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Thread: Upgrade recommendations for X2 CNC open-loop steppers to prevent loss of position?

  1. #13
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    Default Re: Upgrade recommendations for X2 CNC open-loop steppers to prevent loss of position

    Hi,
    great video. The closed loop stepper did better than I thought it would but still not a patch on an AC servo.

    Simply put if you have the budget....why wouldn't you use servos?

    Craig



  2. #14
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    Default Re: Upgrade recommendations for X2 CNC open-loop steppers to prevent loss of position

    Quote Originally Posted by joeavaerage View Post
    Hi,
    great video. The closed loop stepper did better than I thought it would but still not a patch on an AC servo.

    Simply put if you have the budget....why wouldn't you use servos?

    Craig
    Simply put..... Because its not needed. Why spend $1000 when it could be a lot cheaper to diagnose it and find out what is really going on instead of just throwing money at it. It's the same reason I drive a 14 year old truck instead of buying a new one. I could pay cash for a new truck but I drive the old one because it gets the job done.

    Lets look at some numbers:

    1. You stated that the AC servo was 80 oz-in, 200oz-in peak
    2. You posted a graph showing about 50% torque loss for a open loop stepper at 500 rpm

    so lets multiply 381 oz-in by 0.50 and we get 190.5 oz-in at 500 rpm for the stepper, and a max of 200 oz-in for the AC servo. Pretty much the same. We know that the AC servo torque curve is pretty flat, so above 500 rpm, the servo has more torque, and below 500 rpm the stepper has more torque. And the stepper torque is available 24/7. The AC servo can only do that 200 oz-in for a short time, measured probably in seconds.

    So below 500 rpm the stepper has more torque. If the steppers can't push the machine around below 500 rpm, then that means the servo can't either if it is direct drive. 500 rpm on a 5mm pitch screw is 98 IPM which is more than enough for a x2. Since the OP alluded to problems even at low feed rates, presumably much lower than the 98 IPM, the AC servo in a direct drive situation is no better and is actually probably worse. At low feed rates, the stepper should have a much higher available and reserve torque than the servo.

    Servos are commonly belt driven at 3:1 or 2:1 so that the rpm capability can be utilized. However that adds more cost and adds complexity. It is great that these little servos can maintain their torque to 3000 rpm, but it doesn't mean squat if you can't even get to 3000 rpm.

    This is why the OP need to first figure out what is wrong with the system as a whole. If the steppers aren't really performing as they should, there is a reason. If the steppers are ok, then something else is going on that needs to be discovered and addressed.

    I just came in from about 8 hours of working in my shop, using my open loop stepper driven G0704, with one job that had a cycle time of about 1:15, and I didn't have one issue at all.

    About 2 years ago that 14 year old truck of mine had problems. Sometimes wouldn't run right, other times would run fine. Sometimes wouldn't want to start. I could have taken $50,000 out of the bank and bought a new truck. ........Why wouldn't I since I have the budget for it......... That certainly would have settled the issue of not running right, but instead a little diagnosing by the shop and a $250 ECM solved the problem. Total bill was about $800, or $49,200 less than the new truck.



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    Default Re: Upgrade recommendations for X2 CNC open-loop steppers to prevent loss of position

    Hi
    fine, stick with your steppers, when budget permits I'm going to servos.

    Why do industrial machines all have servos.....when steppers are good enough?

    Craig



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    Default Re: Upgrade recommendations for X2 CNC open-loop steppers to prevent loss of position

    Industrial machines have much different requirements that a hobby machine. Industrial machines have tables alone that weigh several times what a entire X2 weighs. They run upward of 800 IPM rapids or more to reduce cycle time. They have linear ways. They have high horsepower high speed spindles. Every second counts when talking industrial machines and production environments. An industrial machine is many many times more rigid and capable, requiring axis drives that are equally as capable.

    None of that applies to an X2. It is a lightweight, slow speed machine that doesn't have enough rigidity or size to need more than about 50 IPM. Properly configured open loop steppers are more than capable.

    Even in industrial machines the design requirements will dictate what is required. I will guarantee that if lower cost options will fulfill the requirements for a product, it will have the lower cost components.

    ...when budget permits I'm going to servos.
    So let me get this right. You don't even have the servos you are proclaiming are the solution to all the OP's problems?



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    Default Re: Upgrade recommendations for X2 CNC open-loop steppers to prevent loss of position

    Hi,
    no, I couldn't afford servos at the time I designed and built my benchtop bed mill.

    What I did was use Vexta 5 Phase Steppers with integrated low lash (< 2 arc min) 10:1 planetary drives. Even second hand they (and the 230VAC input drives)
    were expensive, but at that time were still cheaper than available AC servos. I have been very happy with the steppers, in fact yesterday I spent all day making
    circuit boards with my machine, one of the tasks for which I designed it.

    With the gear reduction my machine has a great deal of axis thrust, over 1500 lb force at stall, which satisfies the other requirement that I wanted, namely the thrust
    and rigidity to machine steels and stainless steels. Because of the gear reduction means my machine is slow, 1200mm/min or 60IPM.
    The cost (off EBay) worked out to about $300 per axis. As I say I am very happy with them and I have had great service from them.

    The DMM servos I linked too are still modestly more expensive again but were not readily available at the time I was designing and building my machine. Were I to build
    my machine today I would use the DMM servos without a doubt.

    One area where I miscalculated/misunderstood when I built my machine was I bought a 24000 rpm 750W Mechatron air cooled spindle from Germany. It a great quality spindle
    and I am very happy with it....I used all day yesterday for instance. It is great for engraving and spinning small tools in aluminum but I had hoped it would work in steel.
    High speed low torque spindles such as this are a poor choice for steel/stainless as I discovered ie my miscalculation/misunderstanding.

    As a consequence I elected to make another spindle using a cylindrical ER25 Rego-Fix toolholder, P4 angular contact bearings being directly driven
    by a second hand 1.8 kW 3500 rpm Allen Bradley servo and drive. I was able to buy the second hand servo and drive in New Zealand for $600 USD (equiv)
    including shipping. I did have to buy/make cables and I also had to buy the servo drive setup software from Rockwell. Still all up the servo and drive cost
    about $1000 USD (equiv). I duly made my high torque spindle and despite being home made and not without its faults I am none-the-less proud of it, and
    it works an absolute treat in steel and stainless. I can swap between my highspeed spindle and my high torque spindle in about 15 minutes.

    It is not as convenient as just having the one spindle that can not only spin fast for engraving but has high low speed torque for steels, but then again
    such a spindle is well beyond my budget.

    When I bought this servo it was the first 'modern' AC servo I'd ever really encountered and used. I was absolutely gobsmacked at the flexibility of the control
    options, monitoring, setup assistance that is conferred by the drive. Quite simply it an order of magnitude better than open loop steppers.
    Additionally now that I have some use of the thing I realize that the servo 'appears' to have very much more grunt than the specs would suggest.
    Clearly that it not a scientific observation but the servo driven spindle seems to consistently outperform my best expectations.

    Since then I have had some experience with Delta servos as an in service replacement for a DC brushed servos in a customers large plasma table.
    The DC brushed servos had the loop closed within the controller and added a lot of complexity in addition to the cost of replacement of the nearly
    obsolescent servos. The AC Delta servos were a breeze to fit and reduced the controller complexity considerably. The smarts built into an AC servo drive
    means it is easy to use and integrate into your machine.

    Delta and DMM are cost effective brands of good quality and considerably cheaper than US and European brands. There are still cheaper Chinese brands
    but I'm hesitant to recommend them.

    I take and understand your assertion that $1000 worth of servos is unwarranted for a $600 machine. Likewise I understand your homily about your truck.
    If there is a fault with OPs machine or setup then it makes sense to address the fault rather than 'waste' money.

    I note that you seem inclined, if you were to upgrade, to go to hybrid closed loop steppers. An intermediate solution costwise from the cheaper open loop
    steppers to the more expensive AC servos. I personally regard the cost of the closed loop steppers to be unwarranted for the performance increase over
    open loop steppers. The scientific evidence for the advantages of close loop steppers is pretty thin. I was therefore surprised at the outcome of the video
    you linked to.

    Personal experience tells me that 'modern' AC servos really REALLY kick anus......and ARE worth the premium (with Delta and DMM brands at least).

    The question to whether OP would be overcapitalizing his machine were he to fit AC servos is as much as anything a matter of opinion. My guess is that OP
    would, as I have become, so enamoured with AC servos that he would pleased no matter the investment and would almost certainly retain those same servos
    for other 'larger' projects down the road. Just my opinion...

    Craig



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    Default Re: Upgrade recommendations for X2 CNC open-loop steppers to prevent loss of position

    There's some confusion here over who posted what.
    109jb isn't pushing hybrid steppers for this application and I posted the video. The real reason i posted was the bit about early G540's.
    Fwiw my X2 is near hardware identical to OP (Sorry:long user name) and it's been OK. Admittedly it's not run hard.

    Anyone who says "It only goes together one way" has no imagination.


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    Default Re: Upgrade recommendations for X2 CNC open-loop steppers to prevent loss of position

    Hi,
    my apologies, I did not thoroughly check whom had posted what.

    To whomever posted the video link I was pleasantly surprised that the hybrid closed loop stepper performed as well as it does. The published torque/speed curves
    would not suggest such an apparent improvement over a closed loop stepper.

    Having said that I would not have expected a servo to perform as well as it does based on the specification alone and yet that is exactly my experience. I should
    not be surprised then that closed loop stepper specifications somewhat understates their performance in the real world.

    Open loop steppers still retain a significant price advantage over either of the closed loop solutions. Steppers, more generally known as reluctance motors, exhibit
    the highest torque density (Nm per kg motor weight) than any other motor design at slow speeds. The problem is that at modest speeds they lose their authority.
    The exact speed at which they become subject to losing steps is highly dependent on the machine, the cutting forces, the driver and its power supply. It is difficult in
    advance to predict the effective performance envelope of a stepper. The video suggests that a closed loop stepper extends that envelope despite scientific
    evidence to the contrary.

    Servos on the other hand are well defined. While they have less torque ( at low speeds) than a stepper of the same size the overall performance envelope
    exceeds steppers. An AC servo is an example of a Permanent Magnet Synchronous machine and have the highest power density (kW per kg motor weight)
    of any conventional electric machine.

    Excepting AC servos, synchronous AC machines are not that common in every day experience however all the electricity available to us on the public power grid
    is generated by synchronous generators, albeit DC excited rather than permanent magnet armatures. The capability, reliability and ruggedness of synchronous
    machines is very much the subject of vast swathes of technical engineering literature. That the same flexibility and ruggedness should be found in small synchronous
    machines, aka our AC servos is no surprise.

    It is the flexibility and ruggedness of AC servos that impress me greatly and I am delighted that quality cost effective brands such as Delta and DMM are bringing the
    prices down to within my range. Such was not the case as little as five years ago, or at least it has improved over that time.

    Craig



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    Default Re: Upgrade recommendations for X2 CNC open-loop steppers to prevent loss of position

    Cyclestart is right. I am not pushing any kind of new hardware for the OP until it is determined that the existing hardware won't fulfill his needs. There is another recent thread with a person having essentially the same problems as described here, missed steps even at low feeds, etc. The problem turned out to be that he was using the wrong dip switch setting on his open-loop stepper drivers. A few clicks and as that person described, he has more performance than he needs. The G540 that this OP has may just have the wrong 10 cent current set resistors installed. We don't know yet.



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Upgrade recommendations for X2 CNC open-loop steppers to prevent loss of position?

Upgrade recommendations for X2 CNC open-loop steppers to prevent loss of position?