Ronald G.K.M. Aarts

Penetration control in laser welding of sheet metal

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:Yttrium–aluminium–garnet laser source, measures the intensity of the weld-pool radiation through the optical fiber. 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 modeled 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 artificial power fluctuations and sudden speed changes. This feedback controller opens the possibility to optimize the welding speed without risking lack of penetration

LIMPACT:A Hydraulically Powered Self-Aligning Upper Limb Exoskeleton

The LIMPACT is an exoskeleton developed to be used in identifying the reflex properties of the arm in stroke survivors. Information on joint reflexes helps in designing optimal patient specific therapy programs. The LIMPACT is dynamically transparent by combining a lightweight skeleton with high power to weight ratio actuators. The LIMPACT is supported by a passive weight balancing mechanism to compensate for the weight of the exoskeleton and the human arm. Various self-aligning mechanisms allow the human joint axes to align with the axes of the exoskeleton which ensure safety and short don/doff times. The torque-controlled motors have a maximum torque bandwidth of 97 Hz which is required for fast torque perturbations and smooth zero impedance control. The LIMPACTs weight is reduced five times as gravitational forces are lowered using a model-based gravity compensation algorithm. The impedance controller ensures tracking of a cycloidal joint angle reference. A cycloid with an amplitude of 1.3 rd and a maximum velocity of 6.5 rd/s has a maximum tracking error of only 7%. The LIMPACT fulfills the requirements to be used in future diagnostics measurements for stroke patients.

Dynamic Simulation of Planar Flexible Link Manipulators using Adaptive Modal Integration

In this paper a modal integration method is proposed for analyzingthe dynamic behavior of multi-link planar flexible manipulators. Anon-linear finite element method is employed to derive theequations of motion in terms of a mixed set of generalizedcoordinates of the manipulator with rigid links and deformationparameters that characterize flexible deformations of the links.Using a perturbation method the vibrational motion of themanipulator is modeled as a first-order perturbation of thenon-linear nominal rigid link motion. For that purpose theflexible dynamic manipulator model is split into two parts. Arigidified model describes the nominal rigid link motion. Alinear system linearized about the nominal trajectory describesthe vibrational motion. In order to reduce the dimension of thelinearized system, a modal reduction technique is proposed. Thenmodal integration can be applied using only a small number of lowfrequency modes. The mode-acceleration concept is used to accountfor the pseudo static contribution of the high frequency modes.Applied to the motion of a manipulator mechanism the method isreferred to as ‘adaptive modal integration’ since thetime-varying nature of the mode shape functions is taken intoaccount.A flexible two-link manipulator is analyzed to illustrate theperformance of the solution method. Comparisons between solutionsutilizing non-linear and perturbation analyzes with and withoutmodal integration show a good agreement. In a simulation with onlya few modes the accuracy is kept, whereas the computation time isdrastically reduced.

Keyhole shapes during laser welding of thin metal sheets

Camera observations of the full penetration keyhole laser welding process show that the keyhole shape is elongated under certain welding conditions. Under these unfavourable circumstances, the welding process is susceptible to holes in the weld bead. Existing models of the pressure balance at the keyhole wall cannot explain this keyhole elongation. In this paper a new model is presented, accounting for the doubly curved shape of the keyhole wall. In this model, the surface tension pressure has one term that tends to close the keyhole and another term that tries to open it. Model calculations show that when the keyhole diameter is of the same order as the sheet thickness, the latter part can become dominant, causing the keyhole to elongate. Experiments on thin aluminium (AA5182) and mild steel (DC04) sheets verify these model calculations. As the keyhole radius depends on the radius of the focused laser spot, it was found for both materials that the ratio of the spot radius and the sheet thickness must be above a critical value to prevent keyhole elongation. These critical radii are 0.25 for AA5182 and 0.4 for DC04, respectively. Furthermore, differences in appearance of the weld bead between the circular and the elongated keyhole welds could be explained by this model.

Closed Loop Control of Penetration Depth during CO2 Laser Lap Welding Processes

In this paper we describe a novel spectroscopic closed loop control system capable of stabilizing the penetration depth during laser welding processes by controlling the laser power. Our novel approach is to analyze the optical emission from the laser generated plasma plume above the keyhole, to calculate its electron temperature as a process-monitoring signal. Laser power has been controlled by using a quantitative relationship between the penetration depth and the plasma electron temperature. The sensor is able to correlate in real time the difference between the measured electron temperature and its reference value for the requested penetration depth. Accordingly the closed loop system adjusts the power, thus maintaining the penetration depth.

A Perturbation Method for Dynamic Analysis and Simulation of Flexible Manipulators

This paper presents a perturbation method for the dynamicsimulation of flexible manipulators. In this method thevibrational motion of the manipulator is modeled as a first-orderperturbation of the nominal rigid link motion. For that purposethe flexible dynamic model is split into two parts. A rigidifiedsystem describes the nominal rigid link motion of themanipulator. A linear system linearized about the nominaltrajectory describes the vibrational motion. These equations arecomputationally more efficient than the non-linear dynamicequations and offer more insight in the dynamic phenomena of thesystem. The method is based on a full non-linear finite elementformulation which treats the general case of coupled largedisplacements motion and small elastic motion. A planar one linkmanipulator and a spatial two link manipulator with flexiblelinks are used for case studies. A comparison is made between thenon-linear and the perturbation analyzes. The perturbation methodappears to be a very efficient approach for dynamic analyzes offlexible manipulators and yields accurate results even forsystems with low frequency elastic modes.

Assessment of Multi-Joint Coordination and Adaptation in Standing Balance: A Novel Device and System Identification Technique

The ankles and hips play an important role in maintaining standing balance and the coordination between joints adapts with task and conditions, like the disturbance magnitude and type, and changes with age. Assessment of multi-joint coordination requires the application of multiple continuous and independent disturbances and closed loop system identification techniques (CLSIT). This paper presents a novel device, the double inverted pendulum perturbator (DIPP), which can apply disturbing forces at the hip level and between the shoulder blades. In addition to the disturbances, the device can provide force fields to study adaptation of multi-joint coordination. The performance of the DIPP and a novel CLSIT was assessed by identifying a system with known mechanical properties and model simulations. A double inverted pendulum was successfully identified, while force fields were able to keep the pendulum upright. The estimated dynamics were similar as the theoretical derived dynamics. The DIPP has a sufficient bandwidth of 7 Hz to identify multi-joint coordination dynamics. An experiment with human subjects where a stabilizing force field was rendered at the hip (1500 N/m), showed that subjects adapt by lowering their control actions around the ankles. The stiffness from upper and lower segment motion to ankle torque dropped with 30% and 48%, respectively. Our methods allow to study (pathological) changes in multi-joint coordination as well as adaptive capacity to maintain standing balance.

Brief paper: A computationally efficient algorithm of iterative learning control for discrete-time linear time-varying systems

Iterative Learning Control (ILC) improves the tracking accuracy of systems that repetitively perform the same task. This paper considers model-based ILC for linear time-varying (LTV) systems. The applied feedforward iteratively minimises a quadratic norm of the feedforward update and the error in the next iteration as predicted by the model. The optimal feedforward update can be derived straightforwardly using a matrix description of the system dynamics. However, the implementation of the resulting matrix equation is demanding in terms of computation time and memory. In this paper it is shown that an efficient algorithm can be derived directly from the matrix equation using the associated state-equations. The ILC algorithm is applied to an industrial robot. The configuration dependent robot dynamics can be approximated as LTV for small tracking errors from the large-scale motion along the desired trajectory. It is shown that a substantial reduction of the tracking error at the robots tip can be realised by ILC using an LTV model of the robot dynamics and the same reduction cannot be accomplished using an LTI model that ignores the variation of the robot dynamics along the trajectory.

Experimental observation of keyhole shapes in the laser welding of aluminum blanks

A camera-based in situ monitoring system for the Nd:yttrium–aluminum–garnet laser keyhole welding process of aluminium has been developed. The use of an external illumination source for image formation decouples the performance of this system from many process parameters. A prototype of the monitoring system was used to investigate two different welding modes, which have been observed during the laser welding process of AA5182 alloy sheets. One welding mode results in a circular keyhole shape, while the second mode has a more elliptical shape. Welds resulting from both welding modes also have a different surface structure at the top and bottom weld surface. Mechanical tests showed a marginal difference in mechanical properties of both welding types. However, the welding mode with an elliptical keyhole shape is more susceptible to holes in the weld seam. Experiments showed that this welding process can be visualized well with the developed monitoring system and that both welding modes can be distinguished clearly

Model-based iterative learning control applied to an industrial robot with elasticity

In this paper model-based iterative learning control (ILC) is applied to improve the tracking accuracy of an industrial robot with elasticity. The ILC algorithm iteratively updates the reference trajectory for the robot such that the predicted tracking error in the next iteration is minimised. The tracking error is predicted by a model of the closed-loop dynamics of the robot. The model includes the servo resonance frequency, the first resonance frequency caused by elasticity in the mechanism and the variation of both frequencies along the trajectory. Experimental results show that the tracking error of the robot can be reduced, even at frequencies beyond the first elastic resonance frequency.