Thread Subject:

simmechanics: adjoining CS

Hi all,

I'm learning Simmechanics and I'm trying to figure out how CSs work. I created a simple model with two bodies: one welded to the ground and the other ground connected to the other body with a prismatic joint. The CG position of the first body is at [0 1 0], instead the CG of the second one is at [0 10 0]. I measure the body position of the second body through a body sensor. From it, should I get a position of 10 if the body sensor is connected to a world CS and 9 if the body is connected to a adjoining CS? I don't understand why I always get 10 even if I change the CS origin. May you be so kind to help me, please?

Thanks

Pietro

Hi Pietro,

According to the block reference page, the position vector has its tail at the origin of the World CS. That means that the position vector is always fixed between the World and Body CS origins. This is true regardless of your choice of measurement frame.

Selecting a different measurement frame doesn't change the end points of the position vector. What the measurement frame does do is set the directions of the X, Y, and Z axes used to measure the the [X Y Z] position vector components.

The position of the measurement frame isn't very important here. What matters is the orientation of that frame. The measured [X Y Z] components of the position vector should be the same in the World and Local measurement frames if the two sets of frame axes point in the same directions - and different if they point in different directions.

"pietro " <bracardi82@email.it> wrote in message <k4e6sc$a34$1@newscl01ah.mathworks.com>...
> Hi all,
>
> I'm learning Simmechanics and I'm trying to figure out how CSs work. I created a simple model with two bodies: one welded to the ground and the other ground connected to the other body with a prismatic joint. The CG position of the first body is at [0 1 0], instead the CG of the second one is at [0 10 0]. I measure the body position of the second body through a body sensor. From it, should I get a position of 10 if the body sensor is connected to a world CS and 9 if the body is connected to a adjoining CS? I don't understand why I always get 10 even if I change the CS origin. May you be so kind to help me, please?
>
> Thanks
>
> Pietro

"Tony" wrote in message <k4j2ro$3r9$1@newscl01ah.mathworks.com>...
> Hi Pietro,
>
> According to the block reference page, the position vector has its tail at the origin of the World CS. That means that the position vector is always fixed between the World and Body CS origins. This is true regardless of your choice of measurement frame.
>
> Selecting a different measurement frame doesn't change the end points of the position vector. What the measurement frame does do is set the directions of the X, Y, and Z axes used to measure the the [X Y Z] position vector components.
>
> The position of the measurement frame isn't very important here. What matters is the orientation of that frame. The measured [X Y Z] components of the position vector should be the same in the World and Local measurement frames if the two sets of frame axes point in the same directions - and different if they point in different directions.
>
> "pietro " <bracardi82@email.it> wrote in message <k4e6sc$a34$1@newscl01ah.mathworks.com>...
> > Hi all,
> >
> > I'm learning Simmechanics and I'm trying to figure out how CSs work. I created a simple model with two bodies: one welded to the ground and the other ground connected to the other body with a prismatic joint. The CG position of the first body is at [0 1 0], instead the CG of the second one is at [0 10 0]. I measure the body position of the second body through a body sensor. From it, should I get a position of 10 if the body sensor is connected to a world CS and 9 if the body is connected to a adjoining CS? I don't understand why I always get 10 even if I change the CS origin. May you be so kind to help me, please?
> >
> > Thanks
> >
> > Pietro
Hi Tony,

thanks for your reply, therefore I always get the absolute position of the CS, right? so in order to get the relative position between two CS I should subtract the two positions, right?