1. For downloading SimTools plugins you need a Download Package. Get it with virtual coins that you receive for forum activity or Buy Download Package - We have a zero Spam tolerance so read our forum rules first.

    Buy Download Package Now!
  2. Do not try to cheat our system and do not post an unnecessary amount of useless posts only to earn credits here. We have a zero spam tolerance policy and this will cause a ban of your user account. Otherwise we wish you a pleasant stay here! Read the forum rules
  3. We have a few rules which you need to read and accept before posting anything here! Following these rules will keep the forum clean and your stay pleasant here. Do not following these rules will lead to permanent exclusion from this website: Read the forum rules.

Some interesting takes on Stewart platforms

Discussion in 'DIY Motion Simulator Building Q&A / FAQ' started by Zed, May 25, 2018.

  1. Zed

    Zed VR Simming w/Vive

    Joined:
    Apr 4, 2017
    Messages:
    665
    Location:
    USA
    Balance:
    3,595Coins
    Ratings:
    +687 / 2 / -0
    My Motion Simulator:
    2DOF, DC motor, JRK
    @Jerry Atrick - I can't say for certain about the code. I don't remember seeing anyone mention settings or custom code for linear actuators vs. gearmotors so kind of think it's not an issue. I might have missed it, though. But like worm gear gearmotors, if you limit travel to a smaller arc it's more linear of a response. The same would be true with the sliding mount hexapod. Stay closer to the bottom of the graph and you'll be more linear. But since the effort to move will go way down near the top of the graph, there will be some speedup of the motors As they unload and an effective linearization of the response. How effective? Hard to say.

    Trying to decide if i want to try building this. Leaning towards it though and using a bit from my 3D printed test platform. I think building three petals that bolt together would make it easier to build and move. And just building a single petal at first limits investment in case there are issues. We know it works. You can see that in the videos. I'm looking more for flying in VR though so less movement shouldnt be as much of an issue as it might be for driving. I'm also looking for compact and this could be as compact as a gearmotor sim since the mechanisms could all fit within the base outline.
  2. bruce stephen

    bruce stephen Hammer doesnt fix it, must be electrical SimTools 2.0 Beta Tester

    Joined:
    Jun 7, 2015
    Messages:
    1,277
    Occupation:
    general contractor
    Location:
    michigan
    Balance:
    8,602Coins
    Ratings:
    +1,179 / 7 / -0
    My Motion Simulator:
    2DOF, DC motor, Arduino, Motion platform, 6DOF
    if each side is combined (like the scissor jack example) you will get less control over movement. if you set each motor to move independently you will have a little more control. i think to add this at the back of a traditional 6dof (lever style) would help reduce the rise and fall effect while using TL.
    Last edited: Jun 3, 2018
  3. Zed

    Zed VR Simming w/Vive

    Joined:
    Apr 4, 2017
    Messages:
    665
    Location:
    USA
    Balance:
    3,595Coins
    Ratings:
    +687 / 2 / -0
    My Motion Simulator:
    2DOF, DC motor, JRK
    Hey @bruce stephen - if I understand what you are saying then you might have misunderstood BlazinH's example. Each leg would have its own mechanism and control for it to be a 6 dof. Work them like a scissors jack and it becomes a 3 dof.

    But I don't understand what you mean about adding this to the back of a lever style 6 dof or rise and fall effect of using TL? What is TL?
  4. BlazinH

    BlazinH Well-Known Member SimTools 2.0 Beta Tester

    Joined:
    Oct 19, 2013
    Messages:
    1,470
    Location:
    Oklahoma City, USA
    Balance:
    10,486Coins
    Ratings:
    +1,319 / 28 / -1
    TL for Traction Loss. I don't know if a custom controller would be necessary but it appears most of the time one motor will need to ramp up either slower or faster than the other. So the PID will have to do more work on one side than the other. It may not be an issue but if it is I would make a custom controller that has variable P depending on the current position of the feedback. So P would be smallest at the bottom of the stroke and get larger at it reaches the top where it needs more speed to move proportionally. Likewise I may do the same with "I" but "I" would be largest at the bottom and least at the top.
    Last edited: Jun 3, 2018
  5. Zed

    Zed VR Simming w/Vive

    Joined:
    Apr 4, 2017
    Messages:
    665
    Location:
    USA
    Balance:
    3,595Coins
    Ratings:
    +687 / 2 / -0
    My Motion Simulator:
    2DOF, DC motor, JRK
    I really don't see it as a problem. It would be basically the same situation as with worm gear gearmotors with an output arm. Those are non-linear and those would generally have mismatched output arm positions.

    There could be some spurious rise or fall with the traction loss, but I'd be real surprised if the average user would even notice. I haven't seen people discussing that as an issue with those but maybe I've missed it.
  6. BlazinH

    BlazinH Well-Known Member SimTools 2.0 Beta Tester

    Joined:
    Oct 19, 2013
    Messages:
    1,470
    Location:
    Oklahoma City, USA
    Balance:
    10,486Coins
    Ratings:
    +1,319 / 28 / -1
    I agree but I believe there would be more discrepancy than there is when using worm gear motors; especially when using only 90 degrees of a lever (which if you're using more your lever is to short and your motor is probably under-powered)
    I generally agree here too but apparently its obvious enough to @bruce stephen or he wouldn't have brought it up.
    Last edited: Jun 3, 2018
  7. Zed

    Zed VR Simming w/Vive

    Joined:
    Apr 4, 2017
    Messages:
    665
    Location:
    USA
    Balance:
    3,595Coins
    Ratings:
    +687 / 2 / -0
    My Motion Simulator:
    2DOF, DC motor, JRK
    Bringing it up is one thing. Actually feeling it and being disappointed by it is another. We can correctly note that the mechanism is non-linear but until someone builds one, and it's actually used for a motion sim, we don't know how well it will work in practice. And if you want to restrict the output of a worm gear gearmotor to a more linear region, you can do the same with this setup. You just stay more in the lower region of the figure which also lets you use shorter rails and ball screws.
    • Agree Agree x 1
  8. bruce stephen

    bruce stephen Hammer doesnt fix it, must be electrical SimTools 2.0 Beta Tester

    Joined:
    Jun 7, 2015
    Messages:
    1,277
    Occupation:
    general contractor
    Location:
    michigan
    Balance:
    8,602Coins
    Ratings:
    +1,179 / 7 / -0
    My Motion Simulator:
    2DOF, DC motor, Arduino, Motion platform, 6DOF
    when I use traction loss(TL) to move the rear of my rig in one direction or the other, because i'm on levers it also causes the rig to move slightly up or down (due to the non-linearity and arc). If I used this setup under the rig you could have tl on a lever style 6dof without the up and down. it could be set up separately from all of the other effects.

    EDIT; yes its very noticable. not just a little. it requires a ton of tuning to minimize thus one of the reasons for my current migration to linear actuators. surge is the same but the rear of the rig can be piched to overcome it. sway isnt as bad.
    • Informative Informative x 1
    Last edited: Jun 4, 2018
  9. Zed

    Zed VR Simming w/Vive

    Joined:
    Apr 4, 2017
    Messages:
    665
    Location:
    USA
    Balance:
    3,595Coins
    Ratings:
    +687 / 2 / -0
    My Motion Simulator:
    2DOF, DC motor, JRK
    @bruce stephen - I'm guessing you ar driving mostly? I'm curious what angle you are using on your actuators?

    Thanks for the info. These are the things that matter to choose a particular kind of sim.
  10. bruce stephen

    bruce stephen Hammer doesnt fix it, must be electrical SimTools 2.0 Beta Tester

    Joined:
    Jun 7, 2015
    Messages:
    1,277
    Occupation:
    general contractor
    Location:
    michigan
    Balance:
    8,602Coins
    Ratings:
    +1,179 / 7 / -0
    My Motion Simulator:
    2DOF, DC motor, Arduino, Motion platform, 6DOF
    angles dont matter as much as lever length. short lever = less movement but smaller less noticable arc. I try to stay close to 45degrees on the sides but the rear is set differently for a balance of TL and heave. I use 110mm levers.
  11. Jerry Atrick

    Jerry Atrick this is my sim Gold Contributor

    Joined:
    May 3, 2017
    Messages:
    102
    Occupation:
    Retired
    Location:
    Sydney Australia
    Balance:
    902Coins
    Ratings:
    +33 / 0 / -0
    My Motion Simulator:
    4DOF
    Hi Zed, among other things, all I've done in the past 4 days is measure, plot and sketch this octopod thing trying to fit it into my needs for a platform to do a test on; however to make it compact enough is just troublesome.

    The problem I see is the hight of the platform, by the time I add somewhere to sit it's way to high, (for me that is) and the more I try to lower it the greater the angle is, and it is obvious the greater the power needed to shift the actuators.

    And what didn't help the fantastic servos I'd chosen couldn't be used on AMC1280USB, "so bummer"

    So to cut a long story short; I will stop while I'm behind!

    Back to the drawing board!!!!!!!!

    Regards Jerry.