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Author Topic: Smoothstepper and probing, home and THC  (Read 15878 times)

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Offline stirling

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Smoothstepper and probing, home and THC
« on: September 01, 2010, 12:25:38 PM »
There's been a few threads re: THCs not working with smoothstepper and I think I understand why. But what about probing? doesn't that suffer from exactly the same issues as THC through the smoothstepper? i.e. lack of synchronization between smoothstepper and Mach because of USB. In fact come to that - what about homing? Limits I can understand (kinda) because as long as the thing stops within a few steps of where it should have - who's to know - but probing, THC and homing have to be step perfect.

Just interested if anyone is in the know.

Cheers

Ian

Offline Hood

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Re: Smoothstepper and probing, home and THC
« Reply #1 on: September 01, 2010, 01:18:01 PM »
Dont know about probing but THC doesnt work, however homing does. I think the time sensitive nature of homing is overcome because the homing is done in the SS and then Mach is told.

Hood
Re: Smoothstepper and probing, home and THC
« Reply #2 on: September 01, 2010, 01:20:16 PM »
Quote
THCs not working with smoothstepper and I think I understand why

Why ?  ;D

Amir
**Even a clock that does not work is right twice a day**

Offline stirling

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Re: Smoothstepper and probing, home and THC
« Reply #3 on: September 02, 2010, 04:33:28 AM »
I think the time sensitive nature of homing is overcome because the homing is done in the SS and then Mach is told.
Thanks Hood - that makes perfect sense.

Quote
THCs not working with smoothstepper and I think I understand why
Why ?  ;D

Thanks Amir - thinking I understand it is one thing - trying to explain it is another - but I'll give it a go...

In order for Mach to process inputs in a timely manner there has to be as good as possible "real time" loop between Mach and the hardware it's controling. With the parallel port this is fine and all works well.

BUT with the smoothstepper, the "real time" loop is between the smoothstepper and the hardware. The loop between Mach and the smoothstepper is NOT "real time" because of the very nature of USB (the real clue is in the B for BUS - think bus contention). So this means there is NOT a "real time" loop between the hardware and Mach.

So if by hardware we consider a THC, imagine what happens when the THC commands "UP". It tells the smoothstepper and that communication is "instant". BUT the smoothstepper then has to tell Mach and that communication is NOT instant and will take a period of time that is totally under the control of the USB system. Therefore the "UP" signal will reach Mach late - how late? who knows but late it is. By that time the THC could well be saying "DOWN". Result - We have a variable phase shift between the THC and Mach - Anarchy.

With homing - as Hood has explained, the smoothstepper takes control of the process. I see it working like this: as soon as a home switch is hit, the smoothstepper doesn't JUST tell Mach to stop sending pulses, it stops sending them itself and also tells Mach. It doesn't matter that Mach has sent too many because they're just discarded. The same happens for the backoff part of homing. Of course by this time Mach has effectively lost track of where it is - it doesn't know how many pulses the smoothstepper has discarded, but it doesn't matter - it just sets home to whatever home is and all is back in synch.

Limits is similar except that Mach WILL lose track of where it is - but again that doesn't matter because we accept that hitting a limit more than likely loses position anyway (even with the parallel port) and should be followed by homing.

But - that still leaves me with a problem understanding how probing can work with the smoothstepper. I'd be surprised if it doesn't because I think we'd have heard about it - but at the moment I can't figure out how. On the face of it when it trips, the smoothstepper could just stop sending pulses and tell mach it's tripped - but by then Mach has no idea of where it is. It's different from homing because home is a fixed position known to Mach - but a probe trip is - well - wherever it trips - How can Mach know where that is? Anybody know?

Why am I interested in all this nonesense? I've been working on my own THC design for a while and have a few vague ideas how I can make it work with the smoothstepper.

Well you did ask...  ;D

Cheers

Ian
Re: Smoothstepper and probing, home and THC
« Reply #4 on: September 02, 2010, 07:45:28 AM »
very thx for information
A proposal:
Can we use the Modbus?

Amir
**Even a clock that does not work is right twice a day**

Offline stirling

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Re: Smoothstepper and probing, home and THC
« Reply #5 on: September 02, 2010, 07:55:49 AM »
Re: Smoothstepper and probing, home and THC
« Reply #6 on: September 02, 2010, 07:57:44 AM »
for connect THC to mach3

Amir
**Even a clock that does not work is right twice a day**

Offline stirling

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Re: Smoothstepper and probing, home and THC
« Reply #7 on: September 02, 2010, 08:29:49 AM »
I don't think so - nowhere near fast enough nor synchronized. The thing is, in any feedback loop the outputs cause a change (movement), that changes things (arc voltage), the input comes in, you process it and the result is (hopefully) an output that brings things back to where you wanted. BUT even in a "perfect" system you're allways playing catchup. In Mach this would ideally translate to: every single step in the XY plane may result in a change to arc voltage so before we take another step or at the very least AS we take another step we step the Z. That obviously means that the input is processed for each and every single step i.e. at kernel speed AND in synch.

Ian

...but whatever, I'd still be interested to hear from anyone using a probe with the SS for their thoughts.
« Last Edit: September 02, 2010, 09:00:48 AM by stirling »

Offline Hood

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Re: Smoothstepper and probing, home and THC
« Reply #8 on: September 02, 2010, 01:36:53 PM »
Had a quick chat with Greg, was a bit rushed so never got full details. It seems the way probing works with the SS is when a contact is made the SS sends the info to Mach and once Mach receives it then the SS will go onto the next. Kind of ties in with something another user mentioned when using the SS for probing, he mentioned a 1/2 second delay between probes, all fits into place now.
Hood
Re: Smoothstepper and probing, home and THC
« Reply #9 on: September 02, 2010, 03:59:47 PM »
Quote
1/2 second delay
i think this time is very long.
i build card modbus and speed refresh is about 70ms .if i have free time, 'll Test it

Amir
**Even a clock that does not work is right twice a day**