Page 524 - The ROV Manual - A User Guide for Remotely Operated Vehicles 2nd edition

19.4 Range of motion and workspace
Each joint added to a manipulator adds a degree of freedom (DoF). As a minimum for a work manipulator, it is desirable to be able to translate within and rotate about the x, y, z axes (six DoF) and then grasp something or hold a tool (seventh DoF). Therefore, the heavy-duty work manipula- tors have the seven linear (or rotary joints) DoF. Manipulator manufacturers then provide plan and elevation views of the range of motion of the subject manipulator (Figures 19.20 and 19.21).
The total volume reachable by the end effector (Figures 19.20 and 19.21) is termed the work space. The geometry of the manipulator’s motion as well as the joints’ mechanical constraints defines the workspace. The workspace is further broken down into “reachable” and “dexterous” workspace. The reachable workspace is the volume whereby the end effector is capable of reaching each point within the space in at least one orientation while the dexterous workspace has the end effector capable of reaching all points in all orientations.
These views provide an excellent depiction of the external shell that the tip of the manipulator can reach, that is, its range of motion. However, the warm feeling of such a magnificent range of motion must be tempered with the reality of performing work within that shell. This is not a case of Michelangelo reaching out with a paintbrush toward the ceiling of the Sistine Chapel. The tasks that manipulators are required to complete offshore are complex and may require various joint con- figurations to reach and/or align with the point of interest. Thus, the range of motion of a manipula- tor will be much larger than the practical “workspace” of the manipulator.
There are other factors that affect the real workspace of a manipulator or set of manipulators/ grabbers. The first is the range of motion of the manipulator/grabber that may be used to hold the vehicle in position so that the other arm can perform the assigned task. As an example, if the grabber had a 4.5 ft (137 cm) reach and the working manipulator had a 6 ft (183 cm) reach (and was expected to work on the same vertical surface to which the grabber is attached), then the range of motion of the manipulator would be affected and the overall workspace would probably be reduced.
Now take the previous example a step further. The manipulator has to pick up a large tool and align it orthogonally to that same vertical work plane. The tip of the manipulator has now become the tip of the tool, which again reduces the maneuverability of the manipulator and thus its overall workspace. The bottom line is that the design of a complex work system that uses a variety of tools on a variety of objects is not a simple task. The manipulator(s) and/or grabber(s) specifications must be evaluated in conjunction with the working environment to be encountered and the tools to be used within that environment. The good news is that the leading offshore firms now have or use simulators of the vehicles, manipulators, and expected working environment to ensure that the cho- sen system and configuration can in fact perform the necessary underwater tasks.
19.5 Types of controllers
Manipulator and grabber controllers range from simple on/off toggle switches (rate control) to small replica controllers that match the joint configurations of the working arm (position control). Going to the next level of complexity is the force feedback controller where the forces encountered by the manipulator are scaled and fed back to the operator via the replica controller. These types of controllers are discussed in more detail in the following sections.
19.5 Types of controllers 521