The Development of Neural Network Based System Identification ...

4.3 SYSTEM IDENTIFICATION WITH NEURAL NETWORK 93
Figure 4.8
Offset distance of the IMU with respect to centre of gravity.
of the IMU was placed not exactly at the centre of gravity (CG) of the vehicle, the
accelerometer only senses the acceleration at the attachment point of the avionics
housing. Therefore, the linear accelerations and velocities have to be corrected for
the offset between the IMU position and the CG (Figure 4.8) before being used for
system identification. Tischler and Remple [2006] derived the correction of acceleration
measurement at the centre of gravity of the vehicle using kinematics of relative motion
as follows:
A xcg = A x − ˙qz a ṙy a − q (py a − qx a ) + r (rx a − pz a ) (4.13)
A ycg = A y − ṗz a + ṙx a − p (py a − qx a ) + r (qz a − ry a ) (4.14)
A zcg = A z − ṗy a + ˙qx a − p (rx a − pz a ) + q (qz a − ry a ) (4.15)
When the location of the vehicle’s CG (x a , y a , z a ) was known, the acceleration
measurement from the accelerometer was corrected for the effect of sensor offset. A
simple weight balancing test was carried out to determine the location of the CG which
gave the measured offset values as r = [x a y a z a ] T = [0.043 0 0.153] T m. As
mentioned in Mettler [2003], the effect of acceleration biases due to sensor offset will
introduce a large bias reading in the y-body direction when a lateral cyclic sweep is
applied.