Design Development And Analysis Of A Tactile Display Based On Composite Magnetorheological Elastomers
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Design, Development, and Analysis of a Tactile Display Based on Composite Magnetorheological Elastomers
In minimally invasive surgery, surgeons carry out the operations by employing small tools and viewing equipment into the patient's body by means of small incisions. In manual and robotic minimally invasive surgery, surgeons do not have direct touch and natural sense of touch, due to utilization of long and often flexible instruments, and palpation is a necessity to provide perfect diagnoses. As a potential candidate, magnetorheological elastomers were investigated as a stiffness display for surgical application. To this end, three different samples of magnetorheological elastomers with various volume fraction of iron particles and one non-MRE rubber sample were fabricated. Six composite MREs were made by combining two layers of different fabricated samples. The samples were characterized under compression test and perpendicular to the applied magnetic field (MF). The compression test was carried out with the strain range of (5 - 25%) at magnetic field densities of 0, 143, 162, 198, 238, 287, 365 mT. It was observed that the elastic modulus of one-layered MREs and bi-layered MREs increase with increasing the magnetic fields. Moreover, MR-effect was enhanced via bi-layer composition, e.g. mono-layered 45%vol iron particles: 211%, bi-layered 45%vol iron particles: 253%. Afterward, a solution for the medical need of the tactile display during minimally invasive surgeries was proposed. To this end, a tactile display based on the composite magnetorheological elastomers, MiTouch , was designed and prototyped. Also, the electromechanical parameters of MiTouch were identified through a transfer function optimization and a PID controller was fine-tuned to achieve a desired stiffness. Later, validation experiments were carried out to showcase the feasibility of MiTouch for pulse examinations and maintaining a desired stiffness. The results revealed that MiTouch applied a pulsed contact force of 0.6N to the phantom finger. The results were within the range of reported pulse examination forces, i.e. 0.5-2N. In addition, the system was capable of following a desired stiffness of 4N=mm and maintaining it within a range of 4:07 +/- 0:41N/mm. In the end, results confirmed the hypothesis of the feasibility of the suggested solution for surgical applications.
Identification of Mechanical Properties of Nonlinear Materials and Development of Tactile Displays for Robotic Assisted Surgery Applications
This PhD work presents novel methods of mechanical property identification for soft nonlinear materials and methods of recreating and modeling the deformation behavior of these nonlinear materials for tactile feedback systems. For the material property identification, inverse modeling method is employed for the identification of hyperelastic and hyper-viscoelastic (HV) materials by use of the spherical indentation test. Identification experiments are performed on soft foam materials and fresh harvested bovine liver tissue. It is shown that reliability and accuracy of the identified material parameters are directly related to size of the indenter and depth of the indentation. Results show that inverse FE modeling based on MultiStart optimization algorithm and the spherical indentation, is a reliable and scalable method of identification for biological tissues based on HV constitutive models. The inverse modeling method based on the spherical indentation is adopted for realtime applications using variation and Kalman filter methods. Both the methods are evaluated on hyperelastic foams and biological tissues on experiments which are analogous to the robot assisted surgery. Results of the experiments are compared and discussed for the proposed methods. It is shown that increasing the indentation rate eliminates time dependency in material behavior, thus increases the successful recognition rate. The deviation of an identified parameter at indentation rates of V=1, 2 and 4 mm/s was found as 28%, 21.3% and 7.3%. It is found that although the Kalman filter method yields less dispersion in identified parameters compared to the variance method, it requires almost 900 times more computation power compared to the variance method, which is a limiting factor for increasing the indentation rate. Three bounding methods are proposed and implemented for the Kalman filter estimation. It was found that the Projection and Penalty bounding methods yield relatively accurate results without failure. However, the Nearest Neighbor method found with a high chance of non-convergence. The second part of the thesis is focused on the development of tactile displays for modeling the mechanical behavior of the nonlinear materials for human tactile perception. An accurate finite element (FE) model of human finger pad is constructed and validated in experiments of finger pad contact with soft and relatively rigid materials. Hyperfoam material parameters of the identified elastomers from the previous section are used for validation of the finger pad model. A magneto-rheological fluid (MRF) based tactile display is proposed and its magnetic FE model is constructed and validated in Gauss meter measurements. FE models of the human finger pad and the proposed tactile display are used in a model based control algorithm for the proposed display. FE models of the identified elastomers are used for calculation of control curves for these elastomers. An experiment is set up for evaluation of the proposed display. Experiments are performed on biological tissue and soft nonlinear foams. Comparison between curves of desired and recreated reaction force from subject's finger pad contact with the display showed above 84% accuracy. As a complementary work, new modeling and controlling approaches are proposed and tested for tactile displays based on linear actuators. Hertzian model of contact between the human finger pad and actuator cap is derived and curves of material deformation are obtained and improved based on this model. A PID controller is designed for controlling the linear actuators. Optimization based controller tuning approach is explained in detail and robust stability of the system is also investigated. Results showed maximum tracking error of 16.6% for the actuator controlled by the PID controller. Human subject tests of recreated softness perception show 100% successful recognition rate for group of materials with high difference in their softness.
Evaluating Tactile Fidelity of Resolution, Amplitude, and Algorithms for Grid-based Tactile Sleeve Displays
Tactile displays have the potential to dramatically increase immersion and presence for users in Virtual Reality (VR) applications. For consumer VR systems, there is a need to increase the quality and fidelity of tactile feedback that is produced by light weight and low cost tactile display. In this research, several arm-based tactile sleeve displays were developed to investigate how certain characteristics of vibrotactile display design, such as spacing, resolution, amplitude, and tactile rendering algorithm can affect the fidelity of a tactile display device and the experiences of its users. The design of the tactile sleeve displays used in this research is grid-based, consists of an array of linear resonant actuator (LRA) motors. Linear resonant actuator is a type of voice-coil actuator that relies on the resonance of mass and spring elements to produce vibrations along a central linear axis. An elastic compression sleeve was used for attaching the LRA motors to the upper arm of the users. Several user studies were conducted to determine the interactions between displayed resolution, amplitude, tactile rendering algorithm and their produced tactile fidelity. In the tactile resolution study, the extended Tactile Brush algorithm was used to produce ten single-finger tactile patterns and whole-hand tactile patterns. Two displayed tactile resolutions (4-by-3 and 2-by-2) were compared and the results indicated that the extended Tactile Brush algorithm produces accurate whole-hand tactile patterns, while higher displayed tactile resolutions tend to produce more acceptable tactile patterns than lower resolutions. In the real-time tactile rendering algorithm study, the Syncopated Energy algorithm was developed, and the efficacy of this new algorithm was evaluated by comparing its recognition accuracy and perceived continuity with a traditional Grid Region algorithm. The Syncopated Energy algorithm was generally perceived to produce more continuous tactile motions, and the Grid Region algorithm provided higher recognition accuracy. In the interaction between amplitude and the rendering algorithms study, a highamplitude tactile sleeve display consisting of professional-grade C3-tactors was developed to determine whether higher amplitudes always produce greater recognition accuracy and improved continuity, whether rendering at very high amplitudes has any negative consequences. The results of using the high-amplitude tactile sleeve display indicated that, even at intensive level of amplitudes, recognition accuracy for both the Syncopated Energy algorithm and Grid Region of algorithm improved significantly, however, the perceived continuity was decreased significantly at intensive level of amplitudes. Based on the results of this research, several design guidelines were proposed to form the best practices for grid-based vibrotactile display designs in VR systems.