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I may have found a way to use cheap stepper motors for a 6dof

Discussion in 'DIY Motion Simulator Building Q&A / FAQ' started by Taylor Schweizer, May 26, 2014.

  1. Taylor Schweizer

    Taylor Schweizer New Member

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    Hi,

    I am new to this forum and to the world of simulators in general. However, I am not new at all to the world of robotics and engineering, so when I found these projects they seemed like a great way to learn a lot (emphasis on A LOT) as well as have a tremendous amount of fun. I began toying around with the idea of making a 6dof Stewart Platform simulator, and was instantly appalled at the estimated price. The only motors I found that would work well for this and were relatively cheap (which I believe were presented by a member of this forum) shipped from Australia, and I simply can't afford that. So I began looking for ways to make this cheaper.

    I had the idea to use springs to support the chair, much like the center pivot spring in a 2dof. I noticed that a lot of people didn't think they would work, but the more I crunched the math the more I hoped it would work (and the more I hoped I wasn't just forcing the equations to show me what I wanted to see). I came up with a simple sketch of this idea, shown here (click image to see the full size.)
    [​IMG]




    As you can see, there are three shafts with the same heim joints (rose joints) connected to a sub-base (just a part of the base) which support the platform. This shafts are split by a spring, and use a locating dowel and a linear bearing to stay co-linear.



    I modelled a very simplistic version of this sketch. I applied a 300 lb (1340 N)force to approximate the weight of the chair and me and probably 70 extra lbs to the platform, and simulated a motor rotating at a max speed of close to 120 rpm. There is a video of the simulation (8 seconds) here
    .




    I simulated this motion with and without the springs, and plotted the results of the torque, displacement, velocity, and acceleration of the motors. The results are here [​IMG] .



    As you can see, the use of springs greatly reduces the amount of torque needed by the motors. I did this all as a simple proof of concept, to make sure the equations I was getting on paper were correct, and to make sure I wasn't wasting time on this thought process. With these results, I now have the information needed to tighten up my model and make it much more realistic, include interference detection (some components were hitting each other in that simulation; this didn't affect the torque but it will when I run the accurate simulation), and include a mass/gravity combination instead of just a force.

    In the future, I will hopefully purchase a chair suitable for a simulator this coming week and use it to model a frame so my center of gravity will be close to my center of rotation. I will also do further calculations to make sure I have the best arm length/rod length/spring force possible, and hopefully reduce my torque to below 5 N*m or so. If I can accomplish this, I can use some 24V stepper motors that produce roughly 4N*m of torque at 120 rpm (which cost only $50) to directly drive the arm without the need for any gearbox.

    I apologize for this being such a long wall of text, but I am hoping this information can be of use to anyone and everyone. Also, as I said this simulation was very "down and dirty", so I can't say I'm 100% positive my theory was correct. If anyone notices any shortcomings, or has any advice or any opinion at all I would greatly appreciate it. If you made it this far, thank you for taking the time.

    Edit: Here is a picture showing the springs and the 300 lb force: [​IMG]
    • Informative Informative x 4
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    Last edited: May 27, 2014
  2. RacingMat

    RacingMat Well-Known Member Gold Contributor

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    Hi Taylor!

    Welcome here! and thanks for sharing your ideas :)
    I'll take time to read this carefully!

    Your post would be even more interesting if you insert the pictures inside!
    Lucky day, this forum is the world greatest for this: simple Drag-and-Drog of picture onto the post!! Thanks @RaceRay
    Mat
    • Like Like x 1
  3. Taylor Schweizer

    Taylor Schweizer New Member

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    Thanks for the heads up! I spent a little time formatting, hopefully it is a bit more engaging now.
  4. Historiker

    Historiker Dramamine Adict Staff Member Moderator Gold Contributor

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    Hi Taylor, great starting thread. Do you have a specific stepper motor in mind? What controller/driver are you thinking of using? My CNC table uses a Gecko G540 driver/controller but I have no idea how I would use that with Simtools, lol but after your post I am thinking of cannibalizing it! :D
  5. eaorobbie

    eaorobbie Well-Known Member Staff Member SimTools Developer Gold Contributor

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    This does sound very interesting, we have used stepper motors in the past to drive dashboards and I too have played NEMA units in building a single linear actuator, work well but on issue was loading on the units main shaft caused the stepper to jam. So if you support the shaft where the lever is with a bearing or something simular this wont be a problem, Seen wipers used in 6dof so the torque from the steppers might be fine.
    Cool thinking Well Done.
  6. adgun

    adgun Member

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    This is relative new on the market,but could be great.
    Sheap on ebay
    regards Ad

    Attached Files:

  7. eaorobbie

    eaorobbie Well-Known Member Staff Member SimTools Developer Gold Contributor

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    @adgun yes bigger the better. Cost I think is the issue nema steppers can be driven easily with a cheapo Ard too.
  8. adgun

    adgun Member

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    hi eaorobbie
    I didnt think on a ard.
    But please open the file,this is a cloosed loop stepper system .
    This wil become the standard in stepper driven nc machinery in the future and will become sheaper to.
    regards Ad
    • Agree Agree x 1
  9. Taylor Schweizer

    Taylor Schweizer New Member

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    @Historiker thanks! I'm not entirely sure yet, I've still got to look more at the software, but I'm thinking I would use something like these (or perhaps an ebay special, found a pretty good deal here). But I will control them with a simple driver like this. For a controller, I plan on making my own. I only need to communicate with the driver with 5 volts, so I will probably make my own atmel microcontroller board similar to an Arduino (I make my own circuit boards and have enough components laying around). Like I said, I have to look more at the software and how to work out the configuration first.

    @eaorobbie Thank you for pointing that out! I didn't even think about that. Yea I will definitely support the shaft with a bearing now.

    @adgun That is very similar to what I will make, but a little out of my price range. Most of the differences between that and a plain stepper motor I plan to build myself. But for someone with more funds, a hybrid stepper would definitely be the way to go.

    Thank you everyone for taking the time to read this.
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  10. Historiker

    Historiker Dramamine Adict Staff Member Moderator Gold Contributor

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    Hi Taylor, would you consider basing your controller on an Ardiuno instead? This way it would be much easier for others to build the same rig if it should prove successful. We are all about sharing with the community whenever possible. ;)
  11. Taylor Schweizer

    Taylor Schweizer New Member

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    Historiker, sure! I will try to use something like the Arduino DUE, since it has enough pins and a fast enough clock speed. However, if it isn't fast enough and I need to use a different microcontroller I can definitely post the electronic schematics and all that. But I will try to see if the DUE can work.

    I don't suppose you would know where I can find more information about the software? From what I've seen so far, it looks like the PID is not controlled by Sim Tools, is that correct?

    I also have a few of these laying around, it appears they would be useful for tuning purposes.
    • Like Like x 1
  12. ferslash

    ferslash Active Member

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    man thanks for sharing... but in the animation you posted (nice animation by the way) i did not understood where the springs comes... where are they?

    best regards

    fer
  13. insanegr

    insanegr !N$@n€

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    @fer as i can see from the middle of lower frame to the middle upper frame its like a "suspension" helping the upper frame to lift with free moves. taylor will say for sure nice thought btw
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  14. eaorobbie

    eaorobbie Well-Known Member Staff Member SimTools Developer Gold Contributor

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    PID is done inside the controller you want to use, this is the hard bit in most codes , Most run a simple power scale and not a true pid calculation as this is too much for most to handle, there is a Ard lib that covers this but once you start sending commands( positions) to it at 20ms the Ard can not keep up , with 2dof so a simple power scale is used in control of dc motors

    Heres a little hint: ( I have been investigating stepper use for Sim for a while now, my little side project goes well)
    Use a dedicated stepper driver , they are dirt cheap $10 and this driver will only require 2 pins for one motor control.
    A Dir(Direction) , high go one wat and low go the other way , and a Pulse(Step) to tell the driver to move one step.
    With this kind of control we have had 6 steppers on a dashboard moving very quickly with 6 separate sets of data hitting it at 10ms intervals and no lag in the stepper reading.

    http://www.pololu.com/product/1182
  15. Taylor Schweizer

    Taylor Schweizer New Member

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    @ferslash, @insanegr is correct. Here is a picture that shows the springs and the force: [​IMG]

    @earobbie I actually have a few of those exact drivers laying around! I just might need something with more amperage. The drivers I linked to are similar, they just require two pins for step and two for direction (not sure why, haven't had time to look at it in detail yet).

    As far as the calculation speed, one of the reasons I want to make my own microcontroller board is because the fastest arduino is the arduino due, clocked at 84mhz. Atmel sells a 500mhz 32bit microcontroller for around $15. I could easily use that to run PID, as well as use my favorite filter, the savitzky golay filter.

    As I said, I haven't had much time to look more into the electronics and software yet, but until then I greatly appreciate any advice.

    Sent from my SCH-R530U using Tapatalk
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    Last edited: May 27, 2014
  16. ferslash

    ferslash Active Member

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    my 2 cents... :D

    the most commond motors used here are wipers (i guess :D) and another guy from this forum made a 6dof sim only using 6 wipers (that are the cheapest alternative by the way, and you can get them also "used" easely)... that guy was not using any springs... so...

    if you could use that kind of motors... :D more people would be coping your work (wich is a grate thing)... :D

    best regards

    fer
  17. MWB

    MWB Drive it like you stole it!!

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    This is brilliant, great simulation
  18. SeatTime

    SeatTime Well-Known Member

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    Yes, from 2014, but unfortunately that is as far as it got..Designing and building are two different animals.
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  19. mariano68

    mariano68 Active Member

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    Yes, it is sad to see this kind of threads vanish.
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  20. Avishka Adikari

    Avishka Adikari New Member

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    Hi Taylor,

    I'm an undergraduate studying engineering, Im soo thrilled to see your design and the adaptations that you have made to the general setup.
    But when i tried to make 3D CAD design, i failed in joining the drawn components. I believe this is because of an error i made in calculating the parameters for the Steward platform ( arm lengths, platform dimensions, etc). Ill be much obliged if you could share the parameters used for this drawing. I believe it might come handy in my next attempt.