Dynamic characterization and modification of dynamic properties of a micro scanner

Micro electro mechanical systems (MEMS) are used in many application areas in different disciplines and took their place among the most promising technologies. The performance of such systems is primarily related to their dynamical characteristics. This study presents the dynamic characterization techniques that are used to identify the modal parameters of a MEMS device and the methods that can be implemented to change its dynamic response. An electrostatic scanner is chosen as the case study to demonstrate the developed methodologies. Initially, the micro scanner is characterized using experimental modal analysis techniques to obtain frequency response function, modal damping, resonance frequencies, and mode shapes. Then, velocity and position feedback control loops are implemented to the scanner system to alter the damping and stiffness characteristics. A closed-loop Simulink model of the scanner is developed to verify the experimental measurements. Several curve fitting methods are used in order to have an accurate representation of the scanner system. Using the model, the influence of both position and velocity feedback on the effective damping, resonance frequency and the transient behavior of the scanner is investigated. The stability limits of the scanner under velocity feedback are also studied via numerical simulations. Based on the experimental and simulation results, the methodology developed in this study proves itself to be very efficient to alter the dynamical characteristics of the MEMS structures and it can be easily adapted to other MEMS applications.     

Adaptive Q control for tapping-mode nanoscanning using a piezoactuated bimorph probe

A new approach, called adaptive Q control, for tapping-mode atomic force microscopy (AFM) is introduced and implemented on a homemade AFM setup utilizing a laser Doppler vibrometer and a piezoactuated bimorph probe. In standard Q control, the effective Q factor of the scanning probe is adjusted prior to the scanning depending on the application. However, there is a trade-off in setting the effective Q factor of an AFM probe. The Q factor is either increased to reduce the tapping forces or decreased to increase the maximum achievable scan speed. Realizing these two benefits simultaneously using standard Q control is not possible. In adaptive Q control, the Q factor of the probe is set to an initial value as in standard Q control, but then modified on the fly during scanning when necessary to achieve this goal. In this article, we present the basic theory behind adaptive Q control, the electronics enabling the online modification of the probe's effective Q factor, and the results of the experiments comparing three different methods: scanning (a) without Q control, (b) with standard Q control, and (c) with adaptive Q control. The results show that the performance of adaptive Q control is superior to the other two methods.     

Haptics in minimally invasive surgical simulation and training

Haptics is a valuable tool in minimally invasive surgical simulation and training. We discuss important aspects of haptics in MISST, such as haptic rendering and haptic recording and playback. Minimally invasive surgery has revolutionized many surgical procedures over the last few decades. MIS is performed using a small video camera, a video display, and a few customized surgical tools. In procedures such as gall bladder removal (laparoscopic cholesystectomy), surgeons insert a camera and long slender tools into the abdomen through small skin incisions to explore the internal cavity and manipulate organs from outside the body as they view their actions on a video display. Because the development of minimally invasive techniques has reduced the sense of touch compared to open surgery, surgeons must rely more on the feeling of net forces resulting from tool-tissue interactions and need more training to successfully operate on patients.     

VR-Based Simulators for Training in Minimally Invasive Surgery

Simulation-based training using VR techniques is a promising alternative to traditional training in minimally invasive surgery (MIS). Simulators let the trainee touch, feel, and manipulate virtual tissues and organs through the same surgical tool handles used in actual MIS while viewing images of tool-tissue interactions on a monitor as in real laparoscopic procedures     

Real-Time Finite-Element Simulation of Linear Viscoelastic Tissue Behavior Based on Experimental Data

We propose an end-to-end solution to real-time and realistic finite-element modeling and simulation of viscoelastic soft tissue behavior. We provide an efficient numerical scheme for solving a linear viscoelastic FEM model derived from the generalized Maxwell solid, and present methods for measuring and integrating experimental data on the viscoelastic material properties of soft tissues into the model for realistic display of visual deformations and interaction forces. Our precomputation scheme and multilayer computational architecture enable the model's real-time execution with visual and haptic feedback to the user. Our approach includes time- and rate-dependent effects, which requires considering a node's loading history in our displacement computations at each cycle of the simulation     

Repetitive control of an XYZ piezo-stage for faster nano-scanning: Numerical simulations and experiments

A repetitive controller (RC) is implemented to control the Z-axis movements of a piezo-scanner used for AFM scanning and then tested through scan experiments and numerical simulations. The experimental and simulation results show that the RC compensates phase delays better than the standard PI controller at high scan speeds, which leads to less scan error and lower interaction forces between the scanning probe and the surface being scanned. Since the AFM experiments are not perfectly repeatable in the physical world, the optimum phase compensators of the RC resulting this performance are determined through the numerical simulations performed in MATLAB/Simulink. Furthermore, the numerical simulations are also performed to show that the proposed RC is robust and does not require re-tuning of these compensators when the consecutive scan lines are not similar and a change occurs in the probe characteristics.     

Numerical simulation of nano scanning in intermittent-contact mode AFM under Q control

We investigate nano scanning in tapping mode atomic force microscopy (AFM) under quality (Q) control via numerical simulations performed in SIMULINK. We focus on the simulation of the whole scan process rather than the simulation of cantilever dynamics and the force interactions between the probe tip and the surface alone, as in most of the earlier numerical studies. This enables us to quantify the scan performance under Q control for different scan settings. Using the numerical simulations, we first investigate the effect of the elastic modulus of the sample (relative to the substrate surface) and probe stiffness on the scan results. Our numerical simulations show that scanning in an attractive regime using soft cantilevers with high effective Q factor (Q(eff)) results in a better image quality. We then demonstrate the trade-off in setting Q(eff) of the probe in Q control: low values of Q(eff) cause an increase in tapping forces while higher ones limit the maximum achievable scan speed due to the slow response of the cantilever to the rapid changes in surface profile. Finally, we show that it is possible to achieve higher scan speeds without causing an increase in the tapping forces using adaptive Q control (AQC), in which the Q factor of the probe is changed instantaneously depending on the magnitude of the error signal in oscillation amplitude. The scan performance of AQC is quantitatively compared to that of standard Q control using iso-error curves obtained from numerical simulations first and then the results are validated through scan experiments performed using a physical set-up.     

Virtual environments for medical training: graphical and hapticsimulation of laparoscopic common bile duct exploration

We develop a computer-based training system to simulate laparoscopic procedures in virtual environments for medical training. The major hardware components of our system include a computer monitor to display visual interactions between 3D virtual models of organs and instruments together with a pair of force feedback devices interfaced with laparoscopic instruments to simulate haptic interactions. We simulate a surgical procedure that involves inserting a catheter into the cystic duct using a pair of laparoscopic forceps. This procedure is performed during laparoscopic cholecystectomy to search for gallstones in the common bile duct. Using the proposed system, the user can be trained to grasp and insert a flexible and freely moving catheter into the deformable cystic duct in virtual environments. The associated deformations are displayed on the computer screen and the reaction forces are fed back to the user through the force feedback devices. A hybrid modeling approach was developed to simulate the real-time visual and haptic interactions that take place between the forceps and the catheter, as well as the duct; and between the catheter and the duct