Ben Jonker

Improving the vibration isolation performance of hard mounts for precision equipment

This paper discusses the application of adaptive feedforward control to generate anti-forces in an active hard mount, thereby improving the vibration isolation performance of the mount. The adaptation algorithm is a preconditioned filtered reference least mean squares (FxLMS) algorithm. An integral acceleration feedback controller is used to add artificial damping to the suspension modes and relevant structural modes. Moreover, the feedback control results in a more computationally efficient and faster converging feedforward controller. Simulation and real-time test results on a laboratory setup are presented, which demonstrate the feasibility of the concept.

Modelling and Identification of a Six Axes Industrial Robot

This paper deals with the modelling and identification of a six axes industrial St ¨aubli RX90 robot. A non-linear finite element method is used to generate the dynamic equations of motion in a form suitable for both simulation and identification. The latter requires that the equations of motion are linear in the inertia parameters. Joint friction is described by a friction model that describes the friction behaviour in the full velocity range necessary for identification. Experimental parameter identification by means of linear least squares techniques showed to be very suited for identification of the unknown parameters, provided that the problem is properly scaled and that the influence of disturbances is sufficiently analysed and managed. An analysis of the least squares problem by means of a singular value decomposition is preferred as it not only solves the problem of rank deficiency, but it also can correctly deal with measurement noise and unmodelled dynamics.

THERMAL MODELING OF A MINI ROTOR-STATOR SYSTEM

In this study the temperature increase and heat dissipation in the air gap of a cylindrical mini rotor stator system has been analyzed. A simple thermal model based on lumped parameter thermal networks has been developed. With this model the temperature dependent air properties for the fluid-rotor interaction models have been calculated. Next the complete system has also been modeled by using computational fluid dynamics (CFD) with Ansys-CFX and Ansys. The results have been compared and the capability of the thermal networks method to calculate the temperature of the air between the rotor and stator of a high speed micro rotor has been discussed.

Design of an Experimental Setup for Testing Multiphysical Effects on High Speed Mini Rotors

Recently, there have been numerous research projects on the development of minirotating machines. These machines mostly operate at speeds above the first critical speed and have special levitation systems. Besides, the multiphysical effects become significant in small scale. Therefore, advanced modeling approaches should be developed and innovative experimental rigs with the foregoing requirements should be constructed in order to test the developed techniques. In the current study, the design of an experimental setup for testing the multiphysical effects has been outlined. First, the previously developed multiphysical models (Dikmen, E., van der Hoogt, P., de Boer, A., and Aarts, R., 2010, “Influence of Multiphysical Effects on the Dynamics of High Speed Minirotors—Part I: Theory,” J. Vibr. Acoust., 132, p. 031010; Dikmen, E., van der Hoogt, P., de Boer, A., and Aarts, R., 2010, “Influence of Multiphysical Effects on the Dynamics of High Speed Minirotors—Part II: Results,” J. Vibr. Acoust., 132, p. 031011) for the analysis of small scale rotors are described briefly for background information. Second, an analysis of the effect of the rotor parameters (diameter, length, rotation speed, etc.) on the dynamics of the rotor under multiphysical effects is presented. Afterward the design process which includes the design decisions based on these results, the availability, simplicity, and applicability of each component is presented in detail. Finally, the experimental results have been presented and the efficiency of the design has been evaluated. In summary, the design requirements for an experimental setup for testing multiphysical effects on minirotors have been analyzed. The design procedure and evaluation of the design have been presented.

Real-time trajectory generation for sensor-guided robotic laser welding

Abstract This paper presents a trajectory generator, which generates a smooth robot trajectory in real-time. The trajectory generator is based on cubic interpolation, where Cartesian locations (position and orientation) can be added on-the-fly during the robot motion. This way sensor information obtained during the robot motion can be used to generate the robot trajectory. A general control architecture called trajectory-based control is presented in combination with this trajectory generator. Experiments have been carried out which show the capabilities of the trajectory generator to provide a smooth trajectory for the robot joint motion controller.

Feedback Control for Optimal Production Speed in Laser Beam Welding of Mild Steel

Full penetration of a weld is an important quality demand in many laser welding applications. Furthermore for economical reasons the highest possible production speed is preferred. To achieve both goals simultaneously, process settings close to partial penetration should be applied. Consequently the risk of losing full penetration increases due to external disturbances or gradual changes in process parameters.In this work a feedback controller is described which is able to search for and operate at the highest possible welding speed given a certain maximum available laser power while at the same time full penetration is maintained. The signal from an optical detector which is placed inside the Nd:YAG laser source is used as a feedback signal. The ability of this detector to monitor the transition from a fully penetrated keyhole to a partially penetrated keyhole weld has been used to design a ‘switching’ controller. In a first feedback control loop the laser power is used as an actuator to operate near the edge of partial penetration. The fast response of this controller can cope with sudden disturbances. A second feedback loop maximizes the welding speed utilizing all available laser power.Full penetration of a weld is an important quality demand in many laser welding applications. Furthermore for economical reasons the highest possible production speed is preferred. To achieve both goals simultaneously, process settings close to partial penetration should be applied. Consequently the risk of losing full penetration increases due to external disturbances or gradual changes in process parameters.In this work a feedback controller is described which is able to search for and operate at the highest possible welding speed given a certain maximum available laser power while at the same time full penetration is maintained. The signal from an optical detector which is placed inside the Nd:YAG laser source is used as a feedback signal. The ability of this detector to monitor the transition from a fully penetrated keyhole to a partially penetrated keyhole weld has been used to design a ‘switching’ controller. I...

Penetration Control in Laser Welding of Sheet Metal using Optical Sensors (cd-rom)

For economical reasons it is desirable to apply the highest possible speed during laser welding. Increasing the welding speed at a certain laser power might result in insufficient penetration of the weld. This work describes the design of a feedback controller, which is able to maintain full penetration in mild steel sheets. An optical detector, which is placed inside the Nd:YAG laser source, measures the intensity of the weld-pool radiation through the optical fibre. This sensor signal is used as input of a feedback control system. A model, which describes the dynamic response of the welding process, including the sensor and laser source dynamics, has been obtained, using system identification techniques. The input of this model is the laser power and the output is the modelled sensor signal. Based on this dynamic model a feedback controller is designed and implemented. The laser power is used as an actuator. The controller maintains full penetration during welding of tracks with disturbances like sudden speed changes. This opens the possibility to optimise the welding speed without risking lack of penetration.For economical reasons it is desirable to apply the highest possible speed during laser welding. Increasing the welding speed at a certain laser power might result in insufficient penetration of the weld. This work describes the design of a feedback controller, which is able to maintain full penetration in mild steel sheets. An optical detector, which is placed inside the Nd:YAG laser source, measures the intensity of the weld-pool radiation through the optical fibre. This sensor signal is used as input of a feedback control system. A model, which describes the dynamic response of the welding process, including the sensor and laser source dynamics, has been obtained, using system identification techniques. The input of this model is the laser power and the output is the modelled sensor signal. Based on this dynamic model a feedback controller is designed and implemented. The laser power is used as an actuator. The controller maintains full penetration during welding of tracks with disturbances like sudden...

Flow Induced Instability on High Speed Mini Rotors in Laminar Flow

In this study, a modeling approach is developed to examine laminar flow effects on the rotordynamic behavior of high-speed mini rotating machinery with a moderate flow confinement. The existing research work mostly focuses on the flow-induced forces in small gap systems, such as bearings and seals, in which the flow is mostly laminar and inertia effects are ignored. In other studies, medium gap systems are analyzed, taking the inertia effects into consideration, but the surrounding flow is considered as turbulent. However, in high speed mini rotating machinery, the large clearances and the high speeds make the inertia effects significant, even in the laminar flow regime. In the current study, the flow-induced forces resulting from the surrounding fluid are analyzed and these models are combined with the structural finite element (FE) models for determining the rotordynamic behavior. The structure is analyzed with finite elements based on Timoshenko beam theory. Flow-induced forces, which include inertia effects, are implemented into the structure as added mass-stiffness-damping at each node in the fluid confinement. The shear stress is modeled with empirical and analytical friction coefficients, and the stability, critical speeds, and vibration response of the rotor is investigated for different friction models. In order to validate the developed modeling approach, experiments were conducted on a specially designed setup at different support properties. By comparing the experiments with the theoretical models, the applicability of the different friction models are examined. It was found that the dynamic behavior is estimated better with empirical friction models compared to using the analytical friction models.

A flexible rotor on flexible supports: modeling & experiments

In this study, a flexible rotor with variable support stiffness has been analyzed. Simple support models consisting of mass, spring systems are extracted from modal analysis of the isolated support and by applying static loads to the finite element model of the supports. The derived equivalent models of the supports are then implemented in the finite element based structural model which predicts the dynamic behavior of the rotor. Finally experimental modal analysis of the rotor is performed with different support stiffnesses. The experimental and theoretical results have been compared and different support modeling approaches have been examined.

Perimetric sensor for the detection and following of complex seam trajectories in robotic laser welding

Laser welding is a process that has high positioning accuracy demands. Such processes are usually performed with velocities of up to 250mm/sec. When robots are used to manipulate the laser welding equipment at such velocities, the robot dynamics can significantly influence the positioning accuracy. The addition of sensors can assist in correcting positioning errors. Nevertheless, the use of sensors imposes additional restrictions on the way that robots are allowed to move over a seam trajectory. For several cases the use of sensors can force robots into positioning errors due to robot dynamics. Thus by fixing one problem another one is created.It is therefore desirable for the sensors to detect and follow a seam without forcing the manipulating robot into dynamic positioning errors. Such a sensor is presented in this paper.Laser welding is a process that has high positioning accuracy demands. Such processes are usually performed with velocities of up to 250mm/sec. When robots are used to manipulate the laser welding equipment at such velocities, the robot dynamics can significantly influence the positioning accuracy. The addition of sensors can assist in correcting positioning errors. Nevertheless, the use of sensors imposes additional restrictions on the way that robots are allowed to move over a seam trajectory. For several cases the use of sensors can force robots into positioning errors due to robot dynamics. Thus by fixing one problem another one is created.It is therefore desirable for the sensors to detect and follow a seam without forcing the manipulating robot into dynamic positioning errors. Such a sensor is presented in this paper.