In each Body block, you define the "home configuration" of each coordinate system by specifying
a. In the Position pane, the location vector of the origin of the CS
- This includes a reference origin so SimMechanics can obtain the absolute point in space: [ 0 0 1 ] from the World origin refers to a different point than [0 0 1 ] from a some other origin not coincident with the World.
- This includes a reference CS so that SimMechanics knows how to interpret your vector components: [ 0 0 1 ] refers to different directions in space if the components are measured along the World axes or along the axes of some other, rotated CS.
b. In the Orientation pane, an orientation specification vector
- This includes a convention (Euler-X-Y-Z, etc) to interpret the meaning of your specification vector.
- This includes a reference CS from which to begin the relative orientation you specify.
The above specifies the home configuration for the CS. SimMechanics steps a machine through three configuration stages before the start of simulation:
a. The Home Configuration is the configuration of the machine using only the information entered on the Body and Joint dialogs. Settings entered in the Sensors/Actuators dialogs refer to this state of Joints.
b. The Initial Configuration results from applying any Initial Condition blocks to the Home Configuration
c. The Assembled Configuration is obtained from the Initial Configuration by assembling the Disassembled Joints while holding all Joints with initial conditions fixed.
The joint force/torque actuators apply a force/torque along the current direction of the joint primitive they actuate.
In regards to the specific model above, you have set up the home configuration of the primitives in Planar1 relative to the follower CS. The initial conditions then kick in to determine the state of the machine at the start of simulation; they change the position/angle/velocities of the primitives. In your case, the revolute actuation will rotate the body, but the direction of the prismatics will not change. The force actuation that happens during the simulation occurs along the instantaneous direction of the primitive, which of course may change depending on how the orientation of the bodies evolve.
Also, remember that the primitives act in order from base to follower. Your Planar1 has R1 as the last primitive, i.e. the one closest to the follower. Rotating that primitive does not change the orientation of the P1 and P2 that precede it. If, on the other hand, you switched the joint so that LongX was connected to the follower side and Short Y was connected to the base side, actuating R (which would then be closest to LongX) would affect the directions of P1 and P2 (which would then be closer to ShortY).
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