Mile Ostojic

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.

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.

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.

A Robust Controller for Shape Memory Alloy Actuators

This paper describes a robust mixed-synthesis controller for shape memory alloy (SMA) actuators. The weights for the controller are chosen such that the disturbance rejection criterion is met for low frequencies and robust stability is maintained for high frequencies. Simulation and experimental results show fast and accurate response for strain in the SMA under no perturbation. The performance of the controller is also verified in the presence of load and cooling rate perturbations. The response shows that the controller maintains good disturbance rejection and robust stability. Sinusoidal references are tracked with negligible error in the presence of perturbations. In addition, the controller is also tested for SMA actuators operating in water. The excellent results validate further both the model and the controller used for the SMA actuators

Rapid fabrication of polymethylmethacrylate micromold masters using a hot intrusion process

A method for rapid fabrication of mold masters for soft-molding of polydimethylsiloxane (PDMS) microfluidic devices is successfully developed and tested. The method involves laser micromachining and a hot-intrusion process, and produces mold masters from polymethylmethacrylate (PMMA) substrates. A metallic mask with microchannel line features of various widths (25 to 200 µm) is initially created by laser micromachining a 75-µm-thick brass sheet. Under the hot-intrusion process, a 2-mm-thick solid PMMA substrate is then heated and molded under pressure to force the softened material through the shaped microfeatures in the mask. The height of the extruded microrelief is determined by the pressure, temperature, and time profile of the hot-intrusion process. A mathematical model that characterizes the rapid fabrication process and enables the operator to select appropriate process parameters is described. The derived empirical model is based on experimental observations where extruded microrelief heights were varied from 5 to 75 µm with aspect ratios from 0.1 to 0.46, and radii of the extruded profile from 12 to 270 µm. The proposed model is developed to describe the relationship between key process parameters and the extruded heights of the microreliefs. Furthermore, the model provides the operator with simple guidelines for selecting the process parameters. An example of PDMS microfluidic devices replicated by the rapid micromold fabrication methodology is presented to illustrate the quality of the resultant features of microchannels.

Fabrication of polymer microfluidic devices with 3D microfeatures that have near optical surface quality

A low cost manufacturing method for creating polymer microfluidic devices with microfeatures that have near optical surface quality is described in this paper. The manufacturing method involves laser micromachining, partial hot embossing, and molding (LHEM) to create polymethylmethacrylate (PMMA) mold masters for device replication. A metallic hot intrusion mask with the desired microfeatures is first machined by laser and then used to produce the mold master by pressing the mask onto a PMMA substrate under applied heat and pressure. The resultant 3D micro-reliefs have near optical quality surface finishes. Design parameters such as the height and width of the extruded features are investigated in this study. The experimental results demonstrate that different heights of the extruded features of a mold master can be fabricated using a single mask at a set of process parameters. Examples of curved microchannels of the PMMA mold masters and an integrated microchannel/microlens of the mold master are presented to illustrate the proposed methodology.

Laser-assisted active microfluidic mixer

Many analytical microsystems use molecular diffusion to mix small quantities of different liquids. However, this passive mixing process requires a relatively long microchannel which may impose design restrictions on the physical dimensions of the fluidic network. To shorten the length of the mixing channels, an active micromixer driven by a focused laser beam is described in this paper. The proposed solution improves the mixing rate by using low power laser radiation to heat the disparate fluids being transported through the channels. The operating principle is ba sed on the observation that the rate of molecular diffusion for non-reactive fluids increases with elevated temperatures. Preliminary experiments on a Y-channel micromixer were conducted using a 1mW, 670nm laser. The laser beam was focused on the microchannel using a 100mm focal length objective lens. The laser-assisted mixing of the test fluids showed a 36.4% increase in the average diffusion coefficient value with 1 to 10μL/min flow rates. The maximum percentage difference of diffusion distances had increased by approximately 7.85% over the non-laser-assisted conditions.

Neural network approach to modeling hot intrusion process for micromold fabrication

The rapid fabrication of polymeric mold masters by laser micromachining and hot-intrusion permits the low cost manufacture of microfluidic devices with near optical quality surface finishes. A metallic hot intrusion mask with the desired microfeatures is first machined by laser and then used to produce the mold master by pressing the mask onto a polymethylmethacrylate (PMMA) substrate under applied heat and pressure. A thorough understanding of the physical phenomenon is required to produce features with high dimensional accuracy. A neural network approach to modeling the relationship among microchannel height (H), width (W), the intrusion process parameters of pressure and temperature is described in this paper. Experimentally acquired data are used to both train and test the neural network for parameterselection. Analysis of the preliminary results shows that the modeling methodology can predict suitable parameters within 6% error.