The Automated Remote Manipulator (ARM) System

The ARM system consists of an advanced hydraulic robotic manipulator with six dof (six independent axes o f rotation). This is mounted on an extend/rotate mechanism which provides an additional two degrees of fi’eedom (one rotational and one translational). This is a steel boom which will extend fi'om a tool skid and rotate about its own axis. The tool skid also has three extensible legs with suction feet on the end to enable attachment of the tool skid to a jacket. In addition it carries the controllers and power supplies for the manipulator and any tools.

The tool skid may be carried by a standard, work class ROV to deliver the tools to the inspection site. This configuration is shown in figure 3.1. A real configuration is seen in the photograph in figure 3.2.

A personal computer (PC) based control system on the surface supplies a full three dimensional graphical interface to the user and communicates with the control electronics mounted on the tool skid.

sea surface

extensible legs and

vacuum feet umbilical cable 6 dof manipulator ROV tool tool skid extend/rotate boom

FIG. 3.1 - SCHEM ATIC OF TO OL SKID A TTA C HED TO ROV

3.2.1. Remotely Operated Vehicle (ROV)

The ROV in figure 3.2, for which the tool skid was originally designed, is built by Slingsby engineering. It was designed as a flexible vehicle to which additional units can be added and is called the Multi-Role Vehicle (MRV). It uses a top hat design of tether management system (TMS) as illustrated in figure 3.3. As mentioned in chapter one, the tether management system is necessary to prevent the ROV having to pull the load of the much heavier lifting cable. Instead, the heavy lifting cable is used to lower the ROV to its operating depth. At this point, the ROV disconnects fi-om its tether management system and flies to its work site towing a cable that is only for power and communications but not lifting. It is, therefore, a shorter cable of smaller cross section. As a result, the drag forces due to sea currents and the motion of the ROV relative to the other end of the cable are less as these are roughly proportional to the projected area of cable as seen by the current flow (diameter x length).

The core vehicle measures 2.7m by 1.5m and is 1.56m tall. It uses six hydraulically powered thrusters and is rated to work at depths of up to 1000m. The full specification is included in Appendix A.

tether management system (TMS)

'top hat' style

main lift umbilical cable

ROV neutral buoyancy tether between the TMS

and the ROV

temporary connection released when TMS is in

correct position

manipulator

ROV (MRV)

FIG. 3.3 - TETHER M AN AG EM EN T SYSTEM ON TOP OF THE ROV (TO P HAT STYLE)

3.2.2. M anipulator Arm

The manipulator arm, also built by Slingsby engineering, was designed to be able to reach a high percentage of the welds around a node from a single base position. With its offset cranks and large angular range in many joints, it is highly dextrous. It has a large work envelope with a reach o f 2.5 metres. The data sheet is in Appendix B.

3.2.3. Tool Skid

The tool skid, also built by Slingsby engineering, was designed to be joined to the MRV or any other similarly sized work class ROV. The tool skid extend/rotate mechanism allows an extension of up to 2 metres and a rotation of up to 360 degrees. This considerably increases the reachable workspace and, for most nodes, allows the manipulator to reach almost the entire weld without the need to move and re-attach the ROV to the jacket. The tool skid carries electronics to control the extend/rotate mechanism, the manipulator and any attached tools as well as a hydraulic pump to drive the vacuum feet and the manipulator.

It is common to have some additional cameras fixed to the front of the tool skid as well as some flood lighting.

3.2.4. Control System

The control system, built by Technical Software Consultants, was designed to enable control of the manipulator in a number of different ways. These are:

1. Position Feedback Mode. A manual control method. A miniature model of the real arm is moved by the operator. The real manipulator follows its motions - known as ‘master/slave’ control.

2. Supervisory Control Mode. The manipulator may now be moved in a variety of standard methods such as position control, joint control, cartesian control. The operator is giving relatively high level commands which are interpreted by the supervisory computer which, in turn, controls the manipulator. For cartesian control, the operator may select whether to use global co-ordinates (the ROV co­ ordinate frame), tool co-ordinates or workpiece co-ordinates. For this type of inspection work, the workpiece co-ordinate frame is particularly useful. If the workpiece model has been created in terms of radius and angle from a reference line

along a cylinder centre (polar co-ordinates), then to follow the circumference of that cylinder, only the angle need be adjusted. This is considerably more intuitive than calculating the surface position in cartesian co-ordinates and also calculating the orientation required of the probe.

3. Fully Automatic Mode. In this mode a very high level command such as ‘perform inspection’ is given. The manipulator will then automatically move to the workpiece and perform its task.

The most important aspect of the ARM control system as a solution to the problem of sub-sea inspection, is the presence of low level intelligent modules that relieve the surface operator of certain difficult tasks. That is, the operator is able to specify a task, such as tracking a 20 degree arc of weld, with only a few simple commands. The commands are interpreted by the low level modules and converted into commands, in an appropriate co-ordinate frame, that can actually drive the manipulator. The high level commands enable tasks such as modelling the structure, envisaging complex three- dimensional paths upon that structure and moving the manipulator along the surface of the structure.