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IndustryArena Forum > OpenSource CNC Design Center > Open Source Controller Boards > Open source low cost servomotor controller
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  1. #73
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    Open source low cost servomotor controller



    I posted this picture awhile back in another thread......

    Here are some motors I have for comparison sake and the continuous torque rating for them. You can see the BLDC motors have much more continuous torque than a similar sized brushed servo. Why do I have so many? Some of these are spares for existing machines and others will be used for not yet completed ones. ALL have been purchased for under $40 each on eBay, most were around $25. The biggest one #12, was only $25 shipped!!! All work and most of them are actually new.

    1. Nema17 stepper motor. 76oz-in. Size found in most 3D printers

    2. Brushed servo Yaskawa minertia T01l. 13.9oz-in.

    3. BLDC servo Parker CM162. 56oz-in

    4. Nema23 stepper motor. 286oz-in. Typical stepper size

    5. BLDC servo CMC BNL2310. 100oz-in

    6. BLDC servo Parker CM231. 46oz-in

    7. Brushed servo Clifton. 46oz-in

    8. Brushed servo Minertia T06m. 96oz-in

    9. Brushed servo Minertia RM-04sa. 59oz-in

    10. Brushed servo Electrocraft E661. 140oz-in

    11. BLDC servo CMC BNR3312. 198oz-in

    12. BLDC servo CMC BMR4445. 722oz-in. Huge motor!!!

    Running a servo motor past its continuous torque rating for any length of time will overheat and could damage it, not recommended to do. The 276oz-in stepper motor is pretty small compared to most of the servos and puts out quite a bit more low speed torque than most of these servos do. This is where pulley reduction for servo motors is quite useful. 3:1 or 4:1 reduction will surpass the low speed torque of the stepper motor and give lots more peak torque for machine acceleration.

    Most of these motors are rated to spin at 3000rpm or more. Some of the brushless ones go 10k. With 3:1 reduction, you can still spin a leadscrew 1000rpm. A stepper motor torque starts to drop pretty fast when it spins more than a few hundred rpm. Big advantage to servo motors when you need higher leadscrew rpm.

  2. #74
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    Re: Open source low cost servomotor controller

    Quote Originally Posted by macdowswe View Post
    I agree that the motors will run more efficiently if you do something like 4:1 reduction gearing. But I am interested in discussing what defines the torque requirements. My hypothesis is that you need a lot more torque on a stepper system because you have to deal with even the shortest load spikes with a decent margin to avoid skipping steps under all circumstances. However, with closed loop encoder feedback, I think you can be completely fine with a fraction of the torque.
    As an example, suppose you had 4Nm of torque on a 5mm pitch ball-screw, that gives 8kN of force. I am not very familiar with machining, but I don't think you persistently need almost a ton of force for most applications.


    That said, efficiency is ofc better to run things cooler, so it may be a good idea to use reduction anyway.


    Cheers,
    Oskar
    For small machines like 3d printers, smaller routers, etc, that run on low friction linear rails, you probably could run without a reduction drive as the motor is likely very much overkill. I tend to think more along the lines of milling machines running on dovetail slides that can have a fair amount of friction, and machine heads weighing 50 pounds or more moving vertically at 200 IPM. In those cases, I would surely want the reduction drive for efficiency. Once I get some more funds I intend to buy a unit, motors, and encoders and try it on my mill with the hopes that I can boost the rapids and push up acceleration. As mentioned, the highest torque required is accelerating. I think the motor that you previously showed is more than enough, but at 1kW max at 7000 rpm it is around 190 oz-in. I currently have 570 oz-in steppers and they will stall right off the bat at essentially 0 rpm with the acceleration turned up. I currently have acceleration at 40 in/sec^2 to keep that from happening, or about 0.1g. So if it is taking about 570 oz-in to accelerate at 0.1g then you could surmise that it would be 1140 oz-in to accelerate at 0.2g. With the motor you linked that would be about 6:1 reduction based on 190 oz-in. At max speed for the motor at 7000 rpm the screw would turn 1166 rpm which would give me 233 IPM feed rate. So theoretically I can use that motor and boost my rapid traverse speed a little, but the big benefit is doubling my acceleration. For my machine 233 IPM is plenty, but better accel for certain machining operations would be welcome. That would likely be pushing that little motor to its limits during accel.

  3. #75
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    Open source low cost servomotor controller

    Finding higher pulley ratio gearing wasn't all that cheap at the time when I did my z axis on my small bench mill. I went 4.5:1 using gt2 pulleys. 20tooth 8mm bore for the motor and 90tooth 1/4" bore for the ballscrew. The 90tooth was the expensive one. I think I paid almost $40 with shipping just for that one pulley from stock drive products. If you find a cheaper source, let me know.

    Edit I actually wanted 5:1 or 6:1 but I recall not finding a larger pulley that had 1/4" bore or something like that. It was a few years ago.

  4. #76
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    Re: Open source low cost servomotor controller

    For example, this 4:1 ratio gt2 pulley and belt set is $17 with free shipping

    3D Printer GT2 60T 15T Belt 6mm Timing Pulley Belt set kit Reduction Ratio 4:1 | eBay

    For larger than 60T gt2 belts you would likely need to order them from a specialty house at higher cost, but even at 4:1 I can get a theoretical improvement on my current mill. From my previous example of my machine I would have 760 oz-in of available torque from the Turnigy motor, so I could increase accel from 0.1g to 1.33g and get 350 IPM rapids, with the benefit of a closed loop system. Since it is closed loop I could probably get a bit higher accel than with the stepper since the stepper accel needs more torque reserve margin than a servo would. Also, this is with the motor that was linked as an example. Other motors could be used that have a lower kv and would operate at lower RPM.

  5. #77
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    Open source low cost servomotor controller

    Thanks for the link. I haven't seen that. Although I don't think the 15 tooth pulley has enough there to bore it out to fit my 8mm shaft. Probably why I choose 20 tooth at the time.

    Edit Some of my servo motors have 3/8" or larger shaft diameter. Getting a 4:1 pulley set that fits without spending $$ isn't easy. It costs more for the pulleys than the motor!!!

  6. #78
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    Re: Open source low cost servomotor controller

    A few questions I have about the drive:

    1. It uses a battery as a reserve for high load motions to stabilize the bus voltage. I think this is a pretty smart way of doing it, butt the battery linked on the Odrive page shows a 3S Lipo, does it have to be a 3s Lipo battery or can other size/type batteries be used. For example, a 12V car/motorcycle/lawn tractor battery?

    2. If it has to be a Lipo, is there protection from overcharging? I have seen Lipo batteries combust and it isn't pretty

    3. The documentation says that the bus voltage is 24V. Is the system using a step up converter to achieve this?

  7. #79
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    Open source low cost servomotor controller

    Quote Originally Posted by 109jb View Post
    For example, this 4:1 ratio gt2 pulley and belt set is $17 with free shipping

    3D Printer GT2 60T 15T Belt 6mm Timing Pulley Belt set kit Reduction Ratio 4:1 | eBay

    For larger than 60T gt2 belts you would likely need to order them from a specialty house at higher cost, but even at 4:1 I can get a theoretical improvement on my current mill. From my previous example of my machine I would have 760 oz-in of available torque from the Turnigy motor, so I could increase accel from 0.1g to 1.33g and get 350 IPM rapids, with the benefit of a closed loop system. Since it is closed loop I could probably get a bit higher accel than with the stepper since the stepper accel needs more torque reserve margin than a servo would. Also, this is with the motor that was linked as an example. Other motors could be used that have a lower kv and would operate at lower RPM.
    For comparison sake I'm using 46oz-in continuous torque brushed servo/gecko320, 5tpi ballscrew, 4.5:1 pulley. The continuous leadscrew thrust is 410lbs, the peak leadscrew is almost 1400lbs. 110ipm. If the specs on the Turnigy motor are as advertised, I think you will have more than enough torque and acceleration.

    The original setup was 276oz-in steppers with geckodrive. Easily stall out stepper when plunging/drilling operations. With the servo setup, I have to worry about the spindle stall if I go to fast. Plenty of torque for my machine.

  8. #80
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    Re: Open source low cost servomotor controller

    Quote Originally Posted by jfong View Post
    Thanks for the link. I haven't seen that. Although I don't think the 15 tooth pulley has enough there to bore it out to fit my 8mm shaft. Probably why I choose 20 tooth at the time.

    Edit Some of my servo motors have 3/8" or larger shaft diameter. Getting a 4:1 pulley set that fits without spending $$ isn't easy. It costs more for the pulleys than the motor!!!
    Yeah. the larger shaft sizes will require larger sprockets. The little RC motor that we have been talking about is a 5mm shaft, so works well for this set of sprockets.

  9. #81
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    Re: Open source low cost servomotor controller

    jfong,

    Maybe the ID of a 8mm pulley can be bored to 9.525mm or 3/8"

    3M HTD3M 72T 18T Belt 15mm Timing Pulley Belt set kit Reduction Ratio 4:1 CNC | eBay

    I agree with you, it will cost more than the inexpensive motors we shop for.
    JoeyB

    Quote Originally Posted by jfong View Post
    Thanks for the link. I haven't seen that. Although I don't think the 15 tooth pulley has enough there to bore it out to fit my 8mm shaft. Probably why I choose 20 tooth at the time.

    Edit Some of my servo motors have 3/8" or larger shaft diameter. Getting a 4:1 pulley set that fits without spending $$ isn't easy. It costs more for the pulleys than the motor!!!
    A doughnut a day keeps the doctor away.

  10. #82
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    Re: Open source low cost servomotor controller

    Quote Originally Posted by joeybagadonuts View Post
    jfong,

    Maybe the ID of a 8mm pulley can be bored to 9.525mm or 3/8"

    3M HTD3M 72T 18T Belt 15mm Timing Pulley Belt set kit Reduction Ratio 4:1 CNC | eBay

    I agree with you, it will cost more than the inexpensive motors we shop for.
    JoeyB
    Oh ya!! I have to look up that pulley size on stock drive products website. They give you CAD drawings of all the gears and pulleys they sell. From there I can see if there is enough to bore it out larger. So much cheaper on eBay. Thanks....

  11. #83
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    Re: Open source low cost servomotor controller

    jfong,

    I believe the HTD5M would be more appropriate for your large motor due to the KW rating.

    HTD5M 48/12 Teeth W-21mm Pitch-5mm Timing Pulley Belt set kit Reducer Ratio 4:1

    JoeyB
    A doughnut a day keeps the doctor away.

  12. #84
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    Open source low cost servomotor controller

    Quote Originally Posted by 109jb View Post
    A few questions I have about the drive:

    1. It uses a battery as a reserve for high load motions to stabilize the bus voltage. I think this is a pretty smart way of doing it, butt the battery linked on the Odrive page shows a 3S Lipo, does it have to be a 3s Lipo battery or can other size/type batteries be used. For example, a 12V car/motorcycle/lawn tractor battery?

    2. If it has to be a Lipo, is there protection from overcharging? I have seen Lipo batteries combust and it isn't pretty

    3. The documentation says that the bus voltage is 24V. Is the system using a step up converter to achieve this?
    My brother makes e-bikes using larger RC brushless motors and big lipo packs. He uses high current 12volt Dell server power supplies in series to charge them. I think these power supplies are around 40-50amps and can be bought on eBay for $10 each. One way to to get the high current demand that these RC brushless require.

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