Farhad Aghili

A unified approach for inverse and direct dynamics of constrained multibody systems based on linear projection operator: applications to control and simulation

This paper presents a unified approach for inverse and direct dynamics of constrained multibody systems that can serve as a basis for analysis, simulation, and control. The main advantage of the dynamics formulation is that it does not require the constraint equations to be linearly independent. Thus, a simulation may proceed even in the presence of redundant constraints or singular configurations, and a controller does not need to change its structure whenever the mechanical system changes its topology or number of degrees of freedom. A motion-control scheme is proposed based on a projected inverse-dynamics scheme which proves to be stable and minimizes the weighted Euclidean norm of the actuation force. The projection-based control scheme is further developed for constrained systems, e.g., parallel manipulators, which have some joints with no actuators (passive joints). This is complemented by the development of constraint force control. A condition on the inertia matrix resulting in a decoupled mechanical system is analytically derived that simplifies the implementation of the force control. Finally, numerical and experimental results obtained from dynamic simulation and control of constrained mechanical systems, based on the proposed inverse and direct dynamics formulations, are documented.

Adaptive force-motion control of coordinated robots interacting with geometrically unknown environments

Most studies on adaptive coordination of multi-robot systems assume exact knowledge of system kinematics and deal only with dynamic uncertainties. However, many industrial applications involve tasks in which a multi-robot system interacts with geometrically unknown environments. In this paper, we consider a multi-robot system grasping a rigid object in contact with a geometrically unknown surface. The proposed adaptive hybrid force-motion controller guarantees asymptotic tracking of desired motion and force trajectories while ensuring exact identification of constraint Jacobian matrix without persistency of excitation condition. The control signal is smooth and does not depend on contact force derivative. The proposed adaptive controller is robustified against environmental friction and nonparametric uncertainty in environment geometry. Simulation examples are presented to illustrate the results.

A Prediction and Motion-Planning Scheme for Visually Guided Robotic Capturing of Free-Floating Tumbling Objects With Uncertain Dynamics

Visually guided robotic capturing of a moving object often requires long-term prediction of the object motion not only for a smooth capture but because visual feedback may not be continually available, e.g., due to vision obstruction by the robotic arm, as well. This paper presents a combined prediction and motion-planning scheme for robotic capturing of a drifting and tumbling object with unknown dynamics using visual feedback. A Kalman filter estimates the states and a set of dynamics parameters of the object needed for long-term prediction of the motion from noisy measurements of a vision system. Subsequently, the estimated states, parameters, and predicted motion trajectories are used to plan the trajectory of the robots end-effector to intercept a grapple fixture on the object with zero relative velocity (to avoid impact) in an optimal way. The optimal trajectory minimizes a cost function, which is a weighted linear sum of travel time, distance, cosine of a line-of-sight angle (object alignment for robotic grasping), and a penalty function acting as a constraint on acceleration magnitude. Experiments are presented to demonstrate the robot-motion planning scheme for autonomous grasping of a tumbling satellite. Two robotics manipulators are employed: One arm drifts and tumbles the mockup of a satellite, and the other arm that is equipped with a robotic hand tries to capture a grapple fixture on the satellite using the visual guidance system.

Task verification facility for the Canadian special purpose dextrous manipulator

As a partner in the International Space Station, Canada is responsible for the verification of all tasks involving the special purpose dextrous manipulator (SPDM). In this paper, the concept of an SPDM task verification facility (STVF) is described. The verification process involves three complementary stages. First, a real-time software simulator is used to verify the complete nominal and malfunction procedures. Next, the feasibility of a task involving contact is verified with the help of a hardware-in-the-loop simulator. Finally, a non real-time simulator is used to perform detailed parametric studies given the known tolerances on the components. The paper describes all three stages but emphasizes on the hardware-in-the-loop simulator, the only new facility within the STVF.

Fault-Tolerant Torque Control of BLDC Motors

Fault tolerance is critical for servomotors used in high-risk applications, such as aerospace, robots, and military. These motors should be capable of continued functional operation, even if insulation failure or open-circuit of a winding occur. This paper presents a fault-tolerant (FT) torque controller for brushless dc (BLdc) motors that can maintain accurate torque production with minimum power dissipation, even if one of its phases fails. The distinct feature of the FT controller is that it is applicable to BLdc motors with any back-electromotive-force waveform. First, an observer estimates the phase voltages from a model based on Fourier coefficients of the motor waveform. The faulty phases are detected from the covariance of the estimation error. Subsequently, the phase currents of the remaining phases are optimally reshaped so that the motor accurately generates torque as requested while minimizing power loss subject to maximum current limitation of the current amplifiers. Experimental results illustrate the capability of the FT controller to achieve ripple-free torque performance during a phase failure at the expenses of increasing the mean and maximum power loss by 28% and 68% and decreasing the maximum motor torque by 49%.

Driftless 3-D Attitude Determination and Positioning of Mobile Robots By Integration of IMU With Two RTK GPSs

This paper focuses on the integration of inertial measurement unit (IMU) with two real-time kinematic global positioning system (GPS) units in an adaptive Kalman filter (KF) for driftless estimation of a vehicles attitude and position in 3-D. The observability analysis reveals that 1) integration of a single GPS with IMU does not constitute an observable system; and 2) integration of two GPS units with IMU results in a locally observable system provided that the line connecting two GPS antennas is not parallel with the vector of the measured acceleration, i.e., the sum of inertial and gravitational accelerations. The latter case makes it possible to compensate the error in the estimated orientation due to gyro drift and its bias without needing additional instrument for absolute orientation measurements, e.g., magnetic compass. Moreover, in order to cope with the fact that GPS systems sometimes lose their signal and receive inaccurate position data, the self-tuning filter estimates the covariance matrix associated with the GPS measurement noise. This allows the KF to incorporate GPS measurements in the data fusion process heavily only when the information received by GPS becomes reliably available. Finally, test results obtained from a mobile robot moving across uneven terrain demonstrate driftless 3-D pose estimation.

A Reconfigurable Robot With Lockable Cylindrical Joints

This paper presents a new conceptual design for reconfigurable robots. Unlike conventional reconfigurable robots, our design does not achieve reconfigurability by utilizing modular joints. Rather, the robot is equipped with passive joints, i.e., joints without actuator or sensor, which permit changing the Denavit-Hartenberg (DH) parameters such as the link length and twist angle. The passive joints will become controllable when the robot forms a closed kinematic chain. Also, each passive joint is equipped with a built-in brake mechanism that is normally locked, but the lock can be released whenever the parameters are to be changed. Such a versatile and agile robot is particularly suitable for space application for its simple, compact, and light design. The kinematics and recalibration of this kind of reconfigurable robot are thoroughly analyzed. A stable reconfiguration-control algorithm is devised to take the robot from one configuration to another by directly regulating the passive joints to the associated, desired DH parameters. Conditions for the observability and the controllability of the passive joints are also derived in detail.

Design of a Hollow Hexaform Torque Sensor for Robot Joints

This work describes the design of a new one-axis torque sensor. It achieves the conflicting requirements of high stiffness for all six force and torque components, high sensitivity for the one driving torque of interest, and yet very low sensitivity for the other five force/torque components. These properties, combined with its donut shape and small size, make this sensor an ideal choice for direct-drive robotic applications. Experimental data validate the basic design ideas underlying the sensor’s geometry, the finite element model used in its optimization, and the advertised performance.

Experimental characterization and quadratic programming-based control of brushless-motors

A new torque control strategy for brushless motors is presented, which results in minimum torque ripple and copper losses. The motor model assumes linear magnetics, but contains a current limit which can delimit the onset of magnetic saturation, or be the motor amplifier current limit, whichever is reached first. The control problem is formulated and solved as a quadratic programming problem with equality and inequality constraints to find the nonlinear mapping from desired torque and position to the motors phase currents. The optimal solution is found in closed form using the Kuhn-Tucker theorem. The solution shows that, unlike the conventional commutation with a fixed current-position waveform, the waveforms of the proposed controller vary in order to respect the current limitation in one phase by boosting the current in the other phases. This increases the maximum torque capability of the motor - in our particular system by 20% - compared to fixed waveform commutation. Experimental data from our brushless direct-drive motor demonstrates that the controller produces virtually ripple-free torque and enhances remarkably the tracking accuracy of the motion controller.

Coordination control of a free-flying manipulator and its base attitude to capture and detumble a noncooperative satellite

This paper focuses on the guidance of a robot manipulator to capture a tumbling satellite and then bring it to state of rest (detumbling). First, a coordination control for combined system of the space robot and the target satellite, which acts as the manipulator payload, is presented so that the robot tracks the optimal path while regulating the attitude of the chase vehicle to a desired value. Subsequently, two optimal trajectories for the pre- and post-capture phases are designed. In the pre-capturing phase, the manipulator maneuvers are optimized by minimizing a cost function which includes the time of travel and the weighted norms of the end-effector velocity and acceleration, subject to the constraint that the robot end-effector and a grapple fixture on the satellite arrive at the rendezvous point with the same velocity. In the post-grasping phase, the manipulator dumps the initial velocity of the tumbling satellite in minimum time subject to the constraint that the magnitude of the torque applied to the satellite remains below a safe value. Simulation and experimental results are appended.