Results and Discussion
6.1 General discussion
6.4.2 Discussion for the path-following using multiple opti- opti-mizationsopti-mizations
6.4.2 Discussion for the path-following using multiple opti-mizations
At first, the initial guesses for theL0- and movingL0-controller were the output from the L1-controller performing a path-following maneuver from (0◦,0◦,0◦) through (30◦,45◦,15◦) to (0◦,0◦,0◦), but these initial guesses yielded poor re-sults. In other words, the spacecraft did not reach the desired attitude and an-gular velocity when applying the output from theL1-controller as initial guesses for the L0- and moving L0-controller. Therefore, different initial guesses than what were first used were applied to theL0- and movingL0-controller. The new
Chapter 6. Results and Discussion 97
Figure 6.16:Euler angles for path-following using multiple optimizations, from (0◦,0◦,0◦)through(30◦,45◦,15◦)to(0◦,0◦,0◦).
initial guess applied for theL0-controller was the output from theL1-controller performing a single-axis maneuver from(0◦,0◦,0◦)to(75◦,0◦,0◦). The new ini-tial guess applied for the moving L0-controller was the output from the L1 -controller performing a single-axis maneuver from (0◦,0◦,0◦) to (90◦,0◦,0◦).
From Fig. 6.16 and Fig. 6.17 it is clear that the spacecraft follows the path and stops rotating for these initial guesses, although they were not expected to yield better results than the original initial guesses. A possible explanation to why the first initial guesses did not yield satisfactory results is that the optimiza-tion solver might have become stuck in a local minimum, and is therefore not able to find the optimal solution. The first initial guesses may have provided a starting point close to a local minimum, and the optimization stops at this point. Contrary, the second initial guesses may have provided a starting point
98 Chapter 6. Results and Discussion
Figure 6.17:Angular velocity, ωbob, for path-following using multiple optimizations, from(0◦,0◦,0◦)through(30◦,45◦,15◦)to(0◦,0◦,0◦).
closer to a better solution. This confirms that the optimization solver, IPOPT, is sensitive to the choice of initial guesses. The results also illustrates the im-portance of choosing good initial guesses for the L0- and moving L0-optimal control problems. The sensitivity of theL0-optimal control problem to different initial guesses was explored in Section 6.1.1.
Each of the two subsequent optimizations lasts fort= 70s, and the orienta-tion trajectories resulting from theL1-controller andL0-controller are symmet-rical about t = 70 s, excluding the propagation time at the end of the second optimization. The same goes for the control signals computed by the two con-trollers. One explanation to why the symmetrical results make sense, is because the same amount of torque is required to move the spacecraft from (0◦,0◦,0◦) to (30◦,45◦,15◦), and from (30◦,45◦,15◦) to(0◦,0◦,0◦). Fig. 6.18 and Fig. 6.19 show that the control torques for the two optimizations have the same size, and
Chapter 6. Results and Discussion 99
Figure 6.18:Control input body frame for path-following using multiple optimiza-tions, from(0◦,0◦,0◦)through(30◦,45◦,15◦)to(0◦,0◦,0◦).
thus supports the suggested explanation.
On the other hand, the results for the movingL0-controller are rather asym-metrical. One way to explain the asymmetrical results for the movingL0-controller could be that the choice ofhN makes it cheaper to apply control torques if they occur withint = 28 s andt = 42 s for the movingL0-controller for each of the two optimizations. This means withint = 28s tot= 42s andt= 98s tot= 112 s in the plots. For the first maneuver from(0◦,0◦,0◦)to(30◦,45◦,15◦), two con-trol inputs occur within this interval and one occur at the end of the optimiza-tion interval. A close-up study of the movingL0-control signals in Fig. 6.18 and Fig. 6.19 reveals that the first and second control torques have slightly different magnitudes. Therefore an additional control input has to be applied to compen-sate for the difference between the two first control torques, i.e., the spacecraft
100 Chapter 6. Results and Discussion
Figure 6.19:Control input wheel frame for path-following using multiple optimiza-tions, from(0◦,0◦,0◦)through(30◦,45◦,15◦)to(0◦,0◦,0◦).
requires the same amount of torque to stop spinning as it requires to start. For the second optimization, i.e., the maneuver from (30◦,45◦,15◦) to (0◦,0◦,0◦), the control torques occur at three instants, all within the interval t = 98 s to t = 112, which differs from the first maneuver. One might have expected that the control inputs would occur at the same time instants for the optimizations for both maneuvers, considering they are opposite maneuvers and the optimiza-tion is performed over the same number of control intervals. Contrary to this, the moving L0-control signals in Fig. 6.18 and Fig. 6.19 are asymmetrical. For each of the optimizations, i.e., the optimization of the maneuver from(0◦,0◦,0◦) to(30◦,45◦,15◦)and the optimization from (30◦,45◦,15◦) to(0◦,0◦,0◦), the op-timization procedure aims to satisfy the constraints and reach the final state values while minimizing the cost function. If it is not possible to reach this goal
Chapter 6. Results and Discussion 101 by applying control input within the cheap interval specified byhN, some or all of the control input will occur outside this interval. Therefore, control inputs may occur outside the interval defined byhN. This, together with the fact that the two maneuvers are different, could explain why the control signals differs for the first and second part of the path-following maneuver for the movingL0 -controller. Another possible explanation to why the first and second part of the path-following maneuver for the moving L0-controller differ, is that the con-troller behave differently based on initial guesses, as discussed in Section 6.1.1.
For the moving L0-controller and the second optimization, i.e., the maneu-ver from(30◦,45◦,15◦)to(0◦,0◦,0◦), control torques occur at three instants, all within the intervalt = 98 s to t = 112 as outlined above. The control torques that occur att≈102s and the control torques that occur att ≈112s make the reaction wheels saturate, as shown in Fig. 6.19. An explanation to why the ac-tuators saturated could be that all the control torques occur within an interval of ∆t ≈ 14 s for the second optimization, compared to an interval of ∆t ≈ 40 s for the first optimization. As discussed in Section 6.2.1; if the control torques occur within a small time interval the values of the torques have to be larger than if there was more time between each control torque, in order to perform a specified maneuver. For the first optimization, i.e., for the maneuver from (0◦,0◦,0◦)to (30◦,45◦,15◦), the two first control inputs are close to the satura-tion limits ofτlimit =±3·10−3 N·m. However, for the first optimization, a third control input occurs at the end of the optimization interval. Although the con-trol torques occur within a relatively large time interval for the first maneuver, two of them are close to the saturation limits. The spacecraft has to rotate the same distance for the two maneuvers, i.e., from(0◦,0◦,0◦)to(30◦,45◦,15◦), and from (30◦,45◦,15◦)to (0◦,0◦,0◦). Since the control torques applied to steer the spacecraft from (30◦,45◦,15◦) to (0◦,0◦,0◦) occurs closer in time than for the first maneuver, it could make sense that the actuators saturate. If the control torques were even closer together, yet another control torque might be required to perform the maneuver as discussed in Section 5.5.3.