### Demo 1 – standard test on robustness

Let’s switch off Cheetah solver and load file *Demo1_Ac.dwg* from the folder *Demo1 – Six Circles Inside Angle *(it is the subfolder of the *Cheetah.Solver*).

In this model (with standard AutoCAD constraints) you have an angle with six circles. These 8 entities (six *circles* and two *line segments*) are connected by 15 *tangent constraints* and one *coincident constraint*.

This is the classical test on *robustness* of constraints solver. The term “*robustness”* means that, if you change “a little” any parameter of the constrained model, then the whole constrained model should change “a little” as well.

Let’s drag a little the angle point (bottom left corner on the image below):

We can see that after a small drag of the angle point the radius of one of the circles became very big. This is the typical example of the *non-robust behavior* of the solver.

Now let’s switch from the *AutoCAD mode* to *Cheetah mode* (see section 5.1). If you didn’t close the file *Demo1_Ac.dwg *yet, then AutoCAD constraints of this file will be converted to the Cheetah constraints:

Now let’s drag the angle point in the same way as we did it in AutoCAD mode:

You can see a very *robust behavior of Cheetah solver* – a little drag of the angle point results in very little changes (blue lines) of the whole model.