Meyer Nahon

Kinematic Analysis and Optimization of a New Three Degree-of-Freedom Spatial Parallel Manipulator

A study of the kinematic characteristics of a three degree-of-freedom (dof) parallel mechanism is presented. The architecture of the mechanism is comprised of a mobile platform attached to a base through three identical prismatic-revolute-spherical jointed serial linkages. The prismatic joints are considered to be actuated. These prismatic actuators lie on a common plane and have radial directions of action. The mechanisms inverse displacement solution is obtained. Since the mechanism has only 3 dof, constraint equations describing the inter-relationship between the six motion coordinates are derived. These constraints allow the definition of parasitic motions, i.e., motions in the three unspecified motion coordinates. Architecture optimization of the device is undertaken demonstrating that specific values of design variables allow minimization of parasitic motion.

Real-time force optimization in parallel kinematic chains under inequality constraints

The real-time control of cooperating manipulators, mechanical hands, and walking machines involves the optimization of an underdetermined force system subject to both equality and inequality constraints. The inequality constraints arise as a result of passive frictional contacts in systems that depend on these for force transmission or when taking into account the limited torque or force capability of the actuators. Since the results of the force optimization are used to provide force or torque setpoints for the actuators, they must be obtained in real time. A technique for solving a quadratic optimization problem with equality and inequality constraints is presented for this application. The technique is compared with linear programming schemes to show its superior performance in terms of the speed and quality of the solution. The technique is well suited to problems where the solution can change discontinuously in time, as is the case with the systems considered due to changes in their topology. >

Response of airline pilots to variations in flight simulator motion algorithms

The use of physical motion in flight simulation is still a much debated topic. This paper investigates the more narrow issue of its application in commercial jet transport simulators. We have attempted to quantify the perceptions of airline pilots about the quality of motion possible when a number of different motion-drive algorithms are tested on a simulator employing a state-of-the-art six-degrees-of-freedom motion-base. Four broad categories of algorithms were tested: classical washout, optimal control, coordinated adaptive, and no-motion. It was found that although there was little impact of algorithm type on performance arid control activity, there was a definite effect on how the pilots perceived the simulation environment. Based on these findings, it appears that the coordinated adaptive algorithm is generally preferred by the pilots over the other algorithms tested, there was almost unanimous dislike of the no-motion case.

Force optimization in redundantly-actuated closed kinematic chains

The authors establish why redundant actuation situations arise, the relationship of redundant actuation to time-varying topologies, and the desirability of redundant actuation. Various techniques are then outlined for solving the optimal force distribution problem, to obtain force setpoints for the controller in real time. Although direct substitutions is by far the fastest of these, its implementation requires a careful monitoring of numerical degeneracies. Solution of the problem using orthogonal decomposition is recommended as it provides the opportunity to introduce inequality constraints, which are important in many problems. An effective technique is introduced for smoothing the discontinuities in the optimal solution, which result from the discontinuous topology.<<ETX>>

Development and validation of a lumped-mass dynamics model of a deep-sea ROV system

Abstract A one-dimensional finite-element lumped-mass model of a vertically tethered caged ROV system subject to surface excitation is presented. Data acquired during normal operation at sea are used with a least-squares technique to estimate the coefficients required by the model. The model correctly predicts: (a) the motion of the cage and the tension in the tether at the ship; (b) the spectrum of cage acceleration in the wave-band; (c) the transfer function between the vertical motion of the ship and cage; and (d) the natural frequency of the system and its harmonics. A simple model of the wake of the cage was added to the model simulation and this reduced the error in the calculated motion and tension by almost a factor of two and brought the calculated transfer function within the 95% confidence interval of the measurements. By increasing ship motion slightly, the model accurately reproduced eight snap loads and their non-linear characteristics—a regularly spaced series of rapid increases in the records of the acceleration of the cage and tension at the ship—that occurred during the measurements used for validation.

Exergy analysis of a fuel cell power system for transportation applications

An exergy analysis of a solid polymer fuel cell power system for transportation applications is reported. The analysis was completed by implementing the derived fundamental governing second law equations for the system into a fuel cell performance model developed previously. The model analyzes all components of the system including the fuel cell stack and air compression, hydrogen supply, and cooling subsystems. From the analysis, it was determined that the largest destruction of exergy within the system occurs inside the fuel cell stack. Other important sources of exergy destruction include irreversibilities within the hydrogen ejector and the air compressor, and the exergy emission associated with the heat rejected from the radiator. To increase the accuracy of the model and extend the results, a more comprehensive model for the fuel cell stack should be developed, and an investigation should performed into the effects of varying the operating parameters of the system. The results may aid efforts to optimize fuel cell systems.

Motion Simulation Capabilities of Three-Degree-of-Freedom Flight Simulators

We present the results of a study aimed at determining the simulation realism that might be achieved using reduced-degree-of-freedom flight simulator motion bases. More specifically, the quality of motion produced by two different three-degree-of-freedom platforms was compared to that produced by a conventional six-degree-of-freedom Stewart platform. The three-degree-of-freedom motion bases investigated were a spherical mechanism allowing only rotational motions, as well as a motion base capable of heave, pitch, and roll motions. To compare the different motion bases, three characteristic maneuvers were simulated using a nonlinear model of a Boeing 747. The aircraft motions were then simulated on nine different combinations of virtual motion platforms and motion base drive algorithms. The motion cues (specific forces and angular velocities) produced in this manner were then graphically compared. The analysis revealed that, in most cases, a three-degree-of-freedom simulator is capable of producing motion simulation quality comparable to that produced by a six-degree-of-freedom Stewart platform

Dynamic Analysis of a Macro-Micro Redundantly Actuated Parallel Manipulator

In this paper the kinematic and Jacobian analysis of a macro–micro parallel manipulator is studied in detail. The manipulator architecture is a simplified planar version adopted from the structure of the Large Adaptive Reflector (LAR), the Canadian design of the next generation of giant radio telescopes. This structure is composed of two parallel and redundantly actuated manipulators at the macro and micro level, which both are cable-driven. Inverse and forward kinematic analysis of this structure is presented in this paper. Furthermore, the Jacobian matrices of the manipulator at the macro and micro level are derived, and a thorough singularity and sensitivity analysis of the system is presented. The kinematic and Jacobian analysis of the macro–micro structure is extremely important to optimally design the geometry and characteristics of the LAR structure. The optimal location of the base and moving platform attachment points in both macro and micro manipulators, singularity avoidance of the system in nominal and extreme maneuvers, and geometries that result in high dexterity measures in the design are among the few characteristics that can be further investigated from the results reported in this paper. Furthermore, the availability of the extra degrees of freedom in a macro–micro structure can result in higher dexterity provided that this redundancy is properly utilized. In this paper, this redundancy is used to generate an optimal trajectory for the macro–micro manipulator, in which the Jacobian matrices derived in this analysis are used in a quadratic programming approach to minimize performance indices like minimal micro manipulator motion or singularity avoidance criterion.

A simplified dynamics model for autonomous underwater vehicles

This paper deals with the derivation and validation of a simplified dynamics model for streamlined underwater vehicles. The model is derived in a form which only requires the specification of the vehicles geometry, and the lift and drag characteristics of its constituent elements. The vehicle is decomposed into these elements, including hull, individual control surfaces, and propulsion system. The forces and moments acting on the vehicle are determined through a summation of the component effects, with correction factors to account for interference effects. Because the model is not linearized, it retains the vehicles fundamental nonlinear behaviour. The approach is validated by comparing the model results to those measured on an existing AUV.

Dynamics and control of a towed underwater vehicle system, part I: model development

Abstract Autonomous vehicles are being developed to replace the conventional, manned surface vehicles that tow mine hunting towed platforms. While a wide body of work exists that describes numerical models of towed systems, they usually include relatively simple models of the towed bodies and neglect the dynamics of the towing vehicle. For systems in which the mass of the towing vehicle is comparable to that of the towed vehicle, it becomes important to consider the dynamics of both vehicles. In this work, we describe the development of a numerical model that accurately captures the dynamics of these new mine hunting systems. We use a lumped mass approximation for the towcable and couple this model to non-linear numerical models of an autonomous surface vehicle and an actively controlled towfish. Within the dynamics models of the two vehicles, we include non-linear controllers to allow accurate maneuvering of the towed system.