Skip to main content

differential equations - Finding a 3d curve from torsion and curvature with NDSolve


I'm trying to use the Frenet–Serret formulas to find the curve that matches the torsion and curvature I specify numerically with an InterpolatingFunction.


The problem is that the system is overdetermined, and have been getting all kinds of errors from Mathematica. If I don't tell NDSolve about the definition of N as the normal vector and B as the cross product of T and N, then I have a 9x9 system, which will solve on occasion, but with results don't seem right.



The curve should be a Mobius strip with length $2\pi$, so I also have the boundary conditions of $T(0)=T(2\pi), N(0)=N(2\pi), B(0)=B(2\pi)$, but when I include those conditions, I get



"There are significant errors {0., 0., 0., 0., 0., 0., 2.49764*10^-6, 0., 0., 
-2.06667*10^-6, 0., 0.} in the boundary value residuals. Returning the best solution
found."

Some errors are to be expected, and $10^{-6}$ is not significant to me, but the resulting curve is a straight line from $(0,0,0)$ to $(\pi/2,0,0)$.


When I try specifying $N(s)$ as $\frac{T'(s)}{\|T'(s)\|}$ and $B(s)$ as $T\times N$, and only using the first 2 equations on the Wikipedia page, I get:



NDSolve::ntdvdae: Cannot solve to find an explicit formula for the derivatives. 

NDSolve will try solving the system as differential-algebraic equations.`

NDSolve::ndcf: Repeated convergence test failure at s == 0.; unable to continue.

As far as I can tell, NDSolve doesn't support vector equations, so I have been breaking everything down into the component equations.


I'd be happy to share the points that form the interpolating functions for torsion and curvature, but I don't know where to post them: it's almost 1500 points.



Answer



First@NDSolve[{
x''[t] == k1[t] nx[t], nx'[t] == -k1[t] x'[t] - k2[t] bx[t], bx'[t] == k2[t] nx[t],
y''[t] == k1[t] ny[t], ny'[t] == -k1[t] y'[t] - k2[t] by[t], by'[t] == k2[t] ny[t],

z''[t] == k1[t] nz[t], nz'[t] == -k1[t] z'[t] - k2[t] bz[t], bz'[t] == k2[t] nz[t],
x[ti] == iniPos[[1]], y[ti] == iniPos[[2]], z[ti] == iniPos[[3]],
x'[ti] == iniDir[[1]], y'[ti] == iniDir[[2]], z'[ti] == iniDir[[3]],
nx[ti] == iniNor[[1]], ny[ti] == iniNor[[2]], nz[ti] == iniNor[[3]],
bx[ti] == iniBin[[1]], by[ti] == iniBin[[2]], bz[ti] == iniBin[[3]]},
{x, y, z, nx, ny, nz, bx, by, bz}, {t, ti, te}]

k1[t] is curvature, k2[t] is torsion, {x[t],y[t],z[t]} is path, {nx[t],ny[t],nz[t]} is normal, {bx[t],by[t],bz[t]} is binormal, arc length is (te-ti)*norm[iniDir].


This is signed curvature...


Comments

Popular posts from this blog

front end - keyboard shortcut to invoke Insert new matrix

I frequently need to type in some matrices, and the menu command Insert > Table/Matrix > New... allows matrices with lines drawn between columns and rows, which is very helpful. I would like to make a keyboard shortcut for it, but cannot find the relevant frontend token command (4209405) for it. Since the FullForm[] and InputForm[] of matrices with lines drawn between rows and columns is the same as those without lines, it's hard to do this via 3rd party system-wide text expanders (e.g. autohotkey or atext on mac). How does one assign a keyboard shortcut for the menu item Insert > Table/Matrix > New... , preferably using only mathematica? Thanks! Answer In the MenuSetup.tr (for linux located in the $InstallationDirectory/SystemFiles/FrontEnd/TextResources/X/ directory), I changed the line MenuItem["&New...", "CreateGridBoxDialog"] to read MenuItem["&New...", "CreateGridBoxDialog", MenuKey["m", Modifiers-...

How to thread a list

I have data in format data = {{a1, a2}, {b1, b2}, {c1, c2}, {d1, d2}} Tableform: I want to thread it to : tdata = {{{a1, b1}, {a2, b2}}, {{a1, c1}, {a2, c2}}, {{a1, d1}, {a2, d2}}} Tableform: And I would like to do better then pseudofunction[n_] := Transpose[{data2[[1]], data2[[n]]}]; SetAttributes[pseudofunction, Listable]; Range[2, 4] // pseudofunction Here is my benchmark data, where data3 is normal sample of real data. data3 = Drop[ExcelWorkBook[[Column1 ;; Column4]], None, 1]; data2 = {a #, b #, c #, d #} & /@ Range[1, 10^5]; data = RandomReal[{0, 1}, {10^6, 4}]; Here is my benchmark code kptnw[list_] := Transpose[{Table[First@#, {Length@# - 1}], Rest@#}, {3, 1, 2}] &@list kptnw2[list_] := Transpose[{ConstantArray[First@#, Length@# - 1], Rest@#}, {3, 1, 2}] &@list OleksandrR[list_] := Flatten[Outer[List, List@First[list], Rest[list], 1], {{2}, {1, 4}}] paradox2[list_] := Partition[Riffle[list[[1]], #], 2] & /@ Drop[list, 1] RM[list_] := FoldList[Transpose[{First@li...

plotting - How to draw lines between specified dots on ListPlot?

I would like to create a plot where I have unconnected dots and some connected. So far, I have figured out how to draw the dots. My code is the following: ListPlot[{{1, 1}, {2, 2}, {3, 3}, {4, 4}, {1, 4}, {2, 5}, {3, 6}, {4, 7}, {1, 7}, {2, 8}, {3, 9}, {4, 10}, {1, 10}, {2, 11}, {3, 12}, {4,13}, {2.5, 7}}, Ticks -> {{1, 2, 3, 4}, None}, AxesStyle -> Thin, TicksStyle -> Directive[Black, Bold, 12], Mesh -> Full] I have thought using ListLinePlot command, but I don't know how to specify to the command to draw only selected lines between the dots. Do have any suggestions/hints on how to do that? Thank you. Answer One possibility would be to use Epilog with Line : ListPlot[ {{1, 1}, {2, 2}, {3, 3}, {4, 4}, {1, 4}, {2, 5}, {3, 6}, {4, 7}, {1, 7}, {2, 8}, {3, 9}, {4, 10}, {1, 10}, {2, 11}, {3, 12}, {4, 13}, {2.5, 7}}, Ticks -> {{1, 2, 3, 4}, None}, AxesStyle -> Thin, TicksStyle -> Directive[Black, Bold, 12], Mesh -> Full, Epilog -> { Line[ ...