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.
In many micro- and nano-scale technological applications high sensitivity displacement sensors are needed, especially in ultraprecision metrology and manufacturing. In this work a new way of sensing displacement based on radio frequency resonant cavities is presented and experimentally demonstrated using a first laboratory prototype. The principle of operation of the new transducer is summarized and tested. Furthermore, an electronic interface that can be used together with the displacement transducer is designed and proved. It has been experimentally demonstrated that very high and linear sensitivity characteristic curves, in the range of some kHz/nm; are easily obtainable using this kind of transducer when it is combined with a laboratory network analyzer. In order to replace a network analyzer and provide a more affordable, self-contained, compact solution, an electronic interface has been designed, preserving as much as possible the excellent performance of the transducer, and turning it into a true standalone positioning sensor. The results obtained using the transducer together with a first prototype of the electronic interface built with cheap discrete elements show that positioning accuracies in the micrometer range are obtainable using this cost-effective solution. Better accuracies would also be attainable but using more involved and costly electronics interfaces.
The performance and accuracy of micro-and nanopositioning systems are directly linked to the measurement device used to close the associated control loop. In this work we propose, design, and test an electronic interface for a new position and displacement transducer based on resonant cavities. This type of transducer has been proven to achieve resolutions in the nanometer range when the detection is performed using a laboratory network analyzer. The proposed electronic interface is intended to provide a more affordable and compact solution, while preserving as much as possible the excellent performance of the transducer, thus, turning it into a true standalone positioning sensor. The operation of the interface establishes a self-resonance in the cavities and detects the resonance frequency (which is directly related to the position to be measured) by analyzing the attenuation produced by a low pass filter. The results obtained in a prototype of the interface built with discrete elements show that the obtainable positioning accuracy using this cost-effective solution is about 5 micrometers.
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