摘要
随着伺服系统应用范围的不断扩大,很多应用场合对现代伺服系统的低速甚至超低速性能要求也越来越高。比如高精度数控机床工作台伺服进给系统、雷达卫星天线自动跟踪系统、工业机器人、光学加工等领域要求伺服系统在低速运行时有良好的跟随特性和抗干扰性能,即电机在给定转速变化、负载变化的条件下能够低速平稳运行。伺服电机系统的性能会受到各种量化误差、采样误差,以及电机转矩脉动等因素的影响。特别是在低速运行时,速度反馈的精度和动态性能都受到极大限制,因此电机往往处于无规律地转和停状态,速度很难控制,进而影响整个系统的低速性能指标。低速运行时,速度的平稳性主要由执行电机的特性和控制器性能决定。因此,对低速电机伺服系统的研究必须从电机本体、控制策略、驱动方式、编码器采样方式等个各方面综合分析和考虑,以提高系统的稳态性能、动态响应特性和抗扰性,满足各种应用场合的要求。
本文首先就低速电机伺服系统的应用背景和相关技术领域进行了阐述,概述了对国内外相关领域的研究现状,分析了本课题的选题背景和意义。
其次,建立了基于DSP的低速电机伺服系统实验平台。平台选取无槽无刷交流永磁电机作为执行电机,以消除齿槽转矩对电机低速运行时的影响,并尽量减少电磁谐波转矩。文中对无槽无刷电机从结构和电磁场性能的角度对其特性做了详细的分析,建立了基于转子磁场定向的数学模型。系统采用定子电压空间矢量电流预测控制方法,以进一步减少电流波形畸变,降低电机转矩脉动。
然后,在实验平台基础上,采用基于位置信号的含滤波器的一阶扰动转矩观测器,构建了低速电机伺服系统。观测器使用一阶伪微分结构,在准确估计扰动转矩的同时,尽量减少微分运算对测量和计算误差的放大。算法结合永磁同步电动机具有的无槽式结构(无齿槽转矩)、气隙大(直轴电流对气隙磁场影响小)、绕组电感及电气时间常数小(动态特性良好)等特点,抑制电机运行时转矩扰动,提高伺服电机转速的抗扰性。系统在采用普通分辨率(1000ppr)的光电编码器,实现了1r/min左右超低速范围内的稳定运行,并具有良好的动态特性。仿真和实验表明,系统能够快速抑制速度误差和扰动转矩对电机低速运行的影响,且容易实现,有较强的实用性。
接着,本文提出了一种基于二阶滑模微分运算的速度计算和补偿算法,算法通过对采样到的位置信号的二次滑模运算,得到加速度信息,然后根据加速度信息计算出速度值,使速度反馈值能够快速跟踪负载的变化。通过采用此算法,使永磁同步伺服电动机可以在低速区域的动态性能获得明显的提高。实验结果表明该方法具有快速的动态性能和对负载扰动有较强的鲁棒性。
提高编码器分辨率是能够提高低速电机伺服系统性能最直接和有效的手段和方法,本文最后介绍一种正弦波编码器信号细分转换方法,使伺服系统能够将采样到的正、余弦波信号值转换为可以用于计算电机速度、位置的高分辨率脉冲信号。方法包括区域分段法和线性函数近似计算法,方法在不提高位置传感器成本条件下,提高了低速电机伺服速度和位置信息检测精度,且具有占用的计算资源少,硬件成本低,精度高,抗干扰性好的特点。
Nowadays, low speed and sometimes even ultralow speed electric machine drive servo systems with exquisite steady and dynamic performance have been highly demanded for wide range of particular applications, such as modern high precision computer numeric control (CNC) machine tools, automatic tracking systems of radar and satellite antenna, industrial robotic systems, and optical processing equipments. The electric machine in those servo systems which require good tracking and anti-disturbance performance in low speed operation should be able to operate smoothly at certain speed or load changes. Normally, the performances of electric machine drive servo systems are inevitably deteriorated by various quantization, sampling, and measurement errors as well as machine torque ripple. Furthermore, the precision and dynamic response of the speed feedback in such servo system might be greatly tampered in low speed operation so that high-performance speed control of the machine could be rather difficult to accomplish. Consequently, the low speed performance of such servo system could be greatly compromised as the machine will usually spin and stop randomly. Overall, the smoothness of low speed electric machine drive servo system is determined by two factors:torque characteristics of the machine and performance of the controller. Investigations on electric machine itself, driving scheme, control strategy, and sampling method of position sensor are usually indispensible in order to improve both the steady and dynamic performances of the low speed electric machine drive servo system.
Firstly, the paper has covered background and corresponding technical fields of the low-speed electric machine drive servo system. An overview of the state of the art in this research area has been also presented and meanwhile both the context and significance of such topic have been declared.
Secondly, the experimental platform of low speed electric machine drive servo system has been built and introduced. A slotless permanent magnet synchronous machine is employed in this system in order to eliminate the cogging torque and minimize electromagnetic torque ripple. Comprehensive analysis on the structure and electromagnetic field has been carried out to unveil the unique features of the slotless machine so that mathematic model of the machine can be established based on rotor magnetic field. Stator voltage vector control with predictive current method has been employed to alleviate current distortion and hence torque ripple.
Thirdly, a first-order disturbance observer based on filtered position signal has been presented and implemented in the proposed experimental platform. A first-order low pass filter, which filters the output value of the disturbance observer instead of stator winding current, is employed to achieve pseudo-differential operation so that the computational speed error caused by position quantization error of the optical encoder can be reduced over the low speed operation region. Besides cogging torque free, the slot less permanent magnet synchronous machine has a relatively larger air gap and hence small inductance and electrical time constant, resulting in excellent dynamic performance. Consequently, the low speed slotless permanent magnet synchronous machine drive servo system with proposed disturbance torque observer can effectively suppress torque disturbances and improve the drive performance. A stable low speed region (around lr/min) has been achieved in the proposed servo system equipped with a moderate resolution (1000ppr) encoder. Both the simulation and experimental results have revealed that the proposed disturbance torque observer can be easily implemented in practical applications to suppress the corresponding effects of the disturbance torque and computational speed error.
Fourthly, a speed calculation method based on second-order sliding mode differentiator has been proposed. The method imposes twice sliding mode operations on the position sampling signal to retrieve the rotor acceleration information. Thus, the rotor speed can be accordingly obtained accurately with on compromise on the effectiveness and timeliness in low speed region. The dynamic performance of the servo system in low speed region (20r/min-40r/min) can be effectively improved by the proposed second-order sliding mode differentiator together with the current compensation algorithm of disturbance torque. The experimental results have shown that the proposed method cannot only improve the dynamic response and robustness of the servo system against the load disturbance.
The most direct and effective approach to improve low speed performance of a servo system is to increase the resolution of the encoder. Last, a novel interpolation method for sinusoidal quadrature encoder, which involves approximate-linear function and interval division algorithm, has been introduced. The sine and cosine wave signals received by the servo system can be converted to high-resolution pulse sequence by this method, so that the speed of the machine can be accordingly calculated. The method can be easily implemented in the system to achieve high precision and anti-interference in speed calculation with little extra computational resource but no additional hardware expense.
引文
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