Simulink accelerometer flipping y axis?

Hi, I was looking into the IMU model from the aerospace blockset, and within the accelerometer model there's the following blocks. This is to compute the measured acceleration from a rotating body when the accelerometer is offset from the CG. My question is, why is there a gain block with [1 -1 1]? This seems to be inverting the Y coordinate of the IMU's location, and I don't understand why that's necessary. Thanks!

Answer 1

[Paul]

I think the reason is ....

The CG and ACC location vectors are resolved in "Flight Gear Coordinates (FGC)" (or a frame parallel to FGC) as described here: link

However, the other vector inputs to the block are resolved in "Body Coordinates (BC)" as described here: link

So the way the model should work is to first compute the vector from the CG to the ACC in the FGC frame and then resolve it in the BC frame so that it's compatible with the other inputs. In fact, that's the exact process described here.

However, for some reason the block actually comptues the vector from the ACC to the CG in the FGC frame, which needs to be 1. negated, and 2. resolved in the BC frame. Those two steps together would be an element-wise multiplication by -1*[-1 1 -1], or as implemented in the block by the element-wise multiplication with [1 -1 1].

[Adam Wiktor]

Perfect, that makes sense. I missed that the CG and ACC were in FGC. Thank you!