A two-layer architecture for force-reflecting bilateral teleoperation with time delays

We propose a two-layer control architecture for bilateral teleoperation with communication delays. The controller is structured with an (outer) performance layer and an (inner) passivity layer. In the performance layer, any traditional controller for bilateral teleoperation can be implemented. In the passivity layer, the output of the performance layer is modified to guarantee that, from the operator and environment perspective, the overall teleoperator is passive: the amount of energy that can be extracted from the teleoperator is bounded from below and the rate of increase of the stored energy in the teleoperator is bounded by (twice) the environment and operator supplied power. Passivity is ensured by modulating the performance layer outputs and by injecting a variable amount of damping via an innovative energy-based logic that follows a principle of energy duplication and takes into account the effects of the delays. In contrast to the traditional teleoperation approach, where the master and slave controllers implement an as-stiff-as-possible coupling between the master and slave devices, our scheme is specifically designed for direct force-reflecting bilateral teleoperation: the slave controller mimics the operator action, while the master controller reflects the slave-environment interaction. We illustrate the effectiveness of the approach in a challenging simulation example with a round-trip delay of 100 ms, while making and breaking contact with a static environment having a stiffness of 50.000 N/m.\u3cbr/\u3

Direct force-reflecting two-layer approach for passive bilateral teleoperation with time delays

\u3cp\u3eWe propose a two-layer control architecture for bilateral teleoperation with communication delays. The controller is structured with an (inner) performance layer and an (outer) passivity layer. In the performance layer, any traditional controller for bilateral teleoperation can be implemented. The passivity layer guarantees that, from the operator and environment perspective, the overall teleoperator is passive: The amount of energy that can be extracted from the teleoperator is bounded from below and the rate of increase of the stored energy in the teleoperator is bounded by (twice) the environment and operator supplied power. Passivity is ensured by modulating the performance layer outputs and by injecting a variable amount of damping via an energy-based logic that follows the innovative principle of energy duplication and takes into account the detrimental effects of time delays. In contrast to the traditional teleoperation approach, in which the master and slave controllers implement an as-stiff-as-possible coupling between the master and slave devices, our scheme is specifically designed for direct force-reflecting bilateral teleoperation: The slave controller mimics the operator action, whereas the master controller reflects the slave-environment interaction. We illustrate the performance of the two-layer approach in a challenging experiment with a round-trip communication delay of 300 ms while making and breaking contact with a stiff aluminum environment. Finally, we also compare our controller with the state of the art.\u3c/p\u3

IO linearization, stability, and control of an input non-affine thermoelectric system

\u3cp\u3eWe consider a novel control architecture for an input non-affine thermoelectric system, which is used to control the temperature of an object subject to unknown thermal disturbances. A key component in this architecture is given by an input-output linearizing feedback controller to deal with the nonlinear dynamics associated with the input. This enables us to use linear control techniques with the associated performance guarantees. Using a Lyapunov-based stability analysis we derive sufficient conditions for asymptotic stability in the nominal operating regime. To prevent instability outside the nominal operating regime, e.g. in the face of large disturbances where stability is inevitably compromised, we propose to saturate the control input by using state-dependent bounds. These bounds automatically trade-off performance and stability, thereby avoiding the need for complicated stability analysis per application, and as such, allowing the designer to focus on performance. The effectiveness of the nonlinear control approach is demonstrated through measurement results.\u3c/p\u3

Force control of the interaction while skinning a pig fore-end

The skinning and defatting of pig legs is a labor intensive task, which the DeboFlex project focuses on to automate by replacing human workers by robotic manipulators. In this internship the necessity and influence of force control is investigated on the contact dynamics between the skinning tool and pig leg. An intuitive and simplified 6DOF (Degree of Freedom) model is derived to perform continuoustime simulations using a PERA (Philips Experimental Robotic Arm). An inner and outer control loop is used to respectively control the motion of the end-effector using independent joint control and the interaction force exercised on the pig leg using an impedance controller. Moreover a discrete-time implementation is realized in Gazebo, a robotic simulator. The developed software is compatible for real-time implementation. An impedance controller has been developed to shape the dynamic contact properties between skinner and pig leg. The controller parameters are dependent on the contact properties of the pig leg. Stability and robustness of the control structure is investigated. Furthermore, discrete-time simulations are performed for practical implementation. This research has shown several scenarios where force control shows huge potential and advantages. But before a fair assessment can be made whether force control is necessary, the contact properties and their variations have to be investigated

Guaranteeing stable tracking of hybrid position-force trajectories for a robot manipulator interacting with a stiff environment

This work considers the control of a manipulator with the aim of executing desired time-varying motion–force trajectories in the presence of a stiff environment. In several situations, the interaction with the environment constrains just one degree of freedom of the manipulator end-effector. Focusing on this contact degree of freedom, a switching position–force controller is considered to perform the hybrid motion–force tracking task. To guarantee input-to-state stability of the switching closed-loop system, a novel stability result and sufficient conditions are presented. The switching occurs when the manipulator makes or breaks contact with the environment. The analysis shows that to guarantee closed-loop stability while tracking arbitrary time-varying motion–force profiles with a rigid manipulator, the controller should implement a considerable (and often unrealistic) amount of damping, resulting in inferior tracking performance. Therefore, we use the stability analysis technique developed in this paper to analyze a manipulator equipped with a compliant wrist. Guidelines are provided for the design of the wrist compliancy while employing the switching control strategy, such that stable tracking of a motion–force reference trajectory can be achieved and bouncing of the manipulator against the stiff environment can be avoided. Numerical simulations are presented to illustrate the effectiveness of the approach

Switched position-force tracking control of a manipulator interacting with a stiff environment

This work proposes a control law for a manipulator with the aim of realizing desired time-varying motion/force profiles in the presence of a stiff environment. In many cases, the interaction with the environment affects only one degree of freedom of the end-effector of the manipulator. Therefore, the focus is on this contact degree of freedom, and a switching position-force controller is proposed to perform the hybrid position-force tracking task. Sufficient conditions are presented to guarantee input-to-state stability of the switching closed-loop system with respect to perturbations related to the time-varying desired motion-force profile. The switching occurs when the manipulator makes or breaks contact with the environment. The analysis shows that to guarantee closed-loop stability while tracking arbitrary time-varying motion-force profiles, the controller should implement a considerable (and often unrealistic) amount of damping, resulting in inferior tracking performance. By redesigning the manipulator with a compliant wrist and employing the designed switching control strategy, stable tracking of a motion-force reference trajectory can be achieved and bouncing of the manipulator while making contact with the stiff environment can be avoided