Design and application of a cellular, piezoelectric, artificial muscle actuator for biorobotic systems

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2010.Cataloged from PDF version of thesis.Includes bibliographical references (p. 219-227).One of the foremost challenges in robotics is the development of muscle-like actuators that have the capability to reproduce the smooth motions observed in animals. Biological muscles have a unique cellular structure that departs from traditional electromechanical actuators in several ways. A muscle consists of a vast number of muscle fibers and, more fundamentally, sarcomeres that act as cellular units or building blocks. A muscle's output force and displacement are the aggregate effect of the individual building blocks. Thus, without using gearing or transmissions, muscles can be tailored to a range of loads, satisfying specific force and displacement requirements. These natural actuators are desirable for biorobotic applications, but many of their characteristics have been difficult to reproduce artificially. This thesis develops and applies a new artificial muscle actuator based on piezoelectric technology. The essential approach is to use a subdivided, cellular architecture inspired by natural muscle. The primary contributions of this work stem from three sequential aims. The first aim is to develop the operating principles and design of the actuator cellular units. The basic operating principle of the actuator involves nested flexural amplifiers applied to piezoelectric stacks thereby creating an output length strain commensurate with natural muscle. The second aim is to further improve performance of the actuator design by imparting tunable stiffness and resonance capabilities. This work demonstrates a previously unavailable level of tunability in both stiffness and resonance. The final aim is to showcase the capabilities of the actuator design by developing an underwater biorobotic fish system that utilizes the actuators for resonance-based locomotion. Each aspect of this thesis is supported by rigorous analysis and functional prototypes that augment broadly applicable design concepts.by Thomas William Secord.Ph.D

Micro motion amplification – A Review

Many motion-active materials have recently emerged, with new methods of integration into actuator components and systems-on-chip. Along with established microprocessors, interconnectivity capabilities and emerging powering methods, they offer a unique opportunity for the development of interactive millimeter and micrometer scale systems with combined sensing and actuating capabilities. The amplification of nanoscale material motion to a functional range is a key requirement for motion interaction and practical applications, including medical micro-robotics, micro-vehicles and micro-motion energy harvesting. Motion amplification concepts include various types of leverage, flextensional mechanisms, unimorphs, micro-walking /micro-motor systems, and structural resonance. A review of the research state-of-art and product availability shows that the available mechanisms offer a motion gain in the range of 10. The limiting factor is the aspect ratio of the moving structure that is achievable in the microscale. Flexures offer high gains because they allow the application of input displacement in the close vicinity of an effective pivotal point. They also involve simple and monolithic fabrication methods allowing combination of multiple amplification stages. Currently, commercially available motion amplifiers can provide strokes as high as 2% of their size. The combination of high-force piezoelectric stacks or unimorph beams with compliant structure optimization methods is expected to make available a new class of high-performance motion translators for microsystems

ELECTROMECHANICAL MODELING OF A HONEYCOMB CORE INTEGRATED VIBRATION ENERGY CONVERTER WITH INCREASED SPECIFIC POWER FOR ENERGY HARVESTING APPLICATIONS

Innovation in integrated circuit technology along with improved manufacturing processes has resulted in considerable reduction in power consumption of electromechanical devices. Majority of these devices are currently powered by batteries. However, the issues posed by batteries, including the need for frequent battery recharge/replacement has resulted in a compelling need for alternate energy to achieve self-sufficient device operation or to supplement battery power. Vibration based energy harvesting methods through piezoelectric transduction provides with a promising potential towards replacing or supplementing battery power source. However, current piezoelectric energy harvesters generate low specific power (power-to-weight ratio) when compared to batteries that the harvesters seek to replace or supplement. In this study, the potential of integrating lightweight cellular honeycomb structures with existing piezoelectric device configurations (bimorph) to achieve higher specific power is investigated. It is shown in this study that at low excitation frequency ranges, replacing the solid continuous substrate of a conventional piezoelectric bimorph with honeycomb structures of the same material results in a significant increase in power-to-weight ratio of the piezoelectric harvester. In order to maximize the electrical response of vibration based power harvesters, the natural frequency of these harvesters is designed to match the input driving frequency. The commonly used technique of adding a tip mass is employed to lower the natural frequency (to match driving frequency) of both, solid and honeycomb substrate bimorphs. At higher excitation frequency, the natural frequency of the traditional solid substrate bimorph can only be altered (to match driving frequency) through a change in global geometric design parameters, typically achieved by increasing the thickness of the harvester. As a result, the size of the harvester is increased and can be disadvantageous especially if the application imposes a space/size constraint. Moreover, the bimorph with increased thickness will now require a larger mechanical force to deform the structure which can fall outside the input ambient excitation amplitude range. In contrast, the honeycomb core bimorph offers an advantage in terms of preserving the global geometric dimensions. The natural frequency of the honeycomb core bimorph can be altered by manipulating honeycomb cell design parameters, such as cell angle, cell wall thickness, vertical cell height and inclined cell length. This results in a change in the mass and stiffness properties of the substrate and hence the bimorph, thereby altering the natural frequency of the harvester. Design flexibility of honeycomb core bimorphs is demonstrated by varying honeycomb cell parameters to alter mass and stiffness properties for power harvesting. The influence of honeycomb cell parameters on power generation is examined to evaluate optimum design to attain highest specific power. In addition, the more compliant nature of a honeycomb core bimorph decreases susceptibility towards fatigue and can increase the operating lifetime of the harvester. The second component of this dissertation analyses an uncoupled equivalent circuit model for piezoelectric energy harvesting. Open circuit voltage developed on the piezoelectric materials can be easily computed either through analytical or finite element models. The efficacy of a method to determine power developed across a resistive load, by representing the coupled piezoelectric electromechanical problem with an external load as an open circuit voltage driven equivalent circuit, is evaluated. The lack of backward feedback at finite resistive loads resulting from such an equivalent representation is examined by comparing the equivalent circuit model to the governing equations of a fully coupled circuit model for the electromechanical problem. It is found that the backward feedback is insignificant for weakly coupled systems typically seen in micro electromechanical systems and other energy harvesting device configurations with low coupling. For moderate to high coupling systems, a correction factor based on a calibrated resistance is presented which can be used to evaluate power generation at a specific resistive load

Piezoelectric Devices in the Sustainable Society

Our 21st century faces to a “sustainable society”, which enhances (a) usage of non-toxic materials, (b) disposal technology for existing hazardous materials, (c) reduction of contamination gas, (d) environmental monitoring system, (e) new energy source creation, and (f) energy-efficient device development in the piezoelectric area. With reducing their size, the electromagnetic components reduce their efficiency drastically. Thus, piezoelectric transducers with much less losses are highly sought recently. Piezoelectric devices seem to be all-around contributors and a key component to the above mentioned five R&D areas. Some of the efforts include: (a) Since the most popular piezoelectric lead zirconate titante ceramics will be regulated in European and Asian societies due to their toxicity (Pb2+ ion), lead-free piezoelectrics have been developed. (b) Since hazardous organic substances can easily be dissolved by the ultrasonic irradiation in water, a new safe disposal technology using piezoelectric transducers has been developed. (c) We demonstrated an energy recovery system on a hybrid car from its engine’s mechanical vibration to the rechargeable battery. (d) Micro ultrasonic motors based on piezoelectrics demonstrated 1/20 reduction in the volume and a 20-time increase in efficiency of the conventional electromagnetic motors. This paper introduces leading piezoelectric materials, devices, and drive/control methods, relating with the above “sustainability” technologies, aiming at further research expansion in this area

Leading the Charge in Bone Healing: Design of Compliant Layer Adaptive Composite Stacks for Electrical Stimulation in Orthopedic Implants

The overall aim of this research is to develop a robust, adaptable piezoelectric composite load-bearing biomaterial that when integrated with current implants, can harvest human motion and subsequently deliver electrical stimulation to trigger the natural bone healing and remodeling process. Building on the preclinical success of a stacked piezocomposite spinal fusion implant, compliant layer adaptive composite stacks (CLACS) were designed as a scalable biomaterial to increase efficiency of power generation while maintaining mechanical integrity under fatigue loading seen in orthopedic implants. Energy harvesting with piezoelectric material is challenging at low frequencies due to material properties that limit total power generation at these frequencies and brittle mechanical properties. Stacked generators increase power generation at lower voltage levels and resistances, but are not efficient at low frequencies seen in human motion. CLACS integrates compliant layers between the stiff piezoelectric elements within a stack, capitalizing on the benefits of stacked piezoelectric generators, while decreasing stiffness and increasing strain to amplify power generation. The first study evaluated CLACS under compressive loads, demonstrating the power amplification effect as the thickness of the compliant layer increases. The second study characterized the effect of poling direction of piezoelectric discs within a CLACS structure under multiaxial loads, demonstrating an additional increase in power generation when mixed poling directions are used to create mixed-mode CLACS. The final study compared the fatigue performance and power generation capability of three commercially fabricated piezoelectric stack generators with and without CLACS technology in modified implant assemblies. All configurations produced sufficient power to stimulate bone growth, and maintained mechanical strength throughout a high load, low cycle fatigue analysis, thus validating feasibility for use in orthopedic implants. The presented work in this dissertation provides a robust experimental understanding of CLACS and a characterization of how piezoelectric properties and composite structures can be tailored within the CLACS structure to efficiently generate power in low frequency, low impedance applications. The main motivation of this work was to develop a thorough understanding of CLACS behavior for implementation into medical implants to deliver therapeutic electrical stimulation and accelerate rate of bone growth, helping patients completely heal faster. However, the ability to tune composite stiffness by changing compliant material properties, type of piezoelectric material and poling direction, or volume fractions could benefit the energy harvesting potential in fields ranging from civil infrastructure to wind energy, to wearables and athletic equipment

EMBEDDED STRAIN SENSOR WITH POWER SCAVENGING FROM BRIDGE VIBRATION

The Objective of this thesis is to investigate an embedded strain sensor system with power scavenged from highway bridge vibration for structure health monitoring. A power scavenging scheme is developed to power the sensor system so as to eliminate the dependence of batteries. Three mechanisms (PZT patch, cantilever beam and PZT stacks/slugs) are described that are suitable for the remote sensor system to scavenge mechanical energy at different locations in a bridge. Calculations and experiments are taken to examine the feasibility of the idea, and predicted power generation is compared to solar/light power. A scheme of an embedded strain sensor using resonant MEMS beam structure is introduced along with the theoretical analysis and simulation. The last consideration is the signal propagation attenuation in concrete. Measurements are taken to determine the RF signal attenuation at 900 MHz range transmitted through a concrete slab. Theoretical calculations are validated by experimental results

Numerical and experimental characterization of a piezoelectric actuator for microfluidic cell sorting

Piezoelectric actuators offer great opportunities for precise and low-cost control of fluids at the microscale. Microfluidic systems with integrated piezoelectric actuators find application as droplet generators, micropumps, and microsorters. To accelerate device design and optimization, modeling and simulation approaches represent an attractive tool, but there are challenges arising from the multiphysics nature of the problem. Simple, potentially real-time approaches to experimentally characterize the fluid response to piezoelectric actuation are also highly desirable. In this work, we propose a strategy for the numerical and experimental characterization of a piezoelectric microfluidic cell sorter. Specifically, we present a 3D coupled multiphysics finite-element model of the system and an easy image-based approach for flow monitoring. Sinusoidal and pulse actuation are considered as case studies to test the proposed methodology. The results demonstrate the validity of the approach as well as the suitability of the system for cell sorting applications

Optimal Material Selection for Transducers

When selecting an active material for an application, it is tempting to rely upon prior knowledge or commercial products that fit the design criteria. While this method is time effective, it does not provide an optimal selection. The optimal material selection requires an understanding of the limitations of the active material, understanding of the function, constraints and objectives of the device, and rigorous decision making method to ensure rational and clear material selection can be performed. Therefore, this work looks into the three most researched active materials (piezoelectrics, magnetostrictives and shape memory alloys) and looks at how they work and their difficulties. The field of piezoelectrics is vast and contains ceramics, plastics and cellular structures that couple the mechanical and electrical domain. The difficulty with piezoelectric ceramics is their small strains and the dependence of their coefficients on the ferroelectric domains. Giant magnetostrictives materials couple the mechanical and magnetic domains. They are generally better suited for low-frequency operations since they properties deteriorate rapidly with heat. Shape memory alloys are materials that couple thermal and mechanical domains. They have large strain but are limited in their force output, fatigue life and cycle frequency. Optimal material selection requires a formalized material selection method. In mechanical material selection, the formal material selection method uses function, constraints and objectives of the designer to limit the parameter space and allow better decisions. Unfortunately, active materials figures of merit are domain dependent and therefore the mechanical material selection method needs to be adapted. A review of device selection of actuators, sensors and energy harvesters indicates a list of functions, constrains and objectives that the designer can use. Through the analysis of these devices figures of merit, it is realized that the issue is that the simplification that the figures of merit perform does not assist in decision making process. It is better to use decision making methods that have been developed in the field of operational research which assists complex comparative decision making. Finally, the decision making methods are applied to two applications: a resonant cantilever energy harvester and an ultrasound transducer. In both these cases, a review of selection methods of other designers provides guidance of important figures of merit. With these selection methods in consideration, figures of merit are selected and used to find the optimal material based upon the designer preference

Thin-Film PZT Scanning Micro-actuators for Vertical Cross-sectional Imaging in Endomicroscopy

The advancement of optics and the development of microelectromechanical systems (MEMS) based scanners has enabled powerful optical imaging techniques that can perform optical sectioning with high resolution and contrast, large field of view, and long working distance to be realized in endoscope-compatible form factors. Optical endomicroscopes based on these imaging techniques can be used to obtain in vivo vertical cross-sectional images of dysplastic tissues in the hollow organs before they progress to mucosal diseases such as colorectal cancer. However, existing endomicroscopic systems that use imaging modalities compatible with the use of fluorescent biomarkers are not capable of deep vertical sectioning in real time. This work proposes a unique MEMS-based scanning mechanism to be incorporated into endoscopic microscopes for real-time in vivo deep into-tissue scanning for early cancer detection. For this task, a class of novel multi-axis micro-scanners based on thin-film lead-zirconate-titanate (PZT) has been developed. Leveraging the large piezoelectric strain coefficient of PZT, the prototypes have demonstrated more than 400 μm of out-of-plane displacement with bandwidths on the order of 100-200 Hz in only a 3.2 mm-by-3.2 mm footprint, which meets the requirements for this application. The scanners have a central rectangular-shaped reflector, whose corners are supported by four symmetric PZT bending legs that generate vertical translation. This design gives the reflector a three-axis motion. The challenges to fabricate high performance piezoelectric actuators are discussed with device failure mechanisms observed during the fabrication of the 1st generation scanners. Improved fabrication steps are presented that solve the issues with the 1st generation devices and enhance the robustness of the scanners for instrument integration. Remaining non-ideal fabrication outcomes cause MEMS devices to produce unwanted motions, which can degrade imaging quality. To overcome this problem, a method to drive MEMS actuators having multiple vibration modes with close frequencies to produce a desired motion pattern with a single input is presented, and was used to generate a pure vertical motion for imaging. Two-photon based vertical cross-sectional images of mouse colon was obtained in real time for the first time using a thin-film piezoelectric microscanner. To understand the effects of fabrication non-idealities on the device behavior and produce more robust scanner performance, analytical models that describes large vertical and rotational motions including multi-axis coupling were developed. A static model that was initially developed for design optimization was calibrated, along with a transient model, using experimental data to incorporate the effects of dimensional variations and residual stress. This models can be used with future integrated sensors and feedback controllers for more precise and robust motion of the scanner. This calibration technique can be useful in developing analytic models for MEMS devices subject to fabrication uncertainty. In addition, nonlinear dynamic behavior due to large vertical stroke in the presence of fabrication non-idealities is captured by linearizing an expanded dynamic model about different static positions obtained by numerically solving the expanded nonlinear model.PHDMechanical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttps://deepblue.lib.umich.edu/bitstream/2027.42/137124/1/jongs_1.pd