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Strange occurence in Mach 4
« on: October 05, 2017, 05:40:06 PM »
I have been having an issue with my mill. The servo motors seem to have a slight oscillating when the machine is running. Lets say I jog the X axis from left to right. The axis will move as commanded but when it stops sometimes it will oscillate for anywhere from a few seconds or until I move the axis in the other direction slightly. It usually only happens with my Z axis, but now I've noticed it with my X axis as well, although still not as much as Z axis. Now this afternoon I had my controller running and PC running but Mach 4 was not running for about 2.5 hours. During this time I had not noticed the issue, event when I tried to move the axis by hand. It seemed very stable. Does anybody have any insight as to what could possibly be the problem?


Offline ger21

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Re: Strange occurence in Mach 4
« Reply #1 on: October 05, 2017, 06:52:02 PM »
Servo tuning maybe?

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Re: Strange occurence in Mach 4
« Reply #2 on: October 05, 2017, 07:36:39 PM »
as ger21 says your servo tuning is slightly off.

What servos and drives are you using? For a situation like yours where the setting is very close to right try backing off
the integral component by 5%. You may need to up the differential component by a similar amount. Variations of a few percent
either way around the current settings should suffice. If you find you need a more radical adjustment then I would suspect
some other fault.

Make sure you write the settings down before you start fiddling.
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Re: Strange occurence in Mach 4
« Reply #3 on: October 05, 2017, 10:46:06 PM »
I'm using the DG4S-08020 DC Servo Drive by cncdrive.com also sold by cnc4pc.com. https://cnc4pc.com/dg4s-08020-dc-servo-drive.html The servos I am using are made by kelinginc they are NEMA34 850 oz/in Dual Shaft DC Servo (KL34-170-72) SKEWED ROTOR DESIGN https://www.automationtechnologiesinc.com/products-page/nema-34-dc-servo-motor/kl34-170-72/ . Ok I will give that a try thanks for the quick reply.
Re: Strange occurence in Mach 4
« Reply #4 on: October 05, 2017, 11:32:46 PM »
I take it you have the servo configurator from CNCDrive? Looks like its reasonably easy to experiment with the settings.
Do write down what you start with so that you can go back and start again.

Theres a reason control engineers are bald, it comes from pulling their hair out over PID settings. Oh boy, are you
in for a treat...LOL.

My wife left with my best friend...
     and I miss him!

Offline mark4

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Re: Strange occurence in Mach 4
« Reply #5 on: October 06, 2017, 12:53:32 AM »
hi was it working better before. i ask because how is your oiler does your ball screw and ways have enough lube. i have seen allot of problems because somebody decided to use motor oil not way oil or similar. i would turn the machine completely off and turn the screws manually how do they feel tight loose. to tight is bad too loose is also bad you want the middle ground. if you aren't getting enough lube you will kill a ballscrew one of the machines i rebuilt had the oiler tube get broken for the x ballscrew by the time it got to me i didn't need any tools to tell it needed to be replaced. oil is your friend. here is another trick i have used with gummy oil orifices take kerosene pump it through the oiler till everything loosens up then follow with proper way oil.
Re: Strange occurence in Mach 4
« Reply #6 on: October 06, 2017, 04:12:47 PM »
Joeaverage, LOL! I've had plenty of pulling my hair out over servo tuning when I initially installed them. I had them running fairly well. The only one I had a problem with was the Z axis and it was only going in the down position. Once I moved the Z back up a few clicks it would stop the oscillating, but now it has since creeped into the X axis as well and I'm not understanding how this has just started getting worse without me changing anything. I am new to servos. My previous installations and experience has been strictly with steppers. Which I never had these problems with them, but I was told that I should be using servos as they are far superior. So I decided that with this build I will give them a try and for the most part they work well, except for this oscillation issue developing.
Re: Strange occurence in Mach 4
« Reply #7 on: October 06, 2017, 05:00:51 PM »
I don't know whats changed but something has. The norm when setting up PID is to go for the fastest response with least following error.
Those conditions also put you right on the verge of instability. Any small change will often see low level oscillatory motion. When you set up
you tuned for absolute maximum performance now you need to de-tune for reliability.

If you were modelling this with Matlab you would tune for desired performance and then do a Monte Carlo analysis. The Monte Carlo analysis
varies all the parameters in a usually small but statistically random manner while observing one or more performance indicators. Requires a lot
of computing power. What is does do is show up elements in your system which with minor variance either manufacturing tolerance or wear
will cause a large deviation in desired performance. You can make tuning allowances or redesign that component or whatever...

The upshot is that it could be the aging of a capacitor or resistor inside the drive nut in the feedback loop. It may also be about the mechanical load,
either the weight has increased, you know that added dial gauge or extra thick vice jaws etc or a change in frictional forces due to wear or lack of

A while ago I bought an Allen Bradley servo and drive off Ebay for use as a spindle motor. I ended up buying the Ultraware software that is used
to program the drive. Must say I didn't appreciate having to pay for it but I needed it and bit the bullet. The advantage of the software is that
the EXACT model of my servo was alredy in the memory, including phase resistance and inductance, magnetic losses at various currents, rotational
inertia and more. There is more info in the software than I can find in the specs sheet and certainly way more than I could reasonably be expected
to measure or estimate. All that info meant I just set up how I wanted the servo to behave, max speeds, torques etc and that's it! All the PID stuff
was already done! Amongst the facilities of the software is an oscilloscope. I've used it to track my actual servo response and its bloody good, I could
make it worse by fiddling with the PID settings but doubt I could make it better.

Have to commend the idea that if you buy a servo buy the matching drive from the same manufacturer with that manufacturers software will result
in optimum performance with the least frustration.

My wife left with my best friend...
     and I miss him!
Re: Strange occurence in Mach 4
« Reply #8 on: October 06, 2017, 07:28:31 PM »
Thanks for the explanation. I wish this was a matching setup servo to drive. Unfortunately these two items are from two separate manufacturers. To make matters worse the instructions are lacking as far as I'm concerned. I followed the instructions but can't seem to get the desired results and when contacting tech support, forget about that. I have tried to contact the drive manufacturer and he never returns my emails. Although some others have stated that his support is excellent. All I can say is that maybe the case previously but not anymore. I have also tried to go through his US distributor CNC4PC and he is also very hard to get in tough with. What ever happened to support. When I used to have a problem I would call support and they would gladly walk me through the problem and get the issues resolved. Now everything is "send me an email and I will return it if and when I feel like it" meanwhile all this equipment you paid good money for is going to waste and costing you money if that is what you are using to make your living with. I guess this is why people pay more money to go with companies like Haas, Fadal, Fanuc, etc.. I really wish there was a better alternative to the servo drives than Gecko, cncdrive, and all the stuff that is currently out there that is lacking usability and support. Sorry I didn't mean this to turn into a rant but these issues are really frustrating and not being able to get real help just pisses me off. rant is over. Joeaverage and the rest of the guys on here who have lended support this is in no way meant for you. I appreciate all the help you have provided me. It is meant towards the guys who take your money and then forget you exist when you have issues.
Re: Strange occurence in Mach 4
« Reply #9 on: October 06, 2017, 07:51:06 PM »
I think the solution is in your own hands, the servo you selected and bought is a brushed DC servo and is pretty old hat design wise. The drive is also
old, tried and trued technology. Don't get me wrong, these devices when setup correctly and assuming they have been adequately specified from the outset
will give many years of faithful service.

The current servo design technology is AC servo, they have a few different names but they all hark back to a concept first published in the engineering
literature of the 60's call 'field oriented control'. If you are interested in engineering history it worth a read. In the early days various analogue techniques
were used to achieve field oriented control. Good quality analogue tends to be expensive. As a consequence the technology was not very widespread prior
to the 90's. With the advent of powerful industrial microprocessors with sophisticated hardware periphials the floodgates opened and now field oriented control
is the norm.

Along with the microprocessors came EPROM and equally inventive modes of operation all available by program. As the programming became greater the
need for a PC interface and the rise of 'manufacturers software'.

Sorry to say it but that PC/ drive firmware/servo combination is commonplace with the current generation products whereas yours date back to the prior
level of technology where that sort of integration didn't exist.

My wife left with my best friend...
     and I miss him!