Unsolved [3.4b5] Motor Brake Feedback / Question

[oozeBot]

@dc42 - we wanted to provide feedback on the new motor brake system you've implemented. Overall, it's great! We've found no scenarios where it isn't properly protecting our printers. However, when first initializing the steppers and the brakes are released, there isn't enough time for the brakes to be energized before the steppers first move which results in missed steps.

Would it be possible to add a momentary delay between energizing the brakes and first movement of the steppers?

[CNCModeller]

@oozeBot, my Z axis brake should be arriving soon so its good to know the feature is working.

Is this something that is just an issue when you initialise the axis with the brake on? Just trying to clarify in my mind what I might expect when I install one on my printer.

Many thanks
Barry M

[oozeBot]

@cncmodeller Correct - just at initialization when the brakes are engaged. I haven't tried, but I'm guessing M17 would work-around the issue. However, we are hoping to see this corrected to fix it for real.

[T3P3Tony]

@oozebot ultimately a configurable delay would be good.

[dc42]

@oozebot what is the scenario in which you are getting missed steps? Are you disabling the motors often? Is this happening on a main board or on an expansion board?

One way you could avoid missed steps is to use M17 to enable the motors, followed by a short G4 delay, before you try to move them.

In most cases there is already a short delay between the motors being enabled and the start of movement, however this may not be the case before G1 H1 and G1 H2 moves.

Adding a configurable delay is unfortunately not simple because it would require changes to the planner, for example to handle the case in which the motors are initially disabled, and then a sequence of moves uses at first just one motor and then two.

[oozeBot]

@dc42 Good timing as I just sat down to work on this a bit. It's happening when homing the machine. Our Z-axis is belt driven and uses two steppers with brakes. When homing X or Y (and Z is unhomed) the gantry is raised 5mm for protection. It was that move (G92 Z0 followed by G1 Z5 F6000) that would first energize the Z steppers, release the brakes, and begin moving it all at the same time. This was resulting in one of the two steppers missing steps which throws the gantry out of alignment until it was homed.

To correct this, I've just added M17 Z followed by G4 P500 prior to the first moves. This appears to be working well and will protect most cases outside of manually moving the machine before homing.