Minimum Distance
9.1 Experimental Setup
The experiment was conducted in Dorabassenget in Trondheim, June 2, 2020. Testing of software and addressing hardware related problems on ReVolt were done at NTNU Gløshaugen before it was shipped down to Skippergata, where ReVolt was ejected into the water. As an escort boat, NTNUI Dykkergruppa’s Fjøset II was rented. The experiment was done together with Knut Turøy and Simen Sem Øvereng, who also performed experiments on ReVolt the same day. Two pictures from the experiment are shown in Figure 9.1.
A path consisting of 14 waypoints forming an 8-shaped path, given in Appendix D, was created. The optimization approach was used as the path generator. The same tuning parameters as for the S-shaped simulation in Section 8.1.2 was used, except for the corridor widthζ set to 4m.
For the control system, the same tuning parameters used in the simulations, given in Section 8.2, was used. In the experiment, the vessel was put to rest in a dynamic position (zero speed) at the first waypoint with heading in the direction of the second waypoint. Then, the vessel was commanded online to move along the path with a desired speed ud = 0.25 m/s. When the vessel was close enough to the next waypoint, a new path segment was generated by the path generator. Figure 9.2 shows the computation graph of the control system running on the onboard computer.
(a) (b)
Figure 9.1: Figure(a)shows me controlling the ReVolt remotely from the escort boat with ReVolt in the background. Figure(b)shows Knut Turøy, Simen Sem Øvereng, and his biceps observing ReVolt’s behavior during the experiment.
The sea trial was conducted under calm weather conditions with practically no waves present. However, it was evident that some current and light breeze was affect-ing the vessel. The environmental load was documented by puttaffect-ing the vessel to rest, with the bow facing the wind. A rosbag was recorded to see how the vessel drifted away from the initial point. See Figure 9.3. One can see that the vessel starts drifting away towards the southwest. Notice how the Munk moment turns the vessel such that the port side is facing towards the direction of the average environmental load.
1http://wiki.ros.org/rqt_graph
9.1 Experimental Setup
Figure9.2:Thefigureshowsthecomputationgraphofthecontrolsystemrunningontheonboardcomputerduringtheexperiment.The verticesinthegraphrepresentROSnodes,whiletheedgesbetweenthemareROStopicswhichthenodescommunicatethrough.The graphisgeneratedbytheGUIpluginrqt_graphpackage1 .TheCMEnodeisthe“ControlModeExecutor”,whichisthesuperiornode decidinginwhichcontrolmodethevesselshouldbe.TheXsensandvector330nodeistheIMUandvector,respectively,providingthe necessarymeasurementstotheobserver.
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+1.141e3 Position in the Horizontal Plane
pn
Figure 9.3: Environmental forces acting on the vessel under the experiment. Figure (a) shows position in horizontal plane. Figure(b)shows position versus time for each DOF.
9.2 Problems
Physical testing always presents unforeseen issues related to hardware and purely practical problems. Initially, the test was set to May 29, but problems related to the battery and stern thrusters were discovered. Thus, the experiment had to be post-poned. During the winter, changes in the location of specific equipment were done.
This led to problems with getting heading signals from the VS330 vector. It was found that the settings on the device were outdated and had to be changed physically.
Also, several problems were discovered related to the thrusters. A safety sensor seemed to be damaged during the shipping down to Skippergata. It was blocking all signals going to the bow thruster and thus had to be disabled. It was found that constraining the bow thruster to 90◦ in the code did not match 90◦ physically. A workaround was to constrain the bow thruster to 120◦. A malfunction in the bow thruster made it incapable of applying any thrust below 50% of maximum thrust.
This will affect the results, especially related to heading. Further, hysteresis related to rotating the stern thrusters were discovered during the initial test day, May 29.
Some duct tape, together with WD-40 spray, lubricated the thrusters and resolved the issue during the test day.
Lastly, it was discovered that ReVolt took in some water during the test day. All these problems should be resolved for ReVolt to be correctly operating in the future.
9.3 Results
9.3 Results
Three robag recordings were done when performing the experiment. All three de-livered similar result. The results shown in Figures 9.4 to 9.9 are from the second recording. The vessel starts in approximatelyWP0 = (179 m,1129 m) and per-forms an 8-shaped maneuver ending up inWP13= (184 m,1129 m).
From Figure 9.4, one can see that the guidance system produces stepwise a smooth 8-shaped path to be followed. From Figures 9.6 and 9.7, one can see that ReVolt can track the desired path, but struggles to satisfy the speed assignment. On the straight lines between the corners, it traces the path precisely. However, one can see that is it struggles to trace the path accurately in the corners. One reason for this is the environmental load acting on the vessel. Another one is the physical problems found with the thrusters, explained in Section 9.2.
However, notice how all signals are lost and freezes after approximately 100 s in Figures 9.7 to 9.9. Also, see the jump in the estimated north position of about 5 min Figure 9.7 around 160 s. One can see the oscillating effect of this on the desired and actual control forces applied to the ReVolt around 160s in Figure 9.9.
This indicates that the experiment was occasionally prone to poor signals from the measurement system, which affect the results. Notice the difference in continuity in signals between the simulations and the experiment. The simulation has continuous smooth signals, while the signals in the experiment have similarities with zero-order-hold sampled signals, as expected.
The experiment indicate that the control design does not need 100% numerically correct values of the hydrodynamic parameters to work. This illustrates the robust-ness of the control system.
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Figure 9.4:Axy-plot of desired path constructed. The B´ezier curve is drawn in red on top of its control polygon.
9.3 Results
0 2 4 6 8 10 12 14
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0 0.5 1
0 2 4 6 8 10 12 14
-20 0 20
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0 0.05 0.1 0.15
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Figure 9.5: The direction, curvature, rate of change in curvature, and the speed profile and its respective derivatives for the desired path in Figure 9.4.
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Position in the Horizontal Plane
pnd pn Vessel
Figure 9.6:Position in horizontal plane.
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Figure 9.7: Position versus time for each DOF.
9.3 Results
Figure 9.8:Speed versus time for each DOF.
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Figure 9.9:Forces versus time for each DOF.