Cobot Implementation of 3D Virtual Surfaces


Cobots are devices for humadrobot interaction, in which axes of motion are coupled to one another by computercontrolled continuously variable transmissions rather than individually driven by servomotors. We have recently built a 3R parallelogram cobot shown in Figure 1. Here we present a control method for the display of virtual surfaces and for fiee-mode in which the cobot endpoint moves as if it were unconstrained. We show experimental results on the performance of the cobot in these modes. Introduction Robotic researchers are discovering that many tasks can be improved through the use of robot created virtual constraints and surfaces that redirect undesirable user motions to useful directions. For example, Akella [ 13 found that constraining the motion of a wheeled cart with virtual guides rails could dramatically decrease the effort involved in material handling operations. Rosenberg [2] has shown that teleoperation tasks such as remote peg in hole can be improved through the use of virtual walls that have the effect of filtering out user forces that would drive the slave end-effector off of an acceptable approach. Z-KAT, a surgical robotics firm, is using the WAM robot from Barrett Technology to render surfaces that constrain the motion of surgical instruments to regions determined preoperatively [3]. Industrial designers are also finding uses for virtual surfaces. For example, Stewart et a1 [4] of Ford Research Labs has developed a system that virtually renders an automobile dashboard so that the accessibility of controls can be examined. One quality measure of a virtual display system is its ability to produce surfaces that are hard and nearly frictionless. A hard virtual surface, which does not deform under user forces, gives the impression that one is interacting with a constraint not unlike those one is accustomed to in the real world. A nearly frictionless surface, which does not dissipate system energy, is better able to redirect user forces to useful directions. It is difficult to create virtual surfaces with these characteristics using traditional robots and haptic displays. Creating a virtual surface with a traditional manipulator requires that the joint motors be actuated to create end point forces that resist undesired user motion. To produce the correct forces, the controller must sense the endpoint location and the user force. The combined sampling and zero-order hold of the real time sensor data can result in the user being able to extract energy from virtual surfaces that are supposed to appear passive [5 ] . As a user probes a surface, the repeated cycle of energy build up and loss may cause the manipulator to go unstable. Maximizing sensor resolution and raising the sampling rate can decrease instabilities, but creating constraints that feel hard and smooth remains a difficult proposition. Figure 1: 3-Joint Arm Cobot Like stability, it is difficult to guarantee safety when a robot is used to create a virtual environment. The controller for a traditional (nonbackdrivable) manipulator can give the impression that the user is supplying the motive power that pulls the end point through space, but in reality, computer controlled motors are actuating the joints such that they move in response to user forces. These motors are capable of creating large end point velocities. In the event of a computer malfunction, the 0-7803-7272-71021$17.00

DOI: 10.1109/ROBOT.2002.1013726

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@inproceedings{Moore2002CobotIO, title={Cobot Implementation of 3D Virtual Surfaces}, author={Carl A. Moore and Michael A. Peshkin and J. Edward Colgate}, booktitle={ICRA}, year={2002} }