4.7.1 Simulator Dynamic Model

The dynamic model used for the Ranger NBV flight simulator is almost identical to the one presented in Section 4.1.1. The only difference is that buoyancy offset has not yet been added to the simulation.
The values of the simulator physical parameters were determined experimentally by comparing simulated and actual vehicle angular acceleration and maximum velocity about each axis, and iterating until the simulation matched the actual system performance.
The simulator models thruster maximum capacity (or saturation). The simulator is normalized to use a maximum torque of 1 N-m about each axis. Assuming (incorrectly) this torque level for Ranger NBV yields the inertia matrix used by the simulator.


kg-m^2

(4.58)

The off diagonal elements are zero indicating that Ranger NBV is symmetric about the vehicle axes. These values can be modified to model asymmetries caused by various manipulator configurations.
Combining (4.58) with the thruster performance data from Table 2-7 gives an estimate of the actual underwater inertia matrix. The underwater inertia matrix is different than the inertia observed on the surface because it also includes the virtual mass created by the water inside of and surrounding the vehicle.


kg-m^2

(4.59)

The normalized values of the drag matrix used in the simulation are


kg-m^2.

(4.60)

These values were determined by observing the terminal angular velocity about each of the vehicle axes. The values chosen in the drag matrix approximate the vehicle with the manipulators stowed. This symmetric configuration causes the off diagonal elements to be zero.
The normalized values in (4.60) are matched to the simulator which has a maximum moment capability of 1 N-m about each axis. Again using the propulsion system performance data we find an approximation of the actual drag of Ranger NBV. After continued testing the drag figures have been determined to be about 30% to 50% too high.


kg-m^2

(4.61)