IEEE Robotics & Automation Magazine - September 2017 - 48

(a)

(b)

(c)

(d)

Figure 8. The connector plug/unplug: (a) approach, (b) grasp, (c) extract, and (d) insert.

Valve Turning
The intervention task is carried out through the following phases:
● Detection: In field operations, detection discovers the
location of the panel and establishes visual contact [7],
enabling visually based navigation with respect to the
panel [14]. For the experiments reported, the robot is
requested to move through a sequence of waypoints
while avoiding obstacles; during the path following, the
panel is localized.
● Inspection: Inspection is the homing operation of the AUV
to the panel. It enables high-accuracy visually based navigation, providing a good estimate of the valve pose. The EE
is guided to a Cartesian-space-defined pose located in front
of the panel.
● Grasp planning: To perform the intervention, one must
specify the desired valve pose. Then, according to the
workspace analysis method explained in the "Workspace
Analysis" section, the grasp pose is computed.
● Valve turning: Valve turning involves the following two
steps: 1) approach to the panel and 2) grasp and turn. The
approach step is aimed at getting close to the panel at a
predefined distance, in the direction normal to the panel.
This guarantees the pose of the EE before grasping. The
second step consists of the actual intervention operation.
The implemented method is based on the MoveIt! pick
pipeline. The frame of reference for the approach is the
valve, and the approach direction is normal to the subsea
panel z axis. Because there is no gripper to open/close, the
grasp closure pose is used to perform the turn action. The
wrist joint is specified as the posture joint, and the turning
degree is provided as the joint value.
Figure 7 shows the I-AUV during several phases of the
experiment. In Figure 7(a), (c), (e), (g), (i), and (k), the
experiment in the water tank is shown, and in (b), (d), (f),
(h), (j), and (l), the 3-D ROS visualization details the robot,
environment, and AUV and EE trajectories. A plot of the
3-D AUV and EE trajectories is presented in Figure 9(a).
The green box represents the obstacle. The panel with the
three valves is also shown, along with the detection, inspection, and grasping points, showing how the I-AUV went
through all of these steps. Figure 9(b) displays the trajectory
of the arm joints along the mission. The valve-turning
action can be appreciated in the wrist joint changing 90c at
the end of the trajectory. The Cartesian AUV trajectory is
shown in Figure 9(c). The dotted bold gray line corresponds
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IEEE ROBOTICS & AUTOMATION MAGAZINE

SEPTEMBER 2017

to the planned path, and the colored lines (red, green, blue,
and orange) correspond to the actual X, Y, Z, } path, respectively. The last plot [Figure 9(d)], presents the EE position and orientation, along with the desired waypoints. The
vertical black dotted lines show the boundaries of each
phase. Here, it can be appreciated that the EE is reaching the
required positions and orientations. In this scenario, the
workspace analysis was integrated and performed online.
When the inspection point was reached, a call to the workspace analysis service was performed, while the I-AUV held
its current configuration. This period of time is indicated by
dotted black lines in the plot of Figure 9(c).
Connector Plug/Unplug
A model inspired by a scaled-down version of a hot-stab
connector, similar to one used in the oil and gas industry,
was used. As is common in this type of connector, the model
uses passive compliance to account for a possible small misalignment, thus helping to complete the insertion process.
Nevertheless, an EE accuracy of better than 12.25 mm (the
difference between the inside and outside radii of the funnel-shaped female connector) was required to complete the
goal. To perform this task, the V-shaped EE was substituted
by the three-finger hand that is described earlier in "The
Three-Finger Hand" section. The task starts with the connector plugged in and then requesting the robot to unplug it,
move away, and then replug it. The intervention procedure is
similar to the procedure previously described but with the
valve-turning step replaced by the following steps:
● Unplug: The hand is requested to grasp the connector by
closing the fingers. Similar to the valve turning, the MoveIt!
pick pipeline is used. First, an approach direction perpendicular to the plane of the connector is specified and the
pregrasp pose is chosen to open the hand before grasping.
The connector pose corresponds to the pick pose, and the
grasp closure pose is used to close the hand after holding
the connector. Once the grasping is successful, the unplugging is accomplished by performing a retreat backward.
The retreat direction is specified along the -z axis in the
connector frame.
● Plug: The MoveIt! place pipeline is used to plug the connector to its original location. Once the connector reaches
its original location, the hand is opened, leaving the connector plugged in.
This experiment allowed us to validate an advanced intervention operation using the three-finger hand. Figure 8

Table of Contents for the Digital Edition of IEEE Robotics & Automation Magazine - September 2017