Abhijit Ganguli

Active damping of chatter in machine tools: demonstration with a “hardware-in-the-loop” simulator

Abstract The motivation of the work is twofold: (a) understand the physics behind regenerative chatter and the influence of structural damping and (b) demonstrate an active damping technique based on collocated actuator/sensor pairs. A numerical stability analysis is performed using the root locus method and it is shown that, along with the structural poles, eigenvalues due to the delay parameter may contribute to instability. Since experimental demonstration of chatter in real machines is difficult, an alternative way of demonstration via a mechatronic simulator is presented, using the ‘hardware-in-the-loop’ concept. The mathematical model of the regenerative cutting process in turning is simulated in a computer and this is interfaced to a beam, representing the structural dynamics of the machine, via a displacement sensor and force actuator. In this way, a hardware and a software loop are combined. In a second step, an additional control loop is added, consisting of an accelerometer sensor and a collocated inertial actuator. Numerical and experimental stability lobe diagrams are compared, with and without active damping.

Active vibration control of piezolaminated stiffened plates

The aim of this work is to present active vibration control of stiffened plates. A stiffened plate finite element with piezoelectric effects is formulated. The characteristic feature of the stiffener is that it can have any shape in plan and need not pass through the nodal lines of the finite element mesh. The coupling between the direct and the converse piezoelectric effects is neglected for simplicity. A velocity feedback algorithm is employed in the active control. Numerical examples for vibration control of isotropic and orthotropic stiffened plates have been presented.

Defect detection around rebars in concrete using focused ultrasound and reverse time migration

Experimental and numerical investigations have been performed to assess the feasibility of damage detection around rebars in concrete using focused ultrasound and a Reverse Time Migration (RTM) based subsurface imaging algorithm. Since concrete is heterogeneous, an unfocused ultrasonic field will be randomly scattered by the aggregates, thereby masking information about damage(s). A focused ultrasonic field, on the other hand, increases the possibility of detection of an anomaly due to enhanced amplitude of the incident field in the focal region. Further, the RTM based reconstruction using scattered focused field data is capable of creating clear images of the inspected region of interest. Since scattering of a focused field by a damaged rebar differs qualitatively from that of an undamaged rebar, distinct images of damaged and undamaged situations are obtained in the RTM generated images. This is demonstrated with both numerical and experimental investigations. The total scattered field, acquired on the surface of the concrete medium, is used as input for the RTM algorithm to generate the subsurface image that helps to identify the damage. The proposed technique, therefore, has some advantage since knowledge about the undamaged scenario for the concrete medium is not necessary to assess its integrity.

Simulation and Active Control of Chatter in Milling via a Mechatronic Simulator

This paper presents a 2 degree of freedom “Hardware in the Loop” mechatronic simulator for the study of regenerative chatter in milling. The main motivation behind the construction of the simulator is to propose active damping as a strategy for control of chatter in milling. A good comprehension of regenerative chatter in milling is essential for that purpose. Characterization of chatter in a real machining environment may be difficult, and the mechatronic simulator provides an alternative way to conduct investigations on chatter in milling in a laboratory environment, without conducting actual cutting tests. The simulator is found to realistically reproduce various kinds of bifurcation phenomena normally expected in milling operations. Active damping is then implemented on the system to investigate its effect on stability. It is shown experimentally that stability lobes are enhanced by the application of active damping, which fulfils the original objective of proposing active damping as a chatter stabilizing strategy.

Localized Condition Monitoring Around Rebars using Focused Ultrasonic Field and SAFT

The objective of this article is to assess the feasibility of detection of delaminations around rebars in cement mortar using focused ultrasound. The focused field is used to insonify the medium and subsequent image reconstruction is done using the Synthetic Aperture Focusing Technique. The attenuation and scattering of ultrasonic waves by cement mortar can be partially circumvented by focusing the ultrasonic energy towards a particular region of interest. This increases the chances of detection of delaminations around rebars in the chosen region of interest. Artificial delaminations of submillimeter dimensions wrapped around rebars are found to be detectable using this approach. The operational time associated with focusing with an ultrasonic array and subsequent image formation is shown to be less in comparison to the Total Focusing Method. This makes the proposed method an attractive diagnostic technique for rapid nondestructive evaluation of cement-based materials.

Experimental Investigation of Ultrasound Wave Focusing in Attenuative Solids

This paper presents an experimental study on ultrasonic focusing in an attenuative solid made of cement mortar. The objective is to comparatively investigate the performance of various techniques for wave focusing. Standard attenuation measurements are performed on small cement mortar samples to generate a preliminary idea about the loss in the material. Subsequently, focusing experiments are conducted on a larger mortar block fabricated with the same mix design parameters. The time-delayed and time-reversal methods are employed to perform focusing. A synthetic aperture approach is used to generate the unfocused and focused fields. The time-reversal focusing method generates a narrower focal spot in comparison to the time-delayed method, indicating better focusing action. The experimental results are also compared with an analytical model of focusing in an attenuative fluid with an array of point sources, and a good agreement is confirmed.

Optimal ultrasonic array focusing in attenuative media.

This paper presents a parametric study on the efficiency of ultrasound focusing in an attenuative medium, using phased arrays. Specifically, an analytical model of ultrasound wave focusing in a homogeneous, isotropic and attenuative fluid with point sources is presented. Calculations based on the model have shown that in an attenuative medium, an optimum frequency exists for the best focusing performance for a particular size of aperture and focal distance. The effect of different f numbers on the focusing performance in the attenuative medium is further investigated. The information obtained from the analytical model provides insights into the design and installation of a phased transducer array for energy efficient wave focusing.

Seismic Analysis of Weightless Sagging Elasto-flexible Cables

There exists considerable literature which deals with the dynamic response of cables with distributed self-weight and some lumped masses, if any. Seismic response of single weightless cable structures has not yet been sufficiently investigated. In this Paper, seismic response of a single weightless planer elasto-flexible sagging cable with lumped nodal masses is studied. This investigation is informed by the appreciation that weightless flexible cables lack unique natural state. Rate-type constitutive equation and third order differential equations of motion have been derived earlier. Using these equations, the dynamic response of such cables subjected to harmonic excitation has also been studied by the Authors. Configurational response is distinguished from the elastic response. The scope of the present Paper is limited to prediction of vibration response of a weightless sagging planer two-node cable structure with lumped masses and sustained gravity loads subjected to horizontal and vertical seismic excitations in the presence of sustained gravity loads. The horizontal and vertical seismic excitations are predicted to cause predominantly configurational and elastic displacements from the equilibrium state. Also, the tensile forces in the inclined and horizontal segments are caused predominantly by these excitations respectively. Cross effects due to mode coupling are predicted. No empirical validation of the theory is attempted. The theoretical predictions are validated by comparing with the seismic response of heavy cable nets predicted by other researchers. The theoretical significance of the approach followed here is critically evaluated.

Investigation of born approximation applied to non-destructive evaluation of concrete media

The accuracy of the Born Approximation as a forward model of elastic wave scattering in the context of simulating Impact Echo tests of reinforced of concrete is investigated in this paper. The ability of a forward model to realistically simulate the physics of a system can be important when such a model is used as part of an inverse solution. Synthetic data of scattering by air void defects that are typically present in damaged civil engineering structures is generated by a two-dimensional Finite Difference in Time Domain (FDTD) model for elastic wave propagation in an infinite, homogeneous and isotropic concrete medium. Horizontal elongated cracks and air voids with compact shapes are considered in this study for comparison between the synthetic and the Born approximated data. It is observed that the Born Approximation simulates a compact air void better than a horizontal elongated one. This knowledge provides insight on Born Approximation as part of an inverse solution towards imaging of air voids of various shapes in a damaged civil engineering structure.

Linearized Dynamic Analysis of Weightless Sagging Planar Elastic Cables

Nonlinear dynamic analysis of elastic structures is known to be much more complex than their linear analysis. There are many sources of nonlinearity of the structural response of elastic cables, viz., physical nonlinearity due to nonlinear tension–extension relations, geometric nonlinearity associated with finite elastic displacements and nonlinearity of nodal load–displacement relations due to the presence of self-weight. Incremental second-order differential equations of motion are used to predict the vibration amplitudes relative to the equilibrium state caused by additional dynamic forces. Generally, the tangent stiffness matrices are determined by adding the tangent elastic and geometric stiffness matrices. Many a time, an approximate linearized dynamic analysis is attempted. In this paper, the initial tangent stiffness matrix corresponding to the equilibrium state is used in the second-order linear differential equation of motion. The dynamic response relative to the equilibrium state of the structure subjected to additional dynamic loads is predicted. The predictions of linearized dynamic analysis are generally considered acceptable for small elastic displacements from the equilibrium state. The validity of such linearized dynamic analysis for elasto-flexible cables obeying third-order differential equation of motion is explored.