Abstract
In this paper, an improved finite control set model predictive current control (FCS-MPCC) is proposed for a two-phase hybrid stepper motor fed by a three-phase voltage source inverter (VSI). The conventional FCS-MPCC selects an optimal voltage vector (VV) from six active and one null VVs by evaluating a simple cost function and then applies the optimal VV directly to the VSI. Though the implementation is simple, it features a large current ripple and total harmonic distortion (THD). The proposed improved FCS-MPCC builds an extended control set consisting of 37 VVs to replace the original control set with only seven VVs. The increase in the amount of VVs helps to regulate the current more accurately. In each control period, the improved FCS-MPCC takes advantage of deadbeat control to calculate a reference VV, and only the three VVs adjacent to the reference VV are predicted and evaluated, which decrease the computational workload significantly. Build waveform patterns for all VVs in the unbalanced circuit structure to modulate the optimal VV using discrete space vector modulation, which improves the current quality in reducing current ripple and THD. The comparative simulations and experimental results validate the effectiveness of the proposed method.
Highlights
Hybrid stepper motors (HSM) are widely used in industrial and consumer applications such as textile machines, robotic arms, scanners, printers, etc., due to their low manufacturing cost, high reliability, and open-loop control capabilities [1]
Three important conclusions have been obtained: (1) the total harmonic distortion (THD) of the improved FCS-MPCC is smaller than the THD of the conventional FCS-MPCC in the whole speed ranges, and huge differences exist at low-speed ranges; (2) compared with the traditional dual H-bridges proportional integral (PI) method, the THD of the improved FCS-MPCC is slightly higher, which is mainly caused by the unbalanced circuit structure; (3) the THD of the improved FCS-MPCC fluctuates at 10.0 when the rotor speed is higher than 420 rpm, so its THD is approximately independent of the speed
The minimum ripple can be observed in the improved FCS-MPCC, which contributes to improving current quality and decreasing THD
Summary
Hybrid stepper motors (HSM) are widely used in industrial and consumer applications such as textile machines, robotic arms, scanners, printers, etc., due to their low manufacturing cost, high reliability, and open-loop control capabilities [1]. A cascade structure is employed, i.e., the outer loop generates reference electromagnetic torque firstly; it uses maximum torque per ampere, direct torque control, and field-oriented control (FOC), etc., to modulate a reference current in real-time according to rotor position and speed; the inner current controller regulates stator winding current according to the reference current. [22] presents a synchronous frame current control strategy for a two-phase linear stepper motor fed by a three-phase VSI Though they improve the space-vector pulse width modulation (SVPWM) to match the unbalanced circuit structure, the maximum winding voltage is about 70% of the power supply, which is low energy efficiency. An improved finite control set model predictive current control (FCS-MPCC) is proposed for a two-phase HSM fed by a threephase VSI.
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