A rigid (micro) robot mounted serially to the tip of a long, flexible (macro) manipulator is often used to increase reach capability, but flexibility in the macromanipulator can interfere with positioning accuracy. A rigid manipulator attached to a flexible but un actuated base was used to study a scheme to achieve positioning of the micromanipulator combined with enhanced vibration damping of the base. Ineltial interaction forces and torques acting between the robot and its base were modeled and studied to determine how to use them to damp the vibration. One issue is that there are locations in the workspace where the rigid robot loses its ability to create interactions in one or more degrees of freedom. These "ineltial singularities" are functions of the rigid robot's joint variables. A performance index was developed to predict the ability of the rigid robot to damp vibrations and will help ensure the robot is operating in joint space configurations favorable for inertial damping. It is shown that when the performance index is used along with the appropriate choice of feedback gains, the inertia effects, or those directly due to accelerating the robot's links, have the greatest influence on the interactions. By commanding the robot link's accelerations propOitional to the base velocity, vibration energy will be removed from the system. This signal is then added to the rigid robot's position control signal.Simulations of a three-degree of freedom anthropomorphic rigid robot mounted on a flexible base were developed and show the effectiveness of the control scheme. In addition, results from two degree of freedom vibration damping are included.

This paper presents practical aspects of implementing a dead zone inverse on a hydraulic wrist. A dead zone occurs over a range of small input values for which a system does not respond. It was desirable to use the most straightforward method available to achieve improved system performance while requiring the least amount of modification to the controller. Thus a fixed parameter dead zone inverse (DZI) was added to an existing proportional-integral (PI) controller. First, the parameters of the dead zone were characterized from open loop testing. These parameters are the break points, or input values between which the system does not respond at all, and the slope of the system’s response just outside the break points. The DZI augments the PI signal input to the plant, effectively adding or subtracting a constant equal to the size of the dead zone break points and scaling the input by its slope. Simulations predicted perfect system tracking, but implementation on the hardware revealed several practical issues. First, the dead zone slope parameters vary throughout the robot’s workspace. Overestimation can lead to non-ideal system performance, but the more extreme problem is underestimation, which effectively increases control loop gain and can lead to system instability. However, performance is not affected significantly unless these parameters are off by an order of magnitude. Overall the system performance is relatively robust to modeling errors in the slope parameters. The second issue is that noise can be magnified by the dead zone inverse and cause chattering. This problem was very noticeable in the wrist when the estimated dead zone break points were used in the DZI. This problem can be eliminated by reducing the dead zone break points or reintroducing a small artificial dead zone back into the control loop to envelope the expected noise level. The requirements for successful implementation of the DZI were found to be a basic characterization of the dead zone and an understanding of practical system issues that can be accentuated by its use. The effectiveness of the technique was tested through simulations and experiments on the wrist.

Il Q).J d ("~.s I L4 !lO'V.I7-1. 2 l-etfJ z. ABSTRACT A rigid (micro) robot mounted serially to the tip of a long, flexible (macro) manipulator is often used to increase reach capability, but flexibility in the macromanipulator can interfere with positioning accuracy. A rigid manipulator attached to a flexible but un actuated base was used to study a scheme to achieve positioning of the micromanipulator combined with enhanced vibration damping of the base. Ineltial interaction forces and torques acting between the robot and its base were modeled and studied to determine how to use them to damp the vibration. One issue is that there are locations in the workspace where the rigid robot loses its ability to create interactions in one or more degrees of freedom. These "ineltial singularities" are functions of the rigid robot's joint variables. A performance index was developed to predict the ability of the rigid robot to damp vibrations and will help ensure the robot is operating in joint space configurations favorable for inertial damping. It is shown that when the performance index is used along with the appropriate choice of feedback gains, the inertia effects, or those directly due to accelerating the robot's links, have the greatest influence on the interactions. By commanding the robot link's accelerations propOitional to the base velocity, vibration energy will be removed from the system. This signal is then added to the rigid robot's position control signal.Simulations of a three-degree of freedom anthropomorphic rigid robot mounted on a flexible base were developed and show the effectiveness of the control scheme. In addition, results from two degree of freedom vibration damping are included.

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