New Open Source PC-Crash clone: PyCrash!

General Questions related to the Momentum Based Analysis programs
#pc-crash #virtualcrash #crash
MSI
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New Open Source PC-Crash clone: PyCrash!

Post by MSI »

There's a new kid in town on the 'instantaneous momentum exchange' software horizon.
  • Pycrash: An Open-Source Tool for Accident Reconstruction
    • From Abstract:
      • In this paper, the current capabilities of Pycrash are illustrated and its accuracy is assessed using matching PC-Crash simulations performed using PC-Crash. The results indicate that Pycrash is wellequipped to perform fundamental accident reconstruction analyses, including impact related effects, but its simplified suspension model is a limitation. It is hoped that others within the accident reconstruction community will use Pycrash, make suggestions for its improvement and develop additional capabilities.
        There are basically three main categories of applications within Pycrash:
        1. Fundamental equations and methods for general reconstruction,
        2. Planar vehicle motion and,
        3. Planar impact mechanics.
        The goal of this paper is to illustrate the current functionality of Pycrash along with a comparison to PC-Crash simulations and crash test data to illustrate its capabilities and provide validation data for future use.
    • From Fundamental Calculations
      • Pycrash can be used to perform various fundamental calculations as you would use a spreadsheet application. These modules allow the user to evoke equations described in accident reconstruction publications for tasks such as deriving A and B stiffness values from crash test data and solving for closing speeds in using impulse-momentum equations. Additionally, processing the load cell and acceleration data from the ascii files obtained from the NHTSA Crash Test Database is automated. Pycrash users can take advantage of higher-level open-source scientific packages (e.g., Scipy) to perform a wide range of analyses on the results produced by Pycrash.
    • From Vehicle Motion
      • Pycrash will simulate vehicle motion when provided initial conditions and driver inputs (brake, steer, throttle). Tire forces are calculated using a linear model as described by Steffan et al. for PC-Crash. Ackerman steering is used to determine the steer angle of each tire based on a prescribed steering wheel angle and steering ratio. Weight-shift due to longitudinal and lateral acceleration is accounted for using a simple pendulum model based on a prescribed center of gravity (CG) height. The CG height within Pycrash is the height at which the vehicle mass acts rigidly with the vehicle chassis. Therefore, the greater the assumed CG height, the more weight shift will be applied for a given level of vehicle acceleration.
        This model assumes a linear growth of tire cornering forces using a slope set by the program at 0.1 g per 1 degree of slip. This growth saturates at the user defined global friction value. Throughout the comparison simulations, the Kinetics simulation model at a 5 ms integration time step was used. Simulations were conducted assuming flat terrain, and global friction was set to 0.76 g. All vehicle motion simulations were performed with a time step of 0.01 seconds
    • Here is a link to the open source Python pyCrash program
    COMMENTS:
    • BRAVO! Congratulations!
      Great paper and great tool to add to the crash reconstruction community!
      • It illustrates that although pc-crash has been around since 1998,
        • it is an extremely simple program and pc-crash have done very little to try to improve upon the limitations of the simplifying assumption of 'instantaneous momentum exchange'.
        • And this is not to take anything away from the great accomplishment of the authors of pycrash as this was obviously a lot of work and a great accomplishment to create this tool
      • The authors compare their results to pc-crash results
        • We have on several occasions started a discussion of the 'validation' of pc-crash.
          • However like the CRASH3 momentum solution simplified momentum with 'instantaneous exchange' is very limited in accuracy in many applications.
          • In our 1997 CRASH paper we improved on the simplified momentum solution of CRASH3 (The Trajectory solution procedure, it also has damage analysis) by
            • adding approximations for external forces,
            • adding approximations for movement during the collision and
            • adding automatic iteration of SMAC from separation to position of rest to get the best separation angle
            Here's a comparison of results with these enhancement implemented in CRASH-97
            Note the improvement of the CRASH trajectory solution procedure
            (which is essentially a instantaneous momentum exchange):
            CRASH97 Speed comparison.jpg
            CRASH97 Speed comparison.jpg (79.67 KiB) Viewed 5113 times
            CRASH97 DeltaV comparison.jpg
            CRASH97 DeltaV comparison.jpg (77.51 KiB) Viewed 5113 times
        Begs the question: why hasn't pc-crash investigated adding some of these features?
        What have they done to try to simplify and/or automate the very subjective user input requirement of the 'point & angle' of momentum exchange these past 30 years?
    • With consideration of the power and capabilities of modern computers McHenry Software decided to focus on msmac3D and added in 2003 the automatic iteration of SMAC
    • We have split off a forum thread on which we will add some more detailed analysis of the pc-crash "Validation" papers, etc.
  • The authors also mention Brach planar momentum program
    • which is extremely simplified and limited by arbitrary and subjective inputs, for example "The energy partitioning process is intuitive, and does not follow directly from any principle of mechanics"
  • BRAVO to the authors to creating this great tool for the crash reconstruction community.
    The paper demonstrates how extremely simple 'instantaneous momentum exchange' is
    • obviously a lot of work for the authors to create this tool, so excellent work and not taking anything away from your accomplishment!
    The authors brilliantly created a simple Open Source program to mimic pc-crash (with additional enhancements!)
BRAVO!
Most importantly it adds to the crash reconstruction community a simple tool for initial analysis of crashes.
pycrash fig 12 14.png
pycrash fig 12 14.png (133.28 KiB) Viewed 5612 times
pycrash fig 13 15.png
pycrash fig 13 15.png (158.08 KiB) Viewed 5612 times

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