Shape Memory Alloy Actuators and Sensors for Applications in Minimally Invasive Interventions

Reduced access size in minimally invasive surgery and therapy (MIST) poses several restriction on the design of the dexterous robotic instruments. The instruments should be developed that are slender enough to pass through the small sized incisions and able to effectively operate in a compact workspace. Most existing robotic instruments are operated by big actuators, located outside the patient’s body, that transfer forces to the end effector via cables or magnetically controlled actuation mechanism. These instruments are certainly far from optimal in terms of their cost and the space they require in operating room. The lack of adequate sensing technologies make it very challenging to measure bending of the flexible instruments, and to measure tool-tissue contact forces of the both flexible and rigid instruments during MIST. Therefore, it requires the development of the cost effective miniature actuators and strain/force sensors. Having several unique features such as bio-compatibility, low cost, light weight, large actuation forces and electrical resistivity variations, the shape memory alloys (SMAs) show promising applications both as the actuators and strain sensors in MIST. However, highly nonlinear hysteretic behavior of the SMAs hinders their use as actuators. To overcome this problem, an adaptive artificial neural network (ANN) based Preisach model and a model predictive controller have been developed in this thesis to precisely control the output of the SMA actuators. A novel ultra thin strain sensor is also designed using a superelastic SMA wire, which can be used to measure strain and forces for many surgical and intervention instruments. A da Vinci surgical instrument is sensorized with these sensors in order to validate their force sensing capability

Material Design, Processing, and Engineering Requirements for Magnetic Shape Memory Devices

For magnetic shape memory (MSM) alloys, a magnetic field stimulates a shape change. We use the shape change to build devices such as micro-actuators, sensors, and microfluidic pumps. Currently, (as a novel technology,) devices suffer from some material and magnetic driver shortcomings. Here we address the issues related to operating temperature, repeatability, failure, and magnetic driver development. To increase the operating temperature of the MSM material, we alloyed Fe and Cu to Ni-Mn-Ga. We showed that the element-specific contribution to the valence electron density as parameter systematically determines the effect of each element on the variation of the martensite transformation temperature of the 10M phase. To stabilize the material, we developed a micro-shotpeening process that adds stresses to the material surface, thereby inducing a fine twin microstructure. The treatment allowed nearly full magnetic-field-induced strain, and extended fatigue life of the material from only one thousand cycles in the electropolished state to more than one million cycles in the peened state. We measured the effect of the peening process on material actuation when in MSM pump configuration. In the polished state, the deformation was stochastic, with a sharp-featured, faceted shrinkage. In the treated state, the deformation was smooth and repeatably swept along the surface akin to a wave. To actuate the MSM micropump without electromotor, we developed a linear electromagnetic actuation device and evaluated its effectiveness in the switching mechanism of the material. By compressing the magnetic field between opposing coils, we generated a strong magnetic field, which caused a localized region to switch at selected poles. In the next iteration of the drive, we inserted the MSM sample between two linear pole arrangements of high pitch density to approximate a moving vertical field. The incremental stepping of the vertical field between poles caused translation of the switched region. The results of this dissertation demonstrate the suitability of MSM alloys for high-precision, persistent, and reliable actuators such as micropumps

Electromagnetic and thermal analyses of high performance magnetic shape memory actuators for valve applications

Magnetic shape memory (MSM) alloys are relatively new “smart” alloys which have enormous potential to be used in actuators, sensors and other electrical devices. Their large strain and considerable stress output can be controlled by magnetic fields or mechanical stresses. Maximum magnetic field-induced strain varies from 6 to 12% of the MSM element’s length depending on its microstructure. However, very low operational temperature limit is one of the main drawbacks of conventional MSM alloys. This makes their application in high performance actuators challenging due to considerable power losses. This paper discusses different MSM actuator designs optimized particularly for large force output for pneumatic electromagnetic (EM) valve applications. The thermal problem is addressed through analyzing the heat transfer conditions of each particular design and the effects of different cooling systems. An energy-efficient operating cycle for varying actuator load that takes advantage of the shape memory effect is also proposed. This allows minimization of energy losses resulting in acceptable increase in temperature ensuring stable continuous actuation

Modelling and design of high-speed, long-lifetime and large-force electromagnetic actuators based on magnetic shape memory alloys

The main topic of this research is modelling and design of high-speed, large-force and long life-time electromagnetic actuators based on Magnetic Shape Memory (MSM) alloys. These relatively new “smart” alloys that change shape in magnetic fields possess very promising properties such as large strain, considerable output stress and potentially very long fatigue life. However, there is still lack of a consistent design methodology for MSM-based devices which can be implemented using techniques common for engineering design. In order to bridge this gap, a modelling approach for MSM element in actuators is developed in which the complete magnetic circuit of MSM actuator is included into a single finite element (FE) model. This approach also allows accurate representation of MSM permeability change during the shape change capturing its effects on total reluctance of the magnetic circuit. Moreover, this approach allows studying the magnetic field distribution in the MSM element in single, two and multi-variant states in magnetic fields of varying strength. The modelling results show striking non-homogeneity of the magnetic field distribution, providing new insights on the magneto-mechanical behaviour of the MSM element. The modelling approach is verified through comparing the calculated MSM permeability change with previously reported results obtained by measurement. Using this modelling approach, electromagnetic analysis is conducted for eleven MSM actuators. The actuators are designed and optimised for a particular 0.1mm strain (displacement) and 10N force output for implementation in food-sorting machines. The conducted analysis also ensures robustness of the designs and stable multi-billion cycle operation. The very long lifetime is achieved through careful analysis of the magnetic circuit and the behaviour of the MSM element during operation. Finally, thermal analysis is conducted for the designed actuators in order to ensure their thermal stability. In order to overcome challenges associated with a very low operating temperature limit of the MSM element in actuators, different available cooling conditions are studied. Moreover, an energy-efficient operation cycle is developed that takes advantage of the shape memory effect of the MSM element also taking into account the pressure change in the pneumatic valve of a sorting machine. The analysis shows multiple regimes which allow thermal stability in a 300Hz pulsed excitation cycle. Implementation of the developed operating cycle also leads to the considerable increase in overall efficiency

Ferromagnetic Shape Memory Alloys: Foams and Microwires

Ferromagnetic shape memory alloys exhibit martensite transformation (MT) and magnetic transition and thus may be actuated by thermal and magnetic fields. The working frequency of these alloys may be higher than conventional shape memory alloys, such as Ni-Ti, because the magnetic field may operate at higher frequency. This chapter focuses on some fundamental topics of these multifunctional materials, including the composition-structure relationship, the synthesis of the foams and microwires, the martensite transformation and magnetic transition characters, the properties (magnetic-field-induced strain (MFIS), magnetocaloric effects (MCEs), shape memory effects, and superelastic effects), and applications. The improvement of the magnetic-field-induced strain due to the reduced constraint of twin boundary motion caused by grain boundaries in polycrystalline Ni-Mn-Ga foams and the size effects of the superelasticity and magnetocaloric properties in Ni-Mn-X (X = In, Sn, Sb) microwires are detailed and addressed

Development of Microactuators Based on the Magnetic Shape Memory Effect

The giant magneto-strain effect in Ni-Mn-Ga alloys is particularly attractive for actuator applications. Two different approaches are being pursued to develop MSM microactuators. To observe large deflections of Ni-Mn-Ga microactuators, the material should be exhibiting low twinning stress and large magnetic anisotropy. In addition, design rules and boundary conditions for operating the Ni-Mn-Ga actuator material are having significant importance for evolution of performance characteristics

Investigation of actuators with smart links

Principal schemes of actuators with smart links have been designed. Equations are presented for describing motion of a pneumatic actuator with viscous magnetorheological liquid and the vibration actuator with a smart link with shape memory. Amplitude-frequency characteristics of the vibration actuator and the pressure developed by the magnetorheological liquid vane have been determined. Application areas of the devices are propose

A comparative review of artificial muscles for microsystem applications

Artificial muscles are capable of generating actuation in microsystems with outstanding compliance. Recent years have witnessed a growing academic interest in artificial muscles and their application in many areas, such as soft robotics and biomedical devices. This paper aims to provide a comparative review of recent advances in artificial muscle based on various operating mechanisms. The advantages and limitations of each operating mechanism are analyzed and compared. According to the unique application requirements and electrical and mechanical properties of the muscle types, we suggest suitable artificial muscle mechanisms for specific microsystem applications. Finally, we discuss potential strategies for energy delivery, conversion, and storage to promote the energy autonomy of microrobotic systems at a system level

Directed Energy Deposition of Ni-Mn-Ga Magnetic Shape Memory Alloy

Processing of magnetic shape memory alloys – active materials that show strains of up to 12 % in a magnetic field and that are being targeted for application as actuators, sensors and energy harvesters – suffers from challenges including time intensive production and macrosegregation that leads to reduced yield. Furthermore, the brittle mechanical behavior of these materials largely eliminates the possibility of machining for a desired shape. This work explores directed energy deposition, an additive manufacturing or “3D printing” process, as an alternative processing route for Ni Mn Ga magnetic shape memory alloy. The magnetic properties, transformation behavior, and composition of the feedstock powder and deposits resulting from a laser metal deposition process are investigated against varied laser power. All samples are seen to possess favorable magnetic behavior and potentially favorable phase for magnetic field induced strain (MFIS) to take place. Additionally, the microstructure of the deposited samples is observed and its special features that may aid MFIS are discussed. Most notably, this thesis presents possible evidence of twin variants crossing deposition layers and of twin boundary motion in a magnetic field. Finally, a connection between lower laser power and reduced loss of Mn is considered