Mohammad Elahinia

Nonlinear Control of a Shape Memory Alloy Actuated Manipulator

This paper presents a nonlinear robust control algorithm for accurate positioning of a single degree of freedom rotary manipulator actuated by Shape Memory Alloy (SMA). A model for an SMA actuated manipulator is presented. The model includes nonlinear dynamics of the manipulator, a constitutive model of Shape Memory Alloy, and electrical and heat transfer behavior of SMA wire. This model is used for open and closed loop motion simulations of the manipulator Experiments are presented that show results similar to both closed and open loop simulation results. Due to modeling uncertainty and nonlinear behavior of the system, classic control methods such as Proportional-Integral-Derivative control are not able to present fast and accurate performance. Hence a nonlinear, robust control algorithm is presented based on Variable Structure Control. This algorithm is a control gain switching technique based on the weighted average of position and velocity feedbacks. This method has been designed through simulation and tested experimentally. Results show fast, accurate, and robust performance of the control system. Computer simulation and experimental results for different stabilization and tracking situations are also presented.

Metals for bone implants. Part 1. Powder metallurgy and implant rendering.

New metal alloys and metal fabrication strategies are likely to benefit future skeletal implant strategies. These metals and fabrication strategies were looked at from the point of view of standard-of-care implants for the mandible. These implants are used as part of the treatment for segmental resection due to oropharyngeal cancer, injury or correction of deformity due to pathology or congenital defect. The focus of this two-part review is the issues associated with the failure of existing mandibular implants that are due to mismatched material properties. Potential directions for future research are also studied. To mitigate these issues, the use of low-stiffness metallic alloys has been highlighted. To this end, the development, processing and biocompatibility of superelastic NiTi as well as resorbable magnesium-based alloys are discussed. Additionally, engineered porosity is reviewed as it can be an effective way of matching the stiffness of an implant with the surrounding tissue. These porosities and the overall geometry of the implant can be optimized for strain transduction and with a tailored stiffness profile. Rendering patient-specific, site-specific, morphology-specific and function-specific implants can now be achieved using these and other metals with bone-like material properties by additive manufacturing. The biocompatibility of implants prepared from superelastic and resorbable alloys is also reviewed.

An enhanced SMA phenomenological model: I. The shortcomings of the existing models

This paper provides an enhanced phenomenological model for shape memory alloys (SMAs), to better model their behavior in cases where the temperature and stress states change simultaneously. The phenomenological models for SMAs, consisting of a thermodynamics-based-constitutive and a phase transformation kinetics model, are the most widely used models for engineering applications. The existing phenomenological models are formulated to qualitatively predict the behavior of SMA systems for simple loadings. In this study, we have shown that there are certain situations in which these models are either not correctly formulated, and therefore are not able, to predict the behavior of SMA wires or the formulation is not straightforward for engineering applications. Such cases most often occur when the temperature and stress of the SMA wire change simultaneously, such as the case of rotary SMA actuators. To this end, a rotary SMA-actuated robotic arm is modeled using the existing constitutive models. The model is verified against the experimental results to document that the model is not able to predict the behavior of the SMA-actuated manipulator, under certain conditions.

On the development of high quality NiTi shape memory and pseudoelastic parts by additive manufacturing

Additive manufacturing provides an attractive processing method for nickel–titanium (NiTi) shape memory and pseudoelastic parts. In this paper, we show how the additive manufacturing process affects structural and functional properties of additively manufactured NiTi and how the process parameter set-up can be optimized to produce high quality NiTi parts and components. Comparisons of shape recovery due to shape memory and pseudoelasticity in additively manufactured and commercial NiTi exhibit promising potential for this innovative processing method.

An enhanced SMA phenomenological model: II. The experimental study

In an earlier study, using an SMA-actuated robotic arm, the authors showed that the existing phenomenological models are not able to predict the behavior of the material under certain conditions. There, it was shown that such cases most often occur when the temperature and stress of the SMA wire change simultaneously. In this part, the existing model discrepancies are further studied experimentally using a dead weight, which is actuated by an SMA wire. Subsequently, an enhanced phenomenological model is developed. The enhanced model is able to predict the behavior of SMAs under complex thermomechanical loadings.

An Automotive SMA Mirror Actuator: Modeling, Design, and Experimental Evaluation

Shape memory alloys (SMAs) provide compact and effective actuation for a variety of mechanical systems. Automotive applications of these materials, although very attractive, have not been fully explored. The lack of activities in this area is partly due to the complex thermomechanical behavior of these materials. In this work a two degree of freedom SMA actuator has been developed to orient the position of a rearview mirror for automotive applications. This paper presents the design, modeling, and experimental evaluation of this SMA mirror actuator. To evaluate the performance of the actuator a prototype SMA actuated mirror is designed and fabricated. Experimental results are presented to demonstrate the effectiveness of the SMA actuator in positioning the mirror.

Nonlinear Stress-based Control of a Rotary SMA-actuated Manipulator

this paper a nonlinear stress-based controller is designed to position a singledegreeof-freedom shape memory alloy (SMA) actuated manipulator. A three-part model was constructed based on the dynamics/kinematics of the arm, the thermomechanical behavior of SMAs, and an assumed heat transfer model consisting of electrical heating and natural convection. Both sliding mode control and inverse dynamics control are used to calculate a desired stress, based on the position error. The desired stress is compared with the actual stress which is computed using an Extended Kalman Filter. The stress error is then used for control via a proportional-integral controller. Numerical simulations are performed to investigate tracking performance as well as other issues such as robustness. The results demonstrate that the variable structure controller designs are highly accurate in tracking both stationary and variable input signals.

Stable Walking Pattern for an SMA-Actuated Biped

In this paper, a walking pattern filter for shape-memory-alloy (SMA)-actuated biped robots is presented. SMAs are known for their high power-to-mass ratio as well as slow response. When used as actuators, the SMA speed limitation can potentially lead to stability problems for biped robots. The presented filter adapts the human motion such that an SMA biped robot maintains a stable walking pattern. The zero moment point (ZMP) is used as the main criterion of the filter to guarantee the stability of the motion. The SMA actuators are designed based on the dynamics and kinematics of the motion. The response time of each SMA actuator is modeled in order to estimate the behavior of the actuator in realizing the given trajectory. After applying the delay times to the motion, the new trajectories are generated and evaluated by the filter for the ZMP criterion. Using simulations, it is shown that the filter can generate smooth trajectories for the SMA-actuated biped robots. The filter furthermore guarantees the stability of a robot mimicking the human walking motion.

Position Control of a Three-link Shape Memory Alloy Actuated Robot

A simple and robust position controller is proposed for a small planar three- degree-of-freedom robot arm actuated by two shape memory alloy (SMA) actuators and a servomotor. A simple model of the robot is used for controller development. The model combines robot kinematics and dynamics with crude models of SMA wire heat convection, constitutive law, and phase transformation. It is then used to estimate ‘optimal’ parameters for the position controllers. The controllers are based on variable structure control approach and their development is of an evolutionary nature starting with a simple switching surface of position tracking errors and followed by the addition of velocity and integral tracking errors, respectively. The final controller is shown to be particularly fast since it heats the SMA wires with maximum available voltage, but avoids overshoot thereby avoiding the slow natural cooling process compared. Several experiments have been performed with a desktop prototype of the robot. The experimental results verify the effective and robust performance of the controllers despite significant modeling inaccuracies during the controller parameter development process. An additional advantage of the controller is that it can be implemented on a controller board with very limited computation capacity.

Design of a Kalman filter for rotary shape memory alloy actuators

Measuring the state variables of systems actuated by shape memory alloys (SMAs) is normally a difficult task because of the small diameter of the SMA wires. In such cases, as an alternative, observers are used to estimate the state vector. This paper presents an extended Kalman filter (EKF) for estimation of the state variables of a single-degree-of-freedom rotary manipulator actuated by an SMA wire. This model-based state estimator has been chosen because it works well with noisy measurements and model inaccuracies. The SMA phenomenological models, that are mostly used in engineering applications, have both model and parameter uncertainties; this makes the EKF a natural choice for SMA-actuated systems. A state space model for the SMA manipulator is presented. The model includes nonlinear dynamics of the manipulator, a thermomechanical model of the SMA, and the electrical and heat transfer behavior of the SMA wire. In an experimental set-up the angular position of the arm is the only state variable that is measured besides the voltage applied to the SMA wire. The other state variables of the system are the arms angular velocity and the SMA wires stress and temperature, which are not available experimentally due to difficulty in measuring them. Accurate estimation of the state variables enables design of a control system that provides better system performance. At each time step, the estimator uses the SMA wires voltage measurement to predict the state vector which is corrected as necessary according to the measured angular position of the arm. The input and output of the model are used for the EKF simulations. The state variables collected through model simulations are also used to evaluate the performance of the EKF. Several EKF simulations presented in this paper show accurate and robust performance of the estimator, for different control inputs.