Pid Tuning for Multi-channel Structural Fatigue Testing System

A proportion-integration-differentiation (PID) controller is a generic control loop feedback mechanism widely used in multi-channel aircraft structural fatigue test. The PID parameter is obtained by test setup method in which PID is tuned after the test setup is actually performed, so that the tuning period becomes overlong. And the dynamic characteristic of mechanical and hydraulic equipment isn’t considered as a crucial factor to decrease working capability of loading equipment. at the same time, there is coupling effect among channels, which lower control accuracy. For this, based on multi-channel fatigue test of cantilever beam, the coupling mathematics model of testing system is presented and linearized reasonably. The basic procedure of a multi-channel fatigue test is analyzed systematically. The PID parameter of every channel is simulated by stability margin and bandwidth analysis. The dynamic characteristic of hydraulic equipment and test article are analyzed. Finally, a double channel fatigue test of cantilever beam is implemented. The results of simulation and test show that the new method is effective. This method not only has the ability to predict PID parameter rapidly and the dynamic characteristic of test equipment, but it can also reduce effectively the control errors. 1 Introduce Full-scale structural testing of aircraft structures has been and still is the task of the aircraft industry when new aircrafts are developed. In recent years, as China has been developing the aviation industry and the research has been entering into a new stage, the requirements of aircraft structure fatigue test become increasing. Aircraft structure fatigue test employ multi-channel coordinated loading control system to drive the hydraulic system to apply load spectrum to aircraft structure. Fatigue test system adopts PID control. Namely, the testing system calculates the control quantity by PID algorithm based on system error. Currently, the tuning to PID of test system is completed using experience after test setup installation, so that the PID prediction is difficult to implement to increase tuning time to impact test cycle. And dynamic characteristics of the mechanical and hydraulic equipment have not considered fully, and the capacity of loading equipment is not enough. Worse, the coupling phenomenon among the actuator cylinders has been occurring. Due to the above problems, aircraft structural test cannot be run smoothly, and the test results cycle are affected seriously. For this, in 2010, Eenkhoorn N.C. of Delft University of Technology in the Netherlands has carried out the PID simulation in single channel structural fatigue[1]. Despite controller, hydraulic system and test article are coupled, Dr E has not yet researched multi-channel test system, and the methods have not been applied HONGWEI ZHAO, SHIHUI DUAN & JIANMIN FENG 2 in engineering. In 2012, Xiasheng Sun of aircraft strength research institute of China have studied the PID simulation of control channel for MTS control system[2]. This method is just focus on the controller, not the hydraulic system and test article. In 2015, Shihui Duan of aircraft strength research institute of China has simulated PID parameter of the cantilever beam structure test system[3]. The control system, hydraulic system and mechanical system are coupled to tune PID of the testing system. This method is applied to structural tests. Unfortunately, the method is based on single channel test system. The above researches are almost related to single channel fatigue test system, so the multi-channel test system research is increasingly important. Therefore, in 2010, professor Huaihai Chen of nanjing university of aeronautics and astronautics has studied variable parameter PID control method for multiple input multiple output random vibration test[4]. The basic procedure of multiple input multiple output random vibration test is analyzed systematically in the article. A definition of control error under logarithmic coordinates is provided, a new PID control strategy is introduced into the control algorithm. But the method does not take into account the hydraulic system and test article effecting to PID and control error. In 2013, professor Jianyu Zhao of jinan university has studied multi-channel coordinated loading control method[5]. This approach is based on neural network algorithm. But it is not involving the hydraulic system and test article of testing system. The other researching refer to the hydraulic system[6~7] or control system[8~10] or developing to control algorithm[11~14] or the coupling simulation between two systems[15~17] or the coupling simulation among the hydraulic systems, control system and test article[18~19]. Currently, The research about single-channel fatigue test is more than multi-channel test system’s. So the thesis considers multi-channel cantilever beam structure fatigue test system as the research object. The mathematical model of subsystem is presented. The coupling characteristic of test system is analyzed. The PID parameter of every channel is simulated to compensate coupling effect among channels. Dynamic characteristic of hydraulic system is described to improve capacity to testing system.