5.3.2 PD+Nonlinear Compensation Tracking Performance

This section examines the performance of the PD plus nonlinear feedforward dynamics controller when tracking single axis sinusoidal trajectories. The magnitude of the specified trajectory was 51.2° peak to peak with a period of 20.6 seconds. This specified trajectory commands the vehicle to rotate at a maximum angular rate of 7.8°/sec. The system parameters used in the nonlinear feedforward terms were determined by comparing the behavior the dynamic vehicle simulation with that of the actual vehicle (Section 4.7.1). Tracking responses are recorded for trajectories about the roll, pitch, and yaw axes using the PD plus nonlinear tuned gain settings: =18, =1.

Figure 5-22 PD+NL roll tracking response ( =18, =1)

Figure 5-23 PD+NL pitch tracking response ( =18, =1)

Figure 5-24 PD+NL yaw tracking response ( =18, =1)

The average angular errors were: roll - 2.1°, pitch - 5.1°, and yaw - 3.9°. Vehicle buoyancy offset in these single axis tracking tests produced almost a 2° steady state attitude error during attitude hold. While the controller can compensate for buoyancy offset, the buoyancy parameters were set to zero for these tests. This inaccuracy is evident in these results. The yaw tracking angular error from the previous section was only 1.8°. To reduce further inaccuracies due to lack of repeatability in vehicle balance, the auto balance algorithm from Section 4.8 was then implemented.