# correct rotation and translation matrices

I wrote a C++ program that can calculate the magnetic field $\bar{B}$ generated by a circular coil that is placed in the origin, for a given point $\bar{P}$ in 3D space.

The coil is in the $x$,$y$-surface, with $z=0$.

I want to describe multipoles now so I would have to be able to transform the coordinates of $\bar{P}$ to the coordinate system of a coil that is shifted and rotated $\bar{P}^\prime$.

The coils that are shifted and rotated would have their coordinate systems like so:

1. the origin of the coil system is at $O^\prime=(R\cos\phi,R\sin\phi,0)$

2. The $z^\prime$-axis points to the origin of the normal system, is also in the $x$,$y$-surface and makes an angle $\phi$ with the $x$-axis of the normal system.

3. The $y^\prime$-axis is parallel to the $z$-axis

4. the $x^\prime$-axis follows (all systems being orthonormal).

Thus all coils (the number of coils is arbitrary) lie on a circle with radius $R$ and their central axis $z$.

My approach has been this so far:

1. transform $\bar{P}$ to $\bar{P}^\prime$ in the coil's system
2. calculate $\bar{B}^\prime$ using my program
3. transform $\bar{B}^\prime$ back to the normal system (only rotation has to be undone, since $\bar{B}$ is a free vector and I only need it's components).

I've been using a transformation matrix $M=R_y(\phi)R_x(\pi/2)$ that looks like this: $$M=\begin{bmatrix}\cos\phi&0&\sin\phi\\0&1&0\\-\sin\phi&0&\cos\phi\end{bmatrix}\begin{bmatrix}1&0&0\\0&0&-1\\0&1&0\end{bmatrix}=\begin{bmatrix} \cos\phi&\sin\phi&0\\0&0&-1\\-\sin\phi&\cos\phi&0\end{bmatrix}$$

so $\bar{P}^\prime=M\bar{P}$, after first translating $\bar{P}$ to $\bar{P}_t$ by: $$\bar{P}_t=\bar{P}-O^\prime$$

And $\bar{B}=M^\prime \bar{B}^\prime$ with $M^\prime=M^T$.

My results are fautly, so what am I doing wrong?

Also, let me know if this question is too concrete to post on a math forum.

Added image for clarity: The gray circle is where the coil always is in the system that's being used for my C++ program, the dark circle is the coil where it should be.

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Ok I know for sure the matrix $M$ is wrong! Also for inverse transformations one should take the inverse and not the transposed as I have done. Some help on the correct transformation matrix would be great. Also, should I first rotate or first shift? –  romeovs Apr 28 '11 at 17:13
The transpose and the inverse of a rotation matrix are the same thing... –  Ｊ. Ｍ. Apr 28 '11 at 18:47
Hm yes your right.. Wow this really isn't my domain. –  romeovs Apr 28 '11 at 18:55
$M$ is definitely not a rotation-matrix. How did you arrive at it? –  Gottfried Helms Apr 28 '11 at 19:16
I tried two different methods so far: 1. multiply rotation matrices $R_x$, $R_y$ and $R_z$ and 2. put the components of the $a^\prime_i$, calculated in $a_i$ in a matrix. 3. mess around with a graphical program, I think that's were I got this. Was kind of busy so might have messed things up. –  romeovs Apr 28 '11 at 20:22

I don't understand how you arrived at your rotation matrix, so I'll just go by your description of the transformed system and the image, which I think I understand.

If we choose $\phi=0$, $O'$ lies on the $x$-axis, so the $z'$-axis should be the negative $x$-axis, the $y'$-axis should be the $z$-axis, and the $x'$-axis should correspondingly be the negative $y$-axis. That corresponds to the transformation matrix

$$\left(\begin{array}{ccc}0&-1&0\\0&0&1\\-1&0&0\end{array}\right)\;.$$

This is not what your expression for the rotation matrix yields for $\phi=0$. Since the $y'$-axis always points along the $z$-axis and only the $x'$-axis and the $z'$-axis are rotated into each other if we change $\phi$, we can immediately infer the general form of the rotation matrix as

$$\left(\begin{array}{ccc}\sin\phi&-\cos\phi&0\\0&0&1\\-\cos\phi&-\sin\phi&0\end{array}\right)\;,$$

where only the sign of the sine was still to be determined and can be fixed by considering $\phi=\pi/2$.

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Okay, I've been trying to fix my program, recalculating and recalculating.. Always coming to this result. Something seems wrong though, my program keeps given faulty results.. –  romeovs May 13 '11 at 17:40