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Re: Following Error
« Reply #20 on: June 29, 2018, 08:06:12 PM »
assume you mean dynamic motor motion stuff..  and the delta servo system.. your way more in price then a clearpath servo plus they are step direction they have built in drivers  and the feedback is built in  .. stepper killer ..  your motor selection would depend on what you are powering  mill plasma or a router
Re: Following Error
« Reply #21 on: June 29, 2018, 08:30:02 PM »
No, sorry for not being more clear, what I meant was swapping my closed loop drivers for leadshine AM882 drivers (regular open loop) which I already own, thus avoiding closing the loop with to different devices which is what average Joe was advising against. Presently I have the loops closing within  the drives and the 7766, although the perimeters are set much closer in the 7766 than the drives. That being the case, the drives never get within an error condition. But as I understand from Craig’s explication is that this is far from ideal because of many reasons he described and probably more. It’s just simplifying an overly complex setup.

Thanks,
Lou
Re: Following Error
« Reply #22 on: June 29, 2018, 08:48:26 PM »
Hi,

Both DMM and Delta are genuine AC servos but have a separate but matching drive, not built in like Clearpath. They are Step/Direction like Clearpath.

DMM= Dynamic Motor Motion

A DMM 60V servo drive $138
A DMM 200W servo      $137
Total                           $275

Clearpath CPM-SDSK-2331S-RLS (193W cont) $333

And if you want to get more power then you'll have to go to the SDHP series:
Clearpath CPM-SDHP-3421P-ELN  (350W cont)     $618

Whereas the DMM offering:
A DMM 60V servo drive   $138
A DMM 400W servo        $147
Total                             $285

So for a 350W/400W servo the clearpath offering is over twice the DMM equivalent.

The DMM have 16bit  absolute encoders whereas Clearpaths have 6000 count incrementals, and 16bit absolute beats 6000 count incremental any day of the week.

Make no mistake that Clearpaths are good, and they play the marketing card very well capturing customers who want to upgrade from steppers
but they are in fact nothing special. All good AC servos can claim all and more than Clearpath and for cheaper too.

Craig
My wife left with my best friend...
     and I miss him!
Re: Following Error
« Reply #23 on: June 29, 2018, 08:52:18 PM »
Hi,
to OP: still cant work out why you persist on using the 7766 to close the loop, the stepper/drive are a matched pair and will do better than the 7766.
Additionally if you use the closed loop stepper drive you'll have the software driven scope to tune your setup whereas you wont with the 7766.

Craig
My wife left with my best friend...
     and I miss him!
Re: Following Error
« Reply #24 on: June 29, 2018, 09:08:32 PM »
Hi,
right at the beginning of this thread you asked about following error, how it affected your setup and why it seemed to trip out to early.

The software scope in the tuning/setup software that accompanies your closed loop stepper drivers is by far  the best way to answer your question.
I you chose to close the loop with the 7766 then that opportunity is gone and you'll have to  do the 'by guess and by God' method.

Craig
« Last Edit: June 29, 2018, 09:10:07 PM by joeaverage »
My wife left with my best friend...
     and I miss him!
Re: Following Error
« Reply #25 on: June 29, 2018, 09:34:10 PM »
Craig,
I’m not opposed to using the drives to close the loop, but there is a bit of rewiring to go that way . The outputs j2 thru j4 would have to be used on the 7766 as opposed to j8 which carries all step and direction as well as all encoder signals. J8 connects through a 24 pin cable to a 7737 encoder board which collects encoder signals and outputs step and direction signals through rj45 Ethernet cable connections. (It is the j8 circuitry that is activated with the closed loop purchase from VSI.) Without the use of  j8 I’m not sure how to wire the encoders. Just simpler at my level to use the 7766, not saying that there isn’t a workaround for the encoder inputs, l just haven’t dug into it yet. It may be possible to output step and direction through j2 - j4 while collecting encoders through j8 thus not closings the loop in the 7766, but I don’t yet know that. As far as using the built in software scope, that has never displayed any motor information at all, are you suggesting that maybe that function only works with the 7766 in an open loop condition? That would be good news indeed.

Thanks for your interest and any input you have to offer is much appreciated,
Lou
Re: Following Error
« Reply #26 on: June 29, 2018, 10:24:00 PM »
Hi,
the scope software is a part of the tuning software and it derives its data from the closed loop stepper drive. No closed loop stepper drive...no scope.

If you use the 7766 as the loop closing device, and contrary to simplicity sake retained the closed loop drives so you can use the scope, the scope would display
the error between the stepper drive input and the stepper encoder, ie it would NOT tell you anything about the dynamics of the 7766 nor would it be able to display the commanded input to
the 7766.

If you wish to use the software scope then the 7766 has to be open loop, ie just a command generator.  If it has dynamics of its own then the scope wont help you tune much as part of the tuning effort
is not observable.

An ordinary scope wont help you either as one input to the scope would be the encoder output which as you know is two signals in phase quadrature. Likewise the input to the control loop,
if the 7766 even makes it available is step/direction. Step/direction is similar to Delta modulation familiar to most communication engineers. A Delta de-modulator would be simple enough
to build. The encoder output would have to be converted to step/direction and that result feed through another Delta de-modulator.

How's you electronics? It is possible but there would be quite an effort to produce signals that could usefully be displayed on a scope.

I still think your best alternative is let the stepper driver close the loop, not the 7766. Either will work but one alternative lets you use the diagnostic smarts built in whereas to other
alternative does not.

Running the 7766 in open loop manner will be easy, after all everyone who has bought one but who has not bought the closed loop activation HAS to run open loop!
If you do run open loop the encoders don't have to be plugged into the 7766 AT ALL! All in all it should be a simpler wiring than what you have at the moment.

Craig.
My wife left with my best friend...
     and I miss him!
Re: Following Error
« Reply #27 on: June 29, 2018, 11:03:36 PM »
Craig,
Your correct on all points as usual, and it might be that you have won me over completely. It’s just an attempt on my part to justify the money I spent on this product to start with, notwithstanding explaining to my wife why I didn’t need to do that (as I said briefly before, my best friend has NOT run away with my wife).
 
I understand that I don’t need to give the 7766 any encoder info at all but if I do then the DRO’S in Mach4 will display the actual machine position based on encoder input as opposed the commanded position. Just seems there has to be some value in that.

Thanks
Lou
Re: Following Error
« Reply #28 on: June 30, 2018, 12:12:27 AM »
Hi Lou,

Quote
I understand that I don’t need to give the 7766 any encoder info at all but if I do then the DRO’S in Mach4 will display the actual machine position based on encoder input as opposed the commanded position. Just seems there has to be some value in that.
Sorry to burst your bubble but Mach is not a feedback controller. The Hicon unit does not routinely relay information back to Mach. If it were to do so the communication lag
would mean the DROs would always be out of date. Mach can read an MPG or encoder and display it in a DRO provided you don't mind it being a tenth of a second out of date.

Hooking the encoders to the 7766 is for the 7766's benefit only. The 7766 can use that to close a loop in realtime whereas Mach cannot, its way way WAY TO SLOW!
A communication loop in Mach3 is of the order of 25ms one way! Mach4 can do a lot better than that, about 10ms as default and you can push it to probably less than 5ms
but that is still to slow for genuine feedback control.

Just to give you some background:  a feedback loop has a refresh rate (sometimes called bandwidth and various other terms). So every so often the control loops get a measurement
of the current error between commanded position verses actual position. Lets say that happens 100 times a second, ie every 10 ms.

The Nyquist sampling theorem assures us that the maximum frequency signal that can be represented by a 100 sample/second stream is half that or 50Hz.  As a control engineer you
yawn, yes its theoretically possible to represent a 50 Hz signal with 100 sample/second but you get no 'discrimination', that is to say the fideility is very poor. You might get fair
representation of a 5 Hz signal with 100 samples per second, but even that would be very grainy or poor resolution. A feedback control loop with a 100 Hz refresh rate will have
at best a 5Hz bandwidth, ie it will be as slow as a wet week! And even a 5Hz bandwidth is pretty sketchy and grainy.....

A servo loop will have refresh rates of the order of 20kHz, for a Nyquist rate of 10 kHz, and an acceleration loop bandwidth of about 1kHz and a position loop bandwidth of about 100Hz.
Thus you can see Mach3 with a refresh rate of about 25 per second and even Mach4 at 100 per second is a long LONG way short of the speed necessary for a high fideility closed loop
control system.

I do understand the inclination to use the activation that you paid for and especially doing what is politic as far as 'her indoors' is concerned..... I would try the method I have outlined
given that it is mainly keystrokes and maybe a bit of wiring. Using the scope feature will teach you stuff in one afternoon that 100 books/videos/forum posts will never do.
Once you have that knowledge/understanding under your belt THEN try using the 7766 to close the loop, either with or without the closed loop stepper drive. That experience based
understanding of feedback control will allow you to dial in the 7766 really well, maybe as good as the stepper and its even remotely possible you could do better. Without that knowledge
you may find as many others have found that PID loop tuning is a Black Art never to be mastered!

Years ago at University I studied this stuff. We had whole labs set up with various servo systems and all manner of measuring and test gear. We were required to do certain experiments
but were encouraged to do a whole lot more than that. Basically we could fiddle with things as much as we liked....all of it adding to the intuitive understanding of feedback control.
Add that intuitive understanding to the mathematics, and there was a s*********t load of that....

Craig
My wife left with my best friend...
     and I miss him!
Re: Following Error
« Reply #29 on: June 30, 2018, 12:34:23 AM »
Craig

Got it, it’ll take some time. Do you want to know how it comes out?
Thanks
Lou