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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. #61
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    Post Re: Open source low cost servomotor controller

    This 6,000 r.p.m. encoder also looks promising and can be found for $25.00 or less.
    See the data sheet for more info.

    JoeyB
    A doughnut a day keeps the doctor away.

  2. #62
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    Re: Open source low cost servomotor controller

    Care needed here.
    Those encoders are only quoted to 2000 ppr, which is 500 lines/rev, and a maximum of 100 kHz output. Those are fairly basic specs. Yes, the bearings can take 6,000 RPM, but you won't be getting any signal at that speed.

    Cheers
    Roger

  3. #63
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    Re: Open source low cost servomotor controller

    Roger,

    Will the encoders I posted work reliably within the r.p.m. limitations of a 5tpi ballscrew?

    JoeyB
    A doughnut a day keeps the doctor away.

  4. #64
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    Re: Open source low cost servomotor controller

    Quote Originally Posted by joeybagadonuts View Post
    Roger,

    Will the encoders I posted work reliably within the r.p.m. limitations of a 5tpi ballscrew?

    JoeyB
    The encoder should go on the motor. Not the screw. With fast turning motors it is unlikely they would be direct drive. For a 7000 rpm motor, you would probably have about a 4:1 ratio.The screws can take upwards of 2500 rpm depending on size. If geared 4:1 then the motor could be spinning upwards of 10,000 rpm. So you could exceed the rpm capabilities of that particular encoder.

  5. #65
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    Question Re: Open source low cost servomotor controller

    109jb,

    Correct me if I am wrong. Pulleys and belts or some sort of gear reducers would add a lot of cost to this arrangement?
    I was under the assumption this was going to be some sort of low cost project.
    JoeyB
    A doughnut a day keeps the doctor away.

  6. #66
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    Re: Open source low cost servomotor controller

    Quote Originally Posted by macdowswe View Post
    I checked ebay very briefly, and I found that there are kits available without any motors or controllers for $800, Link, and that kits with motors, driver, spindle and spindle driver are about $1300, Link. Supposing someone currently has to spend $300 on a G540 on top of the full kit (totaling $1600) to get a usable system, that leaves a $800 space to compete in.

    One approach is to just buy a VFD and a spindle motor, and subtract that off the $800 margin, and start from there. For the motion, I know there exist appropriate motors, appropriate dirt cheap encoders, and the missing piece would be some mechanical bits to put it all together.

    However, in a more interesting scenario, we go one step further: We could use two ODrives: three channels for motion and the 4th channel to drive the spindle. For the spindle, you could do something like this: Link.

    In both cases, I think we have a chance to do something interesting here ;D

    Cheers,
    Oskar
    Those are worst case 6040's which barely meet the rigidity requirements however, stiffeners on the gantry sides help and replacing the lower outer frame assembly for a steel version and the machine improves but cutting 7075-T6 becomes problematic since the table isn't rigid enough to take the down force due to the individual extrusion strips used to make up the table, solid aluminum T's bolted to a 7/16in cold-rolled steel plate solves that problem and adding a couple more rail supports stops the rail flex during plunging.

    I also don't think those specific 6040's are worth the cost since I can get a solid table and cast frame kit for $200.00 more but if you're really hard up for cash it's a good starting place if you're willing to do some modifications.

    Personally I'd go to a larger DIA round rail or use HiWin 4-way loading linear rails on a 3/8in cold rolled flat-stock (with stiffeners) lower frame assembly so cutting mild steel is possible and use one of Dale Walsh's integrated air-cooled 220V 3KW BT20 ATC spindle with 2500PPR encoder (I think he still makes it) or maybe a cheap water cooled 2.2KW ISO20 ATC spindle from china.


    Enough about frames, back to ODrive and package variants.

    Since some gantry machines like the 6040 and larger employ dual drives to move the gantry to prevent skewing, perhaps adding two jumpers wouldn't be a bad idea so that a single axis signal (such as y-axis) can drive both motors (daisy-chain) and the second jumper to invert the direction signal of the secondary channel (so both sides of the gantry move in the same direction) which is good for dual belts, rack and pinion and dual ballscrew gantry setups.

  7. #67
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    Re: Open source low cost servomotor controller

    Pulleys and belts or some sort of gear reducers would add a lot of cost to this arrangement?
    Everything adds cost. Everything.
    Whether the cost of belts and pulleys would be worth while would depend almost entirely on what specs you want. For sure, toothed belts are usually cheaper and work better than gears.

    Cheers
    Roger

  8. #68
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    Re: Open source low cost servomotor controller

    Quote Originally Posted by joeybagadonuts View Post
    109jb,

    Correct me if I am wrong. Pulleys and belts or some sort of gear reducers would add a lot of cost to this arrangement?
    I was under the assumption this was going to be some sort of low cost project.
    JoeyB
    These type motors have the power, but at higher rpms, not at low rpm. They are going to need some kind of reduction system to operate efficiently. If the example 7000 rpme motor is capable of 1kW, that is about 190 oz-in. If we assume a perfectly flat torque curve that means it only has 192 oz-in of torque at low rpm too. So, I'd say that motor needs a belt reduction or something on it. A belt drive doesn't have to cost a lot. I have 2 belt drives on my current stepper project and the timing belts and sprockets were bought on e-bay for about $12 per set. A suitable set of pulleys and belts for this type of servo drive would probably be a bit more money, but certainly less than about $30 per set.

    I personally don't envision this as a lower cost than a stepper system, or even the same cost as a stepper system, but rather a low cost for the potential performance. As far as servo drives go I think it has that potential at a much lower cost than other servo drive systems and am watching the development to see how it flushes out.

  9. #69
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    Re: Open source low cost servomotor controller

    RCaffin &109jb,

    Thank you for taking the time to explain in layman's terms, it's refreshing to get a positive point of view.

    JoeyB
    A doughnut a day keeps the doctor away.

  10. #70
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    Re: Open source low cost servomotor controller

    Quote Originally Posted by RCaffin View Post
    websrvr makes a very good point about the ability of the copper foil on a PCB to support the peak currents. Take note of his concerns.
    I have met problems here myself. It's not just the current carrying ability, its also about how you connect TO the copper. Abrupt transitions between heavy connectors and light foil can be problematic, as shown here (commercial unit):
    Attachment 343960
    I did wonder why the driver had stopped working!
    But there were other faults on the PCB as well. Eventually I replaced the 3 units with 3 Gecko drivers.

    Anyhow, there are design guides on the web for the current carrying capacity of copper tracks.

    Cheers
    Roger
    Thanks for the heads up.
    I have tried to make sure that there is as much copper as possible in the current path that is expected to carry the motor current. The copper track impedance going around the motor current loop is approximately 1 milli ohm. This is 5 times smaller than the impedance through the MOSFETs in the same current path, so hopefully the amount of copper should be fine.
    I have designed the board so that you can add a single heatsink for all the MOSFETs. There is also a thermistor placed as close as possible to each of the MOSFET group (3 thermistors total). The control algorithms will also have thermal control, so hopefully it should be fine with the current design. Of course if it turns out that things get too hot, we can always beef up the tracks at the cost of a larger PCB, or thicker copper at the cost of price.

  11. #71
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    Re: Open source low cost servomotor controller

    Quote Originally Posted by 109jb View Post
    These type motors have the power, but at higher rpms, not at low rpm. They are going to need some kind of reduction system to operate efficiently. If the example 7000 rpme motor is capable of 1kW, that is about 190 oz-in. If we assume a perfectly flat torque curve that means it only has 192 oz-in of torque at low rpm too. So, I'd say that motor needs a belt reduction or something on it. A belt drive doesn't have to cost a lot. I have 2 belt drives on my current stepper project and the timing belts and sprockets were bought on e-bay for about $12 per set. A suitable set of pulleys and belts for this type of servo drive would probably be a bit more money, but certainly less than about $30 per set.


    I personally don't envision this as a lower cost than a stepper system, or even the same cost as a stepper system, but rather a low cost for the potential performance. As far as servo drives go I think it has that potential at a much lower cost than other servo drive systems and am watching the development to see how it flushes out.

    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

  12. #72
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    Re: Open source low cost servomotor controller

    Greatest torque requirements tend to be when accelerating up to speed. This is completely dependent on the specific machine, and the performance that the user is looking for. If you want 1G accel rates, then you'll need a lot more torque than if you're fine with 0.1G accel.

    This seems like it might be a nice solution for a small, high performance machine.
    Gerry

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    (Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)

  13. #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.

  14. #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.

  15. #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.

  16. #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.

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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!!!

  18. #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?

  19. #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.

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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.

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