# New Hydraulic High-Pressure Impulse Generator for Long-Duration Impulse Tests
## 1 Introduction  
The failure of hydraulic components can have serious consequences, especially when advanced hydraulic systems applied in the aeronautics and astronautics areas involve a complex system structure for meeting the strict standards of compactness [1]. Hydraulic pressure impulses are known as a critical cause of damage to hydraulic components [2]. These impulses are generated by an instantaneous compression of the oil, for instance, at the moment of pumping oil through a volumetric pump [3; 4; 5], opening/closing valve orhing, or changing the flow direction of oil [6]. Since the hydraulic pressure impulse is obviously undesirable but inevitable in any hydraulic component, it is reasonable, particularly in industries, to perform impulse tests on randomly selected samples to guarantee the reliability of an entire batch of products. The American Society of Automotive Engineers (SAE) and ISO consider the water hammer wave or the hydraulic shock as one main pressure impulse wave standard for the impulse testing of hydraulic hoses, tubing, and fitting assemblies [7; 8; 9]. The standard pressure/time curves of the wave are shown in Figure 1. Since the produced trace of the dynamic pressure impulse should be mandatorily confined within the shaded area shown in Figure 1[8], the development of the pressure impulse generator is increasingly important for achieving an acceptable hydraulic pressure impulse output. Further, the build-up standard of the pressure impulse generator proposed by the SAE [10] is widely used by researchers. Generally, pressure impulse generators can be divided into two types based on their controllability and adjustability characteristics. The first type focuses on realizing the produced pressure impulse as close as possible to the hydraulic impulse occurring under actual working conditions. A proportional valve or a servo valve driven by an electromechanical actuator can help establish a close-loop feedback system to obtain the required output pressure impulse signals. [PERSON] et al. set up hydraulic-impulse-testing equipment with an oil pump controlled by a solenoid servovalve and modeled the transfer function of the whole system [11]. [PERSON] et al. designed a hydraulic-impulse-testing system with a second-stage servo valve and used the pressure difference feedback signal to achieve accurate output control [12]. In another study, [PERSON] et al. used accumulators for energy regeneration to reduce the energy consumption of the testing system for long-term usage [13]. Recently, [PERSON] et al. used a flow control valve to establish a pressure pulse generator [14]; by employing a fuzzy logic controller, they simultaneously obtained the required pressure square wave and a continuously square flow rate. Adopting a different approach, other research groups focused on providing acceptable pressure impulse waves with different frequencies for long-duration industrial tests such as reliability and performance tests. Unlike the approaches listed in the previous paragraph, these groups attempted to extend the lifetime of the pressure impulse generator and reduce the manufacturing cost. For instance, it is hard to guarantee that the sevro/proportional valve would have a longer life time than the tested hydraulic component when a close-loop feedback system is applied for generating a million pressure impulse waves in a reliability test; further, there exists the concern of the potential high expenditure of the valve. Therefore, a simple and open-loop system is desirable. To simplify the structure, [PERSON] used a profiled cam to drive a single-rod hydraulic cylinder to generate the pressure impulse wave; the rotational speed of the cam indicates the frequency of the output signal [15]. Drop-weight method too has been commonly applied to develop impulse pressure generators [16; 17]; however, this method can hardly provide even a rough water hammer wave.