How articulation sensor work?

following this question, I wonder how articulation sensor work? Or what force it will sense? stress, or external force.

I build a manipulator like this, with upper arm mass 1kg:

But I’m not sure what’s the force sensor has read.
When upper arm stand with joint 0 degree, the force sensor get -9.8N:

But when upper arm lie down with joint 90 degree, the force sensor get -2.77N

considering external force, this doesn’t make sense, neither nor inner force like stress.

And I change the joint position of upper arm from side to center, get more strange result:

The force get 9.8 at any degree.

Next, I put upper arm at higher postion with joint at the side of higher postion:

But the force sensor also get -2.77N. It seems that the reading is related to the relative position of upper arm and it’s joint.

So I want to know what value does this sensor read?

For convenience, I upload my test model to help everyone test the function of articulation force sensor.
sensor_test2.usd (5.5 KB)

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I analysis the 90 degree situation using Mechanics of Materials. But -2.77N is incompatible with the compressive or shear stress, so how to get understanding about the articulation force sensor?

@Hammad_M @ltorabi