Aghil Jafari

Input-to-state stable approach to release the conservatism of passivity-based stable haptic interaction

Passivity has been a major criterion on designing a stable haptic interface due to many advantages. However, passivity has been suffering from its intrinsic conservatism since it only represents a small set of the whole stable region. Therefore, there was always limitation to increase the performance due to the small design margin from the passivity criterion. In most of the cases, stability and performance has trade-off relationship. In this paper, we propose a less conservative control approach for stable haptic interaction based on Input to State Stable (ISS) criterion. The proposed approach is inspired from the analogy between virtual environments and systems with hysteresis nonlinearities. A system with hysteresis nonlinearity has sector bounded property, which allows us to guarantee that only a finite amount of energy can be extracted from the system, which leads the system to be dissipative [1] and also the states to be bounded by a function of the input [2]. Since the finite amount of energy is allowed to be extracted from the system, the proposed ISS approach has less conservative constraint compared with passivity-based approaches. Moreover, the proposed approach has a simple structure and does not use any system parameters which make it suitable for practical implementations. Experimental evaluation validates the effectiveness of the proposed approach.

Hybrid force-motion control of coordinated robots interacting with unknown environments

This paper presents a unified framework for system design and control in cooperative robotic systems. It introduces a highly general cooperative system configuration involving any number of manipulators grasping a rigid object in contact with a deformable working surface whose real physical parameters are unknown. Dynamics of the closed chain mechanism is expressed based on the objects center of mass, and different robust controllers are designed for position and force control subspaces. The position controller is composed of a sliding mode control term, and involves the position and velocity feedback of end-effector, while the force control is developed based on the highest derivative in feedback methodology. The force controller does not use any derivation of the force signal as well as the internal force controller induced in the system, and it appears to be very practical. Simulation results for two three joint arms moving a rigid object are presented to validate the theoretical results.

A stable perturbation estimator in force-reflecting passivity-based teleoperation

Many researchers have focused on force tracking improvement in teleoperation systems. Various methods have been utilized to reach this aim. This paper focuses on force-reflecting passivity-based architecture. Adding a force error term to conventional passivity-based architecture, a controller has been introduced to improve transparency. A condition has been obtained, which guarantees the stability of the system, while it holds. Moreover, a sliding mode perturbation estimation algorithm has been proposed. The method proved to be asymptotically converging to perturbations. The transparency of system has been compared in simple and force-reflecting passivity-based architectures, and also with and without perturbation estimator. Experiments on a 2-DOF non-linear delayed teleoperation system were conducted to investigate the system’s performance. Experimental results have shown that the controller is stable and force tracking has been improved compared with previous research. Furthermore, the perturbation in the system has been successfully estimated and cancelled. Conclusively, the paper introduces a controller that is stable and compares its performance in force tracking with previous research.

Independent force and position control for cooperating manipulators handling an unknown object and interacting with an unknown environment

Abstract This paper presents a unified framework for system design and control in cooperative robotic systems. It introduces a highly generalized cooperative system configuration that involves any number of manipulators grasping a rigid object in contact with a deformable working surface whose real physical parameters are unknown. The dynamics of the closed chain mechanism are expressed based on the object׳s center of mass (CoM), and different robust controllers are designed for position and force control subspaces. The position controller is composed of a sliding mode control term, and it involves the position and velocity feedback of an end-effector, while the force controller is developed based on the highest derivative in feedback methodology. The force controller does not use any derivation of the force signal or internal force controller induced in the system, and it appears to be suitable for practical implementation. Using a Lyapunov stability approach, the controller is proven to be robust with varying system dynamics. The position/orientation and the force errors are also demonstrated to asymptotically converge to zero under such conditions. The simulation results for two-joint arms moving a rigid object are presented to validate the theoretical results.

Sliding mode hybrid impedance control of robot manipulators interacting with unknown environments using VSMRC method

In the present paper, the objective of hybrid impedance control is specified and a robust hybrid impedance control approach is proposed. Based on the concept of hybrid control, the task space is decomposed into position and force controlled subspaces. Impedance control is used in the position controlled subspace. Desired inertia and damping are applied in the force controlled subspace to meliorate the dynamic behavior of robot manipulator. Robust controller using the variable structure model reaching control (VSMRC) is introduced that can realize the objective impedance in the sliding mode in finite time. In order to overcome the chattering effect due to sliding mode approach, fuzzy logic methodology is employed in the control system. In addition, the reaching transient response is undertaken with prescribed quality. Simulating the control system for a 6DOF PUMA560 robot confirms the validity and effectiveness of the proposed control system.

An Adaptive Hybrid Force/Motion Control Design for Robot Manipulators Interacting in Constrained Motion With Unknown Non-Rigid Environments

In the present paper, the objective of hybrid control is specified and an adaptive hybrid force/motion control approach is proposed. Based on the concept of hybrid control, the task space is decomposed into position and force controlled subspaces. An adaptive scheme is presented which makes the controller robust when the robot is in interaction with an unknown non-rigid environment. By using the classical Lyapunov method, it is demonstrated that the proposed control law ensures the tracking of the unconstrained components of the desired end-effector trajectories, with regulation of the desired contact force along the constrained direction. Simulation results verify the effectiveness of our prosperous adaptive hybrid control in robot-environment interaction.© 2012 ASME

Multi Degree-of-Freedom Input-to-State Stable approach for stable haptic interaction

Passivity has been the most often used constraint for the controller design of haptic interfaces. However, the designed controller based on passivity constraint has been suffering from its conservatism, especially when the user wants to increase the maximum achievable impedance. To overcome this problem, our group have proposed Input-to-State Stable (ISS) approach [1], which reduce the design conservatism of the passivity-based controller by allowing bigger output energy from the haptic interface compared with the passivity-based controller while guaranteeing the stability. However, the previous paper was limited to single Degree-of-Freedom (DoF) systems. This paper extends the ISS approach for multi-DoF haptic interaction. For multi-DoF haptic interaction, penetration depth-based rendering method using Virtual Proxy (VP) is adopted, and VP allows us to decouple the interaction into each axis. Although the interaction can be decoupled, previous ISS analysis “cannot” be directly implemented because the decoupled system, unlike to the previous case, has unconstrained end point, that is a moving Virtual Environment (VE). To include the moving VE into the ISS approach, we extend the previous one-port ISS approach to two-port ISS approach, and generalize this into multi-DoF ISS approach by augmenting each two-port analysis. Proposed approach is experimentally verified with Phantom Pre. 1.5, and showed the effectiveness of the proposed multi-DoF ISS approach.

Increasing the impedance range of admittance-type haptic interfaces by using Time Domain Passivity Approach

This paper proposes a method to increase the impedance range of admittance-type haptic interfaces. Admittance-type haptic interfaces are used in various applications that typically require interaction with high impedance virtual environments. However, the performance of admittance haptic interfaces is often judged by the lower boundary of the impedance that can be achieved without stability problem; in particular, minimum displayable inertia. It is well known that rendering the low value of inertia makes the admittance-type haptic interfaces unstable easily. This paper extends Time Domain Passivity Approach (TDPA) to lower down the minimum achievable inertia in an Admittance-type haptic interface. To use the well-developed TDPA framework, an admittance haptic interface should be represented in network domain with clear energy flows, which was not straightforward due to unclear causality. Therefore by introducing dependent effort and flow source concept, the admittance type haptic interface is represented in electrical network domain. This network representation allows us to have clear causality, and consequently allowing to implement TDPA. The proposed idea was experimentally verified, and found successful in bringing down the minimum inertia 10 times lower than without TDPA case.

6-DOF extension of memory-based passivation approach for stable haptic interaction

This paper extends previously proposed memory-based passivity approach (MBPA) to 6 degrees-of-freedom (DOF) haptic interactions. By introducing 6-DOF virtual proxy and virtual object, connected with haptic interaction point (HIP), we can extend MBPA to 6-DOF, including torque vs. orientation passivity. To find a passive relationship between positions vs. force graph, instead of position itself, we use position error between the center of mass of the virtual proxy and virtual object. For the angle vs. torque graph, we use angular displacement between the unit vector of the virtual proxy and virtual object instead of angle itself. After the extension, position/angular resolution became a problem, since it depends on configuration of the haptic device. Position resolution should be fixed, and real-time monitoring of every single position change is strongly required in previous MBPA. However, those are difficult requirements when the approach is extended to multi-DOF, especially 6-DOF including orientation. This paper revises the data saving procedure and force bounding mechanism to allow variable position resolution. Position value instead of simple incremental integer is saved together with matched force data into memory space, which allows to interpolate corresponding force value at the releasing trajectory even though it is not in the saved data list. This generalized framework of 6-DOF haptic rendering is evaluated according to simulation results.

Sliding Mode Hybrid Impedance Control of Robot Manipulators Interacting With Unknown Environments Using VSMRC Method

In the present paper, the objective of hybrid impedance control is specified and a robust hybrid impedance control approach is proposed. Based on the concept of hybrid control, the task space is decomposed into position and force controlled subspaces. Impedance control is used in the position controlled subspace. Desired inertia and damping are applied in the force controlled subspace to meliorate the dynamic behavior of robot manipulator. Robust controller using the variable structure model reaching control (VSMRC) is introduced that can realize the objective impedance in the sliding mode in finite time. In order to overcome the chattering effect due to sliding mode approach, fuzzy logic methodology is employed in the control system. In addition, the reaching transient response is undertaken with prescribed quality. Simulating the control system for a 6DOF PUMA560 robot confirms the validity and effectiveness of the proposed control system.Copyright