Halscope scaling - Axis Acceleration behaviour

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19 May 2020 20:14 - 20 May 2020 01:24 #168295 by rodw
This is the first time, I really want to measure elapsed time in Halscope. So if it says 200 ms per division, is that between the smallest tick or is it between the vertical lines.


The graph shows current velocity for a move on my plasma machine from a standing start to 10240 mm/min (170.667 mm/sec)

So the question is, how long does it take to get to speed? I read this as 2.3 seconds (assuming the smallest tick = 200 ms)
But maybe its 230 ms

Help please?
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Last edit: 20 May 2020 01:24 by rodw.

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19 May 2020 20:34 - 19 May 2020 20:35 #168298 by PCW
Replied by PCW on topic Halscope scaling
The major divisions are what time/division refers to.
The small divisions (tics) are 10 per division (so every 20 ms)
(so it gets to speed in 230 ms)
Last edit: 19 May 2020 20:35 by PCW.
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19 May 2020 20:51 #168299 by tommylight
Replied by tommylight on topic Halscope scaling
Just did some tests, came to the same conclusion as PCW, as i noticed you used a different sample rate, so was not sure if that affects the readings, but as it should, it changes the time per division.
I was wondering why you were mentioning that same 2.3 seconds before somewhere else when all i could see was 230ms, but since you used halscope more than me, i did not doubt your results. I should have, shame on me ! :)
What is the acceleration set at?

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19 May 2020 21:11 #168300 by rodw
Replied by rodw on topic Halscope scaling
Thanks guys. it was my bad.

But the next question is that if I have the acelleration in my ini file set to 1500mm /s/s so it should have got to speed in 114 ms.
So why in hte real world am I only getting half? Is this where a better quality stepper drive would come into play?
Sounds like I need to change my ini file to be 750 mm/s/s and retest.

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19 May 2020 22:07 #168306 by tommylight
Replied by tommylight on topic Halscope scaling

This is the simplest i could find on the net explaining acceleration.
Also found this, looks more like what we need, so i blatantly copied/pasted here:
Acceleration (A) at a given time (t) when acceleration is constant is given by
(I) A = a where a is the acceleration at time t = 0

Velocity (V) at a given time (t) when acceleration is constant is given by
(ii) V = at + b where b is the velocity at time t = 0

Distance (S) at a given time (t) when acceleration is constant is given by
(iii) S = ½at² + bt + c where c is the distance from the origin at time t = 0

As the initial velocity is zero and we are measuring from the start point b and c become zero giving
(iv) V = at
(v) S = ½at²

Rearranging (iv) to give t becomes
(vi) t = V ÷ a

Substituting (vi) into (v) to get S in terms of V and a gives
(vii) S = ½at² = ½a (V ÷ a)² = V² ÷ 2a

So the time taken to reach the target velocity of 40mm/s with an acceleration of 1500mm/s² starting from a standstill is given by
t = 40mm/s ÷ 1500mm/s² = 0.2667s to 4sf

Distance covered during this acceleration is given by
S = (40mm/s)² ÷ (2 x 1500mm/s²) = 0.5333mm to 4sf

Regards,
Martin
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19 May 2020 22:20 - 19 May 2020 22:21 #168309 by rodw
Replied by rodw on topic Halscope scaling
Tommy, yes, thats the same maths I did but without the proof
And I checked myself on this calculator
www.smartconversion.com/unit_calculation...tion_calculator.aspx
So restating it, by halscope, the acelleration was about 742 mm/s/s when I had 1500/mm/s/s in the ini file
So where did the other 50% go?
Last edit: 19 May 2020 22:21 by rodw.

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19 May 2020 22:31 #168311 by tommylight
Replied by tommylight on topic Halscope scaling
Noticed the same thing when we were doing those accel tests, moving the mouse over the halscope clearly shows 750 during accel.
Had to much today so my gray matter is all mushy, will resume tomorrow.
Have a nice one.
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20 May 2020 01:22 - 20 May 2020 01:25 #168330 by rodw
So I got to work and had to check this out.
At first I thought it may have been some legacy external offset ratios in the INI file so I played around for a bit with that. They worked as expected. Then I took deleted the OFFSET_AV_RATIO from the X & Y axes
So I halved the velocity to 750 mm/s/s and the time to speed doubled
So then I doubled the velocity to 3000 mm/s/s and the time to speed halved

But it is not going to speed at the max acceleration
So I set the STEPGEN_MAXACCEL to 6000 (double 3000) in case that was limiting but it made no difference
It is consistently only acellerating about 50% of the maximum you allow.
So does it keep some acceleration in reserve for dealing with short sharp segments while cutting?
Any Ideas welcomed.
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Last edit: 20 May 2020 01:25 by rodw.

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20 May 2020 02:28 #168338 by rodw
Actually, reloading one of those calculators, that acceleration is way faster than the ini file setting. I tried another one to check So I need to look at Tommy's formulas. (eyes glaze over....)
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20 May 2020 05:11 #168342 by rodw

rodw wrote: Actually, reloading one of those calculators, that acceleration is way faster than the ini file setting. I tried another one to check So I need to look at Tommy's formulas. (eyes glaze over....)


Well it all works! I just keep getting confused about the required conversion from mm/min to m/sec before doing the calculations.
So its clear that the acceleration used is only about half of what the INI file settings are.

If anybody who knows the LCNC internals better than I, can explain it would be appreciated.

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