Faster means slower

Hi, not sure if this is the right cat. to post this question… but, if i ‘speed up’ my machine significantly to reduce job time or to get an effect wanted (say, faster speed on glass to reduce fracturing) then it actually takes much more time to do the same job. This is due to the head ‘over travel’ and slow down then speed up again to do the next line. I understand why this is needed in principal (physics). From watching you tube vids of other machines going at much higher traverse speeds they don’t seem to have this large head over travel…? Is there some setting in the controller (Ruida) that can tweak this issue. Most work I run at 100mm/sec… if i go to 300mm/sec the job time will baloon out to ‘time waster’ factor. Worse still, If the job is close to the machine bed border it will not engrave due to ‘frame slop’ or similar (old ruida software term… i.e. pull hair out and start smashing anything at random in frustration…you know the drill :,) Thanks for any help

I saw something a while ago about increasing scan acceleration settings to reduce overscan and overcome this type of thing. If you have headroom to increase acceleration you may be able to increase speeds while reducing overall job time.

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From LB documentation: :slight_smile:

When engraving, the head is scanning back and forth. Because the laser has to accelerate and decelerate the head at the beginning and end of each line, on machines with limited power control this can cause darker edges than expected.

Overscanning generates extra moves, past the ends of each line, switching the laser off before it fully stops, or even before it begins to decelerate, allowing the entirety of the engraving to happen at the desired head speed and then decelerating while the laser is not burning. The overscan number is a percentage of your cut speed - the default setting is 2.5%, meaning a cut at 100mm/sec will move an additional 2.5mm past the last cut with the laser off.

Note, Overscan is applied automatically by DSP hardware, like Ruida and Trocen controllers. This setting is only available for gcode based systems which do not do this automatically.

https://github.com/LightBurnSoftware/Documentation/blob/master/Operations.md

I kicked mine up to 1500mm/s and set the accel to 10,000. That’s about the max I can do without faulting the motors.

For a raster like 100mm wide, a 750mm/s raster can actually be faster. But if it’s like 500mm wide, the 1500mm/s is definitely faster. There’s a fixed time per line needed for the extra extend space, and you don’t see the benefit of faster line speed until you’ve got wider lines.

You can do some tradeoff. The motors are capable of greater accel at lower speeds. If you cap the speed, you can safely kick up the accel as far as the motors are concerned. However I can’t guess how well your frame and belts handle the extra accel.

The distance is a direct result of the acceleration and speed chosen. Ruida’s firmware calculates the distance it will take to reach full speed at the acceleration set in firmware, and adds that much overshoot to each side. Higher acceleration or lower speed means less overshoot.

As Danny suggests above, the limit will be a function of your motors, drivers, motor current settings, and machine mass. Increasing the accel will reduce the time, but at some point things will slip, wobble, or just fail, so increase a little, test, and repeat.

Hi, Thanks for the info… I understand what you are getting across. Will look into it. Cheers

Thanks for the info… all replies contribute something… it could be that my accel. rate is too slow - so it takes way longer to get up to speed and thus needs further over travel to achieve this…(If i understand correctly)… so could look at this and - as you say - experiment carefully. Yes I understand re machine limits with the physics .

This might be getting into the weeds a little, but the math looks like this:

Distance = (Accel * (Speed / Accel)²) / 2

So, for example, if your acceleration is 2000 mm/sec²:

  • 100 mm/sec gives 2.5 mm of overshoot
  • 200 mm/sec gives 10.0 mm of overshoot
  • 300 mm/sec gives 22.5 mm of overshoot
  • 400 mm/sec gives 40.0 mm of overshoot
  • 500 mm/sec gives 62.5 mm of overshoot

If you double your acceleration, all of those numbers cut in half.

Generally speaking, you won’t hurt it. It may make ‘noise’ at you, but it’s just the fields moving faster than the motor can respond.

My original acceleration was 8,000mm/s^2, I think… It’s now 45,000mm/s^2 after removing a bit of mass from the X axes… it will run 1650mm/s, fun to play with. It’s not really good for much as most of the work I do is in the 250 - 500 mm/s speeds.

What is does do, is lower the overscan tremendously, speeding up the job…

Good luck

:smile_cat:

Great. Thanks for the info. Will have a play.

Thanks for the extra details… Yes at 300mm/sec 20+mm of overtravel, slowdown then accel. is just too much ‘extras’ and job takes longer etc. Will have a careful experiment and see what we can do. Appreciate the forum help - as always.

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