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Virtual Reality: Scientific and Technological Challenges
In order to discuss the application of hazardous waste disposal in more detail, we take as a scenario the removal of hazardous waste from an underground storage tank, such as at Hanford, Washington (Harrigan, 1993), which has been demonstrated in part in laboratory settings. Sensors and manipulators are needed that are immune to the chemical, radiation, and thermal stresses that would destroy ordinary video camera lenses and sensing elements, manipulator lubricants and mechanisms. Moreover, the hazardous nature of the materials being handled places a premium on high reliability. Manipulators for gaining access to storage vats must be large yet have the ability to work in confined spaces. Such systems will be extremely expensive. Since there are many waste sites to handle, advances in supervisory control are required to speed up operations.
The first problem is that the contents of each tank is not known. Hence vision and proximity sensors have to be inserted by a manipulator to map the tank's contents. In this mapping phase, the manipulator is manually controlled; proximity sensors are used to approach surfaces without collisions using local control. In this operation, it is critical that the tank sides not be contacted to avoid rupture. From these data, a representation of the tank's contents is constructed with operator assistance. For recognizable objects such as pipes, an object model can be specified. For waste surfaces and nonrecognizable objects, a three-dimensional surface representation is employed.
The model of the tank's contents is then used to generate a graphical image and to perform simulations of operations. Tasks and robot motions are planned through real-time graphical simulations: sequences are developed, operations are dynamically simulated, and collisions and joint limits are checked. Just prior to a real operation, a simulation can be run to verify the expected result.
For the actual task, the operator relies heavily on the graphical display and simulation to control the telerobot because of poor visibility in the tank. Sensing during the operation is employed to update and modify the graphical model. As a motion develops, the simulation is run simultaneously to check for emerging problems such as collisions or other unsafe operations. Approaches to surfaces and objects will be conducted in local mode, using proximity and contact sensors in the end effector. Wherever possible, autonomous control will be employed to speed up such operations as grasping objects, cutting pieces of the structure loose, and conveying waste from the tank.
Achievement of remote manipulation from the earth to the moon dates from the 1967 United States Surveyor and the Russian Lunakhod,