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Author Topic: Jerky dual axis movement  (Read 1524 times)

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Re: Jerky dual axis movement
« Reply #10 on: April 21, 2018, 05:42:39 PM »
Hi,
I agree the PoKeys should handle this no trouble. I'm wondering if the signal is being transmitted faithfully to the driver.

As you know when using RF you have to use coaxial cables or other transmission lines to convey RF energy from one spot to another. Well 100kHz would be
VLF radio but there are radio signals that low and I'm thinking that you may be required to give some thought to the cabling between your controller and the stepper drives.
It may or may not be an issue and might be simple enough to experiment to prove one way or another.

Craig
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Re: Jerky dual axis movement
« Reply #11 on: April 23, 2018, 11:49:17 AM »
There are no radio signals or similar in my basement.
I exclusively used shielded cables, both for power and signal.

Reducing the  step per revolution didn’t change anything except increasing the noise level and induction of vibrations.
I stay with it: for the most part it‘‘s the toolpath generation. The installed motors are quite big, this leads to a increased noise generation. If I wouldn‘t have heard it, I would not have noticed this at all. Probably I‘ll dampen the motors‘ screw joints by adding some rubber layers between, that should minimize the noise to level where I don‘t notice it at anymore.
Re: Jerky dual axis movement
« Reply #12 on: April 23, 2018, 02:37:12 PM »
Hi,
I wasn't talking about radio signals interfering but rather that your 100kHz signal is a VLF radio signal. LORAN-C is 100kHz for example.

My suggestion is that you need to be careful that such a high frequency signal is cabled in such a manner that it is transmitted faithfully.

Craig
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Re: Jerky dual axis movement
« Reply #13 on: April 23, 2018, 03:14:10 PM »
Yes, I understood that and agree with you that at these high frequencies usually coaxial cables should be used. I only use shielded cables, though. But I think it works just fine.
I thought you talked about external radio signals interfering with my control signals.
 
All cables in my electric cabinet are arranged in a way that signal and power cables are spatially divided from one another. I did the best I could for avoiding interferences and I think it's sufficient. I mean, I don't have any real disturbances or unpredictable movements. They always happen at the same code lines and all motors behave like that, the dual axis even synchronously. If they would be out of phase, the portal would change its orientation on the linear rails by little. But I let the Gcode run several times now and measured all angles between my axes afterwards. They didn't change a bit.

It get's a lot better though by increasing the speed and playing with different accelerations.

I don't there's a fast solution now, it's probably something that has to be worked out step by step. It could be everything. Next step is that I'll update my CAD/CAM solution the next days. I see a good chance that this could vanish this weird movement as Autodesk did some severe changes to their software.
I'll take note of every change I make on my machine in the future and check if anything has changed regarding this. If so, I'll let you know.

I want to thank you, Craig, for your time and your willingness to help. I really appreciate it, many thanks!
Re: Jerky dual axis movement
« Reply #14 on: April 24, 2018, 02:59:07 PM »
Hi,
glad you are making some progress.

My concern about cabling and the high frequency of pulses is not about interference, either interference leaking in to the step circuit nor interference leaking out of it
but rather that the signal may be attenuated.

Consider most coaxial cables have input capacitance of 60-90 pf per m. Thus two meters of cable between the PoKeys board and the drivers puts a load of 150pf on
the output of the PoKeys. The output is a low power TTL gate, excess capacitance could easily attenuate its output, further it would be worse at high frequencies than low.
A lot of very small moves as is often encountered in toolpaths in the vicinity of corners and the increase in frequency of the pulses might result in lower than expected
signal level at the drivers.

I'm not saying that it is the case but it could be. You will no doubt have noted that servo drives often specify an upper limit of 100kHz- 200kHz single ended signal but the same
drive will accept up to 500kHz when differentially signaled. Thus signaling a drive at high frequencies is not trivial.

Craig
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Re: Jerky dual axis movement
« Reply #15 on: April 24, 2018, 03:54:32 PM »
Hi,

frankly, I didn't think about the "cable load" and also didn't pay much attention to frequency depending signal disturbance. I know that HF signals usually should be transmitted via coax cables, and for avoiding signal reflexions the wave impedance of the cable should match with the connected device. But I didn't use them because there aren't coax ribbon cables available (I took the best I could get: shielded (ribbon) cables) and I never heard that not using coax cables in applications like this caused problems. If so, most people (and even many companies) would be in real trouble. It sounds little overkill to me.
I only tried to prevent any interferences from happening.
As far as I can tell though, I don't have problems with the signals itself, otherwise I would probably see some really bad and more random behavior and it would occur at other tool paths (also very fast and small little corners), too, but it doesn't.
Nevertheless I'm gonna hook up an oscilloscope (as soon as I have my own) to the drive inputs in order to see if the signal level drops when the PoKeys is firing with commands :-D

Thanks for this interesting input!
Re: Jerky dual axis movement
« Reply #16 on: April 24, 2018, 04:15:15 PM »
Hi,
I have a servo as a spindle motor and was signaling single ended at 150kHz and I hooked up a my scope and was surprised to see how rounded the signal envelope had become.

I had about 4m of cable, shielded twisted pair and the output of the BoB is regular LS TTL. It did signal the drive ok but I was suprised at the signal degradadtion.
I rearranged the electronic gearing to reduce the signal rate to just under 50kHz and shortened the cable to 1.5m and now the signals look good.

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