An Auto Bandwidth Boost Tuning Method of Current Control for Servo Motor Drives Considering Time Delay

This paper proposes an automatic bandwidth boost method for the current control loop in servo motor drives, specifically targeting the adverse effects of time delay. In digital control systems, time delays caused by PWM updates, computation latency, and current sensing significantly degrade the bandwidth and stability of current regulation. To address this issue, three main contributions are presented. First, the impact of observer-based current prediction on the effective control loop delay is thoroughly analyzed, showing that it reduces control latency and increases phase margin. Second, an FFT-based estimation method is proposed to estimate the compensated time delay by measuring the phase shift between the sensed and observer currents. Third, an autotuning method based on the Magnitude Optimum criterion is developed to automatically adjust controller gains using the estimated delay, thereby maximizing the control bandwidth while maintaining sufficient stability margin. Experimental validation on a 400 W servo motor drive demonstrates that the proposed method improves the –90° bandwidth from 1.48 to 2.17 kHz and from 4.67 to 6.71 kHz at 10 kHz and 30 kHz PWM frequencies, respectively. Phase margins remain above 69°, and dynamic stiffness increases from 8.4 to 14 dB and from 15.9 to 20.7 dB. Robustness is confirmed under motor parameter variations, where the proposed method maintains stable operation and outperforms the original controller design without gain retuning. These results fully support the effectiveness of the proposed techniques for servo motor drives.