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  • Re: Inv.Dynamics w. DADS

    On Apr 8, 7:19pm, Arnim Henze wrote:
    >
    > since about one year ago besides other simulation software we have
    > the simulation system DADS in our institute. We increasingly use it
    > for different kinds of projects - direct and inverse dynamics.
    >
    > Now, I wonder whether we are the only ones, that have problems using
    > DADS for invers dynamics:
    >
    > DADS uses as input data only the positions and orientations of the
    > segments w.r. to time and calculates the 1st. and 2nd. derivatives
    > itself. The problem is, that all input data are interpolated with
    > cubic splines. The 2nd. derivative of these splines are straight
    > lines from time-step to time-step. These zig-zag 2nd. derivatives
    > lead to zig-zag forces and torques calculated by DADS. DADS's results
    > strongly oscillate around those calculated by other simulation software
    > or 'handmade inverse dynamics'.
    >
    > To those of you, who do inverse dynamics with DADS: Do you also
    > have this problem and if so, how do you work around it without
    > too much effort?
    >
    > Or - having DADS - do you use other tools to do inverse dynamics?
    >
    > Thanks in advance for any information,
    >
    > Arnim Henze.
    >

    Hi Arnim,

    At present I'm using a Mechanical Analysis package called ADAMs to calculate
    joint torques. The data input into the model, like yours, is joint angles
    w.r.t. time. The software then fits a cubic spline to this data....hence I have
    the same problem.

    The only solution I can suggest, and I'm about to try this myself, is a two
    stage approach. I.e., step one use the displacement data to calculate the
    velocities, step two, use the calculated velocity to drive your model, and fit
    the spline to -this- data.

    Alternatively, hastle the software company into writing a higher order curve
    fitting function !

    If I have any luck I'll let you know,

    Gail Jeffries.




    --
    Gail Jeffries Fax: +44 (0)1784 472879
    Design Researcher Phone: +44 (0)1784 431341 Ext. 244
    Brunel University Email: gail.jeffries@brunel.ac.uk
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