Raster Gcode

[infamous_panda]

@dc42 said in Raster Gcode:

I can only conclude that either GRBL is not honouring acceleration and jerk (or junction deviation) limits

This may be it. I'll do some digging

[infamous_panda]

@dc42 said in Raster Gcode:

Your config.g file sets the maximum A speed to 4000 units/min in the M201 command

I thought M203 was max speed. Which we set at 80,000. I'll try setting M201 much higher tonight and test it again

[dc42]

I thought M203 was max speed. Which we set at 80,000. I'll try setting M201 much higher tonight and test it again

You are correct, M203 is max speed. I'm sorry, I was reading the X value instead of the A value.

I will continue looking into this, because the simulation time is still greater than I expect. I suspect that jerk policy 1 is not being applied between some moves.

[dc42]

I found the main cause of the problem. The movement queue has limited length (39 moves on the Duet WiFi). When assembling moves, RRF needs to know that if moves suddenly stop being fed to it, it can decelerate to standstill without violating acceleration and jerk limits. When you have a lot of small moves, this has the effect of limiting the top speed. In your example, taking the first 39 moves in the file, they total about 22 units for the A axis. So the maximum speed the machine can reach (in units/min) is 60 * sqrt(2 * A * 22) + J, where A is acceleration, and J is allowed A jerk. That works out as 8735 mm/min which is well below the 80000mm/min maximum speed that you set.

By increasing the A acceleration to 400000, I was able to get the simulated time down to 4 minutes. Increasing it further to 100000 bright the simulated time down to 3 minutes. I didn't change the X parameters, or the A jerk limit.

If GRBL has a similar mechanism to protect against uncontrolled deceleration, then I guess that you have configured it for a much higher acceleration limit. Perhaps it has a longer move queue as well, however as top speed only increases as the square root of the length of the queue, I don't think that can be the whole story.

Even with such a high acceleration, the top speed doesn't reach the A speed limit, and it occasionally drops right down. I suspect that this is because the files contains a few moves in which X moves but A either doesn't move or moves hardly at all, although I didn't spot any in the parts of the file that I looked at.

HTH David

[infamous_panda]

@dc42 said in Raster Gcode:

The movement queue has limited length (39 moves on the Duet WiFi)

Is there a useful improvement to this on the Duet 3? Or a way to change this limit?

I am wary of increasing the global acceleration and jerk to get the raster time down. We have other non raster operations and even just jogging which I think can be catastrophic if we tried to accelerate 30kg at the values you tested.

[infamous_panda]

I received a Teensy 4.1 yesterday and using "stock" grbl-HAL was able to achieve a 9:21 time for our test file. I tweaked one setting in the firmware called BLOCK_BUFFER=250 in the planner.h file by increasing the value to 1024 and was able to achieve a stable 50,000 mm/minute etch resulting in a 1:20 time. I left my standard acceleration settings at 400 for both X and A axis.

I am not technical enough to state how this information can apply but it seems that Duet should be capable of this kind of performance. I much prefer working RRF and DWC over grbl but for now I think I have to just reserve it for 3D printing.

Just wanted to provide an update in case this helps in future RRF/Duet development for high speed laser engraving.

[dc42]

Looks like BLOCK_BUFFER set the maximum number of queued moves in grbl. So 250 is already much more than standard RRF.

I have it on my work list to allow the number of DDAs and DMs per DDA to be configured in config.g. That would allow you to increase the length of the movement queue without needing to recompile the firmware.

[infamous_panda]

@dc42

Awesome I have laser at home which I would love give Duet another go in the future.

[dc42]

@infamous_panda, we've just released RRF 3.2RC1. In this release you can increase the length of the movement queue without recompiling the firmware, using the new M595 command. Using this command, you may be able to overcome the difficulty you had in achieving the required movement speed.

[infamous_panda]

@dc42 Thanks, I will have to give this another review