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MDPS (motor driven power steering) systems have been widely used in vehicles due to their improved fuel efficiency and steering performance when compared to conventional hydraulic steering. However, the reduction of torque ripples and material cost are important issues. A low resolution position sensor for MDPS is one of the candidates for reducing the material costs. However, it may increases the torque ripple due to the current harmonics caused by low resolution encoder signals. In this paper, the torque ripple caused by the quantized rotor position of the low resolution encoder is analyzed. To reduce the torque ripples caused by the quantization of the encoder signals, the rotor position and the speed are estimated by measuring the frequency of the encoder signals. In addition, the compensating q-axis current is added to the current command so that the 6 th order torque harmonic is attenuated. The reduction of torque ripples by applying the estimated rotor position and the compensated q-axis current is verified through experimental results.

Sound navigation and ranging (SONAR) systems detect a target in the front direction by using acoustic signals. A switching-type power conversion system is used to improve power efficiency, and an impedance matching circuit is used to decrease reactive power. A low-pass filter is used to improve the quality of acoustic signals. To achieve the desired voltage level for a SONAR transducer, a transformer is connected in series with a low-pass filter. In conventional design methods, design value errors occur because the components are designed independently and later combined. Moreover, if parameters that considerably impact operating characteristics are ignored in the design process, these errors will increase. Hence, time and cost losses are incurred during refabrication because operational characteristics differ from design values. To solve this problem, this study proposes the simultaneous design of a low-pass filter and impedance matching circuit, which includes critical design parameters, utilizing the particle swarm optimization algorithm. Moreover, conventional design methods were examined, and the superiority of the proposed design method to conventional methods was verified through analyses and experiments in terms of overall impedance phase and filter blocking characteristics.