The output of the 'Transform Sensor' is based on axis–angle representation of rotation in Euclidean space. Please refer to the following documentation in Wikipedia about the axis-angle representation:
In essence, any arbitrary rotation in Euclidean space can be represented as two values: a unit vector and a scalar angle. The resultant rotation is obtained by taking the dot product of these two quantities.
Hence, in the 'Transform Sensor', the output 'q' represents only the angle, which is half the information. To get the complete rotation, you can enable the axis output and use the dot product of the two to obtain correct angular rotation between the base and follower. The attached model ('SimplePendulum_AxisAngle.slx') shows the correct output for the simple pendulum model.
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