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  1. #37
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    Default Motor rpm max

    Hi Lucky and springlake.
    I just wanted to update you on how things are going. I have been making parts for 6 hours between yesturday and today and it had been flawless, except fo the ~30 year old home switch that is iffy.

    One thing that you can't see with the gecko drives, at least easily, but can see with my dspmc is a graphic display of the programed move vs the actual move. Plus I can set the error to what ever I want.

    A large error of 1000 counts would let you do things to show how the motor was way out of position during a rapid. What I saw on the bench as opposed to on the machine was that a well oiled axis, not set to too high of an acceleration, would run about the same as bench testing. I could run the accel faster on the bench, but the motor runs out of steam at a given voltage. As a general rule of thumb a brushed dc motor makes max power at 50% of its no load speed. If it will run 4000 rpm on 100v, then at 2000rpm it make max power for a short period. At Max rpm the motor has almost no reserve power as any load will cause it to slow down.

    From my electric flying stuff we used to shoot for about 75% of max speed on the ground which kept the motor in a nice part of the power band and wouldn't burn up the brushes.for some reason If I have trouble with these Renco encoders, and for my lathe conversion, I am going to try the ones from digikey that Mariss recomended.

    Mike

    Warning: DIY CNC may cause extreme hair loss due to you pulling your hair out.


  2. #38
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    Quote Originally Posted by Mariss Freimanis View Post
    ...It used to be their encoders were rock-solid. Now we have been dealing with stuff like "The G320 passed 'encoder test' but goes nuts when I connect the motor!".

    Begin rant:

    Something must have changed at USdigital. 40uA is not something in good conscience I would ever use to drive a cable! I would even think twice to even drive a 0.1" long pcb trace with that current. 8mA is barely acceptable but not 40uA.
    ...
    Mariss
    I have about 20 of the HEDS encoders, but they are all at least 5-7 years old. They have been working fine for the most part (except for a couple that have died in the past), with shielded encoder cable from 3' to 8' lengths. You mentioned that something must have changed at USD.. Does this mean the older HEDS work correctly? These are so old that they have the metal encoder disk (500 count) with physical slots cut in them (quite delicate!)



  3. #39
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    Please see the attached gifs:

    These pictures were taken of a HEDS-9100 driving a 25' (8 meter) long shielded 4-conductor cable back to the G320. The shield was unterminated and the NEMA-34 motor was not grounded back to the G320. The supply voltage was 72VDC and the motor load was >10A.

    Picture 1 shows the HEDS signal while the motor is being powered by a DC supply and not the G320. This is an OK signal.

    Picture 2 shows the HEDS signal while the motor is powered by the G320 and under load. Another encoder is providing the G320 feedback. This is a not OK signal.

    Picture 3 shows the HEDS signal after a 1uF 25V ceramic capacitor is placed across the HEDS +5V and GND terminals at the encoder. This is an OK signal now.

    Put a ceramic 1uF capacitor across the HEDS power supply terminals at the encoder. It makes all the difference. I will take and post pictures of how to do this easily on a HEDS-9100 encoder when time allows. Use Digikey BC1151CT-ND capacitors. They cost $1.80 for a package of 10 pieces.

    Mariss

    Attached Thumbnails Attached Thumbnails G320 losing position-1k-pull-gif   G320 losing position-500mv-vcc-noise-gif   G320 losing position-470nf-vcc-bypass-gif  


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    Thats pretty good if the bypass compacitor is allowing this signal with the 25 foot cables. I have about 8ft of cable on my HEDS encoders and the signal is every bit as good as the first or last pics. I have never seen a signal get the way the center pics show out of these encoders, of course I am not running 25ft cables either.

    I am glad you posted the pics. The lower encoder signal has the little tick at the midway point of the top of the square wave. What is this about, I am guessing it is normal if you get the same thing but I was wandering why its this way. The pic of the encoder losing its signal is great, I wasn't a %100 sure what I would be looking for when testing if they was giving trouble but that answers the question for sure there. From the info from my scope I am good but I do want to add the caps just to be sure they will stay stable. I did a quick test with some caps and I could not get any better signal but I was not having trouble either. This sounds like good insurance to add the caps.


    For soom reason I was thinking it wasn't a ceramic copacitor that was needed to be used. Caps are cheap is there a better cap that could be used or will these be just fine? I have plenty of caps, even some of the 104 candy coated looking caps they use to bypass the supply for IC's with. My local store has anything I would possibly need so if there might be something that could work or last better jsut list it and I will get them. I do plan to run some even longer cables just so I can run my wiring the best/safest way possible. Plus I just bought some cable to use when I add extra lenth and it is one guage heavier than the factory US Digital cables, I am thinking this will be harder to drive and the caps should help if so.


    Really after looking at the pics closer I might be getting better signals than this. I will post a pics of my scope with it reading the signal at 120,000 counts per second with 8ft cables.


    Thanks Jess

    Last edited by LUCKY13; 04-04-2009 at 06:38 AM. Reason: adding coment


  5. #41
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    I don't have a scope available to verify, but I have a couple of smaller NEMA23 servo motors With HEDS 9100 500cpt encoders that would lose position while running if the motor's power cable and shielded encoder cable were next to each other (4' long cables). I added the 1uF capacitor across the power pins on the 9100, and no more position loss. Thanks for the recommendation Mariss!



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    1) The cable length doesn't make the middle picture look the way it does. Noise on the encoder +5V pin makes it do that. I intentionally coupled 20kHz 100nS noise pulses to the +5V pin; when the noise amplitude reached 500mV the encoder went berserk.

    2) The 'notch' in the yellow trace is caused by the blue trace signal going from 5V to 0V. The wires in the 25' cable have capacitance between them. The longer the cable the greater the capacitance. The signal change on the blue trace gets capacitively coupled to the yellow trace producing a notch that is R * C seconds long. R is 1K at a logic 1 and 20 Ohms at a logic 0. That is why you don't see a notch on the blue trace when the yellow signal goes from 5V to 0V.

    3) Ceramic capacitors have extremely low ESR (equivalent series resistance) and minimal series inductance. These are the qualities needed to bypass noise and render it harmless. If a 1uF MLCC (multi layer ceramic capacitor) is used then it is sufficient by itself. Otherwise a it has to be paralleled with an aluminum electrolytic if the MLCC is much smaller (say 100nF like the 104 you mention).

    Mariss



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    nicad,

    Noise pulses have a distribution of amplitude and width. The drive is the source of noise pulses and they increase in amplitude as load on the motor increases. That is why some people erroneously said the G320 isn't good above 4A. When you have something that is on the verge of not working, it is the occasional rare high-amplitude noise pulse that injects an error count into the system.

    A 1uF MLCC completely eliminates the encoder's sensitivity to noise. It is an absolute necessity because the HEDS-9100 doesn't have any bypass capacitors at all. The third picture in the series shows the effectiveness of bypassing. Note the 100mv 'fuzz' riding on the waveform. That is noise that has been rendered harmless in both amplitude and rise/fall time. The capacitor integrates (in the calculus meaning of the word 'integral') the noise.

    Mariss



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    This last weekend I decided to install the bypass caps Mariss recommends. I'm running Homeshopcnc servos, which have the HEDS encoders.

    There have been people using the same setup who say they're having no problem whatsoever, but it seemed like cheap insurance, so I went ahead and did it.

    3 allen head bolts will remove the access cover on the servos:



    You can see there is a little wiring harness in there. It's dead easy to just bend the leads of the tiny little capacitor to piggyback into place:



    I bypassed pins 1 (Gnd) and 4 (+5V). With the cap in place, just put the harness back onto the encoder. You don't need to remove the assembly as I did to get a better picture, just unplug, insert cap leads, replug, and button it up.

    Total cost was about $2 as Mariss suggested. Digikey was only too willing even for the small order to send me the parts right away. Took about 10 minutes of my time to plug them in.

    What was the result?

    The servos were immediately quieter at idle and they needed a little retuning, not much, and I just touched them up by ear. However, I got a couple of immediate benefits. First, some odd little glitches I used to get when jogging (especially small jogs, such as when measuring backlash) just went away. The system was much smoother. Second, I've been fiddling with a part trying to tune up my accuracy and this made a big difference there too.

    It was pretty clear that I had been getting noise on these encoders, though I never did look at them via o-scope or anything.

    This mod is so easy, I highly recommend it just as noise "insurance".

    Cheers,

    BW



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    I had the same issue until last night.

    After two nights of bashing my head into a wall, and swapping out everything, the last thing that I could account was the position of the G320 in relation to the others.

    In my installation, I have 3 G320's installed right next to each other, and the fourth 1" away. The drive that was sandwiched between the other 2 was constantly jumping and loosing position. All the encoder wires were segregated from the drive wires. It seems that the other 2 drives on either side of it puke loads of EMF into the center drive glitching it out. (My guess is the main power wires are then too close to the encoder wires on the next drive)

    I would pull the center drive away from the other 2, and there was no more issue.

    I installed the cap today, and no more problems. Thanks!

    As a note to others: Mount your drives at least 1" apart to avoid this issue. I hope this helps somone.



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    Quote Originally Posted by BobWarfield View Post
    This last weekend I decided to install the bypass caps Mariss recommends. I'm running Homeshopcnc servos, which have the HEDS encoders.

    There have been people using the same setup who say they're having no problem whatsoever, but it seemed like cheap insurance, so I went ahead and did it.

    3 allen head bolts will remove the access cover on the servos:



    You can see there is a little wiring harness in there. It's dead easy to just bend the leads of the tiny little capacitor to piggyback into place:

    Cheers,
    BW
    Bob , did you make that Encoder housing for the motor? Does it hold on by the 2 4-40 screw or is the other screws ?
    Nice design ! I need one for our wood routers to keep the dust out.

    Larry K

    Manufacturer of CNC routers and Viper Servo Drives
    www.LarkenCNC.com and www.Viperservo.com


  11. #47
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    Quote Originally Posted by Larken View Post
    Bob , did you make that Encoder housing for the motor? Does it hold on by the 2 4-40 screw or is the other screws ?
    Nice design ! I need one for our wood routers to keep the dust out.

    Larry K
    Larry, homeshopcnc makes those housings.

    Cheers,

    BW

    Try G-Wizard Machinist's Calculator for free:
    http://www.cnccookbook.com/CCGWizard.html


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    I just wanted to follow up on this with my setup. All though I had to let things set for a long time before I got to finish this project I now have my system ready to bolt onto the mill.


    I keep having a intermiting problem and after adding the caps per this thread it totally cured the problem. If you have these encoders dont even try and run without this mod, I could see the cure on the scope and recreate the problem by removing the caps. What was really strange if one day the system would be stable and the next it would start faulting out of no where. I guess resistance of electronics can be effected by many things we dont think of and these differences from one day to the next would allow this system to work some times but not others.

    ANyway its a 100% fix, and even if your system is working this will make it work better/smoother, and probably even allow a retune to a better level of performance.


    Jess

    GOD Bless, and prayers for all.


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G320 losing position

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