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IndustryArena Forum > CNC Electronics > Servo Motors / Drives > BIG Servo Drive Recommendation
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  1. #1
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    BIG Servo Drive Recommendation

    I'm currently involved in a complete tear down and CNC upgrade on a very large gantry machine (7.5 HP saw, dual 15 HP routers, dual air drills) that I affectionately refer to as The Beast. It has old Siemens brushless DC servo motors that we're keeping, but we're scrapping the old SiemoDrive servo amps. This is a big machine, and the typical servo drives from Gecko, Automation Direct, etc. don't quite cut it.

    X: 20 HP (50 A)
    Y: 10 HP (25 A)
    Z: 5 HP (13 A)

    Any suggestions? I'd like something modern and reliable, but we're on a budget. I don't mind paying for quality, but we don't need a lot of features and I don't want to pay extra if it's only for a name brand logo.

    LinuxCNC will be operating the servo drives via MESA Electronics 5i25 and 7i77 boards.

    Thanks in advance.

  2. #2
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    I sent u a PM with info. often 'large gantry' means rack/pinion dual x motors and drives, which in turn means dual cnc axes that work together. simple master-slave typically does not work due to following error delays as speed increases. If more than a few thousands positioning is required then antibacklash with dual pinions on each side in addition. any of this apply to your x axis?

  3. #3
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    Quote Originally Posted by mike_Kilroy View Post
    often 'large gantry' means rack/pinion dual x motors and drives, which in turn means dual cnc axes that work together. simple master-slave typically does not work due to following error delays as speed increases. If more than a few thousands positioning is required then antibacklash with dual pinions on each side in addition. any of this apply to your x axis?
    The X and Y axes are just large one-motor-per-axis designs, albeit with large motors. The servos turn large ball screws.

    This machine cuts wood, not metal. While the tolerances are on the high end for wood, we don't need the sort of tolerances typical of metalworking.

    Thanks for the info.

    For anyone interested in the project, I'll start a separate thread specific to this project and hopefully share some of the info to help others contemplating a similar large CNC conversion project.

    The gantry structure is fixed. The table moves in the X axis beneath the fixed gantry, and the massive tool head moves in Y and Z axes. The tool head has a 7.5 HP radial arm saw with 4th axis for blade tilt angle and a 0-90-180-270 degree semi-5th axis pneumatic contraption to select the blade pivot angle for the orthogonal direction of cut. The tool head also has two huge 15 HP routers and two air drills. That's why we call it The Beast!

  4. #4
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    Not super cheap, but I have collected a few 7.5kW and 10kW Ultra3000 drives from ebay for a big lathe I am working on. They are made by Allen-Bradley and are definitely quality.

    Matt

  5. #5
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    this is in the middle range of servo motors and drives we work with daily. pm or email or call us with the exact motor specs and we will send you a quote. As I also mentioned, some of the older Siemens motors used 3 phase AC tachs or resolvers instead of encoders for feedback so the exact motor part nos are required to be sure a given drive will work. Sometimes they also provided geared resolvers integral on the motors for position feedback to the old cnc control, so you will need to see if they have those; if so, a lot of times a new size 11 encoder can be stuck in place of the old resolver. From the sound of your control card part nos you are looking for +/-10v speed command sig to drive?

  6. #6
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    Quote Originally Posted by keebler303 View Post
    Not super cheap, but I have collected a few 7.5kW and 10kW Ultra3000 drives from ebay for a big lathe I am working on. They are made by Allen-Bradley and are definitely quality.

    Matt
    I'm certainly not beyond buying good surplus equipment. I had actually been shopping around eBay a while ago and didn't see many options in our power range. I just took another look and things look better. Maybe I should have set search parameters and been a long term player in the eBay market for the best surplus deal.

    Thanks for the very good suggestion. I'll keep looking.

  7. #7
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    Quote Originally Posted by mike_Kilroy View Post
    As I also mentioned, some of the older Siemens motors used 3 phase AC tachs or resolvers instead of encoders for feedback so the exact motor part nos are required to be sure a given drive will work. From the sound of your control card part nos you are looking for +/-10v speed command sig to drive?
    These old Siemens three phase brushless DC servos have a three phase tachometer, implemented with Hall Effect sensors, to tell the servo drive when to switch phases. I believe newer servo drives sense the phase relationship using counter EMF and are capable of smoother operation, and better speed, power and efficiency than the simple ON-OFF phase switching. From my experience, newer servo drives are also more fault tolerant and much less likely to burn up a servo motor or self destruct the drive itself.

    We have encoders on the sevo motors for accurate position feedback to the CNC controller. On my small home CNC lathe project, the servo system uses Gecko drives and I'm planning to run the high resolution encoder signals to the servo drive and buffer the same encoder signals to the LinuxCNC controller. That should avoid the need for two encoders per axis, and should prevent any arguments between where LinuxCNC wants the axis to be and where the Gecko drive thinks it is.

    Yes, my preference would be to use +/-10 VDC as the analog servo velocity control signal.

  8. #8
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    If the motors have Hall effect you could use A-M-C BLDC drives, the large 100amp peak ones, they don't come up on ebay as much as the lower current types.
    Too bad, I just blew three away for a song to a local company.
    Al.
    CNC, Mechatronics Integration and Custom Machine Design

    “Logic will get you from A to B. Imagination will take you everywhere.”
    Albert E.

  9. #9
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    that's what I expected when you got down to details on the motors - IHU51 series probably. Anyway, no servo drive I know of in existence today can use the AC tach feedback for anything. Although they may have been generated from halls, they are analog sigs that do not resemble any hall sigs used today. That is why we implimented a program to pull those out and replace them with integral resolver feedback in their place.

    In addition, although many drives can do a crude 'wake & shake' routine to find absolute rotor/stator position without an absolute feedback, and although all our industrial drives have this capability, as well as sophisticated 'wake no shake' routines, I would absolutely not go into a project where I spent thousands on drives where I had to rely on the sometimes unreliable wake & shake startup if I had a chance to design it right before hand as you do now.

    Your incremental encoders do not provide the absolute info required for industrial brushless servo drives of this size today. Your Gecko drives are DC so they have a commutator inside with brushes to do the alignment for you; once you go to AC or Dc brushlesh motors, your present encoder is no longer enough without additional channels of absolute information, and then some servo drives accept that type of signal and some don't. So be sure to ask here before plucking money down on surplus drives to be sure they will work with your motors.

  10. #10
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    Quote Originally Posted by mike_Kilroy View Post
    Your incremental encoders do not provide the absolute info required for industrial brushless servo drives of this size today.
    I searched this entire web page, and your quote above is the only reference to "incremental". I sometimes say the wrong thing, so I wanted to make sure I wasn't thinking "absolute" and typing "incremental".

    Our encoders on The Beast are high resolution absolute encoders. [I meant to say incremental encoders with index pulses]

  11. #11
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    I think you have to accurately define what you have there in the way of feedback device, (absolute encoder?) and if this also serves the purpose of commutation, or is the commutation device separate?
    If these are in fact absolute encoders, you may have a problem with 3rd party drives, as you most likely are looking at a drive that will accept (unlikely) the absolute input/format and output a pseudo differential encoder output for the controller.
    One other solution is to fit modern incremental encoders on the motor with simulated hall tracks for commutation.
    Al.
    CNC, Mechatronics Integration and Custom Machine Design

    “Logic will get you from A to B. Imagination will take you everywhere.”
    Albert E.

  12. #12
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    OK. I managed to confuse myself.

    The encoders are incremental quadrature encoders with index pulses, and NOT absolute encoders.

    Sorry about that. Caffeine isn't always your friend.

    As I did correctly state before confusing myself, there are three Hall Effect devices in each servo motor that could be used for commutation. I've seen the output and it's an ugly square wave. Run it through a Schmitt trigger and it'd clean up to be a nice square wave, but it's a rough indication of motor phase. With induction effects, the commutation should optimally be adjusted based on motor speed, and newer servo drives have smart tuning that can do that. As I also mentioned before, I'm no expert, but I'm under the impression that modern servo drives can analyze the voltage they apply and the current that flows into the motor over time, and determine the proper phase commutation at any motor speed based on the back EMF. In other words, the servo drive can determine the angular position of the motor without any external sensors, based entirely on the V/I relationship on the motor leads that results from the mutual induction of the windings in the motor.

    If not, I could install an external sensor on each motor shaft that would provide better angular data to be used for commutation than the three phase Hall Effect tach.

  13. #13
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    The commutation via hall effect should not be adjusted but accurately aligned with each stator phasing.
    Hall effect devices for BLDC purposes, although originally effective are not considered as accurate as optical tracks on an encoder for e.g.
    Step/dir Drives such as Granite, et al, take a electronic picture of the motor at power up and from then on, use the differential encoder as commutation timing.
    Most encoder manufacturers offer differential encoders with optical commutation tracks, you need to know the pole count of the motors, one way to find this is to short out two of the windings and count the 'bumps' as you rotate one turn of the shaft.
    PDF of alignment using double beam 'scope.
    A 1k resistor is connected to each phase and the other ends connected together to form a virtual neutral, this is the common point for one of the 'scope channel leads, the motor is back fed at ~200rpm.
    Al.
    Attached Files Attached Files
    CNC, Mechatronics Integration and Custom Machine Design

    “Logic will get you from A to B. Imagination will take you everywhere.”
    Albert E.

  14. #14
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    Quote Originally Posted by Al_The_Man View Post
    The commutation via hall effect should not be adjusted but accurately aligned with each stator phasing.
    I think I'm still not communicating that concept clearly.

    The Hall Effect sensors, one per phase, are fixed relative to the motor coil phases. The adjustment that I was describing is advancing the electronic timing of the commutation in the servo drive firmware by a small amount to compensate for faster motor RPM. The timing could also be adjusted a little bit to compensate for instantaneous torque requirements. It's roughly analogous to the timing of ignition and fuel injection in an car's engine. Modern electronics can factor in a lot of different variables to determine the optimal timing under different circumstances. Old engines had a distributor. That's technologically similar to the SiemoDrive from the late 1980s that we're pulling out of this machine, but of course the SiemoDrive used solid state electronics rather than contacts. The next engine timing innovation was the vacuum advance, where increased vacuum pressure (proportional to engine load) resulted in the distributor timing being advanced a few degrees. Now we have electronic engine controls with engine maps that determine the correct ignition and fuel injection timing and dwell based on engine RPM, engine torque output, air density, fuel octane rating, oxygen in the exhaust system, etc. I assume servo drives are getting to be that intelligent as well.

    Thank you for the rest of your post. Excellent info. Interesting and educational.

  15. #15
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    i believe u r communicating this clearly.

    once u design the schmittd trigger and analyze the ac tach analog signal into a repeatable digital signal and have a controllable 3 hall 5v signal that aligns repeatably with the motor poles, please consider posting the data and schematics so others can also use it. no one has done so yet; iirc, the reason this is called an ac TACH is that the amplitude of the ac sig also increases with speed so be sure to take that into account in ur electronics design; iirc, this sig may go from .1vpp to 100+vpp so u need a really cool schmidtt trigger giszmo.

    with enough time and effort I have no doubt this can be done and sold. in the meantime, it is not available on the market. replace the ac tach with our resolver or put a commutating encoder on the motor with pole count that matches each motor and just buy industrial hardware that works out of the box.

    if u buy new industrial drives at 230v 50amp continuous level (is that 50a rms or 50a peak or 50a pp?) expect to pay about $ 5000 each. is this even in the equation for your project? ur into the absolute micro details and may not know the macro picture details?

  16. #16
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    Quote Originally Posted by Liberty4Ever View Post
    The Hall Effect sensors, one per phase, are fixed relative to the motor coil phases. The adjustment that I was describing is advancing the electronic timing of the commutation in the servo drive firmware by a small amount to compensate for faster motor RPM. The timing could also be adjusted a little bit to compensate for instantaneous torque requirements. .
    I maybe would question this as to why you would need this over the normal tuning of a BLDC motor via a PID loop with an encoder?
    I was informed a long time ago by a motor supplier that I would experience 'cogging' at low rpm with a BLDC motor when used in a servo application, I have for some years now used BLDC motors and analogue drives using the torque mode in CNC applications and operate down to very low rpm rates without position error recorded?
    Al.
    CNC, Mechatronics Integration and Custom Machine Design

    “Logic will get you from A to B. Imagination will take you everywhere.”
    Albert E.

  17. #17
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    Quote Originally Posted by mike_Kilroy View Post
    once u design the schmittd trigger....
    The design is trivial. There are CMOS Schmitt buffers. I wasn't planning on designing it. I assumed that any servo drive that used Hall Effect sensors to externally sense commutation would have Schmitt triggers on those inputs. The servo motors are about 30 feet away, as the cable runs. Even with shielded control cables, they're run next to motor and other power cables and the signals get dirty.



    Quote Originally Posted by mike_Kilroy View Post
    ...the reason this is called an ac TACH is that the amplitude of the ac sig also increases with speed...
    I think the reason these are called TACH is that it was translated from Gerrman and that's what Siemens engineers called them. These Siemens servo motors simply have three Hall Effect sensors, positioned 120 degrees apart. They have a 5 VDC supply and they produce mostly TTL level signals. As I recall, the signals overlap to create something like a quadrature signature. That allows twice the resolution as well as rotational direction sensing.

    A
    AB
    B
    BC
    C
    CA

    They're not a tach generator. Those are essentially little three phase generators and the voltage does vary a lot with the RPM.



    Quote Originally Posted by mike_Kilroy View Post
    ...and just buy industrial hardware that works out of the box.
    Buying modern hardware that works out of the box is the plan. I'm not looking to spend a lot of money, but I'm also not looking to waste a lot of time. I also want something that is fairly standard, so it's maintainable by people other than me. I've done a lot of custom design when there isn't an off-the-shelf solution. This isn't one of those times.



    Quote Originally Posted by mike_Kilroy View Post
    if u buy new industrial drives at 230v 50amp continuous level (is that 50a rms or 50a peak or 50a pp?) expect to pay about $ 5000 each. is this even in the equation for your project?
    $10K for three servo drives is not an absolute budget buster, but given what I'm trying to accomplish, it seems like a needless expense. I'd rather spend a lot less. This isn't a shiny new machine being delivered to a customer who's writing a seven figure check. It's a good old piece of iron (well, the table is aluminum ;-) that needs a "good enough" midlife upgrade. Industrial surplus drives would be fine with me.



    Quote Originally Posted by mike_Kilroy View Post
    ur into the absolute micro details and may not know the macro picture details?
    I wanted to keep this thread at the macro level (problem -> solution) but I brain farted the encoder description and it's been bogging down in theory ever since. I should stop speculating about how modern servo drives work and just stick to part numbers and wiring block diagrams.

    I am VERY appreciative for ALL of the help, though.

  18. #18
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    Quote Originally Posted by Liberty4Ever View Post

    . These Siemens servo motors simply have three Hall Effect sensors, positioned 120 degrees apart. They have a 5 VDC supply and they produce mostly TTL level signals. As I recall, the signals overlap to create something like a quadrature signature. That allows twice the resolution as well as rotational direction sensing.

    A
    AB
    B
    BC
    C
    CA

    They're not a tach generator. Those are essentially little three phase generators and the voltage does vary a lot with the RPM.

    .
    I think you will find that is the same logic sequence as shown in the PDF timing diagram for the 120° commutation, there is one electrical rotation x every pole pair for every mechanical rotation, IOW for a 8 pole motor there is 4 electrical rotations per mechanical rotation, normally hall effect devices are simple switching devices and not analogue output.
    The only reason they overlap is that with a BLDC, only two stator windings are energized at any one time. Unlike the AC sinusoidal where all three windings are powered at any given time.
    Al.
    CNC, Mechatronics Integration and Custom Machine Design

    “Logic will get you from A to B. Imagination will take you everywhere.”
    Albert E.

  19. #19
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    I worked third shift tonight making some parts. Gotta get the laser time when I can. I had some unattended run time where I had to be there to keep an eye on it but didn't have any real work to do, so I renewed my understanding of servos and servo drives. I'm now officially an old engineer, and all of that technical info is starting to fade.

    I also browsed eBay looking for large servo drives. Looks like $500 to $600 each for used drives, or maybe more. It's tedious to shop that way, because most scrappers just list the part number. They don't know any of the technical details. There might be some real deals there if servo drives were your field and you knew the parts from 5-10 years ago. I don't, so I'm stuck trying to search for the technical specs online. I guess TANSTAAFL.

    The odd thing is, the power electronics are cheap. The rest of the drive is probably the same. The cost difference for final drive transistors with 5X the current capacity, a larger heat sink and a larger enclosure isn't much. Ditto for the front end power supply. Larger amperage diodes are cheap, and larger capacitors aren't THAT much more expensive. IMO, the largest reason the big drives are so much more expensive than the smaller servo drives is the low volume and a bit of "whatever the market will bear" pricing. Most people looking for a 15 kW servo drive have some money to spend.

  20. #20
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    Quote Originally Posted by Liberty4Ever View Post
    I also browsed eBay looking for large servo drives. Looks like $500 to $600 each for used drives, or maybe more. It's tedious to shop that way, because most scrappers just list the part number. They don't know any of the technical details. There might be some real deals there if servo drives were your field and you knew the parts from 5-10 years ago. I don't, so I'm stuck trying to search for the technical specs online. I guess TANSTAAFL.
    That is why I recommended the Allen Bradley drives. There is normally a decent selection, you can still get all the manuals and technical data, there is a free commissioning software to use with them called Ultraware. You can expect to pay north of $1k for 15kW unless you just get lucky.

    Note that you can use the 400V class (HV) drives at 200V, but the kW rating will be halved (Current is the limiting factor so half voltage is half power).

    Here are 2 15kW , 200V drives. In my opinion that's a real good deal on them.
    ALLEN BRADLEY 2098-DSD-150-SE 9101-1931 Free Ship | eBay

    10kW, 400V Drive.
    ALLEN-BRADLEY 2098-DSD-HV100-SE ULTRA 3000 SERVO DRIVE | eBay

    Here are 6 22kW, 400V Drives. A decent price as well.
    Allen Bradley, Digital Servo Drive 2098-DSD-HV220-SE | eBay

    15kW, 400V Drive.
    NEW ALLEN BRADLEY 2098-DSD-HV150 ULTRA 3000 SERIES B SERVO DRIVE 2098DSDHV150 | eBay

    Matt

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