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Please Help calculate mechanical energy

  1. Feb 9, 2009 #1
    I am a new PE member. I am not a physicist but would appreciate any and all assistance offered.

    I am attempting to calculate the quantity of mechanical energy available at an automotive shock absorber. The end result of this exercise would be to replace the existing shock with an electrical generator. I would like to break down the exercise into manageable chunks, manageable for me. Please feel free to ask for more information, point out errors, etc. I am new to this and am in over my head.

    Some real world data has been taken as a starting point. A linear measuring device was attached to the existing shock absorber. The zero point was set with the shock at rest approximately at mid travel. Data sampling was conducted at intervals of 1000 per second. This was collected for 65.534 seconds. To summarize. A table consisting of two columns, the first being the time index, the second being the distance from the "'zero" point. Makes up the data set we have.

    The first step, calculate the total travel distance of the sensor during that time span. I separated the positive travel values and the negative travel values into two additional columns. Summed the positive values into a positive travel total, and the negative values into a negative travel total. Removed the negative sign from the negative travel total, summed it to the positive travel total. This gave me the absolute total travel of the sensor for the time span previously mentioned.

    The next step, calculate velocity. With the travel distance of 3316.47 inches during 65.534 seconds resulted in an average velocity of 50.607 inches per second (rounded to three places). Reading the forums here at PE I understand that a mass value is also required. Please use the value of 100 pounds.

    This is where my understanding ends. With an end goal of a power figure in Watts or Joules. how would one get there from here.

    Thank you
  2. jcsd
  3. Feb 9, 2009 #2

    From time to time I run across posts where inventors seek to convert this kind of motion into useable energy, perhaps to partially power the auto. Is that your goal?

    The hysteresis in a shock absorber is essential to its operation, and hysteresis implies a loss of energy so far as I can see. You could, I suppose, replace a shock with a generator feeding into a large variable resistor, though I'm not sure at the moment how to vary that resistance in any controlled manner.
  4. Feb 9, 2009 #3

    Yes that is the ultimate goal.
    Specifically using the Goldner device as described in US patent 6952060.
    We want to calculate the maximum potential energy available.
    We understand that there will be energy losses all along the extraction/conversion process, but as the say you have to start somewhere.
  5. Feb 9, 2009 #4
    You can model the shock absorber as a simple spring as a starting point to get an idea of the system. The potential energy of a simple spring is


    where U is the potential energy, x is the distance traveled from the equilibrium point, and k is the spring constant and is determined by the force required to compress the spring a given distance so the units of k will be lbs/in. This simple model assumes k is a constant. If you want the potential energy units in Joules which then easily converts to watts, use metric units.
  6. Feb 9, 2009 #5
    But, if you take the energy out of a shock absorber, you've got quite a rough ride.
  7. Feb 9, 2009 #6
    I can't answer that definitively, the prototype isn't complete.
    But my gut, if you will, tells me yes.
    In terms of substituting a shock absorber.
    I believe that if no load is placed on the electrical output the spring will oscillate as if there were no damper.
    If 100 percent of the available energy it tapped, the device will lock-up solid.
    Therefor controlling the load placed on the output will in effect control damping.
    We are not to this stage yet, but microprocessor control will allow adjustable real-time damping.
  8. Feb 9, 2009 #7
    Yes, that should work. Are you familiar with the units being developed by Renaissance Technologies in Frederick, MD?
  9. Feb 10, 2009 #8
    No, I am working with Electric Truck LLC developing the patent held by Tufts U.
    We need to put together some rough power estimations, that is why I am here asking.

    The only other unit I am aware of was made by Argonne.
    I have heard it vanished without leaving behind any residual energy, (Read! a feeble attempt at a physics joke)
  10. Feb 14, 2009 #9


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