A step forward for motor torque controllers: In order to reduce oscillations and oversets, a speed loop pseudo derivative feed forward controller is proposed

DTC with Speed Loop PDFF Controller

The schematic of a PMSM drive using a DTC-based speed loop PDFF controller is shown in Fig. 1. This DTC scheme is implemented in stator flux frame, which requires knowledge of stator flux in flux-torque (M-T) axes. The torque axis involves dual loops: outer loop speed control and inner loop torque control. On other hand, stator flux axis consists of single loop. Utilizing a PDFF controller, the outer speed loop creates an electromagnetic torque reference for torque control. In [9], overshoot in rotor speed is discussed for a DTC-based PMSM drive with speed loop PI control, overshoot is unavoidable during system dynamics perturbation. The overshoot and speed oscillations are reduced by the proposed PDFF speed regulator. An error signal of electromagnetic torque and stator flux is generated by comparing measured and estimated values of these quantities. Then, the DTC's switching table is used to generate logic for switching based on the error signals processed through hysteresis comparator.

Effectiveness of Proposed Speed Loop PDFF and PI Controller

In this section, outer speed loop is examined to evaluate the effectiveness of PI-HCC, PI-DTC, and the proposed PDFF-DTC scheme. The simulation results show that the dynamic performance of the proposed system (overshoot and settling time) is superior then the PI controller-based speed regulator. . In PI-HCC and PI-DTC based PMSM drives, the outer speed loop is comprise of a PI controller. Thereby in the analysis, the same PI controller gain values are considered for both PI-HCC and PI-DTC based PMSM drive. By experimenting with different controller gain values, the efficacy of outer loop PI and PDFF controller-based speed regulator is validated. The results are pertained to step response of speed regulator at no load condition i.e., load torque disturbance is considered as zero. It is clear that the PDFF controller overshoots the speed by 7.65% to 0, whereas the PI controller overshoots it by 10.65%. It's clear that, the PDFF controller-based system is more stable and has less overrun than the PI controller-based system. Bode plot of the PDFF and PI speed controller is shown in Fig. 2. Overshoot is mitigated by the PDFF controller for the speed loop, which has a significantly smaller magnitude gain than the PI-based speed loop system.

Laboratory Implementation

This section pertains to demonstration of experimental results for four quadrant operation for a PMSM drive of 1.5 kW. Experimental implementation for the proposed scheme is realized with FPGA controller (ALTERA cyclone II). The signal conditioning and sensing circuitry is built up of Op-Amps with LEM current and voltage sensors using OPA 227P, LAH 25-NP, LV-25P respectively. Conclusively, a 1024 PPR encoder is emloyed for gathering rotor angular speed and rotor position of motor. The experimental setup is depicted in a block diagram (Fig. 3) and the laboratory prototype is illustrated in Fig. 4. The comparative analysis of PMSM drive is performed for speed loop PI controller based hysteresis current controller (HCC), speed loop PDFF and conventional PI controller based DTC under different case studies viz. case (1): reverse to forward braking operation and case (2): forward to reverse motoring operation.

Conclusion

This paper proposes PDFF based speed regulator which aims to reduce overshoot and oscillations in rotor speed, electromagnetic torque and stator current of direct torque controlled PMSM drive. The proposed PDFF-DTC is compared to the state of the art, which includes the PI-DTC and the PI-HCC. With the proposed controller during reverse to forward braking operation the overshoot in the rotor speed and electromagnetic torque response is reduced to 50% and 20% respectively compared to other controllers. While in forward to reverse motoring operation the overshoot in the stator current response is reduced to 30% compared to other controllers. Thus, the experimental studies confirm effectiveness of the proposed PDFF controller based speed regulator for DTC-PMSM drive.