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  • More on Servos?

    I didn't want to hijack the other thread, but all this talk about Servos has got me considering converting my stepper driven b'port S2 CNC boss to servos. I've started doing a lot of plastic machining/engraving on the mill, and the factory steppers are starting to show their limits. I can easily rapid at 100IPM or better, but I had to tune the acceleration so low that I can't even approach these speeds while machining. For a small engraving w/ lots of small moves I normally top out at ~30IPM. I can come closer to 50-60IPM during straight moves like cutting out a part from a sheet, but I would like to get into higher feeds for the plastic stuff.

    I'm trying to educate myself on servos, but there is a huge hodgepodge of information out there, do any of you have any pointers on where to start learning? I found this website DIY DC Servos and it seems interesting, but will this perform similarly to a off the shelf DC servo?

    I was considering just purchasing new steppers, as the consensus in one of my past threads was that there will be an improvement on speed and power over the 30+ year old factory steppers on my mill. I hate to throw more money after steppers though and still not be happy.

    I know that AC servos seem to be the way to go, but in order to afford it I would have to buy surplus, and in order to do that I need to know how to size and match the drives. DC servos seem to be a lot cheaper to pull off, but is there going to be any kind of performance increase by using them?

    I would like to be able to get my feed rates up into the 100IPM range, is this unrealistic for a knee mill w/ a ~42x18 inch table size? And one last question, to keep costs down, can you intermingle steppers and servos? My Z axis is fine with stepper drive, as is my rotary table, the only axis' im interested in speeding up are the X and Y.

    Any info is very much appreciated.


  • #2
    Performance of DC servos will be fairly similar to AC servos so no problems there getting the accel you want. DIY servos versus real ones I would think the likely difference would show at slow speed where the DIY oenes may be prone to cogging. However I really dont know much about the subject so take that with a pinch of salt
    One thing I would say is get servos that are big enough to drive 1:1 as you will get better acceleration performance, but thats just my opinion, I am sure there will be many who disagree.
    As for mixing steppers and servos, if you are meaning with Mach then there is no problem as Mach doesnt know or care what the motor type is.


    • #3
      My mill at home has brushed DC servo motors and I can get rapids up to 400ipm. I think they are 2:1 but they are so massive that there is no acceleration issue here. My little lathe is 1:1.

      As for drives I would go with the Whale3 from

      You should be able to pick up some old motors on ebay.

      There are also a few sellers selling AC brushless servo packages with three motors and drives for a really good price. Usually Yaskawa or Mitsubishi motors. Given the choice I would go with those over DC brushed. They are an integrated set and can tune themselves. I never have had to tune the mitsubishis on my lathe. They tune themselves on the fly.


      • #4
        Here's a couple thoughts

        for Brushless servos you might take a look at Yaskawa (or if you have a good Omron distributor you might look at Omron, they rebrand Yaskawa), Mitsubishi or Automation Direct.

        FOr brushless you will need to buy the Drive (or amp), cables and motor. Most brushless drives can be setup for Step/Direction, Clockwise/Counter-Clockwise or analog control. You can also adjust the number of steps per revolution.

        Seeing as how most brushless motors have a max speed of 3,000 RPM you will probably want some form of gear reduction. Timing belts (inexpensive)are a common method or a planetary gearbox (expensive) are the most common ways of getting a usable shaft speed.

        Brushless motors are MUCH smoother than steppers and you have the ability to set the number of stepps per revolution of the motor so you can get very good resolution, even if you have to change the step/rev because of gearing. Typical is either 1000 or 2000 from the factory, but it is easy to change those number to something that will work better in you application.

        If you are currently using Steppers you will need to use some gear reduction if you use brushless because there is quite a max rpm difference between them.

        I have worked with both the Yaskawa/Omron drives and the Mitsubishi drives. They each had their little quirks and niether was easier/harder to use than the other. You can run drives up to 750W on single phase 220V, bigger drives will need 3 phase. You would need to order an interface cable as they tend to use a connector that is a pain to solder on. They need power and a drive enable signal and they are good to go.


        • #5
          Originally posted by kf2qd
          You can run drives up to 750W on single phase 220V, bigger drives will need 3 phase. You would need to order an interface cable as they tend to use a connector that is a pain to solder on. They need power and a drive enable signal and they are good to go.
          I have 3KW drives that run on single phase also I dont think its that hard for someone with basic soldering skills to wire up cables themselves, my drives have 44pin high density D connectors and its not a problem and a heck of a lot cheaper than paying for pre wired cables..

          Last edited by Hood; 02-01-2010, 01:56 PM.


          • #6
            I would say that 80-90% of the brushless drives on ebay are analog input velocity/torque models. On mitsubishi model look for the model ending with an "A" or "AN" and Yaskawa the model ends with a "P". These drives are positioning drives and they will accept step and direction inputs.

            The Japanese drives do not really allow using other brand of motors with their drives, especially with newer sets as the encoders are connected to the drive by a serial communication method. This relays motor information to the drive and also can provide absolute position indication.

            On the other hand many of the American and European drives will support almost any motor. The drives are programmable in software to the specific characteristics of the motor. I am currently using an Allen Bradley 1398 series drive to control a Parker Compumotor motor. All that was needed was a little adjustment of the encoder to get the timing where it drive was looking for it and it runs fine. I have drives by Aerotech, Glentek, Electrocraft, and SimpleServo (AC Lenze).

            Motors are a different story. I have a whole bunch of different ones sitting around for the event I have a use for them. Some have only hall and tachometer feedback. These will work fin with any analog input drives as they dont need an encoder. The control that runs the drive will look at feedback from an external source. This could be an external encoder, resolver, abscoder, linear scale, etc.

            Motors with resolvers usually require a specific drive for resolver based motors. A resolver is a rotary transformer that looks for a shift in the output to determine position. Resolvers are absolute positioning devices. They are very rugged and rather expensive. Since they are analog the minimum useful resolution is very good, down to several arc-seconds.

            Small motors 400w and under can have top speeds from 3000 to 4500 RPM with intermittant speeds up to 6000 RPM. The larger motors tend be found mostly in the 2000 RPM varieties. Mitsubishi offers larger motors in three speeds, 1000, 2000, and 3000 RPM.

            So far almost all the drives I have messed with that say three phase in run fine off of single phase. They have no phase loss circuity so they dont know the difference. You may have to watch you acceleration times or set the maximum torque setting in the drive to about 70% though. Under heavy acceleration the bus voltage will drop and can trigger an alarm on the drive. You cal also add more bus capacitance to the drive as well. The drives usually have separate bus power supplies from the control section. The control section also usually controls a contractor that applied power to the drive electronics after it passes self test and the drive is enabled. Dont leave this out when you build your final design.

            Bayside makes some nice gear heads. Watch out though. Servos are not standard NEMA frames. They are almost always a metric frame that has a larger shaft, centering boss, and the mounting bolts are on a larger bolt circle. I have modified gearboxes that were intended to fit NEMA 23 sized steppers to attach to the equivalent size servos.

            If you can find them use harmonic gear boxes, they have zero backlash.


            • #7
              I recently finished retrofitting a bridgeport V2E3 which was a servo machine with boss 8 control originally. I used the cncdrives dugong servo drives, I have been extremely happy with them. Their whale servo drive is on the small side for a machine the size of a bridgeport.

              The factory motors are powertron servos, 6000rpm max although with the original 2:1 belt drives and original rapid speed of 250ipm the motors only reach 2500rpm in use.

              As for mixing servos and steppers on the same machine, that is no problem if you are using Mach 3 for control. I have dugong servos for XYZ, a spare drive for future use to power the knee and a stepper running my rotary table 4th axis.

              Having had a stepper bridgeport before, I can tell you the difference in noise and smoothness is unbelievable.

              On your particular present application. If you do a lot of small moves, you probably want to tune for best accelleration at the sacrifice of rapid speed. That would probably make quite a difference in real world machining in your case.


              • #8
                Lots of good info so far. I am trying to keep up.

                The machine already has 2:1 reduction from the steppers to the screws. I am not opposed to going 1:1, but that seems like it will take a monster motor. We are talking about a fairly heavy table here.

                Where do you start on sizing motors? I see that a lot of surplus stuff is rated in Kw or HP, but rarely torque like you typically find with steppers. How do I figure out a baseline of what I will need?

                How much is this likely to cost me? I have lots of amateur electronics experience, I am not afraid of soldering, wiring or reading schematics. I am a pretty good scrounger, and if I can educate myself enough about the subject, I don't think it will be necessary for me to buy anything in "kit" form. Can anyone give me an guess of the cash outlay if I find the majority of what I need on the used/surplus market?

                Does anyone have any more thoughts on the DIY DC brushed servos linked above? This kind of thing is right up my alley of how I like to tackle problems...



                • #9
                  Okay, so I did some more research and I am still confused. Here are some more questions:

                  Is peak/stall torque for a servo similar to holding torque for a stepper? Meaning, if my steppers are 1000 Oz/in holding torque, am I looking for a servo w/ similar "stall torque"? I know that steppers have a "torque curve" and drop torque quickly as speed increases. I also understand that Servos have a more constant torque curve, but it seems quite low. I am looking at these servos from Keling drives (I have had good luck with their products for the first retrofit of the mill) Clicky, first servo in the list, they are rated at 1125 Oz/in peak, but their constant torque is 226 Oz/in. I believe this makes them ~700W motors, is this going to be enough w/ my 2:1 reduction? Will it come even close to cutting it if I go 1:1?

                  I did some of the formulas on the geko faq, and it seems that I should really be looking for a 1.5HP servo, however, anything in that range, even used on ebay is going for $300+, is this really what I will need?



                  • #10
                    There is a poster at both the mach yahoo group and cnczone, I believe it is Himykabbile (hope I spelled that right). He used kelling 850oz peak servos for his bridgeport clone with gecko drivers. I used a belt drive reduction. He got 400ipm rapids but runs it lower than that because he says its scary.
                    (go to cnczone, the bridgeport section, there is a thread he started on a new Z drive, that is the guy)

                    Edit: LOL, I went to the zone and seen you already posted your questions and got replies from the guy I mentioned above.
                    Last edited by Sparky_NY; 02-04-2010, 08:12 AM.


                    • #11
                      Yes, but thank you. I typically try to wring all the information I can out of the available sources

                      Last edited by jacampb2; 02-04-2010, 09:18 PM.


                      • #12
                        Well, I have three 3Kw AC servos coming thanks to a guy on cnczone who pointed them out to me. Once they come, I am going to have my work cut out for me finding a compatible drive. Once again, if anyone can point me in the right direction for research, it is appreciated!



                        • #13
                          I found some big analog servo amps that I purchased on a whim... A-M-C S100A40, I haven't received the servos yet, but these drives should be able to handle any requirement they might have. They are good for up to 400Vdc buss and 100Adc peak current. I only found 2, but I don't think I'll need anything this big for the Z axis, so I'm not really concerned about finding a 3rd drive at the moment, I can always leave the Z stepper driven for now.

                          I know the drives are not positioning drives, the are +/-10V analog velocity or torque control. I intend to build the YAPSC:10 servo amp controller. It is an open source project detailed on CNCzone. It appears to be a lot like the old Pixie controller, but the software interface is excellent and it appears the designer has most of the bugs worked out. I realize that this might not be the best route to take for a newbie to servos and control, but the price is right! I only gave $50 each for the amps, so I am not out much if it works.

                          Let me know your thoughts on my plan. If it is full of holes, I am not out much on the drives, I can still part them out, as the IGBT modules in them are worth far more than I paid for them



                          • #14
                            Awe hell, it turns out these amps are sinusoidal command only... I either misread the data sheet, or read the one for the 100A40 (the S is the key). Is there any control solution that will take step/direction? Any affordable solution at all?



                            • #15
                              The delta-tau control is the only control I know of that will drive them.

                              3kw??? What are you thinking? That would be good for a spindle. Massively overkill for axis control short of a big HMC or boring mill. 3 KW is over 5HP! And that continuos, peak they will run 3 times that. That will twist a ball screw like taffy. No forgiveness.

                              The milltronics VMC we had at work only had 900w motors on it and it was a 40 taper machine.