Research On Highperformance Digital Control Strategies For Permanent Magnet Synchronous Machine Drives

Permanent magnet synchronous machine(PMSM)is a promising technology for industrial drives because of its high efficiency,high torque and power density,and fast dynamic response.In modern motor drives,the control algorithms are executed in discrete time steps.However,traditional control strategies designed in the continuous-time domain do not fully consider the discrete-time characteristics of digitallycontrolled motor drives,such as computational delay and pulse-width modulation(PWM),which can limit the operating speed range.In addition,discretization of the continuous-time controller for digital operation is not an equivalent transformation,which may lead to suboptimal control performance.To fully exploit the performance potential of PMSMs under complex operating conditions,control strategies directly designed in the discrete-time domain can avoid these problems,especially for motor drives with wide speed range,high response bandwidth,low switching frequency,or high frequencies of current harmonics and torque fluctuations.However,challenges remain,including accurate discrete-time modeling of motor drives,dynamic current control with high stability margins,compensation for current harmonics and torque pulsation caused by inverter nonlinearity and non-sinusoidal electromotive force(EMF),and position sensorless control under low carrier ratio.Therefore,further research on digital control technology for PMSM drives and breakthrough of the above key issues are crucial to improve control quality under complex operating conditions and expand the speed range.The fruits of this thesis are as follows:1.The design of digital control strategies for motor drives relies on accurate discrete-time modeling.Different combinations of current sampling and PWM updating will result in various computational delays.By modeling the PWM as a zero-order voltage latch,the discrete-time models of motor drives with computational delay being one,a half,and zero sampling period are established separately.For salient PMSMs with unequal d-and q-axis inductances,the matrix parameters of the discretized model contain many complicated calculations,thus a simplified model is derived to ease the online calculation under variablespeed operation.2.Based on the accurately discretized models,and using pole-zero cancellation and active damping technique,the discrete-time current PI regulators are designed for the motor drives with computational delay of one,a half,and zero sampling periods,respectively.The command tracking,disturbance rejection and relative stability of the current control loop are analyzed,demonstrating the suitability of the derived current regulators for wide-speed-range and low-carrier-ratio operation.To avoid the issues raised by the coupling plant zeros in the single-sampling-single-updating scheme with the computational delay of a half sampling period,a single-sampling-double-updating scheme is proposed to achieve decoupled dq-axes current control while avoiding the introduction of high-frequency oscillation modes.To achieve apparent zero-sample computational delay in real motor drives,a fast-current-control scheme is proposed to reduce delay time between current sampling and PWM updating.This scheme decomposes the current control algorithm into the primary calculation stage and the post calculation stage.The primary calculation stage only performs the steps directly related to calculating the PWM duty cycle and then the PWM command is updated immediately,which minimizes the PWM update latency and thus reduces the waste of bus voltage utilization or interference from switching ripple current sampling,making it feasible to implement in low-cost digital chips.A feedforward compensator is designed for the above discrete-time current regulators to achieve deadbeat control performance without affecting the closed-loop stability.When the current bandwidth is high,the expected output voltage of the current regulator may surpass the capability of dc-bus voltage dynamically.Consequently,the current regulator loses the consistence between its input and output,leading to a degraded current response with overshoot and prolonged settling time.To circumvent this issue,the anti-windup strategies of discrete-time current regulator are studied,and the current controller state variable correction method based on the reachable reference concept is proposed to enhance the robustness of the transient process motor operation.3.To suppress the harmonic current currents caused by harmonic voltage disturbances,such as inverter nonlinearity and nonsinusoidal back-EMF,a proportional-resonant(PR)controller is designed in the zdomain,which is a discrete-time extension of the vector proportional-integral regulator.It contains a controller zero to cancel the plant pole,which shapes the loop response in the low frequency range and achieves decoupled dq-axes current control.A proportional-integral-resonant(PIR)controller is obtained by combining the PR controllers and PI controller,enabling the regulation of fundamental and harmonic components simultaneously without steady-state error at desired harmonic frequencies.However,incorporating resonant control affects the original dynamic performance of PI current control,and several methods are employed to address this issue.A partial-prefilter is designed to improve the command-tracking performance by filtering the current reference value of resonant controller,which avoids exciting the resonant controller when the current reference value is changed.The active damping technique is utilized to enhance the immunity performance of PIR controller in the low frequency ranges.Additionally,the closed-loop poles with PIR controller are optimized to balance the frequency response near the harmonic frequencies.To further improve dynamic response when regulating both fundamental and harmonic currents simultaneously,a multifrequency disturbance observer(MFDOB)is designed in a two-degree-of-freedom framework with disturbance estimation and compensation.The MFDOB can be applied to motor drives with one-sample and zero-sample computational delay,and the latter can realize a wider frequency range of low sensitivity,significantly enhancing current control performance under complex harmonic disturbances and parameter errors.Since the mechanism of current distortion caused by current sampling error differs from that of harmonic voltage disturbance,a modified compensation method for current measurement offset error is developed.By filtering the errors of current prediction and current feedback in the αβ stationary reference frame,the current measurement offset error can be estimated and compensated with little influence on the transient and steady-state current control performance.4.After analyzing the sources of harmonic torque disturbance,the extended state observer(ESO)-based speed control strategy is developed to reduce torque fluctuation and improve the speed control performance.A transfer-matrix-based framework is established to analyze the performance of ESO-based speed control system,which can be applied to different disturbance assumptions of ESO.The study provides an in-depth analysis of the speed control performance and relative stability of ESO under constant disturbance assumptions.To mitigate torque ripples across a wide speed range,an extended harmonic state observer(EHSO)is proposed,which utilizes motor speed and current information to estimate harmonic disturbance online and determine the current for compensating torque ripples.The observer gain of EHSO is optimized to reduce the sensitivity peak of the closed-loop system.The design concept of EHSO is extended to the discrete-time domain and the discrete-time EHSO(DEHSO)is obtained,which avoid the problem of EHSO that requires extra discretization.In the framework of DEHSO,the influence of current control on the effectiveness of speed control and torque ripple compensation is analyzed,and a novel feedforward strategy based on the estimated disturbances is proposed,which can effectively suppress the harmonic torque considering the dynamics of the current loop.Experimental results verify the effectiveness of the proposed method,which successfully reduces torque across a wide speed range without affecting the original speed control performance.5.For middle-and high-speed operation of PMSM drives,the sensorless control scheme acquires the rotor position based on the back-EMF observation.In high-power or high-speed PMSM applications,to reduce the switching losses and increase the drive output,the carrier ratio(ratio of PWM carrier frequency to fundamental current frequency)could be lower than ten.In such scenarios,the conventional back-EMF observing scheme in the αβ stationary reference frame is incapable and will lead to large steady-state and transient position errors.To address this issue,a sensorless control scheme is designed in the estimated γδreference frame which employs a discrete-time ESO for back-EMF observation.With the previous studied discrete-time model,the position estimation error can be analyzed analytically.In addition,an ESO-based phase-lock loop(PLL)is developed for higher position estimation accuracy under rapid acceleration.Considering the average error between the sampled currents and the actual currents under low carrier ratio,a model-based error compensation method is proposed,and the effectiveness of the proposed scheme is verified by simulation and experimental results on two high-speed permanent magnet synchronous motors.