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About the Body Orientation Examples Rotating the Body and Its CG CS Relative to World |

This section shows you a set of examples of orienting a test
body and its attached coordinate systems (CSs). It makes detailed
use of the rotation representations and conversions explained in the preceding section, Representations of Body Orientation, and
shows how to use MATLAB^{®} workspace variables in your SimMechanics™ block
dialogs.

This sequence of examples assumes you are familiar with the basics of setting up and visualizing bodies and machines.

The later examples require a configured body to work with. Here you set up a simple body with one Body coordinate system (CS), located at the center of gravity (CG).

First, create, connect, and visualize the initial body:

Open the SimMechanics block library and a new Simulink

^{®}model window.Drag and drop a Machine Environment, a Ground, and a Body block, as well as any Joint block you want, into your model window.

Open the Body dialog. In the Body coordinate systems area, delete the Body CS named CS2.

Leave the other defaults. The CG CS is located at

**Origin position vector**`[0 0 0]`. Select**Show port**for the CG CS and deselect it for CS1. Delete the CS1 entry. Click**Apply**.Open the Ground and select

**Show Machine Environment port**. Click**OK**. Connect Ground to Machine Environment at the new Machine Environment port.Connect the Body block at its CG CS port, through the Joint block, to Ground at its grounded CS port.

From the

**Simulation**menu, open**Configuration Parameters**to the**SimMechanics**node. In the**Visualization**area, select the**Display machines after updating diagram**check box. Click**OK**.From the

**Edit**menu, select**Update Diagram**. When the window opens, open the**Model**menu and select**Body Geometries > Ellipsoids**. The window now displays the body as a sphere.

Now configure the body's mass and geometric properties:

In the Body dialog's

**Mass properties**area, enter`11.5 kg`for the mass and`[18 0 0; 0 56 0; 0 0 56] kg*m`^{2}for the inertia. Click**Apply**.Update your diagram. The body in the window changes to an ellipsoid, with

*(x,y,z)*axes of length 2.02, 0.885, and 0.885 meters (m), respectively.Switch from ellipsoid display to displaying individual body settings. This defaults back to convex hull display. But here, the CG CS triad alone displays. (There are no other CSs.) The

*x*-,*y*-, and*z*-axes are red, green, and blue, respectively.Turn the ellipsoid back on.

**Test Body Ellipsoid: Initial Orientation, with Detail of CG CS Axes**

In this example, you rotate the body, along with its CG CS axes, with respect to the World CS. The CG CS axes continue to have the same orientation with respect to the body shape. The rotation is a positive turn of 75 deg (degrees) about the axis (1,1,1) in World. In this example, you rotate with a quaternion.

The unit vector ** n** in the (1,1,1)
direction is (1,1,1)/
.
The rotation angle is θ = 75
deg = 1.3090 rad. At the MATLAB command line,
define

q = [sin(th/2)/sqrt(3) sin(th/2)/sqrt(3) sin(th/2)/sqrt(3) ... cos(th/2)] q = 0.3515 0.3515 0.3515 0.7934

To rotate the body and the CG CS coordinate axes together by this rotation,

In the Body dialog, click the

**Orientation**tab in the Body coordinate systems area. Under the**Specified using convention**pull-down menu, select`Quaternion`.Under

**Orientation vector**, enter`q`. Leave the other defaults. The**Relative to coordinate system**field indicates that the rotation represented by`q`is oriented with respect to the World axes. Click**Apply**.Update the diagram. The body and its CG CS rotate together relative to World:

**Test Body Ellipsoid: Body and Its CG CS Axes Rotated Together**Finally rotate the body and its CG CS back to the original orientation by entering

`[0 0 0 1]`under**Orientation vector**and clicking**Apply**. Update your diagram to refresh the visualization.

You can also rotate the body *without* rotating
its CG CS axes. To accomplish this requires leaving the Body CSs unchanged
while rotating the body's inertia tensor relative to the CG CS. Here
you use the same rotation as in the preceding example, but represent
it as a rotation matrix.

The unit vector ** n** in the (1,1,1)
direction is (1,1,1)/
.
The rotation angle is θ = 75
deg = 1.3090 rad. Compute the rotation matrix:

nDotJ = [0 -1/sqrt(3) 1/sqrt(3); 1/sqrt(3) 0 -1/sqrt(3); ... -1/sqrt(3) 1/sqrt(3) 0] nDotJ = 0 -0.5774 0.5774 0.5774 0 -0.5774 -0.5774 0.5774 0 R = eye(3) + nDotJ*sin(th) + nDotJ^2*(1-cos(th)) R = 0.5059 -0.3106 0.8047 0.8047 0.5059 -0.3106 -0.3106 0.8047 0.5059

The components of the inertia tensor that you enter into the
Body dialog are always defined relative to the CG CS axes. If you
hold the CG CS axes fixed and rotate the body by a rotation matrix *R*,
the inertia tensor transforms according to *I*_{new} = *R***I*_{old}**R*^{T}.
Compute this with MATLAB:

I = [18 0 0; 0 56 0; 0 0 56] I = 18 0 0 0 56 0 0 0 56 Irot = R*I*R' Irot = 46.2753 -15.4698 5.9711 -15.4698 31.3911 9.4987 5.9711 9.4987 52.3336

Symmetrize `Irot` by entering `Irot
= (Irot + Irot')/2`. Then rotate the body alone relative
to World and the CG CS.

In the Body dialog's

**Mass properties**area, replace the existing inertia tensor with`Irot`. Click**Apply**.Update the diagram. The body rotates again, as in the preceding example. But unlike that example, the CG CS axes do not change.

**Test Body Ellipsoid: Body Rotated Relative to Its CG CS Axes**Finally rotate the body back to the original orientation by entering

`I`in the**Inertia**field and clicking**Apply**. Update the diagram to refresh the visualization.

In the preceding examples, you work with only one Body CS, the CG CS. In this example, you set up additional Body CSs and learn how to rotate them. It is common in mechanical applications to require extra Body CSs to locate sensors and actuators on bodies. Their axis orientations do not, in general, align with the orientation of the CG CS axes.

You visualize the body here using convex hulls, instead of ellipsoids, to articulate the Body CSs more clearly. Obtaining a full convex hull, with a surface enclosing a volume, requires at least four non-coplanar Body CSs.

This example also shows you how to obtain Euler angles and rotate with them.

To change the visualization display to convex hulls,

To create and visualize the new Body CSs,

Go to the Body dialog's Body coordinate systems area. Add three new coordinate systems to the CS list. Name them CS1, CS2, and CS3.

For these three CSs, change the

**Origin position vector**fields to`[1 0 0]`,`[0 1 0]`, and`[0 0 1]`, respectively. Click**Apply**.Update the diagram. The visualization window now display the body as a triangular surface. All the Body CS triads are oriented the same way, parallel to the CG CS axes and the World axes.

**Test Body Convex Hull with CG CS and Three
Body Coordinate Systems**

The preceding examples used two rotation representations, the quaternion and the rotation matrix, based on the axis-angle representation. In this example, you use the same rotation as before, but represented as a set of Euler angles.

The rotation axis sequence convention is *Z-X-Z*,
with Φ, θ, Ψ as the first, second, and third angles,
respectively, the same as presented in Converting Rotation Representations preceding. To obtain the angles, you
equate the rotation matrix form for that convention:

to the numerical form of *R* you computed in Rotating the Body Relative to Its Center of Gravity preceding:

R = 0.5059 -0.3106 0.8047 0.8047 0.5059 -0.3106 -0.3106 0.8047 0.5059

and invert the resulting equations. The solution for θ and Ψ is

theta = acos(R(3,3)) theta = 1.0404 psi = asin(R(3,1)/sin(theta)) psi = -0.3684

or about 60 and -21 deg, respectively.

Here is a method that yields a unique solution by using the
structure of the Euler convention, *R* = *R*_{1}(Φ)**R*_{2}(θ)**R*_{3}(Ψ).
Multiply *R* on the right by *R*_{3}(Ψ)^{T}**R*_{2}(θ)^{T},
isolating *R*_{1}(Φ):

R3 = [cos(psi) -sin(psi) 0; sin(psi) cos(psi) 0; 0 0 1] R3 = 0.9329 0.3601 0 -0.3601 0.9329 0 0 0 1.0000 R2 = [1 0 0; 0 cos(theta) -sin(theta); 0 sin(theta) cos(theta)] R2 = 1.0000 0 0 0 0.5059 -0.8626 0 0.8626 0.5059 R1 = R*R3'*R2' R1 = 0.3601 -0.9329 -0.0000 0.9329 0.3601 -0.0000 0.0000 0.0000 1.0000 phi = acos(R1(1,1)) phi = 1.2024

or about 69 deg.

Here you rotate one of the Body CS axis triads, using the *Z-X-Z* rotation
axis sequence convention and, for Euler angles, the Φ, θ,
and Ψ you just found.

In the Body dialog's

**Orientation**tab, locate the`CS2`entry.Under

**Orientation vector**, enter`[phi theta psi]`. In the**Units**pull-down menu, select`rad`. In the**Specified using convention**pull-down menu, select`Euler Z-X-Z`. Click**Apply**.Update your diagram. The CS2 axis triad, whose origin continues to be located at (0,1,0), now looks like this:

This rotated orientation of the CS2 axis triad is the same as that of the rotated CG CS in Rotating the Body and Its CG CS Relative to World preceding. The two rotations are the same and produce the same result.

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