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Sloppy Seatmover due to back-drive in gearboxes

Discussion in 'DIY Motion Simulator Building Q&A / FAQ' started by PMK, Mar 19, 2020.

  1. PMK

    PMK New Member Gold Contributor

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    I've just finished putting together my seat mover using 2 of the 200W Motion Dynamics motors. (20NM, 180RPM).

    The motion is quite "sloppy" due to the gearboxes being back-driveable I think. Everything is balanced over the centre of gravity.

    I'm currently using 20mm CTC levers which gives me enough speed (~300mm/s) and enough to torque to throw me around it just feels "sloppy".

    Is the only way to fix this to upgrade the gearboxes with ones that can't be back-driven?

    My current design is similar to the DOF Reality M2 with the motors mounted below the platform.

    20200320_075532.jpg
    Last edited: Mar 19, 2020
  2. Ads Master

    Ads Master

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  3. noorbeast

    noorbeast VR - The Next Generation Staff Member Moderator

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  4. prodigy

    prodigy Burning revs

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    What does "sloppy" really means? Like slow moments? Jerky?

    Can you do a video?
  5. PMK

    PMK New Member Gold Contributor

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    The backlash isn't really an issue. I'll try and record the issue later.

    When I sit in the seat I can lean left/right/forward/back and move the CTC levers. So the lever moves causing the pot to move which results in the PID control to kick in and re-center it. There's no issues re-centering the platform once it's been displaced, it's this initial displacement that is the issue, it's only a few mm but any reduction in play would be best.

    I think this is carried forward to when I'm driving, the motors push me to a specific position and then "stop" but they overshoot very slightly and need to return back.
    I guess I could account for this by turning up my derivative gains?

    I assume with higher ratio gear-boxes this initial displacement won't happen and the gearboxes will lock when in position?
  6. noorbeast

    noorbeast VR - The Next Generation Staff Member Moderator

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    That sounds like a design flaw and/or hardware settings issue, as when powered on the system should be capable of maintaining management of the mass involved.
  7. SeatTime

    SeatTime Well-Known Member

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    I notice that you have a hand made universal joint. Maybe something to look at. When I had a seat mover, the only uni joint that I found that was suitable under load was a big beefy one from a car drive shaft- all others seem to have some slack/flex under load which cause noticeable unwanted movement.
  8. PMK

    PMK New Member Gold Contributor

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    I think this is actually a big issue too. I've noticed it has a few degrees of play in the yaw axis. I've been trying to find a joint from a local car wrecker but no one seems to have any...

    I can buy the actual U joint from somewhere like Supercheap Auto but I don't believe I can buy the yoke arms anywhere.

    I can source a joint from a motorbike such as this:
    https://www.ebay.com.au/itm/Univers...264938?hash=item2cf11548aa:g:NBEAAOSwEOpdQYG6

    Would that be strong enough?
  9. SeatTime

    SeatTime Well-Known Member

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    Hard to tell, but likely better then what you have.. I original had something of a similar size from a car steering column, but it had too much slack/flex under load, which is why I went for the larger one from a car drive shaft - it was rock solid, even under load :).
    • Like Like x 1
  10. PMK

    PMK New Member Gold Contributor

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    Here's a video of the issue.
    https://1drv.ms/v/s!AjztARxhETXYgZdYZFwdedOnYOc1eg

    This is from me just shifting my weight around in the seat back and fourth.

    I can actually reproduce the issue from just pushing on the CTC lever with my thumb, I feel like this is WAY below what these motors should be able to hold?

    https://1drv.ms/f/s!AjztARxhETXYgZdclUgTF_PE05Bycg

    This is my pot setup, the gears have no play between them, roughly 2:1 ratio.
    20200320_204925.jpg

    Here's a photo of the variables plot while I push and pull the lever. Am I reading it correctly that my PWM is only reaching 40% peak? Why is it not hitting 100% when displaced?
    20200320_205506.jpg

    PID settings are: 0.8, 0.001, 1.0
    All the motor settings in the JRK config are set to 100% or no limit
    Last edited: Mar 20, 2020
  11. MarkusB

    MarkusB Active Member Gold Contributor

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    I also once used the Motion Dynamics 25:1 motors, and as far as I remember I was not able to move the levers by hand when the motors were under power.
    The fact that you are able to do this so easily is not good.

    What is the spec of your power supply? Can it be that it is too weak for the motors?
    • Agree Agree x 1
  12. PMK

    PMK New Member Gold Contributor

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    I'm using a 650W power supply, I don't think current is the issues as it can supply 54A on the 12V rail and it doesn't trip.

    I think I've found the issues though!
    My PID period was set to 25ms to account for a high analog sampling value which was pretty stupid for real-time control in hindsight...

    I turned that down to 2ms and I'm now using a sampling value of 64, effectively going from 4hz control to 50hz control... This made a huge impact in the responsiveness.

    The other issue is the potentiometers I'm using. They're terrible and have a pretty big deadzone, they don't even detect the play in the gearboxes.
  13. SeatTime

    SeatTime Well-Known Member

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    I would also like to see bigger gauge cable used for the motor drive/power supply circuit. At least slightly bigger then what is coming out of the motors.
  14. PMK

    PMK New Member Gold Contributor

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    Yup, that's the next step now all these issues are resolved. Would AWG 12 gauge be suitable? The current cables are AWG 16 for reference.