The change in magnetoimpedance (MI) during surface modification of the magnetic sensitive element caused by highly corrosive fluid was studied with the aim of creating a robust method to monitor the surface effects. A MI-based sensor prototype with an as-quenched FeCoSiB or FeCoCrSiB amorphous ribbon sensitive element was designed and calibrated for a frequency range of 0.5–10MHz at an intensity of the current of 10–60mA without bath for fluids. Measurements as a function of the exposure time were also made in a regime where chemical surface modification and sensing were not separated (in a bath for fluids). The MI variation was explained by the change of the surface magnetic anisotropy and the geometry of the sensitive element. A simple model was developed to describe MI change. It was shown that the magnetoimpedance effect can be employed as useful method to probe the electric features of surface-modified magnetic electrodes when the corrosive fluid, the material of the sensitive element, and the detection conditions are properly selected.
The ability of shape memory alloys (SMA) to respond to an external stimulus by modifying their dimensions can be used to generate motion or force in electromechanical devices and micro-machines. It has been often demonstrated that SMA-based devices are serious alternatives to conventional micrometric actuators. We have previously demonstrated that, using a high-quality position sensor, such as a linear variable differential transformer (LVDT), to provide the position feedback, accuracies about 3 μm in position control can be obtained. In this work, we present an actuator prototype based in a SMA wire, conceived to be used in lightweight applications, where the bulky position sensor previously used is replaced with a lighter alternative. The most convenient one, and also the most challenging, is to use the wire’s own resistance as a measure of its position, that is, to implement a sensorless control strategy. We propose to use a neural network to characterize the relation between the resistance of the wire and its strain and introduce this correspondence as the position feedback in a simple PID closed loop. The experimental results show that, in this way, accuracies about 70 μm can be obtained. The great advantage of this procedure is that the actuator is reduced to a single SMA element without any additional sensor, which is of great importance when the main goals are to reduce the overall weight, size, and cost of the actuator.
Ferromagnetic shape memory alloys are promising active elements for actuators. Our work centered on achieving and maintaining an intermediate fixed deformation so that they can be used as precision positioning actuators. For this purpose, a custom actuator was built using a single crystal of NiMnGa. The results show that these alloys can be controlled within less than 5 nm for both precision and accuracy, a result comparable to piezoelectric ceramics. Interestingly, the defect structure plays a fundamental role in achieving such performance. The stochastically distributed defects determine a progressive diminution of the magnetic field strength required to achieve the control.
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