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View Full Version : Re: Inv.Dynamics w. DADS



Gail A Jeffries
04-08-1997, 04:06 AM
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