Approach and Departure Angles in mSMAC?

General questions on the SMAC, msmac3D and other Collision Simulation programs
MSI
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Approach and Departure Angles in mSMAC?

Post by MSI »

Q: I'm looking at the run summary. Can you point out the approach and departure angles? Is it PSI1C and PSI2C for approach and PSEP1 and PSEP2 for departure?

A: yes.
In the View->Run Summary there is listed IMPACT and Separation
  • Here are the values from the sample My First Project:
    IMP and SEP for My Forst project.jpg
    IMP and SEP for My Forst project.jpg (111.65 KiB) Viewed 990 times
A few things to note:
  • 1) As you noted PSI1C and PSI2C are the IMPACT Angle
    2) Note the summary also includes the INITIAL Conditions (see below) which may be the same heading angles as the IMPACT
    • unless pre-impact steer
    • You use term APPROACH angle we use IMPACT angle since vehicles can approach and not hit!
    3) As you also noted, the SEPARATION CONDITIONS list the PSEP1 & PSEP2 which are the separation angles
    • point where the vehicles are no longer in contact
    4) The test for collision interaction in mSMAC is
    • 4a) when the forces of collision interaction which cause accelerations goes above 1 G-unit it is considered IMPACT
    • 4b) When the forces of the collision interaction which produce accelerations goes below 1 G-unit it is considered SEPARATION
    • 4c) This assumption is based on the fact tire forces generally only produce accelerations below 1 G so if the accelerations are greater than 1 G, some other forces are occurring which means a COLLISION
    5) Note that the positions change between impact and separation
    6) If you apply simple momentum to these angles you may get differences from SMAC results
    • momentum generally considers the approach/departure angles applied at same location
    • vehicles move between impact and separation
    7) Lastly note the time listed for IMPACT and SEPARATION. IT IS NOT INSTANTANEOUS! another reason for differences.
    • 0.538 sec - 0.414 sec = 0.124 seconds, NOT INSTANTANEOUS
See our paper CRASH 97 for a discussion of some of the issues of applying simple momentum and why some vendors (pc-crash, virtual crash, etc.) require an arbitrary and subjective 'point of maximum engagement/momentum exchange' to compensate for the simplifications particulalry the assumption of an INSTANTANEOUS EXCHANGE of momentum at an arbitrary and subjective single point!

Here is a discussion of the technique we used in CRASH 97
  • we never created a commercial version of CRASH97 since computer technology advanced to allow us to automatically iterate/optimize SMAC, see paper SMAC 2003.
crash97 logic.jpg
crash97 logic.jpg (150.59 KiB) Viewed 990 times
Here is a discussion of momentum issues:
momentum simplifications.jpg
momentum simplifications.jpg (114.15 KiB) Viewed 990 times

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