It is well known that both martensite and austenite (superelastic) Nitinol Shape Memory Alloys (SMAs) have damping capacities, benefiting from their hysteretic stress-strain relationships. In general, for SMA devices for passive vibration control, martensite SMA has a larger damping capacity; however, it requires external heat to cause a phase transformation to restore its original shape. On the other hand, superelastic SMA has less damping capacity, but it has a strong re-centering force to restore its initial shape and there is little residual strain of the superelastic SMAs. This paper researches the damping capacity of Nitinol in martensite and austenite co-existence phase. Nitinol with the co-existence of both martensite and austenite phases combines advantages of martensite SMAs and superelastic SMAs and has a large damping capacity with self-shape restoring ability. To quantitatively study the damping effect of Nitinol in martensite and austenite co-existence phase, a setup is designed and fabricated. This setup involves a cantilevered steel beam with pre-stressed SMA wires attached to each surface at the remote end of the beam. The SMA wires function as a damper to the cantilevered beam. A piezoceramic patch sensor attached to the beam near its cantilevered end is used to record the data of the vibration of beam and the data is then used to estimate the damping ratio of the system. The percentage of the martensite phase in the Nitinol SMA wires is controlled by electrically heating the wires via a closed-loop control system. Experimental results verify that the Nitinol wires with the co-existence of the both martensite and austenite have the best damping property for vibration suppression. For practical implementation, the transformation temperate of the SMA wire damper can be chosen as the room temperature so that both martensite and austenite co-exist.
KEYWORDS: Shape memory alloys, Actuators, Feedback control, Control systems, Fluctuations and noise, Position sensors, Temperature metrology, Data acquisition, Data modeling, Control systems design
There is no reported research of using shape memory alloy (SMA) actuators for variable area exhaust nozzle for a jet engine in the literature, to the authors' best knowledge. SMA actuators have the advantages of high power-to-weight ratio and can result in dramatic weight reduction as compared to hydraulic systems. However, the difficulty of using SMA actuators for controlling variable area exhaust nozzle lies in the fact that the temperature near exhaust nozzle is far higher than the transformation temperature of an SMA actuator. Due to the flexibility and small volume of SMA wire actuators, they can be remotely replaced in a region where temperature is lower than that of its transformation temperature. By exploiting this fact, this paper presents a novel design of a proof-of-concept variable area exhaust nozzle using shape memory alloy wire actuators. The SMA actuators are remotely placed away from the exhaust nozzle area so that the environmental temperature is below their transformation temperature. By electrically heating the SMA actuators, the exhaust nozzle will experience an area reduction of up to 40%. Bias springs will apply forces to return the fan nozzle to the open-up configuration. A feedback controller based sliding mode method is used to regulate the SMA actuators' position. Experimental results demonstrate that the proposed design meets the desired area variation specifications and show the promise of a lightweight and simple exhaust nozzle design by using shape memory alloy actuators.
KEYWORDS: Shape memory alloys, Actuators, Control systems, Resistance, Neural networks, Feedback control, Position sensors, Control systems design, Signal processing, Crystals
This paper presents the development of a position control system for a shape memory alloy (SMA) wire actuator using the electrical resistance feedback. It is commonly known that an SMA actuator is highly nonlinear and a position sensor is often required to achieve a stable and accurate positioning. And this position sensor often contributes a large portion of the system cost. To eliminate the position sensor in an SMA actuator system, a novel control theme is proposed by utilizing the actuator’s electrical resistance feedback. With an SMA wire test setup, the relationship between the electrical resistance and the displacement is experimentally investigated. However, this relationship is highly nonlinear, and a neural network is employed to model this relationship and predicts the position of the actuator using only its electrical resistance information. To enable feedback control of the SMA wire actuator using only its electrical resistance, a Proportional-Integral-derivative (PID) controller is used. Feedback control experiments are performed and the results demonstrate that the proposed position control system achieves a good control performance without using a position sensor.
KEYWORDS: Shape memory alloys, Actuators, Modulation, Modulators, Control systems, Energy efficiency, Resistance, Control systems design, Smart materials, Power supplies
Shape memory alloy (SMA)s, in particular the nickel-titanium alloy (or Nitinol), have been used as actuators in some astronautic, aeronautic and industrial applications. Future will see more SMA application if less energy is required for actuation. This paper presents the design and experimental results of control of an SMA actuator using Pulse Width (PW) Modulation to reduce the energy consumption by the SMA actuator. An SMA wire test stand is used in this research. Open-loop testing of the SMA wire actuator is conducted to study the effect of the PWM parameters. Based on results of testing results and parameter analysis of the PW modulator, a PW modulator is designed to modulate a Proportional plus Derivative (PD) controller. Experiments demonstrate that control of the SMA actuator using PW modulation effectively save actuation energy whiling maintaining same control accuracy as compared to continuous PD control. PW modulator also demonstrates robustness to external disturbances. A comparison with pulse width pulse frequency (PWPF) modulator is also presented.
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