PyDy and other things

This weekend is Maker Faire, so the lab is going there.  One thing I’ll have is some info on some MEMS sensors from a class project last quarter.  We collected info on the Analog Devices ADXL 345 and Invensense ITG-3200 (tried to use a Honeywell HMC5843, but we couldn’t read it over I2C).  Anyways, I got permission from my group members to post our report online, so here it is.

Report: MAE276 Final Paper

For PyDy/SymPy & GSoC there is also some news.  It looks like the previous separation between UnitVector and Vector classes is going away; in addition they will no longer extend SymPy’s Basic or Expr classes.  There will now be only one Vector class.

It will store a list of lists; the inner list will have the 3 vector measure numbers for a frame (something like [([1],[2],[4]),’b’] ), where the measure numbers will use the SymPy Matrix class (and can have symbols in them).  The outer list will hold each of these lists for each frame.  So the data stored will be something like [[([1],[2],[4]),’b’],[([3*x],[1],[0]),’c’]]… or something like that.  The current plan is to write our own operators for addition, subtraction, scalar multiplication & division, and dot and cross products.  By not using the SymPy classes, we can better control how the Vector class will behave.  One important behavior is to have every operator (except for dot product) take in and return a vector, and not have any confusion between SymPy objects and Vectors when using operators.

There is also some debate as to whether the ReferenceFrame class should store its own basis vectors, or they should be returned upon initialization (see the sympy list post: Question regarding vectors.  I understand the argument to keep them with their frame and the awkwardness that will exist when creating the basis vectors upon ReferenceFrame initialization.  But, I feel that the Vector class already stores information about each frame (in its inner lists), and question whether basis vectors are a property of ReferenceFrames, or a frame is a property of a set of basis vectors.

I guess the orthonormal properties of a set of 3 basis vectors exist independently of a particular reference frame.  I guess what defines a frame?  I think of it as a rotation (and related time derivatives) from on set of basis vectors to another.  So it is defined by both its basis vectors and a rotation.  I still feel like its is preferable to keep the basis vectors outside of the ReferenceFrame class, but not sure my position has a strong enough argument (but I also don’t think it is too weak of an argument).  Hopefully starting to discuss more of the ReferenceFrame class methods will clear this up.


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