Yves Gonthier

Emulation of a space robot using a hydraulic manipulator on ground

To verify the tasks of one of the Space Station manipulator, the Canadian Space Agency is developing a hardware-in-the-loop simulation system. A hydraulic robot manipulating mockup of the space payload reproduces the contact forces occurring during insertion and extraction. The control law allowing a good performance requires a very accurate torque tracking for the hydraulic robot. With good force sensing in the hydraulic cylinder the torque control can be achieved in constraint motion but not in free motion. A controller using a feedforward term to compensate the velocity related effect is presented and tested experimentally. It is shown that this controller achieves very good results both in free motion and in a constrained task.

A Contact Modeling Method Based on Volumetric Properties

A novel contact model is presented which includes normal contact force and damping, rolling resistance torque and tangential friction force. This model is appropriate for the simulation of robotic tasks involving contact between objects of any shapes, occurring at relatively low velocities. It features a contact stiffness proportional to the contact area and leads automatically to the correct selection of the point of action of the force. The contact force magnitude is shown to be proportional to the interpenetration volume. A numerical simulation of a sphere impacting on the inside surface of a cylinder is presented.Copyright

On the development of a real-time simulator for an electrohydraulic forestry machine

This paper focuses on the development of a real-time graphical simulator engine for a forestry machine. The rigid body dynamics of the machines manipulator are integrated with electrohydraulic actuator dynamics and joint controllers. System stiffness introduced by the closing valves, high order hydraulic dynamics, and an interpreted implementation are identified as the prime reasons for slowing down the integration. Successive models are proposed aiming at achieving a faithful machine simulator that can run in real-time. Simulation results obtained show very good prediction of an actual machines behavior, with execution speeds improved by a factor of 35.

On the Implementation of Coulomb Friction in a Volumetric-Based Model for Contact Dynamics

A contact model based on volumetric properties is presented. The model properties are derived assuming the elastic behavior of the contacting objects can be approximated as a modified Winkler elastic foundation model, and that the contact surface between the objects is approximately flat. The resulting model includes friction and features a contact force proportional to the inter-penetration volume. The work shows that the Coulomb friction is affected by the relative motion. The contact model can be used as a general-purpose tool to model contact dynamics for a broad range of object shapes because the volumetric quantities that serve as input to the contact model can be determined for any object shape. A numerical simulation of a Tippe-Top is presented, and the results are shown to be consistent with published data, but with a higher spinning friction torque.Copyright

On manipulator posture planning for large force tasks

Studies the problem of large force/torque application using robotic systems with limited force/torque actuators. For such systems, the available workspace may be smaller than its reachable workspace. It is shown that redundancy increases the force capability and workspace of a robotic system. To plan redundant manipulator postures during force tasks, a new method based on a min-max optimization scheme is used. Unlike other norm-based methods, the proposed one guarantees that no actuator capabilities are exceeded, and that the required force/torque of the most loaded actuator is minimized. Examples that demonstrate the validity and usefulness of the proposed method are included.

On the Development of a Real-Time Simulator Engine for a Hydraulic Forestry Machine

Abstract This work focuses on the development of a real-time training simulator engine for a forestry machine. The rigid body dynamics of the machines manipulator is integrated with electrohydraulic actuator dynamics and joint controllers. System numerical stiffness introduced by the closing valves, high order hydraulic dynamics, and simulator implementation using an interpreted language were identified as the prime reasons for slowing down the integration. Successive models of lower complexity and switching between models for the open and closed phases of the valves are proposed aiming to achieve a satisfactory simulator engine that can run in real-time. Simulation results demonstrate very good prediction of an actual machine behaviour with execution speeds improved by a factor of 35.

On the modeling and control of an experimental harvester machine manipulator

This paper focuses on the modeling, parameter estimation, and model validation in open and closed-loop of an experimental forestry machine manipulator. Symbolic Newton-Euler and linear graph methodologies are used in deriving mathematical models of the swing, boom and stick subsystems. Actuation dynamics are integrated with manipulator dynamics to result in a complete manipulator and actuation model. Identification procedures employed in estimating physical parameters are discussed. Model validation studies show good agreement between model predictions and experiments. The models will be used for designing a controller for coordinated end-point motion and for a real-time graphical training simulator.

An Object-Oriented Contact Modelling Framework for Robotics

An object-oriented framework that facilitates the creation of contact models based on compliance is introduced. Contact models are created by adding contact model components to a standard container class. The geometric components can readily be implemented for objects with simple geometries. The volumetric contact model is then proposed to handle complex objects. It can be used as a general-purpose tool to model contact between objects of any shapes. Numerical simulation results are presented.Copyright

Contact Parameter Estimation With a Space Robot Verification Facility

Computer simulations play an important role in the design and verification of space robotic operations since on-orbit tests are impossible to conduct before launch. Thus, accurate computer modeling and simulation of space robotic tasks is essential. Of particular difficulty are the space manipulator operations, that involve constrained or contact tasks. Here, the contact dynamics capability in the modeling tools becomes critical for high fidelity simulations. This in turn implies a need for accurate determination of contact parameters, which are used as inputs to contact dynamics simulation. In this work, the identification of contact dynamics parameters based on sensor data obtained during robotic contact tasks is considered. In particular, the contact parameter estimation problem is addressed for simple contacting geometries using the SPDM Task Verification Facility Manipulator Testbed (SMT) at the Canadian Space Agency, where SPDM is the Special Purpose Dexterous Manipulator. The SMT is a robotic simulation facility, which also features gravity compensation algorithms to support the emulation of space robots. Single point SMT contact experiments were performed with six different payloads. Eight unique single point contact parameter estimation algorithms were used as part of the process of identifying payload stiffness from SMT experimental data.Copyright

Large force-task planning for mobile and redundant robots

This paper analyzes the application of large force/torques by robotic systems with limited force/torque actuators. It is shown that such system may be able to apply a force/torque in some configurations only; therefore its useful force workspace is limited. To improve the force capabilities of a system, base mobility and/or redundancy can be employed. A planning algorithm is proposed which results in proper base positioning relative to a large-force quasi-static task. To plan redundant manipulator postures during large force-tasks, a new method based on a min-max optimization scheme is developed. Unlike norm-based methods, this method guarantees that no actuator capabilities are exceeded, and that the force/torque of the most loaded joint is minimized. Examples that demonstrate the validity and usefulness of the proposed methods are included.