Present Status and Prospect of High-Frequency Electro-hydraulic Vibration Control Technology
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Electro-hydraulic vibration equipment (EHVE) is widely used in vibration environment simulation tests, such as vehicles, weapons, ships, aerospace, nuclear industries and seismic waves replication, etc., due to its large output power, displacement and thrust, as well as good workload adaptation and multi-controllable parameters. Based on the domestic and overseas development of high-frequency EHVE, dividing them into servo-valve controlled vibration equipment and rotary-valve controlled vibration equipment. The research status and progress of high-frequency electro-hydraulic vibration control technology (EHVCT) are discussed, from the perspective of vibration waveform control and vibration controller. The problems of current electro-hydraulic vibration system bandwidth and waveform distortion control, stability control, offset control and complex vibration waveform generation in high-frequency vibration conditions are pointed out. Combining the existing rotary-valve controlled high-frequency electro-hydraulic vibration method, a new twin-valve independently controlled high-frequency electro-hydraulic vibration method is proposed to break through the limitations of current electro-hydraulic vibration technology in terms of system frequency bandwidth and waveform distortion. The new method can realize independent adjustment and control of vibration waveform frequency, amplitude and offset under high-frequency vibration conditions, and provide a new idea for accurate simulation of high-frequency vibration waveform.
KeywordsElectro-hydraulic vibration equipment High-frequency Vibration control Vibration waveform Twin-valve
Vibration equipment mainly refers to a kind of equipment to generate the corresponding vibration by mechanical, electro-dynamic, electrostrictive or magnetostrictive, electro-hydraulic and other driving principles with the form of shaking table and vibration exciter. Among them, electro-hydraulic vibration equipment (EHVE) is widely used in vibration environment simulation tests, such as vehicles, weapons, ships, aerospace, nuclear industries and seismic waves replication, etc. [1, 2, 3, 4], has large output power, displacement and thrust, adaptive workload and multi-controllable parameters. With the further study of high-frequency EHVE, the vibration frequency has been improved greatly in recent years. But due to the limitations of system bandwidth and influence of nonlinearity, the traditional electro-hydraulic vibration control technology (EHVCT) based on servo control has a deviation between the response signal and expected input signal, the current vibration control requirements of high-accuracy, low-distortion and high-stability are difficult to meet. Therefore, developing a high-frequency EHVCT which can accurately control the EHVE and make the vibration waveforms to meet the above-mentioned vibration control requirements is the key to improve the existing vibration environment simulation testing technology.
This paper describes the domestic and overseas development of high-frequency EHVE, discusses the research status of high-frequency EHVCT, points out the existing problems of current high-frequency EHVCT, and discusses the solutions to some problems.
2 Development of High-Frequency Electro-hydraulic Vibration Equipment
The main excitation forms of vibration equipment at present are mechanical excitation, electro-dynamic excitation and electro-hydraulic excitation. Compared with the other two forms, electro-hydraulic excitation has been widely used due to its unique advantages. With the development of related technology and improvement of testing requirement, promoting the EHVE to develop from medium-frequency and low-frequency to high-frequency and from low-precision to high-precision vibration control. Then according to the types of vibration control element, the EHVE is mainly divided into servo-valve controlled vibration equipment and rotary-valve controlled vibration equipment.
2.1 Servo-valve Controlled Vibration Equipment
Overseas research on high-frequency servo-valve controlled vibration equipment is more early, various vibration environment simulation testing devices composed of electro-hydraulic shaking table (EHST) have been developed and the most products have been series manufactured, formed a certain market scale and industry. The EHST is mainly composed of electro-hydraulic actuators controlled by electro-hydraulic servo-valve. The working principle is that processing the desired signal of the user by a certain algorithm to generate the electronic control driving signal firstly, and then converting it into current signal by an amplifier to excite the electro-hydraulic servo-valve secondly. Finally, the hydraulic oil proportional to the driving signal is input to the hydraulic cylinder to drive the piston rod to generate the desired vibration simulation environment. However, due to the limitations of frequency response ability of servo-valve, the working frequency of EHST is usually lower and suitable only for the medium-frequency and low-frequency vibration environment simulation. In order to improve the high-frequency performance of EHST, one of the ways is to introduce a servo-valve with high-frequency-response. At present, the overseas advanced high-frequency-response servo-valve manufacturers are mainly represented by MTS and MOOG in American, while the EHST manufacturers are mainly represented by TEAM in American, SERVOTEST in Britain, and IMV in Japan.
The performance parameters of different series of MOOG servo-valve
Frequency response (Hz)
Step response (ms)
Flow range (L/min)
Maximum working pressure (MPa)
In addition, there are many companies have also successfully developed various high-frequency electro-hydraulic vibration equipment, such as WYLE and IST company in American, REXROTH and SCHENCH company in Germany, INSTRON company in British, AMSLER company in Switzerland, Mitsubishi and Saginomiya company in Japan. However, due to the impact of embargo, high-price and power restrictions, etc., these equipments have rarely been popularized and applied in China.
By contrast, the research on high-frequency EHVE in China started relatively late, especially in the research and manufacture of large-scale vibration simulation testing equipments. Compared with the overseas advanced level, there is a big gap in both technical and scale in China. The achievements [20, 21, 22, 23] obtained in recent years based on technology introduction and independent research cannot be ignored, but the mature and reliable products are relatively less, and mainly distributed in universities and research institutes.
2.2 Rotary-valve Controlled Vibration Equipment
2.3 Other High-Frequency Vibration Equipment
3 Progress of High-Frequency Electro-hydraulic Vibration Control Technology
3.1 Progress of Vibration Waveform Control
Vibration waveform control research has important practical significance for the development of high-frequency EHVE and EHVCT. Therefore, domestic and overseas scholars have conducted a lot of research on the vibration waveform replication accuracy and distortion of EHVE.
The three dominate parameters of the vibration waveform control of EHVE are displacement, velocity and acceleration, and change with the change of the frequency. In the early displacement control strategy, the low-frequency displacement signal is stronger and the feedback is enough, but the high-frequency signal is weaker, which is equivalent to the open-loop control. The three-parameter control is that the displacement, velocity and acceleration are all involved in the feedback control, which can provide feedback in the whole frequency bandwidth. As the high-frequency phase lag becomes larger and larger, the high-frequency acceleration cannot achieve as good a follow-up effect as the low-frequency displacement feedback. But generally, it is much better than the displacement control. Besides, an important difference between the three-parameter control and traditional displacement control is that the input is acceleration waveform. And a flat acceleration transfer function and a good control effect of acceleration waveform can be obtained while the parameters are appropriate.
In order to accurately simulate the reference acceleration signal, the three-parameter control technology [39, 40] is used to correct the dynamic characteristics of servo control system for improving the frequency response performance of servo system. But still cannot accurately reproduce the expected acceleration signal under high-frequency excitation conditions. The waveform replication accuracy can be further improved by using inverse transfer function equalization technology. Furthermore, the expected signal can be accurately reproduced in a wider frequency bandwidth by using feed forward compensation technology based on the inverse transfer function, which can improve the dynamic response performance of testing system, rather than changing the closed-loop feedback characteristic. So that, as long as the transfer function of the acceleration closed-loop control system is accurately identified and its inverse transfer function is designed, the dynamic characteristics of acceleration can be effectively compensated while the desired acceleration signal can be accurately reproduced through the acceleration closed-loop control system .
However, the EHST servo system has non-linear time-varying characteristic in testing and the acceleration closed-loop system may be affected by non-linear factors. On the one hand, leading to a model deviation between the designed inverse transfer function model and the actual system model, on the other hand, leading to a certain amplitude attenuation and phase delay of acceleration waveform, that is, waveform distortion, which will reduce the waveform control performance and even increase the instability of system. Therefore, it is necessary to consider the non-linearity and interference factors of electro-hydraulic servo system while establishing the mathematical model of EHST servo system. Such as Righettini et al.  established a non-linear empirical model of single-axis EHST. Kim et al.  proposed an output feedback non-linear control method for position tracking of electro-hydraulic vibration system. Wang et al.  considered the factors of uncertainties, non-linearity and time-varying, established a model of parallel driven EHST servo system. Nakata  established a dynamic model of electro-hydraulic single-axis driven seismic simulator. Klimchik et al.  considered the influence of internal force and external interference force of parallel mechanism, established a stiffness model of parallel mechanism. Wei et al.  established a hydraulic system non-linear model of super-redundant EHST and a dynamic coupling model based on space forces. Shen et al.  established a hydraulic system non-linear model of parallel driven 6-DOF EHST. Yao et al.  developed a heuristic algorithm called water cycle algorithm (WCA), which can be used to identify the amplitude and phase of harmonic signals. Bruyne et al.  proposed a multi-axis EHST decoupling vibration control system, which can improve the performance of reference tracking, harmonic distortion and anti-crosstalk. Rana  introduced an intelligent time domain digital acceleration amplitude controller based on fuzzy logic for the sinusoidal vibration testing in the field of aerospace and automobile, which has a good reference tracking performance. Stehman et al.  proposed a new EHST control strategy, which is characterized by using acceleration feedback directly without displacement feedback. Besides, the force feedback is added to ensure the stability of EHST and reduce the drift of the table. As a result, the acceleration tracking performance and waveform accuracy are improved. Besides, Shen et al.  introduced other control algorithms of EHST servo system in the review for improving the replication accuracy of acceleration waveforms.
In addition to the above algorithms, the self-adaption control algorithm and off-line iterative control algorithm are the two most widely used control strategies in the field of EHST control, but both of them have their own advantages and disadvantages. Among them, once the self-adaption algorithm converges to an optimal solution, it can reproduce the reference time-domain waveform accurately. However, many self-adaption algorithms have the problem of slow convergence speed, which makes the self-adaption algorithm unable to be applied well to the EHST control. Meanwhile, the off-line iterative control is the most practical method to realize the waveform reproducing and the most commonly used method in industrial applications. But it may cause the divergence of the algorithm or more iterations when the dynamic characteristics of the sample are changed. Therefore, some scholars put forward various hybrid control strategies to overcome the problem of slow convergence speed of self-adaption control and off-line iterative control . Such as Zhang et al.  proposed a hybrid control strategy to improve the synchronization and tracking control accuracy of double EHST system. Compared with the traditional control strategy, the tracking error can be reduced to 25% and the synchronization error to 16%. Tang et al.  proposed a combined control strategy combining off-line iterative learning control and improved internal model control to improve the time domain waveform replication accuracy of EHST. Yan et al.  aimed at the problem of poor control effect of electro-hydraulic vibration system under high-frequency, proposed a hybrid control strategy combining displacement control and acceleration control. The displacement control is mainly used in low-frequency and acceleration control is mainly used in high-frequency. As a result, not only guarantees the control stability, but also improves the high-frequency performance of electro-hydraulic vibration system and reduces the distortion of acceleration waveform.
The above researches indicate that the current research on vibration waveform of EHVE is based on the traditional servo-valve to improve the vibration waveform control accuracy and reduce the waveform distortion by designing and optimizing the complex control algorithms. But due to the characteristics of servo-valve itself, the vibration frequency cannot make a breakthrough when the vibration waveform has higher requirements. Meanwhile, considering the influence of system nonlinearity, existing a deviation between the electro-hydraulic vibration response signal and input desired signal. Therefore, to reduce the vibration waveform distortion and improve the vibration waveform control accuracy by above-mentioned methods have certain limitations. While the rotary-valve controlled EHVE changes the flow area of the valve port by rotating the spool at a certain speed to generate the vibration waveforms with adjustable frequency and amplitude. Compared with the vibration waveforms of servo-valve controlled EHVE, the waveform distortion of rotary-valve controlled EHVE is relatively small, and it is easy to realize high-frequency excitation, so it has a broader application prospect [56, 57]. So far, many scholars have done a lot of research on the vibration waveforms of rotary-valve controlled EHVE. Such as Ruan et al.  designed a 2D-valve controlled EHVE and studied the vibration waveforms under different working frequencies. Li et al. [59, 60] made the theoretical analysis and experimental research on resonance waveform and vibration waveform of 2D-valve controlled EHVE in the whole frequency bandwidth. Compared with the working frequency of traditional EHVE, the new high-frequency EHVE can be used for vibration tests from low-frequency to high-frequency, and the upper limit working frequency can reach 1000 Hz. Furthermore, Ren et al.  studied the theoretical and experimental vibration waveforms under typical input waveforms, obtained the analytical solutions of vibration waveforms, and derived the mathematical expression of harmonics. Meanwhile, in order to realize the accurate control of the offset of hydraulic cylinder piston vibration center position, a new scheme that parallel mechanism consisted of 2D-valve and electro-hydraulic digital valve is proposed by Ren et al. , and the vibration fatigue tests are carried out in the frequency range of 5‒200 Hz. The results show that the parallel mechanism can not only improve the frequency response, but also realize the separate control of vibration frequency and amplitude of EHVE. Moreover, it can realize independent and accurate control of the offset of hydraulic cylinder piston vibration center position. Shao et al.  applied the DSP technology to the construction of control system of high-frequency EHVE to realize 2-DOF control of 2D-valve. The acquisition signal is processed by digital filtering, which can improve the control accuracy of EHVE. And the tracking control under non-linear distortion conditions can realize by the control algorithm while the motor out-of-step and speed mutation can be eliminated.
Besides, Han et al. [34, 64, 65] studied the influence of working frequency on saturation degree of vibration waveform and the influence of the number of spool grooves on fluctuation degree of vibration waveform. At present, the vibration waveforms of rotary-valve controlled EHVE are usually studied based on rectangular valve port, few research work have been done on the influence of different valve port shapes on vibration waveforms. Therefore, according to the previous design, Wang et al.  designed a kind of rotary-valve controlled EHVE, and respectively established the flow area models under triangular, semi-circular and rectangular valve port. Wang et al.  qualitatively and quantitatively studied the waveform quality of theoretical and experimental vibration waveforms under different oil supply pressures and working frequencies. The results show that the waveform quality is mainly affected by the third harmonic resonance. The waveform distortion of output sinusoidal wave is less than 5% when the vibration frequency is higher than 70 Hz, and in this frequency range, the influence of oil supply pressure on waveform distortion is extremely small. Therefore, in this vibration frequency, the waveform amplitude can be adjusted by changing the oil supply pressure with the waveform quality is almost unaffected. Liu et al.  studied the influence of different valve port structure parameters on the regular wave and obtained the static and dynamic characteristics under different valve port shapes with single-stage rotary-valve controlled EHVE.
3.2 Progress of Electro-hydraulic Vibration Controller
In addition to the large enterprises and institutions above-mentioned, domestic and overseas scholars have done a lot of research on vibration controller as well. Such as overseas scholar Plummer  proposed a new model-based motion control method for multi-axis EHST. Compared with the traditional proportional controller, it has a better performance. For example, the horizontal acceleration frequency bandwidth is nearly six times higher than that of the traditional proportional controller and the hydraulic resonance frequency is also many times higher. Thenozhi et al.  effectively combined the offset elimination and high-pass filtering technology to solve the common problems in the numerical integration of acceleration signals. While the integration accuracy is improved compared with other numerical integrators. Nakata [44, 76] proposed a control method called acceleration trajectory tracking control (ATTC) and a multi-purpose single-axis seismic simulation controller, which can improve the acceleration control performance of seismic simulation controller and increase the flexibility.
Domestic scholar Shen et al.  proposed a method of combining self-adaption controller and off-line compensator to improve the acceleration frequency bandwidth and tracking accuracy of 6-DOF EHST. Compared with the traditional three-variable controller and self-adaption controller, the acceleration tracking control performance, including the convergence speed and acceleration tracking accuracy of LMS algorithm, is improved. Yan et al.  designed a multi-dimensional waveform reproducing controller by studying the multi-dimensional waveform reproducing control algorithm, and realized the six-dimensional waveform reproducing control simulation with a three-axis 6-DOF EHST prototype. The controller has been successfully applied to a low-frequency horizontal seismic simulation EHST and a high-frequency seismic simulation electro-hydraulic vibration system, and their effect of waveform reproducing is satisfactory. Xu  analyzed and studied the random vibration power spectrum reproducing control algorithm in the vibration testing control system, and discussed the method of designing a vibration controller with high-performance DSP chip. The experiment showed that the algorithm is better than the traditional algorithm in the practical application of engineering and the designed vibration controller is actually available. In order to overcome the shortcomings of the analog servo controller, Shu et al.  designed an electro-hydraulic digital servo controller based on FPGA and classical PID control algorithm. The designed controller has good static and dynamic response performance and high performance-price ratio compared with the analog controller. To realize high-frequency and high-magnitude earthquake simulation vibration tests on the centrifuge, Deng et al.  designed a set of digital control system for seismic waveform replication, and reproduced the time-domain seismic waveform on the centrifugal EHST which can meet the testing requirements. While the effect and accuracy of vibration control are satisfied with the technical requirements for system development.
Moreover, Feng et al.  proposed a control strategy based on predictive function control algorithm, and designed a predictive function controller for electro-hydraulic vibration control system. The proposed strategy for the controller has strong robustness and anti-interference ability, which can effectively improve the accuracy of dynamic tracking of an electro-hydraulic vibration control system. Luan et al.  systematically studied the control strategy of 3-stage servo-valve controlled EHST, designed an integrated controller for high-flow EHST, which can simultaneously realize the servo control and vibration control of EHST. Through the integrated controller, the waveform replication tests of 3-stage servo-valve controlled EHST under different time-history and frequency bandwidth are carried out. The results show that the controller for EHST has a good waveform control ability and can realize high-accuracy waveform replication of EHST. Ma et al.  analyzed the three-axis vibration testing system, studied the multi-input and multi-output power spectrum reproducing control algorithm, and carried out the vibration tests with the three-axis EHST and multi-input and multi-output vibration controller integrated with the algorithm. The results show that the HV frequency response function estimation modified iterative control algorithm has good power spectrum reproducing accuracy and engineering practicability.
4 Problems and Prospects
The vibration frequency of rotary-valve controlled electro-hydraulic vibration equipment is easier to improve than that of the servo-valve controlled electro-hydraulic vibration equipment. But due to the structural symmetry of rotary-valve, cannot introduce an offset signal to realize the offset control of vibration center position just like the servo-valve controlled electro-hydraulic vibration equipment. In the absence of offset control, on the one hand, it cannot satisfy some offset vibration conditions. On the other hand, the vibration center position cannot be corrected in time when away from the balance position of the piston.
From the above published, many research achievements have been obtained on output vibration waveform distortion and control accuracy of electro-hydraulic vibration equipment. However, the output vibration waveform distortion control and stability control still have a lot of room for improvement under high-frequency excitation conditions.
The inlet and outlet valve port throttle area of vibration control valves (including servo-valve and rotary-valve) of high-frequency electro-hydraulic vibration equipment is always adjusted by a spool, which cannot realize the independent control of the load port. Therefore, the change of valve port flow area is limited to a certain extent, and it is not easy to control the vibration waveforms flexibly under complex working conditions.
Some domestic manufacturers have developed a series of vibration controllers with good performance, such as VT series and VESSTA series, but there still have gaps in multi-channel vibration control compared with overseas, and most of them are designed for electro-dynamic shaking table. The high-end vibration controllers especially used in electro-hydraulic shaking tables are not perfect. Few mature and reliable products have been put into practical application, still in theoretical research.
Based on the new method and structure above-mentioned, considering the requirements of accurate simulation of high-frequency vibration waveform in large-scale vibration environment simulation tests. Studying the mapping relationships between the high-frequency vibration waveform and the shape and structure parameters of valve port when the two spools synchronous rotating in opposite directions. Revealing the influence of fluid physical properties, rotational speed fluctuation, structural combination form and valve port size error on output vibration waveform. Analyzing the inherent amplitude and phase frequency characteristics of electro-hydraulic vibration system with different structure and control parameters. Revealing the dynamic response law of electro-hydraulic vibration system under high-frequency vibration conditions. Proposing a new method of distributed control for high-frequency electro-hydraulic vibration system. Expecting to break through the limitations of current electro-hydraulic vibration technology in bandwidth and waveform distortion, and provide technical support for the development of high-frequency and high-power electro-hydraulic vibration system.
The high-frequency electro-hydraulic vibration control technology is widely used in large-scale projects, on the one hand, it can carry out high cycle fatigue tests in much less time to shorten the development cycle of products, on the other hand, it determines the level of product performance and structure optimization. Meanwhile, it is not only an important indicator of the level of national industrial technology development, but also indispensable basic research. With the development of national high-end equipment manufacturing industry, the higher requirements on replication accuracy, distortion and stability of vibration waveform under high-frequency vibration conditions are put forward. In order to reduce the import of overseas high-end electro-hydraulic vibration equipment, it is urgent to carry out research on domestic high-frequency electro-hydraulic vibration equipment and corresponding high-frequency electro-hydraulic vibration control technology to improve the control accuracy and stability of electro-hydraulic vibration equipment under high-frequency vibration conditions. In a word, it is of great significance to improve the overall technical level of large-scale vibration environment simulation tests of China and international competitiveness of national equipment manufacturing industry and products.
GG determined the subject discussed and wrote an outline; YL and TW collected and wrote the manuscript; RG assisted with revision and translation. All authors read and approved the final manuscript.
Yi Liu, born in 1985, is currently an associate professor at Ningbo Institute of Technology, Zhejiang University, China. He received his PhD degree from Zhejiang University, China, in 2013. His research interests include electro-hydraulic vibration control and wave simulation control.
Tao Wang, born in 1994, is currently a master candidate at Anhui University of Science & Technology and Ningbo Institute of Technology, Zhejiang University, China. His research interests include electro-hydraulic vibration control.
Guofang Gong, born in 1963, is currently a professor and a PhD candidate supervisor at State Key Laboratory of Fluid Power Transmission and Control, Zhejiang University, China. His research interests include hydraulic transmission and control.
Rujun Gao, born in 1994, is currently a master candidate at Zhejiang University, China.
The authors declare that they have no competing interests.
Supported by National Natural Science Foundation of China. (Grant Nos. 51605431, 51675472)
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