Long wavelength results in the low radiation efficiency of a portable conventional antenna operating at very low frequency (VLF) and below. This has motivated one to develop an innovative approach to design an electrically small antenna in a frequency band lower than VLF. The time-varying electromagnetic fields can be generated by spinning a permanent magnet. In this way, the mechanical energy is converted to the electromagnetic energy, and the impedance matching networks with nonnegligible insertion loss are not required. Therefore, this mechanical antenna with spinning magnet can improve radiation efficiency in a low frequency band. In this paper, we give the detailed analysis procedure for the spinning magnet, which is seldom discussed in other published reports. In order to analyze the electromagnetic characteristics of the spinning magnet, in this paper we use the ampere return circuit theorem to investigate the equivalent relation between a spinning magnet and the orthogonal magnetic dipole. We introduce an initial spinning angle of the magnet into the dyadic green’s function. With this modification, we provide the rigorous analytic formula for field computation of the orthogonal magnetic dipole. Thus the electromagnetic characteristics of the spinning magnet and spinning magnet array can also be analyzed. For a spinning NdFeB magnet with a magnetization of <i>B</i><sub>r</sub> = 0.8 T and a volume of <i>V</i><sub>r</sub> = 270 cm<sup>3</sup> as well as 9600 revolutions per minute, the simulation results reveal that the magnetic field of 15 fT at 1 km in air space can be obtained. But the magnetic field of the spinning magnet decreases quickly to 1 fT at 250 m in sea water. Considering the potential demand for increasing the field strength in the near field region, we recommend to use a magnet array with small-sized elements. The magnet array can be used to control the near field pattern. We take two magnets as an example for studying the performance. It can be found from the simulation results that the magnetic near field is increased by 3 dB with the linear magnet array consisting of two elements. With the initial spinning angle of the magnet element adjusted, the near field pattern of the magnet array can be controlled. This is analogous to beam steering of traditional phased array for high band operation. It can be concluded from our study that the spinning magnet is a possible alternative solution for low frequency small transmitter antenna.
This paper presents a triple rotating coordinate transformed vector control method for dual three-phase permanent magnet (PM) machines. In the proposed scheme, the control variables are converted to three sets of αβ components directly, which are 120° electric degrees different from each other. It omits the complicated six-dimensional transformed matrix and reduces the computation greatly. The relationship with vector space (VSD) control was mathematically analyzed. By ensuring the consistency of control variables in the three stationary reference frames, the suggested method can not only achieve the same fundamental control performance as VSD but compensate for the imbalance current caused by the harmonics in the back electromotive force. In addition, the proposed method belongs to multi redundancy control in theory, which is maybe a good solution for fault-tolerant operation. Finally, a prototype dual three-phase PM machine was tested. The experimental results are in good agreement with the theoretical analysis.
A rotating magnet-based mechanical antenna (RMBMA) is a new promising paradigm which can significantly reduce both the size and the power consumption of the super-low frequency electromagnetic transmitter. To further reveal the effects of the rotational motion on transmission, this paper investigates the performance analysis of MSK (Minimum Shift Keying)-based RMBMA for the first time. Initially, based on the framework of MSK-based RMBMA, both the information loading of RMBMA and the control strategies of the motor are proposed, such that the impacts of the step response induced by the motor on the information loading are characterized. Then, the transmission capacity of the MSK-based RMBMA is derived in the closed form. The results indicate that increasing output torque or decreasing inertia load can enhance the transmission capacity. Finally, numerical simulations using a realistic system model demonstrate the validity of the proposed performance analysis. The simulation results show that when the inertia is less than 0.0128 kg∙m2 and the symbol rate is less than 4 bit/s, the bit error rate is less than 10%, thereby improving the transmission capacity. The proposed comprehensive design principles of RMBMA provide guidance for system design and practical implementation.
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