Overcoming Current Quantization Effects for Precise Current Control using Dithering Techniques

Abstract

Accurate current signals are required for precise current control of AC motor drives. However, current measurement errors that involve metering error and quantization error are unavoidable because of the inaccuracy of current sensors and the quantizing feature of analog-to-digital converters in all digital motion control systems. The quantization error contains harmonic components that can induce torque ripple, thus deteriorating the control performance. Therefore, it is necessary to suppress the quantization effects. In this paper, two dithered systems, namely, the subtractively dithered system and nonsubtractively dithered system, are presented in order to remove the quantization-based harmonic components of the measured current signals by whitening the quantization error. Moreover, for ease of practical implementation, the design of dither signals is analyzed for nonsubtractively dithered system, considering cases where the metering error is uniform noise or Gaussian noise. It is shown that by applying the dither having a suitably chosen probability density function, the total measurement error can be rendered spectrally white. The effectiveness of the proposed methods is verified through simulations and experiments using a high-precision linear stage.