Declan Hughes

Preisach modeling of piezoceramic and shape memory alloy hysteresis

Smart materials such as piezoceramics, magnetostrictive materials, and shape memory alloys exhibit hysteresis, and the larger the input signal the larger the effect. Hysteresis can lead to unwanted harmonics, inaccuracy in open loop control, and instability in closed loop control. The Preisach independent domain hysteresis model has been shown to capture the major features of hysteresis arising in ferromagnetic materials. Noting the similarity between the microscopic domain kinematics that generate static hysteresis effects in ferromagnetics, piezoceramics, and shape memory alloys (SMAs), we apply the Preisach model for the hysteresis in piezoceramic and shape memory alloy materials. This paper reviews the basic properties of the Preisach model, discusses control-theoretic issues such as identification, simulation, and inversion, and presents experimental results for piezoceramic sheet actuators bonded to a flexible aluminum beam, and a Nitinol SMA wire muscle that applies a bending force to the end of a beam.

Vision-Based Sensor and Navigation System for Autonomous Air Refueling

Autonomous in-flight aerial refueling is an important capability for the future deployment of unmanned aerial vehicles, because they will likely be ferried in flight to overseas theaters of operation instead of being shipped unassembled in containers. A reliable sensor, capable of providing accurate relative position measurements of sufficient bandwidth, is key to such a capability. A vision-based sensor and navigation system is introduced that enables precise and reliable probe-and-drogue autonomous aerial refueling for non-micro-sized unmanned aerial vehicles. A performance robust controller is developed and integrated with the sensor system, and feasibility of the total system is demonstrated by simulated docking maneuvers with both a stationary drogue and a drogue subjected to light turbulence. An unmanned air vehicle model is used for controller design and simulation. Results indicate that the integrated sensor and controller enables precise aerial refueling, including consideration of realistic measurement errors and disturbances.

Modeling of a flexible beam actuated by shape memory alloy wires

A thermomechanical model is developed to predict the structural response of a flexible beam with shape memory alloy (SMA) wire actuators. A geometrically nonlinear static analysis is first carried out to investigate the deformed shape of a flexible cantilever beam caused by an externally-attached SMA wire actuated electrically. The actuation force applied by the SMA actuator to the beam is evaluated by solving a coupled problem that combines a thermodynamic constitutive model of SMAs with the heat conduction equation in the SMA and the structural model of the beam. To calculate the temperature history of the SMA actuator for given electrical current input, the heat transfer equation is solved with the electrical resistive heating being modeled as a distributed heat source along the SMA wire. The steps in the formulation are connected together through an iterative scheme that takes into account the static equilibrium of the beam and the constitutive relation of SMAs, thus translating an electrical current history input into beam strain output. The proposed model is used to simulate the experimental results, thus demonstrating the feasibility of using SMA actuators for shape control of active flexible structural systems.

A vision-based DSP embedded navigation sensor

Spacecraft missions such as spacecraft docking and formation flying require high-precision relative position and attitude data. Deep space missions require the use of alternative technologies. One such technology is the vision-based navigation (VISNAV) sensor system developed at Texas A&M University. VISNAV comprises an electro-optical sensor combined with light sources or beacons. This patented sensor has an analog detector in the focal plane with a rise time of a few microseconds. Accuracies better than one part in 2000 of the field of view have been obtained. Simultaneous activation of beacons with frequency division multiplexing is given as part of the VISNAV sensor system. The synchronous demodulation process uses digital heterodyning and decimating filter banks on a low-power fixed point digital signal processor, which improves the accuracy of the sensor measurements and the reliability of the system. This paper also presents an optimal and computationally efficient six-degree-of-freedom estimation algorithm using a new measurement model based on the attitude representation of modified Rodrigues parameters.

Modeling and control of smart structures with bonded piezoelectric sensors and actuators

Smart structure has become an increasingly common term describing a structure embedded or bonded with a large number of lightweight active electromechanical sensors and actuators. The authors consider the modeling and control issues related to smart structures bonded with piezoelectric sensors and actuators. They first apply Hamiltons principle to obtain a linearized equation of motion. The natural modes are then found by solving an eigenvalue problem. From the voltage input to current output, the transfer function is shown to be passive, which allows for any strictly passive feedback controller. Issues related to the design of the passive feedback controller in the presence of model non-idealities and performance augmentation with learning-based feedforward control are then discussed.

Vision Based Sensor and Navigation System for Autonomous Aerial Refueling

Autonomous in-flight aerial refueling is an important capability for the future deployment of unmanned aerial vehicles, because they will likely be ferried in flight to overseas theaters of operation instead of being shipped unassembled in containers. A reliable sensor, capable of providing accurate relative position measurements of sufficient bandwidth, is key to such a capability. A vision-based sensor and navigation system is introduced that enables precise and reliable probe-and-drogue autonomous aerial refueling for non-micro-sized unmanned aerial vehicles. A performance robust controller is developed and integrated with the sensor system, and feasibility of the total system is demonstrated by simulated docking maneuvers with both a stationary drogue and a drogue subjected to light turbulence. An unmanned air vehicle model is used for controller design and simulation. Results indicate that the integrated sensor and controller enables precise aerial refueling, including consideration of realistic measurement errors and disturbances.

Experimental Results of a Learning Controller Applied to Tip Tracking of a Flexible Beam

This paper presents the experimental results of a learning controller applied to the tip trajectory tracking of a flexible beam. The overall controller consists of a feedback portion to ensure closed loop stability and a feedforward portion obtained via off-line learning to improve the tracking performance. Neither the model information nor the states of the flexible beam are required for the feedforward learning. Experimental results have verified that the tracking performance of the tip position is satisfied while maintaining the boundedness of the internal states.

Preisach modeling of piezoceramic hysteresis: independent stress effect

Many of the smart materials being investigated (e.g., piezoceramics, shape memory alloys (SMAs), and magnetostrictives) exhibit significant hysteresis effects, especially when driven with large control signals. Furthermore a single input single output hysteresis model may not adequately capture the corresponding nonlinear effects due to the influence of an unmodeled parameter, and two input hysteresis models may therefore be more appropriate. In this paper the Preisach model and inverse compensator for a piezoceramic sheet actuator is described and experimental data presented. The effect of a large independent applied stress on the observed applied electric field to measured strain hysteresis for a piezoceramic sheet actuator is demonstrated.

A DSP embedded optical navigation system

Six degrees of freedom (6DOF) data estimation has a wide range of applications in navigation, proximity operations, manufacturing and robotic control. This paper presents an optimal and computationally efficient 6DOF estimation algorithm using modified Rodrigues parameters. The analytical results of the estimation algorithm and also its computational results on a low power floating point DSP, the TMS320VC33, are presented. A new approach involving frequency division multiplexing of the beacons for a vision based navigation system and the demodulation on a low power fixed point DSP, the TMS320C55x, is introduced in order to improve the accuracy of the sensor measurements and the reliability of the system.

Hysteresis in shape memory alloy actuators: the control issues

Shape memory alloy has been considered as an actuator for applications that require low bandwidth, high force, and large displacement. Two factors have limited the usefulness of such actuators: hysteresis and bandwidth limitation. This paper considers the hysteresis phenomenon from a control point of view. We first consider the application of the Preisach hysteresis model to describe the SMA hysteresis, and demonstrated experimentally that the two key assumptions: minor loop congruence and wiping-out property hold approximately. We then consider the feedback control of the force exerted by the SMA wire. By using a simple lumped temperature model, we argue that proportional feedback with a suitable range of gains would render the closed loop stable. This is verified experimentally in a simple experimental setup consisting of a flexible aluminum beam and to a Nitinol shape memory alloy wire that applies a bending force to the end of the beam. When the gain is chosen too high, clear instability has been observed despite the low bandwidth of this system (about 1 Hz).