摘要
近年来,随着电液比例控制技术的发展,凭借着成本低、抗污染能力强的优点,电液比例阀在许多场合逐渐取代了电液伺服阀。电液比例方向阀能同时实现流量和方向控制,并且可以方便地实现计算机控制。因此采用计算机控制的电液比例阀控缸位置伺服控制系统具有结构简单、可靠性好等优点,在各个领域尤其是工业领域得到了广泛的应用。
根据本课题中数控液压弯管机的工作需要,选用缸筒固定的单出杆液压缸作为系统的执行元件,由于采用左右两边节流面积相同阀芯的直动式比例方向阀来控制非对称单出杆液压缸,加大了对该系统进行位置伺服控制的难度。本文的工作就是针对该系统的特点,设计高性能的控制器对其进行任意点定位和连续轨迹跟踪控制的研究。主要内容如下:
首先,分析了系统的结构和工作原理,在此基础上搭建了系统硬件实验平台,采用Labview和Matlab混合编程环境实现系统的实时控制功能。针对电液比例位置控制系统的特点,建立了该系统各个环节的数学模型并进行了一些合理简化,该模型表明系统具有高阶非线性。通过大量实验测定了比例阀的实际中位电压和系统的动作死区电压;并对系统的输入输出特性有了较深刻的理解,为系统控制器的设计提供了依据。
其次,为系统设计了PID控制器,分别对阶跃信号、方波信号和正弦信号的系统响应进行了实验研究,发现PID控制器在该非线性系统中的不足并对其进行了改进。在阶跃和方波跟踪响应实验中,控制器在单向点位控制时误差较小,但是在跟踪连续曲线正弦信号时,由于采用对称阀控制非对称液压缸,再加上电液比例换向阀死区的存在,造成在正弦信号拐点附近误差较大,虽然采用分段PID控制器效果有所改善,但控制系统的响应仍未能达到较好的效果。
为解决实验中的上述问题,本文为系统设计了普通模糊控制器和模糊自适应整定PID控制并进行大量的重复实验。结果表明,模糊控制器虽然可以实现快速的方波响应,但难以减小消除稳态误差,这是由模糊控制器本质是PD控制的特点所决定的。故提出了模糊自适应整定PID控制,提高了正弦响应的跟踪精度,并在一定频率范围内都得到了较高精度的正弦跟踪响应,增强了系统的适应能力。因此,本文提出的控制方法是稳定和有效的。
最后,对本论文的研究工作进行了总结,对存在的一些缺点和不足之处提出了进一步研究工作的设想和展望。
In recent years, with the development of the electro-hydraulic proportional control technique, the electro-hydraulic proportional valve gradually replaced the servo valve because of its low cost, good anti-contamination capacity. Due to its simple structure , excellent reliability, the electro-hydraulic proportional valve controlled hydraulic cylinder of the position servo controlled system has abroad application in different fields, especially the industry.
According to the requirements of the numerical control hydraulic bending machine, a single-rod hydraulic cylinder, whose cylinder barrel is fixed, is selected as the actuator, but it's more difficult to carry out servo control because the asymmetric single-rod hydraulic cylinder is controlled by the symmetry valve port direct-acting electro-hydraulic proportional direction-valve. According to these characters, high quality controllers are designed to focus on both point-to-point and continuous tracking control. The main contents are shown as follows:
Firstly, the hardware experiment platform is established on the basis of analysis about the system structure and working principle. The real-time controller is carried out by the software Labview and Matlab. According to the electro-hydraulic proportional control characters, the mathematic models on the different parts of system is established by detailed analysis and then simplified, the model shows that the system is high order and nonlinear. The voltage of the proportional valve middle position and the system acting dead band is also determined during the experiments. The I/O characters are well understood, which is the basis of the controller design.
Secondly, a PID controller is designed for the system, during the experiments of responding the step signal, square wave signal and sine signal, it is found that there are some disadvantages of the PID controller in this nonlinear system. In the step signal, square wave trajectory tracking experiment, the error is very small on the point-to-point control in single direction. But for the continuous tracking control experiment, the error is a little big, especially in the direction reversing process. The PID controller with subsection is developed and some effects are obtained . But generally, the error is still big and the response is unstable.
In order to solve the above problems, a general fuzzy controller and a self-adaptive fuzzy-PID controller are developed and a mass of experiments are carried out. The result shows that the general fuzzy controller isn't able to eliminate the stable error, although which has a fast square wave response. That is decided by the essence of the fuzzy controller. So the self-adaptive fuzzy-PID controller is adopted, the result shows that the precision of sine wave response in a certain frequency bound is improved and the adaptive ability is enhanced. So the fuzzy controller is stable and effective.
Finally, for the shortages and disadvantages still existing in this study, some suggestions and expectations are put forward for further research.
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