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Low damping rubber materials

  1. Aug 2, 2017 #1
    Cross posted this in the materials forum but i received nothing so i'm posting it here.

    << Mentor Note -- OP has been reminded not to cross-post at the PF >>

    So the application is in a pedal set for a racing simulator cockpit. The brake pedal has a high resolution pressure transducer that interprets the hydraulic pressure into an electrical signal via USB into the computer. The problem is that in order to provide a realistic brake force pedal resistance, i have to use rubber cylinders that counteract pedal movement. This is what gives me the pedal feel. The only problem is that the measured brake force lags behind pedal force due to the inherent damping within rubber. I wonder if theres something i can use (rubber or not) that has an extremely low damping characteristics, but is about as hard as a very low durometer polyurethane. I've tried different types of rubber, polyurethane foam, and solid polyurethane. Out of the 3, it seems normal rubber has the least damping, but it is not sufficient.

    Any tips?
     
    Last edited by a moderator: Aug 3, 2017
  2. jcsd
  3. Aug 2, 2017 #2
    How about a low inertia lever with a metal spring under it?
     
  4. Aug 2, 2017 #3
    .... That's actually a really good idea. I imagine a spring won't provide enough force at low deflections and likely too much at higher deflections. I'll have to plot some force/distance graphs of the rubbers to try and emulate that shape with a bellcrank.
     
  5. Aug 2, 2017 #4
    There is a simple way around the difficulty your expect. Use a soft spring (low stiffness), holding the lever against a stop with a large initial spring deflection. There will be no motion of the lever until the torque due to the applied force exceeds the torque of the spring. Then, when movement begins, the increase in resistance will be quite low due to the softness of the spring. Within limits, you can get just about any thing you want with this.
     
  6. Aug 3, 2017 #5
    A small master cylinder who's output goes to a short tube with a bleeder at the end.

    Maybe a clutch master cylinder since it is small.

    Vary the amount of air in the tube to get your brake feel.

    You don't need much.

    From there, you can either use a pressure sensor or hall effect sensors on the moving parts.......whichever one gives you the best "curve".
     
  7. Aug 3, 2017 #6

    JBA

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    On way to achieve the pedal movement/engagement motion might be to use one or more belleville washer springs that have no damping effect at all. You can combine soft, medium and high spring rate washers in the stacked washer set to achieve early light resistance while the brakes are engaging with stiffer washers that will better simulate the higher pedal resistance with little movement during actual braking with no damping effects.
     
  8. Aug 3, 2017 #7
    Stacks of Bellville washers are prone to friction damping as the deform under load.
     
  9. Aug 3, 2017 #8

    Nidum

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    Gold Member

    Use an active system . Look up 'artificial feel for aircraft controls' .
     
  10. Aug 3, 2017 #9
    Sure, always make it more complex when simplicity is available! (\snarkoff)
     
  11. Aug 3, 2017 #10
    Bellvilles are a great idea, but i have no experience using them. What kind of friction levels are typical? I assume the inherent friction/damping would be less than rubber?

    Maybe some electroless nickle coated washers would help out with that friction?
     
  12. Aug 3, 2017 #11

    JBA

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    For the arrangement I am suggesting the belleville washers should be stacked outside edge to outside edge and inside edge inside edge and in that configuration
    they have very little contact to contact friction because the outer diameter edges move together as the washers flatten; however, there can be some friction between the inside washer edges and the center guide rod the washers are stacked around and that centers them. In order to minimize that friction it would be best to use a hard alloy rod with a PNP nitriding (polished/nitrided/polished) coating process rather than plating. (Nickel plating is a relatively soft and could be damaged by the rubbing of the rather sharp edges on the center holes of the washers).
     
  13. Aug 3, 2017 #12
    And all of this is more appealing than a simple bell crank, a soft spring, and a stop that can achieve almost exactly the same force deflection relation? I would never have guessed!
     
  14. Aug 3, 2017 #13
    I had a 240z race car in my basement in the 1990s that was an empty shell with windshield and roof removed so I put 19 inch monitor on a piece of wood on the cowl and used the setup I described above with the factory master cylinder in the car and pots on the steering, brake, and accelerator.

    Was fun to play GP2 and Viper Racing on it.

    Worked great..........I had already tried springs and most springs don't have a rate progression that feels like a brake pedal.

    So.......I went to exact equipment I was trying to emulate.

    Now it is done with a clutch master and a small bit of air to get the feel.
     
    Last edited: Aug 3, 2017
  15. Aug 3, 2017 #14

    JBA

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    The problem with the bell crank and spring solution is that with this there is a smooth progression of increasing force with movement whereas, in actual braking there are three stages of braking with relatively sudden transitioning discontinuous force change points. Stage one is the force required to move the brake pads into contact against the discs (little force relative to pedal movement); stage two is the quick initiation of actual braking with the effects from the flexible brake hoses and parts under pressure (more force and considerably less pedal movement); and, stage three is heavy braking (strong resisting force and almost, but not quite, negligible pedal movement). Those with experience with high performance driving and braking are very aware and sensitive to these transitions in order to provide the necessary modulation of braking at thin edge of tire traction gain and loss experienced during competition driving.
     
  16. Aug 3, 2017 #15
    I am assuming from your description of your system creating a delay in the braking that you are using the rubber in the pedal actuator to create resistance. The rubber in turn is applying force to your hydraulic sensor. This arrangement will cause a lag just as you describe. Could you create a pedal that has a single fulcrum at the bottom but multiple pushrods higher up, possibly even a multi hole arrangement to allow for adjustment. Use one pushrod to direct force to your hydraulic sensor another pushrod would direct force to a simple rubber block for resistance.
    This type arrangement would retain the sensitivity of the hydraulics and allow a travel adjustment via position change. The other rod would have similar effects on the resistance and could be adjusted by travel and durometer changes. It would not however allow one pushrod to direct affect the other and delay sensors.
     
  17. Aug 6, 2017 #16
    Another great idea! De-couple the force feedback from the position sensing.
     
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