to 60 seconds. Now that I understand the cause I can go on. BTW the firmware I am using is the DACB submission but I also checked that the original Makerfarm firmware was 60 seconds as well.

[kablahz]

Thanks all for the responses. Kablahz you nailed it with the DEFAULT_STEPPER_DEACTIVE_TIME statement. I checked my Configuration_Adv.H file and it was set to 60 seconds. Now that I understand the cause I can go on. BTW the firmware I am using is the DACB submission but I also checked that the original Makerfarm firmware was 60 seconds as well.

Glad I could help. Yeah I am using DACB firmware as well but tweaked the HOMING_FEEDRATE for the Z since by default it is a bit slow.