Mobile Manipulator Design December 10, 2007 Reid Simmons, Sanjiv Singh Robotics Institute Carnegie Mellon University
1. Introduction This report provides a preliminary design for two mobile manipulators under development at Carnegie Mellon University. We expect that in the near future that these machines will be integrated and tested in a scenario that involves assembly tasks such as insertion of wiring harness clips into a taskboard. This project will integrate state of the art components a mobile base and a 7 DOF robot arm along with computing, sensors and wireless communication to produce a state of the art mobile manipulator. This function integration will utilize software infrastructure developed previously and demonstrated on several mobile manipulators. Our design is broken out into the following elements: Mechanical configuration Sensing Computing Control Software Below we discuss each of the above in separate sections. 2. Mechanical Configuration The Mobile Manipulator is composed of two main elements the base is a differentially driven mobile robot manufactured by MobileRobots.com called POWERBOT. The manipulator is a 7 DOF arm manufactured by Barrett called the WAM. Figure 1. The design for a mobile manipulator uses a Powerbot for the base and a WAM arm for the manipulator. Mobile Manipulator Design, Dec 10, 2007 1
Figure 2 shows an integrated design for the mobile manipulator. The arm can be mounted in one of two configurations, one of them show below and another in which the base of the arm is moved forward to be flush with front of the top plate on the base. Figure 2. Mechanical configuration of the integrated mobile manipulator. Figure 3 shows a breakout of the main components of the mobile manipulator. Figure 3. Key components on the mobile manipulator. Mobile Manipulator Design, Dec 10, 2007 2
3. Sensing Sensing for the mobile manipulator is accomplished by a number of devices. The first three are shown in Figure 3. Bumblebee stereo cameras. Two camera provide a way to determine relative pose between fiducials. Vistracker. This device provides an alternate means of tracking fiducials. While this device is not as flexible as the bumblebee stereo cameras, it provides tracking using an embedded processor. We expect to use this device especially when the taskboard is mobile. Sick laser scanner. This device is used to safeguard the mobile manipulators from collisions with objects in the environment. This is expected to be useful in the case that there is an unanticipated object in the environment but also if there is an error in trajectory generation Force Sensing. The WAM provides forces sensing and we expect that this will be very useful for insertion tasks. 4. Computing Figure 4 shows the computing architecture that is planned for the mobile manipulator. Figure 4. Computing architecture for the mobile manipulator Mobile Manipulator Design, Dec 10, 2007 3
5. Control We have designed the controller for the mobile manipulator such that it is possible to control each joint individually but it is also possible to specify motion of the end-effector directly. The control scheme ensures that the joints move appropriately to accomplish the desired endeffector velocity. The architecture of this control scheme is shown in Figure 5. Figure 5. Control scheme for coordinated motion of base and arm This control scheme has been validated using a simulator that models the base and the manipulator arm. Two instances of simulation are shown in Figure 6 and Figure 7. In both cases, a target point outside the reach of the manipulator is specified. The control scheme moves both base and the arm to the desired target. In addition to arriving at a target point, it is possible to add additional criteria such as the maximization of a measure such as manipulability. Mobile Manipulator Design, Dec 10, 2007 4
Figure 6. (left) starting configuration and target shown in red circle (right) ending configuration that the mobile manipulator drives to automatically. Figure 7. (left) starting configuration and target shown in red circle (right) ending configuration that the mobile manipulator drives to automatically. 6. Conclusions We have created a design for a mobile manipulator based on previous experience on other platforms. The configuration and components chosen for this design are different from those that we have used in the past but we expect that our designs will generalize to the new configuration. We expect that the added dexterity available from a 7 DOF arm and a robust base will lead to a high performance mobile manipulator. Mobile Manipulator Design, Dec 10, 2007 5