Structure of the Shock JSON spec

[Thorsten Wittenator]

Hi all,

I am currently working on creating a digital twin for an existing car in Chrono. I am currently using the DoubleWishbone suspension template since I faced numerical instabilities with the ReducedDoubleWishbone suspension version. Moving most of the data was not a problem, but I was a little lost at the shock element. What exactly is specified in the Deformation and Map Data attributes? Is it damper travel in "Deformation" that is used to index the "Map Data" columns starting from the second entry? Is the first the deformation velocity? I have a damper that has a constant damping force over the damper travel and only depends on the velocity of the deformation. Would I then only have one or two dummy entries in each row of "Map Data" ? I tried to answer my questions with the documentation but couldn't find too much on that.

Sincerely,

Thorsten

[Radu Serban]

Hi Thorsten,

 

The characteristic of a shock can be specified as:

1. Linear damper:  F = -C * v
   where v is the rate of change of the shock length (= rate of change of shock deformation).
   In a JSON specification file, all you need to provide is the constant C through the key “Damping Coefficient”.
2. Nonlinear damper: F = -mapC(v)
   where mapC is some nonlinear function of v.
   In a JSON file, you provide mapC as a N-by-2 table of pairs [v_i, mapC_i] using the key “Shock Curve Data” (in other words, you provide data at N velocity values and this data will be used for piecewise linear interpolation).
3. General nonlinear map: F = -map(l – l0, v)
   where map is a nonlinear function of both shock deformation and its rate of change (this is needed for some special types of shocks that depend on both).
   To provide this information in a JSON file, I had to somehow encode a 3D table.  This is done by providing M values for the deformation (“Deformation” key) and then a table (key “Map Data”) of size N-by-(1+M) which contains in each row a value v_i and the values map_ij (for j=1,…,M).  This data is used for a 2D piecewise linear interpolation.

 

Note that the underlying implementation of the shock uses a ChTSDA force element which accepts an arbitrary functor (derived from ChLinkTSDA::ForceFunctor). If you write your own C++ class for a concrete wheeled vehicle suspension, you can use that to implement any nonlinear function of both length and length rate of change (by overriding the evaluate() method).  For suspension shocks (and springs) specified in a JSON file, I provide the 3 approximations above (based on scalar or tabular data and using piecewise linear interpolation where needed).

 

ChSubsysDefs.h and ChSubsysDefs.cpp contains several predefined derived functors of type ChLinkTSDA::ForceFunctor (mostly used when reading a suspension specification from a JSON file, but which could also be used in a user-provided suspension implementation file).

 

See HMMWV_DoubleWishbone.cpp or FEDA_DoubleWishbone.cpp for examples of user-provided shock functor.

 

See the JSON files for the HMMWV suspensions for examples of #1 and #3 above.

 

--Radu

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