Close-Loop Current Control With Current-Sensing Resistor for Electromagnet Driven by Full-Bridge PWM Inverter

The full-bridge pulse-width-modulation (PWM) inverter is wildly applied to drive the electromagnet and enables advanced technologies, such as the active magnetic bearing (AMB) and the permanent-electro magnetic suspension (PEMS). However, the characteristic relationship between the current through the electromagnet and the duty cycle of the PWM signal has strong nonlinearity around the zero current. Moreover, the electromagnet possesses the induction that results in the time constant and significantly hinders the change of the current. Hence, the open-loop control of the full-bridge PWM inverter cannot accurately or timely tune the current through the electromagnet, especially around the zero current. This work proposes a closed-loop current control approach by three steps: (1) inserting a current-sensing resistor into the middle of the electromagnet, (2) obtaining the current signal with an analog signal-processing circuit, and (3) generating the PWM signal with the bang-bang control circuit. The proposed approach innovatively arranges the current-sensing resistor to take advantage of the symmetry and to minimize the influence from the high-frequency switching of the inverter, so that the measured current signal is comparable to the hall-effect current sensor. The experimental results demonstrate the effectiveness and efficiency of the proposed closed-loop current control approach, though the weak charging capability of the full-bridge PWM inverter still hinders its performance.