Suwas Nikumb

Modeling and Control of Shape Memory Alloy Actuators

This brief describes a new model for shape memory alloy (SMA) actuators based on the physics of the process and develops control strategies using the model. The model consists of three equations - the temperature dynamics described by Joules heating-convectional cooling, the mole fraction distribution with temperature given by statistics to describe a two state system, and a constitutive equation relating the changes in temperature and mole fraction to the stress and strain induced in the SMA. This model is used to develop two control schemes for controlling the strain in an SMA actuator. The first control scheme describes a gain-scheduled proportional-integral (PI) controller, the gains of which are obtained by means of linear quadratic regulator (LQR) optimization. The second control scheme is an Hinfin loop-shaping controller using normalized coprime stabilization which ensures robust stability by minimizing the effect of unmodeled dynamics at high frequencies. Simulation and experimental results show fast and accurate control of the strain in the SMA actuator for both control schemes.

Experimental study of drilling sub-10 μm holes in thin metal foils with femtosecond laser pulses

Aluminum foils with thickness ranging from 1.5 to 50 μm, and W, Mo, Ti, Cu, Fe, Ag, Au, and Pb foils of 25 μm thickness have been drilled with femtosecond Ti:sapphire laser pulses centered at 800 nm. The influence of laser parameters and material properties on hole drilling processes at sub-10 μm scale has been examined. A simple model is shown to predict the ablation rate for a range of metals.

Robot-assisted Active Catheter Insertion: Algorithms and Experiments

Angioplasty is a frequently performed clinical procedure in which a catheter is inserted into a blood vessel under image guidance to open narrowed or blocked arteries and to allow normal blood flow to resume. This paper is concerned with the development of algorithms for a robot-assisted method for a more accurate, safer and more reliable approach for catheter insertion that can reduce the potential for injury to patients and radiation exposure or discomfort to clinicians. A force control algorithm is presented for a robot to control the force of insertion of a catheter and prevent the catheter from buckling or “bunching up” during insertion. In addition, the paper also describes a master—slave control strategy to precisely control the bending angle of the tip of an active catheter instrumented with Shape Memory Alloy (SMA) actuators. A novel model for SMAs and a robust H∞ loop-shaping controller have been implemented to guarantee robust performance of the active catheter. The algorithms for catheter insertion developed in this paper may help to prevent damage to the epithelial cells of an artery and enable easier guidance of the catheter into appropriate branches. In addition, a robotics-based approach could make it possible for a clinician to remotely perform the insertion of the active catheter from a safe and comfortable environment, thereby reducing exposure to harmful X-ray radiation. Experimental results are presented to illustrate the performance of the algorithms.

Rapid fabrication of tooling for microfluidic devices via laser micromachining and hot embossing

This paper presents a new method for rapid fabrication of polymeric micromold masters for the manufacture of polymer microfluidic devices. The manufacturing method involves laser micromachining of the desired structure of microfluidic channels in a thin metallic sheet and then hot embossing the channel structure onto poly(methyl methacrylate) PMMA substrate to form the mold master. The channeled layer of the microfluidic device is then produced by pouring the polydimethylsiloxane (PDMS) elastomer over the mold and curing it. The method is referred to as LHEM (laser micromachining, hot embossing and molding). Polymers like PDMS are preferred over silicon as the material for building microfluidic devices because of their biocompatibility properties as well as because of their lower cost. The proposed manufacturing method involves fewer processing steps than the conventional soft lithography process and enables manufacture of non-rectangular channels in microfluidic devices. To test the method, a mold for a micro capillary electrophoresis microfluidic chip was fabricated. The experimental results confirmed that high quality (Ra 10 to 100 nm) molds can be fabricated quickly and inexpensively. Advantages and limitations of the proposed method are discussed in the concluding section of the paper.

Modelling and gain scheduled control of shape memory alloy actuators

This paper describes a gain-scheduled controller for a shape memory alloy (SMA) actuator. For accurate control of an SMA actuator, it is important to develop a precise model for the SMA. A model has been proposed based on concepts from physics. The equations include Joules heating - convectional cooling to explain the dynamics of temperature, Fermi-Dirac statistics to explain the variation of mole fraction with temperature, and a stress-strain constitutive equation to relate changes in mole fraction and temperature to changes in stress and strain of the SMA. This model is then used to develop a gain-scheduled controller to control the strain in the SMA. Simulation and experimental results show fast and accurate control of the strain in the SMA actuator

Performance characterization of in-plane electro-thermally driven linear microactuators

Static and dynamic electro-mechanical performance of a microactuator is a key factor in the functioning of an integrated microsystem composed of moving components such as optical shutters/switches, micropumps, microgrippers, and microvalves. Therefore, the development of such systems primarily focuses on the overall design and parameter optimization of an actuator as the major driving element with respect to the desired performance parameters, e.g., displacement, force, dimensional constraints, material, actuation principle, and method of fabrication. This study presents results on the static and dynamic electro-mechanical performance analysis of an in-plane electro-thermally driven linear microactuator. Each microactuator, having a width of 2220 mm and made of 25 mm thick nickel foil, consisted of a pair of cascaded structures. Connecting several actuation units in a series formed each cascaded structure. Several microactuators with a different number of actuation units were fabricated using the laser micromachining technology. The static performance of these microactuators was evaluated with respect to the maximum linear output displacements, actual resistance, applied current, and consumed electric power. The maximum displacements varied approximately from 3 to 44 mm, respectively, depending on the number of actuation units. The dynamic performance was studied as a response function on constant applied current with respect to the output displacements. In addition, the response time was evaluated for different applied currents and for actuators with 2, 4, and 6 actuation units. The microactuators’ performance results are promising for applications in MEMS/MOEMS, microfluidic, and microrobotic devices.

Microgripper: Design, finite element analysis and laser microfabrication

This research is focused on new and innovative design, finite element analysis, precision laser microfabrication, and performance evaluation of a microgripper. The design of the microgripper with overall dimension of 1.4(W)x2.8(L)mm is based on a pair of cascaded structures oriented in a face-to-face direction, to act as microtweezers. Each cascaded structure is formed by connecting several basic actuation units in series. Each actuation unit consists of a constrainer and two semi-circular-shaped actuation beams. The actuation principle is based on the electrothermal effect. On application of electrical potential, the output displacement and the force are generated from the summation of all basic actuation units in these cascaded structures. Finite element analysis (FEA) is applied to simulate dynamic performance of the microgripper and to choose proper operational voltage parameters. Thin nickel foil of a thickness of 12.5 micrometers was used in the laser microfabrication of these prototypes. Dynamic performance of the prototype device was evaluated within 0-1.9 voltage range. The maximum tweezing displacements of up to 30 micrometers were recorded for nickel microgripper prototype. Larger displacements are feasible through the optimization of design parameters.

Geometrical modeling of surface profile formation during laser ablation of materials

Recent advances in laser machining technology have made it possible to fabricate parts and features with high accuracy and precision, using high-powered, short-pulsed, Q-switched lasers. To determine the machining parameters to obtain the desired geometrical quality, an understanding of the relationship between the process parameters and the resulting surface profile is necessary. In the present study, we adopt a geometrical approach which, coupled with the material properties and machining process parameters, yields a method to determine the surface profile of the material ablated by a laser pulse. It is reasoned that the energy incident upon an infinitesimal area of the surface at a given time is transferred in the outward normal direction to the surface, and the volume of ablation, centered about the normal, is determined by the laser–material interaction and the process parameters. The direction and depth of ablation determine the modified surface profile an infinitesimal time later, yielding a nonlin...

Rapid Fabrication of Micromolds for Polymeric Microfluidic Devices

Lab-on-a-chip (LOC) and other microfluidic devices for medical applications need to be mass produced at a low fabrication cost because the disposable device is destroyed after a single use to avoid sample contamination. In this paper, a new method for rapidly fabricating metallic micromold masters for manufacturing large volumes of polymeric microfluidic devices is presented. Polymers are preferred over silicon as the device material due to their better compatibility with biological and chemical substances. The manufacturing method involves laser micromachining of the desired imprint features from thin metallic sheets and then microwelding them onto a substrate to form the final mold master. The polydimethylsiloxane (PDMS) elastomer is then poured over the mold and cured to produce the microfluidic device. The proposed method involves fewer processing steps than the soft lithography, electroplating and molding (LIGA) process. To verify the method, a metallic mold for a passive Y-channel microfluidic mixer was fabricated. The mold master was made from low-cost steel and the mold manufacturing process can be completed within an hour. PDMS elastomer is then poured over the mold and cured to produce the mixer. The channels of the mixer were 75 micrometers wide and 50 micrometers high. The mixer created from the mold was tested by mixing two streams of colored water in it. The maximum flow rate achieved by the prototype was 6.4 microlitres per minute. The experimental results confirm that a viable metallic mold master for microfluidic devices can be created by combining laser micromachining and microwelding processes. Finally, the limitations of the proposed rapid fabrication method are discussed.

H ∞ Loop Shaping Controller for Shape Memory Alloy Actuators

This paper describes a Robust H∞Controller for a Shape Memory Alloy actuator. A formulation based on concepts from physics, has been used to model the Shape Memory Alloy (SMA). The modelling equations include Joules heating - convectional cooling to explain the dynamics of temperature, Fermi-Dirac statistics to explain the variation of mole fraction with temperature, and a stress-strain constitutive equation to relate changes in mole fraction and temperature to changes in stress and strain of the SMA. An H∞loop shaping controller using normalized coprime stabilization is designed such that the gains are high when the model describes the SMA accurately and low at higher frequencies when the model is inaccurate. Simulation and experimental results show fast and accurate control of the strain in the SMA actuator.