Question on Mesa board best suited for what i need to do.

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28 May 2020 22:00 #169257 by machinedude
i have to give Ditron some credit. 4 days to hit customs here is not bad. i should at least have the scales by 06/02 if the date is accurate on tracking. i have lots of time to sort out details i'm just trying to gather what i need to get started. i will try my best to avoid the land mines on the internet regarding information :)

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28 May 2020 22:34 #169259 by tommylight
What length and how much did they cost ?

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28 May 2020 22:37 - 28 May 2020 22:38 #169260 by machinedude
2600 mm 1300 mm and 250 mm about $610 8m cables for 2 and a 5 m cable on the 3rd. i got 5 um if you wanted 1 um add another $55 they are all housed in the aluminum track.
Last edit: 28 May 2020 22:38 by machinedude.
The following user(s) said Thank You: tommylight, bbsr_5a

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29 May 2020 06:56 #169275 by bbsr_5a
is there only 250mm movement on a large table
what is this for a Woodengraver on scale mode
ok it will be very quick to cut to textile

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29 May 2020 07:01 #169278 by machinedude
this was something i came across and it made a lot of sense to me so i thought it was worth sharing. this would be why i did not get caught up in getting the highest resolution scale possible. what you don't want when trying to tune the PID control loop is to be chasing something out of reach.sometimes to much of something is bad. in this case if you have a linear scale with such a fine resolution that it is more than what the rotary encoder can do you end up making the control loop unstable as a result.

*********There are two basic requirements for a system to use dual loop. The first is a load encoder with
resolution at least 2 times the position accuracy required for the system. This is a general rule of thumb
when deciding the appropriate encoder for a system. The second requirement is that the resolution of
the motor encoder should be at least 2 times the resolution of the load encoder, taking any gear
reduction into consideration. This is required because 1 count of motor encoder movement must
account for less than 1 count of movement of the load. If this condition is not met, 1 motor count can
result in more than 1 count of movement of the load, causing dither about the desired position.***********
The following user(s) said Thank You: tommylight, bbsr_5a

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29 May 2020 08:06 #169286 by machinedude
i think i could take this a step further and say if you have a bad match between the rotary encoder and linear encoder when you look at the signal with a scope you could mistake it for oscillation when in fact it's just the driver looking for mark it can't find because of a resolution mismatch on the two encoders.

so i can see how this could cause all kinds of problems if you don't take it into account. so with what i need the machine to do and what i have to work with i am well within reason.

just as an example my machine moves around .800 imperial per rev and with a 5um scale i have around 4000 pulses on the scale and the DMM encoders can do somewhere around 65,000 PPR . these are just ballpark numbers and are not exact. the DMM rotary could do a 1um scale as well but that's overkill for what i'm after. some people don't have the 16 bit encoders so they would not be so lucky.

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29 May 2020 09:14 #169290 by rodw

machinedude wrote: 2600 mm 1300 mm and 250 mm about $610 8m cables for 2 and a 5 m cable on the 3rd. i got 5 um if you wanted 1 um add another $55 they are all housed in the aluminum track.


Wow! They have bought their market share cos when I did my manual lathe (since sold) I paid an dextra $10 per scale for 1 micron scales! I think they are superior to a lot of other options.

I thought for dual loop you did what PCW suggested to the drive and then closed the outer loop (the scales) in LCNC (with the mesa card)

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29 May 2020 09:54 #169294 by machinedude
that was $55 extra on all 3 but still more than $10 each.

at first i thought the dive side was left alone but the further down the rabbit hole i go the more i think everything has to go back to the PID loop in LInux Rod.

i think what he might have been saying was the new drives have an output for the signal? the DMM Drives have a 9 pin connection on the drives for the differential signals from the rotary encoder, this is an access point for the rotary encoder. this how you would use the encoder to keep tabs on spindle speed in the software. but that's about as far as i got messing around with the analog end of things.

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29 May 2020 10:20 #169296 by machinedude

bbsr_5a wrote: is there only 250mm movement on a large table
what is this for a Woodengraver on scale mode
ok it will be very quick to cut to textile


the moving gantry machines are hard to keep rigid if you get to high on the Z axis so i kept it low to keep things solid. i have seen some tangential knife cutting machines and they are pretty neat to watch but i know nothing about them on a technical level :) i was after the length and width for travel reasons not so much on height with the Z axis. the way the machine turned out the tools hardly stick out below the gantry beam when the Z is up all the way. only thing i need to watch is work holding solutions so low profile working holding is going to be something i use a lot of. wood router was in the plans as well as Aluminum cutting at the very least but will see how it does with mild steel as well. it's not light weight and has some mass to it. it did ok with aluminum before i cast 2100 lbs of E.G into the frame it should do a lot better now but i have not tested yet. still tying up loose ends from casting E.G.

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29 May 2020 10:38 #169298 by rodw
Machinedude, was it you in one of your threads that had a link to some diagrams of dual loop feedback? I had a feeling it was. And I thin kit has many answers to the questions you are asking now! Have a think and share the link again.

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