Review of Key Technologies for Aviation Intelligent Pumps

Abstract

The airborne intelligent hydraulic system is an effective way to solve the ineffective power consumption and temperature rise of an aircraft hydraulic system. An intelligent pump that can work in a variety of modes according to the change of flight conditions is an inevitable requirement for the realization of airborne intelligent hydraulic system, and it is also the development trend of aviation pumps in the future. In this paper, key technologies for aviation intelligent pumps are reviewed. This paper briefly describes its development process and summarizes the research on aviation intelligent pumps from the aspects of the system scheme, working mode, structure form, and control strategy. Finally, the conclusions and trends of the research status of intelligent pumps are given, which can provide a reference for subsequent research on further improving the performance of aviation intelligent pumps.

1. Introduction

The development of airborne hydraulic energy systems in the direction of high pressure and high power [1,2] has caused problems such as the increase of ineffective power and the sharp increase of system temperature [3]. At present, the constant-pressure variable displacement pump widely used in airborne hydraulic systems is designed with the highest working pressure during the whole flight process as the system-rated pressure, but the maximum working pressure condition accounts for only 10% of the flight process, which means that a large amount of power is wasted in other flight stages [4,5]. With the development trend of high pressure and high power airborne hydraulic systems, the invalid power is mainly generated in the form of heat, which will lead to a sharp increase in the temperature of the system, reduce the reliability, affect the endurance time of the aircraft, and endanger flight safety.

An airborne intelligent hydraulic system that can match the pump source to the load is an effective way to solve this problem [6]. As the core component of the system, the intelligent pump can provide flow and pressure according to the load requirements of the system under the current working conditions, realize the matching of load and pump output flow, reduce ineffective power consumption and heat generation, and greatly improve the efficiency of the hydraulic energy system [5]. Therefore, an intelligent pump that can work in multiple modes according to the changes in flight conditions is the development trend of aviation pumps in the future [7].

Since the 1980s of the 20th century, scholars carried out research on airborne intelligent pump systems. The main structure of Abex’s airborne intelligent pump, the information interaction scheme with the flight control system, and the dynamic response performance of the pump were comprehensively introduced in [4], and the flight test was carried out on the F-15 Iron Bird Stage. During the same period, the United Kingdom’s intelligent pump DPRV3-115-EAT was used in the system verification test frame, and the relevant aircraft design bureaus of Russia also conducted specific research on the airborne intelligent pump system. However, there are few public reports from the United States, Britain, and Russia on the research of airborne intelligent pumps [8], and the specific progress is not yet known.

Chinese scholars have been late in the research on airborne intelligent pump systems, and the early related research results mainly appeared at Beihang University. Wang et al. [8,9,10,11,12,13,14,15,16,17] began to study the airborne intelligent pump system in the second half of the 1990s of the last century, including the structural scheme, working mode, control strategy, and test system of the intelligent pump system. After entering the 21st century, Song et al. [18,19,20,21] carried out research on the control strategy of intelligent pumps for missiles. Liu et al. [22] carried out research on the load-sensing control method of airborne intelligent pumps, and Li et al. [23] conducted a study of thermal characteristics analysis of various pump forms. Pei et al. [24,25,26,27,28] carried out research on the control strategy of the intelligent pump source system. Wang et al. [29,30,31,32] studied the working mode selection and switching of the intelligent pump system, the pulsation characteristics of the variable-pressure pump and its influence on the pipeline system, and the performance reliability. Qi et al. [33,34] carried out a simulation study on the principle scheme and control method of the electric variable displacement intelligent pump. Li et al. [35,36,37,38,39,40] studied the modeling and control method of the aviation electrohydraulic proportional variable-pressure pump.

This review focuses on peer-reviewed journals, conference papers, research reports, and dissertations between 1986 and 2024 and covers research on airborne intelligent pumps. However, there are fewer research studies on control strategies for airborne intelligent pumps. Therefore, this review extends to the scope of variable displacement piston pumps. The research was performed by querying the Web of Science, Scopus, and CNKI databases with the string TITLE-ABS-KEY (intelligent*pump* OR aviation*pump*). The last query was run on 30 September 2024. Two exclusion criteria were imposed on the results of the search: (i) reports must have been published within 2024, and (ii) the language of the full text must be English or Chinese. The final dataset contained 61 documents, classified as follows: 33 articles, 15 conference papers, 10 dissertations, 2 book chapters, and 1 review. Based on the above process, it can be concluded that this review is in great compliance with PRISMA guidelines.

The structure of this paper is as follows: firstly, the current research on the scheme and working mode of intelligent pump systems is introduced. Secondly, the structure scheme of intelligent pumps proposed by current scholars is summarized. Subsequently, the current research on the control strategy of intelligent pumps is summarized from the two aspects of pressure control and flow control. Finally, the conclusion part summarizes the current research status of intelligent pumps.

2. Research on the System Scheme and Working Mode of the Intelligent Pump

As the core component of the airborne intelligent hydraulic system, the intelligent pump needs to establish an efficient and reasonable communication scheme with the flight control actuation system to achieve the objective of load sensitivity, obtain the load information of the actuation system and the output demand of the pump in time, and simultaneously be able to quickly and effectively switch the mode according to the needs of the intelligent hydraulic system. Therefore, the research on the system scheme and working mode of the intelligent pump is very important to ensure the smooth realization of its load-sensitive function. Scholars have already carried out relevant studies.

The information interaction scheme between the intelligent pump and the flight control system was proposed in [4], as shown in Figure 1. In addition to the pump itself, the intelligent pump system also includes a controller, numerous sensors, and other electronic devices connected to the flight control computer. The controller of the pump receives the pressure demand signal from the flight control processor, and then adjusts the intelligent pump and feeds back the received pump status information to the flight control computer, and the flight control processor interacts with the processor of other intelligent pumps through the system bus and other pump status data. This method can isolate a pump in time when it fails, and at the same time, other pump systems can be reconstructed to meet the performance requirements of the whole machine for the hydraulic system, avoid major accidents, and improve the reliability of the system.

Li et al. [32] summarized the information that needs to be interacted between the flight control computer, the intelligent pump, and the actuator based on the functional analysis of the airborne intelligent pump system. The flight control computer generates the work cycle task profile of the intelligent pump based on the flight status and the load of the rudder. In each work cycle, the flight control computer transmits the command signals from the actuator to the actuator and the controller of the corresponding intelligent pump. After entering the working cycle, the controller of the intelligent pump determines the working mode according to the command signal, combines the flight state parameters and other information, calculates the corresponding reference signal, and converts it into the control signal of the servo valve. Subsequently, the servo valve drives the swashplate for real-time adjustment of the inclination angle, and the intelligent pump outputs the pressure and flow required by the actuating system. After the completion of a work cycle, the next work cycle begins immediately. By completing the above process, the intelligent pump system information interaction can be obtained, as shown in Figure 2.

In terms of command signal acquisition of intelligent pumps, two methods were proposed in [4] to enable the variable-pressure pump to obtain the pressure required by the current system load. One is to calculate the maximum actuating pressure required by sensing the current flight parameters of the aircraft, such as flight altitude, Mach number, etc., and then adjust the pressure supply of the pump. Figure 3 shows a hypothetical relationship between aircraft pressure, flight altitude, and Mach number. Another method is to sense the demand pressure of each actuator in the flight control system and transmit it to the controller of the pump, and then control the pump to generate a pressure slightly higher than the total demand value, as shown in Figure 4 in [41].

In terms of the research on the working mode of the intelligent pump, Li et al. [17] designed the working mode and input settings of the intelligent pump for the flight mission profile of military aircraft, as shown in

Table 1. Intelligent pump operating mode and input settings.

Mission Number	Mission Mode	Percentage of Time/%	Operating Mode	Set Values
1	Take-off	1.9	Constant flow mode	Large
2	Climb and cruise	29.6	Load-sensitive or constant-pressure mode	The differential pressure is set to medium or medium constant pressure
3	Hovering and descending	22.2	Load-sensitive or constant-pressure mode	The differential pressure is set to medium or medium constant pressure
4	Swoop down	2.4	Constant-pressure mode	Large
5	Fight	3.2	Constant-pressure mode	Large
6	Cruise and landing	29.6	Load-sensitive or constant-pressure mode	The differential pressure is set to medium or medium constant pressure
7	Landing	11.1	Constant flow mode	Large

. The management of the operating modes and the setting of the inputs is performed by the intelligent management computer of the onboard common equipment. It forms a stepwise control with the microcontroller of the intelligent pump via the 1553B bus.

Huang et al. [29] proposed the basis and method of mode switching according to the characteristics of the four working modes of the intelligent pump system, as shown in

Table 2. Four working modes of intelligent variable-pressure pump system.

Working Modes	Switching Conditions	Control Method	Applicable Phases
Flow mode	The load pressure is small, and the flow rate is large, or constant speed control is required.	Flow signals	Take-off, landing
Pressure mode	Medium load pressure and flow or failure modes.	Pressure signals	Climbing, descending
Power mode	The power is overrun, or constant power control is required.	Power signals	Assault, fighting
Load-sensitive mode	The load pressure and flow are not large, which does not affect the normal operation.	Pressure drop signals	Cruise, search

. Mathematical models are established for several working modes, and mode-switching modules and control signal sequences are added. Using simulation, the response of each output quantity of the system in the process of working mode switching and in-mode control signal change was analyzed. The response results of the intelligent pump system obtained from the simulation are shown in Figure 5. This result shows that the output flow rate and pressure curves of the pump system can change smoothly in both the flow mode and power mode, and the steady-state ripple is small. This is due to the fact that the use of the output flow rate as a feedback signal in these two modes effectively suppresses the flow fluctuations. The pressure output is generated by the volume effect of the flow through the pump outlet chamber, and the quality of the flow output also affects the quality of the pressure output. The product of these two signals gets the output power, so the characteristics of the two curves can be reflected in the output power curve. After switching to the load-sensitive mode, the system pressure drop is basically kept at the set value of 0.2 MPa because the smaller the value of the pressure drop, the closer the output pressure is to the load pressure and the higher the efficiency of the system. Therefore, it can be seen that the energy-saving effect in the load-sensitive mode is obvious.

On this basis, Li et al. [32] proposed the working mode-switching criterion of the intelligent pump system, as shown in

Table 3. Working mode-switching strategy for intelligent pumps.

Flight Altitude/m	Speed/Ma	Climbing Rate/(m/s)	Descent Rate/(m/s)	Working Mode
<1500	<0.35	N/A	N/A	Flow mode
>1500	<0.9	>5 and <25	>3 and <18	Pressure mode
>1500	<0.9	<5	<3	Load-sensitive mode
N/A	N/A	>25	>18	Power mode

. According to the aircraft work cycle task profile, the switching strategy that is dominated by the aircraft flight speed and altitude and supplemented by the work cycle task pressure and flow demand is considered. In this case, when the aircraft is flying at an altitude of less than 1500 m and at a speed of less than 0.35 Ma, the aircraft is in the take-off or landing phase, in which the rate of climb and rate of descent are not involved. When the climb rate and descent rate are both at very high values, the aircraft is in the extensive maneuvering phase, and the flight altitude and speed are not involved in this phase. Therefore, parameters that are not involved in some phases are indicated in the table by N/A.

In addition to the literature, some industry reports and patents describe the applications of intelligent pumps in aviation. Technical report [4] describes flight tests of Abex’s onboard intelligent pump on an F-15 Iron Bird bench. Fighter pilots involved in the test found the flight experience of the variable-pressure control airplane to be virtually indistinguishable from that of a conventional airplane and that the Abex intelligent pump was consistent with the pumps used in the F-15 in terms of its performance in meeting the needs of the flight control actuators. Laboratory test results for Abex intelligent pumps include step response to pressure and flow, frequency domain response, and pressure pulsation testing. The results of the laboratory and Iron Bird bench tests demonstrate the practical significance of the potential of intelligent pumps to improve the operating efficiency of hydraulic systems and reduce cooling requirements.

The technical report [41] presents a comparative study of the Abex intelligent variable-pressure pump and constant-pressure pump systems, tested separately on the F-15 flight control system Iron Bird bench. The actuator duty cycle was derived from the motion of the flight control rudder surfaces obtained from a combination of USAF and McDonnell Aeronautical Corporation databases, covering the full flight profile of the F-15 for take-off, climb, cruise, combat, and landing. Tests showed that the outlet and drain temperatures of the variable-pressure pump during the cruise phase were 18 to 24 °C lower compared to those of the constant-pressure pump at 56 MPa system pressure, as shown in Figure 6. In the low maneuvering phase, the leakage and heat loss of the intelligent pump with variable pressure is reduced by 50% compared to the constant-pressure pump, as shown in Figure 7. In a two-hour work cycle, the energy consumption of the intelligent variable-pressure pump is reduced by 39% compared to the constant-pressure pump, as shown in Figure 8. A series of experimental results show that the intelligent variable-pressure pump source technology is feasible for reducing heat loss, and it is a very effective method in mainstream energy-saving technology.

Technical report [42] describes a two-stage variable-pressure pump that provides two independent controlled output pressure settings in one pump. Pressure levels are normally selected by external electrical, mechanical, or hydraulic signals set by an onboard computer. The F/A-18E/F pump utilizes this concept and can be selected for either 3000 or 5000 psi operating pressure. Lower pressures are used for most flight conditions, while higher pressures are used for extreme maneuvers requiring larger loads. Dual pressure control extends system life because the high-pressure mode is used for only a small portion of the operating time. During low-pressure operation, the pump draws less power from the engine, while pump and system losses are reduced, and less heat is generated in the system.

In addition, some patents describe the application of intelligent pumps in the aviation field. Ouyang et al. [43] developed an intelligent aviation variable displacement piston pump pressure-flow adaptive control system and proposed an adaptive control selector that can automatically select the pressure closed-loop or the flow closed-loop under different operating conditions, which reduces the complexity of the control and improves the robustness of the system. Xu et al. [44] developed an electronically controlled proportional stepless pressure-regulated variable displacement piston pump to achieve stepless continuous adjustment of piston pump outlet pressure and achieve a better energy-saving effect and control performance. Jiao et al. [45] developed a stepless variable-pressure aero pump based on a proportional direct-drive split spool pressure valve. The pressure valve is directly driven by a wet proportional solenoid, which can realize precise force control, and the spool adopts a split structural design, which has a simple structure and reduces the volume and weight of the variable mechanism while realizing the stepless variable pressure.

From the above literature, it can be seen that for the research on the system scheme and working mode of the airborne intelligent pump, scholars have constructed the information interaction and command signal acquisition scheme between the intelligent pump system and the flight control system, etc., and established a relatively complete working mode-switching criterion. Relevant industry reports and patents demonstrate the initial applications of intelligent pumps in the aviation field.

3. Research on the Structure Scheme of Intelligent Pumps

From the beginning of the research on intelligent pumps, the structural scheme of intelligent pumps has been the focus of scholars. The structural scheme not only fundamentally affects the output dynamic characteristics but also has a great impact on the weight and reliability of the system. It is the purpose of structural research to improve the power-to-weight ratio and reliability as much as possible on the basis of ensuring high performance.

Joseph A. et al. [4] introduced two types of intelligent pumps: servo valve indirect drive and servo valve direct drive. Among them, the indirect drive type controls the compensation valve by the servo valve, then controls the variable displacement cylinder, and drives the swashplate inclination to change the displacement. Figure 9 shows two specific forms of this variable displacement principle, where the casing displacement in Figure 9a is controlled by the nozzle valve and mechanical feedback, and the casing displacement in Figure 9b is controlled by a direct-acting servo valve and electrical feedback. In this configuration, the output pressure of the pump is proportional to the position of the valve sleeve, i.e., to the size of the input current. For most failures caused by mechanical, hydraulic, and electronic, the pump can operate at a preset maximum or minimum pressure. Therefore, it is in a mode of automatic troubleshooting, with high reliability, but the structure is more complex, and the dynamic response is slow.

The servo valve direct-drive structure eliminates the need for a compensating valve, and the servo valve directly controls the piston to change the swashplate inclination, as shown in Figure 10. This form has a simple structure and great dynamic performance. However, when the servo valve has no current input, the position of the swashplate is uncontrollable and does not have the function of automatic troubleshooting. Therefore, it is necessary to increase the mechanical structure to prevent the occurrence of failures.

The overall scheme of the airborne intelligent variable-pressure pump system proposed by Chen [9] is shown in Figure 11. On the basis of the traditional airborne constant-pressure variable displacement pump, a servo valve connected in parallel with the pressure compensation valve is set up. The oil source for the servo valve is provided by an auxiliary pump, which, together with the variable displacement cylinder, forms a position servo system for the control of the swashplate inclination. The position control signal depends on the working mode and output requirements of the pump system, the flight control system sends instructions, and the intelligent pump controller collects signals such as pump outlet pressure, output flow, and load pressure and determines the working mode and generates control signals based on flight status parameters and flight mission requirements. In order to realize the timely isolation and reconstruction of the servo valve part in the case of failure, a normally closed solenoid switch valve is set on the pressure compensation valve circuit. When a fault occurs, the servo valve is kept in the neutral position, the switch valve is opened, and the pump source is converted into a constant-pressure variable displacement pump.

Ma [46] combined the advantages of the two schemes of servo valve indirect-drive and direct-drive intelligent pump introduced in [4] and adopted the form of servo valve direct-drive variable displacement mechanism, as shown in Figure 12. This form has the advantages of a simple structure and good dynamic performance, and at the same time, retains the return spring, which can make the intelligent pump work at the maximum displacement in the event of failure, ensuring the reliability of the pump.

Gao et al. [47,48,49] proposed an electric variable displacement mechanism for the servo pump used in the electro-hydrostatic actuator (EHA), considering that EHA does not have a constant system pressure and is not convenient to use the form of hydraulic servo variable displacement, so the variable displacement mechanism of the servo pump driven by a DC servo motor through the transmission device is used, as shown in Figure 13. The high-performance coreless DC servo motor selected in this scheme is decelerated by the reducer and then directly drives the swing shaft of the swashplate through a sector gear to realize the purpose of variable displacement. The integrated swashplate angle adjustment mechanism design in this scheme gives it a good degree of integration, and the fixed load pressure is omitted compared with the electrohydraulic servo form.

Liu [34] selected the servomotor and reducer schemes after a comparative analysis of five electric servo variable displacement schemes for intelligent pumps with different structural principles. The scheme is characterized by arranging the servo motor driving swashplate on the outside of the pump housing; that is, an electric servo variable displacement mechanism is connected at the outer interface end of the swashplate drive shaft, including a servo motor, a planetary reducer, a flange, and related accessories. After the servo motor is rigidly connected with the reducer, it is fixed with the pump body through the flange, and the output shaft is connected with the interference at the end of the drive shaft of the swashplate so that the swashplate can be driven to rotate through the control of the servo motor, and then the servo control of the output pressure and flow rate is realized. A schematic diagram of its structure is shown in Figure 14. The scheme directly drives the swashplate to rotate through the external electric variable displacement mechanism, and the overall structure is relatively simple. The processing and installation are convenient, and it is easy to realize.

Based on the above analysis for intelligent pump structural solutions, the classification and characteristics of multiple intelligent pump structural solutions can be summarized, as shown in Figure 15.

From the existing related research, it can be seen that in terms of structural form, the current research has proposed a variety of intelligent pumps with different characteristics. Intelligent pumps can generally meet the adjustment needs of pump outlet flow through speed adjustment and displacement adjustment. The speed adjustment scheme using servo motor speed regulation has the advantages of simple structure, energy-saving, and high efficiency [50,51,52], but it is not suitable for airborne engine driving pumps. In the displacement adjustment mode, the electric variable displacement intelligent pump [53,54] adds a servo motor and a transmission device on the basis of the original structure, which also increases the weight and reduces the power-to-weight ratio, and the existence of clearance in the transmission device makes it difficult to realize the precise position control of the swashplate. The electrohydraulic variable displacement intelligent pump [55,56,57] using the auxiliary pump source as the servo valve oil source can provide a stable oil source pressure for the swashplate adjustment mechanism, but the additional pump source will increase the volume and weight of the system and reduce the reliability of the aviation pump. In contrast, the self-supplied intelligent pump that directly draws oil from the outlet of the pump as the servo valve oil source can avoid the above shortcomings and is easy to modify based on the widely used constant-pressure variable displacement pump while still maintaining a high power-to-weight ratio and reliability. Therefore, the self-supplied intelligent pump is the future development trend of aviation intelligent pump sources.

4. Research on Intelligent Pump Control Strategies

After the system scheme and structure of the intelligent pump are determined, the control method becomes an important factor to determine its output performance. The object to be controlled in the output characteristic control of an intelligent pump is a strongly nonlinear and time-varying parameter system. If the linearized swashplate moment model and linear control theory are adopted, the performance of the controller designed based on the nominal model will be degraded in actual use and even become unstable at different operating points. In addition, in the operation process of the intelligent pump, the parameters such as the comprehensive volume elastic modulus and leakage coefficient are not fixed values, and their parameter values change within a certain range and are difficult to measure online, which will affect the effect of the controller and make the dynamic performance of the controlled object unable to meet expectations.

In addition to disturbances within the intelligent pump system, load changes can also cause disturbances to the control of output characteristics. When the flow rate increases dramatically, such as when the load needs to be actuated or run at high speed, the pump output pressure decreases during the transient process, resulting in pressure collapse. Conversely, when the load reduces the flow demand, the pump output pressure will increase rapidly, resulting in a brief oscillation. Therefore, it is of great significance to carry out research on advanced control algorithms with high precision and fast response for intelligent pumps to achieve their expected output performance.

4.1. Mathematical Modeling of Intelligent Pumps

Before carrying out the research on the control method of the intelligent pump, it is necessary to deeply analyze the working principle of the system and establish a well-developed mathematical model. Here, we take the current highly competitive self-supplied aviation intelligent pump as an example on the basis of fully analyzing its principle and structure and giving a mathematical model that accurately describes its dynamic characteristics. This is especially important for analyzing its output characteristics.

The schematic diagram of the self-supplied intelligent pump is shown in Figure 16. The system consists of three main parts: the piston pump body, the variable displacement mechanism, and the electrohydraulic servo valve. Among them, the main body of the piston pump is responsible for generating and outputting the flow and pressure of the hydraulic oil; the variable displacement mechanism changes the displacement by adjusting the inclination angle of the swash plate, thus realizing the control of the output characteristics; and the electrohydraulic servo valve undertakes the task of precise motion control of the variable displacement mechanism to ensure that the intelligent pump can accurately regulate its output flow and pressure.

The pump’s control system receives commands from an upper-level computer and controls the fluid entering the control piston chamber by precisely adjusting a servo valve. This process effectively overcomes the offset spring force and the torque generated by the plunger, which causes the swashplate angle to change, thus realizing the dynamic adjustment of output pressure and flow rate to ensure a fast and accurate response to the command requirements. It is worth noting that the servo valve’s operating medium comes directly from the pump’s own outlet, a design that not only demonstrates self-supplied characteristics but also eliminates the need for additional auxiliary pumps, helping to maintain the system’s high power-to-weight ratio and reliability.

The flow-pressure equation for an electrohydraulic servo valve is as follows:

$${\mathrm{Q}}_{\mathrm{c}}={\mathrm{k}}_{\mathrm{c}}\mathrm{u}\left[\mathrm{S}\left(\mathrm{u}\right)\sqrt{{\mathrm{P}}_{\mathrm{p}}-{\mathrm{P}}_{\mathrm{L}}}+\mathrm{S}\left(-\mathrm{u}\right)\sqrt{{\mathrm{P}}_{\mathrm{L}}-{\mathrm{P}}_0}\right]$$

(1)

where Q_c is the flow in and out of the control piston chamber through the servo valve, called the servo valve control flow; m³/s; $k_c=k_q{k}_u$ is the total flow gain of the servo valve; $k_q=C_{d}w\sqrt{2/\rho }$, C_d is the valve’s flow coefficient; w is the valve’s opening area gradient, m; $\rho$ is the density of hydraulic oil, kg/m³; P_p is the pump outlet oil pressure, Pa; P_L is the control piston chamber’s oil pressure, Pa; P₀ is the oil tank’s pressure, Pa; u is the input voltage, V; and S(u) is an approximate sign function defined as $S(u)=\left\{\begin{array}{c}1 u>0\\ 0 u\le 0\end{array}\right.$.

The flow-pressure equations for the variable displacement mechanism and swashplate assembly is as follows:

$${\dot{\mathrm{P}}}_{\mathrm{L}}={\displaystyle \frac{{\mathsf{\beta }}_{\mathrm{e}}}{\mathrm{V}}}{\mathrm{Q}}_{\mathrm{c}}+{\displaystyle \frac{{\mathrm{A}}_{\mathrm{p}}\mathrm{L}{\mathsf{\beta }}_{\mathrm{e}}}{\mathrm{V}}}\dot{\mathrm{\gamma }}-{\displaystyle \frac{{\mathrm{C}}_{\mathrm{L}}{\mathsf{\beta }}_{\mathrm{e}}}{\mathrm{V}}}{\mathrm{P}}_{\mathrm{L}}$$

(2)

where A_p is the effective area of the control piston, m²; $A_p=\pi d_p^2/4$, d_p is the diameter of the control piston, m; L is the vertical distance from the control piston and offset spring force to the swash plate rotary axis, m; γ is the swash plate inclination, rad; $V$ is the effective volume of the control piston chamber, m³; $V=V_0+A_p{x}_p$, V₀ is the initial volume, x_p is the displacement of the control piston, m; β_e is the volume elasticity modulus of the hydraulic oil, Pa; and C_L is the leakage coefficient of the control piston chamber, m³/(Pa·s).

The kinetic equation of the swashplate is as follows:

$$\ddot{\mathrm{\gamma }}=-{\displaystyle \frac{{\mathrm{A}}_{\mathrm{p}}}{\mathrm{mL}}}{\mathrm{P}}_{\mathrm{L}}+{\displaystyle \frac{\mathrm{K}}{\mathrm{m}}}\left({\displaystyle \frac{{\mathrm{x}}_0}{\mathrm{L}}}+{\mathrm{\gamma }}_{\max }\right)-{\displaystyle \frac{\mathrm{K}}{\mathrm{m}}}\mathrm{\gamma }+{\displaystyle \frac{\mathrm{M}}{\mathrm{m}{\mathrm{L}}^2}}-{\displaystyle \frac{{\mathrm{B}}_{\mathrm{p}}}{\mathrm{m}}}\dot{\mathrm{\gamma }}$$

(3)

where $m=m_p+I_s/L^2$ is the combined mass of the control piston and swash plate assembly, kg; m_p is the mass of the control piston, kg; I_s is the rotational inertia of the swash plate assembly around the swash plate rotary axis, kg·m²; K is the stiffness of the offset spring, N/m; B_p is the viscous damping coefficient of the control piston, N·s/m; and x₀ is the pre-compression of the offset spring, m.

The flow-pressure equation for the pump outlet fluid is as follows:

$${\dot{\mathrm{P}}}_{\mathrm{p}}={\displaystyle \frac{{\mathsf{\beta }}_{\mathrm{e}}}{{\mathrm{V}}_{\mathrm{p}}}}{\mathrm{k}}_{\mathrm{Q}}\mathrm{\gamma }-{\displaystyle \frac{{\mathsf{\beta }}_{\mathrm{e}}{\mathrm{C}}_{\mathrm{p}}}{{\mathrm{V}}_{\mathrm{p}}}}{\mathrm{P}}_{\mathrm{p}}-{\displaystyle \frac{{\mathsf{\beta }}_{\mathrm{e}}}{{\mathrm{V}}_{\mathrm{p}}}}{\mathrm{Q}}_{\mathrm{p}}$$

(4)

where Q_p is the load flow rate of the pump, m³/s; k_Q is the flow coefficient; $k_Q=n\pi d^2{D}_{p}Z/4$, m³/s; n is the spindle speed, r/min; d is the diameter of the plunger, m; D_p is the diameter of the plunger distribution circle, m; Z is the number of plungers; C_p is the leakage coefficient in the pump, m³/(Pa·s); P_p is pump outlet pressure, Pa; and V_p is the pump outlet volume, m³.

By taking state variables ${\left[\begin{array}{c}{x}_1,x_2,x_3,x_4\end{array}\right]}^{\mathrm{T}}={\left[\begin{array}{c}\gamma ,\dot{\gamma },P_L,P_p\end{array}\right]}^{\mathrm{T}}$ and associating the above equations, the state space equation of the aviation intelligent pump system can be obtained as follows:

$$\left\{\begin{array}{c}{\dot{\mathrm{x}}}_1={\mathrm{x}}_2\\ {\dot{\mathrm{x}}}_2={\mathrm{C}}_1{\mathrm{x}}_1+{\mathrm{C}}_2{\mathrm{x}}_2+{\mathrm{C}}_3{\mathrm{x}}_3+\mathsf{\phi }\left(\mathrm{x}\right)\\ {\dot{\mathrm{x}}}_3=\mathrm{g}(\mathrm{x},\mathrm{u})\mathrm{u}+{\mathrm{C}}_5{\mathrm{x}}_2+{\mathrm{C}}_4{\mathrm{x}}_3\\ {\dot{\mathrm{x}}}_4={\mathrm{C}}_6{\mathrm{x}}_1+{\mathrm{C}}_7{\mathrm{x}}_4+{\mathrm{C}}_8{\mathrm{Q}}_{\mathrm{p}}\end{array}\right.$$

(5)

where $C_1=-{\displaystyle \frac{K}{m}},C_2=-{\displaystyle \frac{B_p}{m}},C_3=-{\displaystyle \frac{A_p}{mL}},C_4=-{\displaystyle \frac{C_L{\beta }_e}{V}},C_5={\displaystyle \frac{A_{p}L{\beta }_e}{V}},C_6={\displaystyle \frac{k_Q{\beta }_e}{V_p}},C_7=-C_p{\beta }_e/V_p$, $C_8=-{\beta }_e/V_p$, $\phi (x)={\displaystyle \frac{M(x)}{m{L}^2}}+{\displaystyle \frac{K}{m}}({\displaystyle \frac{x_0}{L}}+{\gamma }_{max})$, $M(x)$ is the other torque M, subjected to the swashplate, $g(x,u)={\displaystyle \frac{{\beta }_e}{V}}{k}_c[S(u)\sqrt{P_p-P_L}+S(-u)\sqrt{P_L-P_0}]$.

Since the elastic modulus of the oil fluid and the pump leakage coefficient are affected by factors such as air content and temperature in the oil fluid, which are time-varying parameters, and combined with Equation (5), it can be seen that the self-supplied aviation intelligent pump is a fourth-order nonlinear time-varying system. A more detailed mathematical modeling process can be found in reference [58].

4.2. Pressure Control Strategies

In order to meet the requirements of fast and accurate output pressure and overshoot within a certain range in the pressure regulation mode and effectively suppress the internal parameter perturbation and external load flow disturbance, the research on the pressure mode control method of the intelligent pump system is very important to meet the aviation standards and realize the practical application.

Pressure regulation through spindle speed control is a type of implementation mode of intelligent pumps. Song et al. [18] studied the flow compensation under the pressure control of the intelligent pump with a fixed displacement of the missile-borne hydraulic servo, and the system model was approximated by the offline trained RBF neural network identifier and combined with the servo control deviation to predict the output flow rate at the next moment and compensated to eliminate the interference of the flow rate on the pressure as much as possible. In Ref. [19], the fuzzy control method is introduced for the pressure control of this type of intelligent pump. The switching function and its change law of sliding mode control are fuzzy, and the fuzzy reasoning rules are set. The simulation results show that the introduction of fuzzy theory allows the sliding mode control to maintain good robustness and effectively weaken its jitter problem. In Ref. [21], the feedback linearization process was used to transform the intelligent pump with compound speed and displacement into a controllable linearization system, and the stability analysis showed that the internal dynamic subsystem was stable after the feedback linearization treatment. Liu [28] adopted a double-loop control structure of the speed of the inner loop motor and the output pressure of the outer loop pump for the fixed displacement and variable speed intelligent pump system and designed an active disturbance rejection controller for the control of both loop controllers, as shown in Figure 17. The simulation results show that the tracking and anti-disturbance performance of the speed loop meet the requirements, and the pressure loop can also ensure the control effect when the load flow changes greatly. However, speed adjustment to change the output pressure is not suitable for applications where the engine drives the pump.

Most of the existing electrohydraulic airborne intelligent pumps use auxiliary pump sources as the servo valve oil source and do not have the characteristics of self-supply. Guo et al. [55] designed an adaptive robust control method for the time-varying and uncertain nonlinearities of the parameters in the pressure control of variable displacement pumps for construction machinery and simulated and verified that the preset transient performance and tracking accuracy can be realized. Wei et al. [59] designed an interference observer for the unknown time-varying load flow interference in the pressure control of the pump control system in construction machinery. They used the sliding mode control to compensate for the estimation error and designed the feedforward and feedback control to stabilize the system, and the experimental results proved that it has good pressure tracking performance. Helian et al. [60] designed an adaptive robust backstepping control strategy for the nonlinearity and parameter uncertainty in variable displacement axial piston pumps, and simulated and experimental results verified that it has higher pressure tracking accuracy and performance under dealing with dynamic uncertainty and time-varying disturbances. However, the research objects in these studies do not have the characteristics of self-supply, so they do not have the characteristics of variable gain.

Some scholars have carried out research on the pressure control of self-supplied variable displacement piston pumps. Kemmetmüller et al. [61] designed a pressure control strategy for a self-supplied variable axial piston pump for injection molding machines, using feedforward and feedback combined with the strategy of load estimator to verify the high dynamic tracking of the pump outlet pressure under the condition of unknown load. Guo et al. [62] designed a switching control scheme combined with output redefinition to solve the problems of unknown time-varying load flow interference, switching characteristics, and non-minimum phase system of self-supplied variable displacement axial piston pump, and the experimental results showed the effectiveness of the control strategy. Li et al. [63] designed a PID control strategy based on a neural network to adjust parameters online in order to solve the problems of rapid demand change, unknown load, and time-varying parameters in aviation variable-pressure pump, and the simulation showed that it had great pressure tracking performance. However, these studies did not consider the effect of the time-varying outlet pressure of the self-supplied pump on the forward channel variable gain of the system.

In summary, scholars have carried out research on the control strategy of output characteristics of intelligent pumps with various structural forms. In terms of pressure control of intelligent pumps, the research on the control strategy of motor speed regulating intelligent pumps and electrohydraulic intelligent pumps with auxiliary pump sources is not fully applicable to the situation of airborne hydraulic systems. The research on self-supplied servo pumps does not pay attention to the problem of forward channel variable gain in the system, which will have a great impact on the response of the system.

4.3. Flow Control Strategies

Flow control is a common working mode of an airborne intelligent pump, which is mainly used to meet the flow needs of hydraulic users such as actuation systems with changing flight conditions.

In the existing research on flow servo control of variable displacement axial piston pumps, Li et al. [64] designed an intelligent learning control method based on a fuzzy neural network in order to solve the problems of time delay, nonlinearity, and time-varying characteristics of electrohydraulic proportional variable displacement pumps, and the experimental results show that the proposed method has faster response and stronger anti-interference ability than the traditional PID control in the displacement adjustment of servo pumps. Fan et al. [65] designed a fuzzy logic controller for the flow control of electrohydraulic proportional variable displacement pumps, and the simulation results showed that the output flow rate of the pump with fuzzy logic controller had a smaller steady-state pulsation amplitude, but the dynamic response performance in flow regulation was not verified. Guo et al. [66] proposed a T-S fuzzy control algorithm for the position adjustment of the control piston of the electrohydraulic proportional variable displacement pump, and the simulation results show that the proposed method can achieve faster response and smaller overshoot than the traditional PID method. Su et al. [67] designed a double closed-loop control strategy with the angular displacement control of the swashplate as the inner loop and the flow control as the outer loop control for the flow control of the axial piston pump controlled by the high-speed solenoid on-off valve, the integral separation PID control for the inner loop and the two-stage fuzzy incremental control for the outer loop, and the effectiveness of the proposed control method was verified by simulation. Wang [8] also adopted a double-loop cascade structure for the electrohydraulic variable displacement intelligent pump, the outer loop is the output flow control, and the inner mold principle is used to control it. The experimental study shows that the internal mode control can maintain static-free tracking of the reference signal even under the condition that the leakage coefficient undergoes perturbation, and the control scheme is shown in Figure 18.

However, the subject of the above study is not a self-supplied pump with a high power-to-weight ratio, so it does not have the characteristics of variable gain. In addition, most studies directly compare the flow control of the pump outlet to the displacement control of the swashplate inclination or the variable displacement control piston, ignoring characteristics such as leakage in the pump, and it is difficult to achieve accurate flow regulation. In addition, the traditional PID or fuzzy control methods that do not depend on the accurate model of the system are mostly used in the current research, and there is still space for further improvement in the dynamic response performance and anti-interference performance of flow regulation.

4.4. Simulation and Experimental Research

4.4.1. Simulation Research

After establishing the mathematical model of the system as well as proposing the control strategies used to improve the performance, scholars have conducted various simulation studies on the dynamic characteristics and control strategies of aviation intelligent pumps.

Han et al. [58] used simulation to investigate the effects of key structural parameters and operating conditions on the output characteristics of a self-supplied aviation intelligent pump. Their developed MATLAB/Simulink simulation model is shown in Figure 19. The simulation model is mainly divided into four parts: the electrohydraulic servo valve input voltage part, the load part, the intelligent pump body part, and the intelligent pump state display part.

Based on this simulation model, the effects of key parameters such as offset spring stiffness, control piston diameter, spindle speed, and servo valve input voltage on the output characteristics of the intelligent pump were investigated. For example, the effects of different control piston diameters on the output characteristics of the intelligent pump are shown in Figure 20 and Figure 21. The results show that when the control piston diameter is too small, the variable displacement mechanism cannot push the swashplate. Increasing the diameter can improve the quickness of the variable displacement mechanism, but it will reduce the quickness if it is too large. This conclusion can guide the determination of the diameter of the control piston when designing the intelligent pump.

Since the research object of this study is a self-supplied intelligent pump. When the load is constant, and the inclination angle of the swashplate decreases, the pump displacement decreases, and thus, the oil pressure at the pump outlet decreases. When the oil pressure decreases to a certain extent, the variable mechanism can no longer push the swashplate, and the swashplate will eventually stabilize at a certain angle, which is a major feature of the self-supplied intelligent pump. Therefore, in the simulation results, the swashplate is ultimately stabilized at a certain angle, and the larger the diameter of the control piston, the larger the force of the control piston rod on the swashplate, so the swashplate is ultimately stabilized at a smaller angle.

Li et al. [63] designed a PID control strategy based on a neural network to adjust the parameters online and established a MATLAB/Simulink simulation model for validation in order to address the problems of rapid demand change, unknown load, and time-varying parameters in an aviation variable-pressure pump. The responses to sinusoidal signals and pressure signals under complex operating conditions are shown in Figure 22 and Figure 23, respectively. From Figure 22, it can be seen that compared with the traditional PID controller, the system with a BP neural network PID controller has no oscillation in the output pressure when the system is just started, and the system can follow the input signal well without hysteresis.

In order to simulate the aircraft hydraulic system where the load pressure and flow rate usually vary over a wide range, a simulation under complex operating conditions is performed. In the simulation, the load demand pressure is set to suddenly increase from 4 MPa to 16 MPa at 1s, and the load demand flow rate is suddenly doubled at 2 s. As can be seen in Figure 23, the system with a BP neural network PID controller has no oscillation in the output pressure when it is just started, the regulation time is reduced by 50% compared to the PID controller, and there is no overshooting phenomenon. When the pressure suddenly increases, the regulation time is reduced by 25% compared with PID control, about 0.15 s, and there is no overshooting phenomenon. Meanwhile, under the control of the BP neural network PID controller, the output pressure of the pump can be adjusted faster when the load flow rate increases suddenly. Therefore, the system with a BP neural network PID controller has a faster response speed, smaller overshooting, and better robustness.

4.4.2. Experimental Research

The aviation intelligent pump system contains a variety of nonlinear characteristics. While some parameters will change with the system pressure and temperature, it is difficult to fully reflect its real characteristics only through simulation studies, and it is also difficult to fully verify the effectiveness of the designed control strategy. In order to further explore the output characteristics of the aviation intelligent pump, verify the accuracy of the established system mathematical model and the effectiveness of the control strategy, scholars developed a variety of intelligent pump principle prototypes as well as experimental platforms and conducted experiments on the output characteristics of the intelligent pumps and the control experiments of the pressure and flow rate under different working conditions, and analyzed the experimental results.

Han et al. [58] built a self-supplied aviation intelligent pump prototype and testbed, as shown in Figure 24, in order to verify the output characteristics of the aviation intelligent pump and the designed control method. The intelligent pump prototype is transformed from an aviation constant-pressure variable displacement pump, which can be realized by replacing the valve core of the pressure regulator and connecting the servo valve pipeline outside the pump. The experimental platform mainly consists of two major systems: one is the mechanical–hydraulic system, mainly including an oil tank, drive motors, cooling fans, throttle valves, relief valves, a variety of switching valves, as well as flow and pressure sensors, and the other is the measurement and control system, which mainly consists of the power cabinets, signal cabinets, a junction box, and human–computer interaction equipment.

Based on this experimental platform, Han et al. investigated the effects of different rotational speeds and servo valve input voltages on the output characteristics, respectively. For example, the experimental results of the flow rate and pressure of the intelligent pump at different voltages are shown in Figure 25. From the results, it can be seen that as the voltage increases, the time for variable displacement regulation of the intelligent pump is significantly shortened. The outlet pressure and output flow rate of the pump have lower limits, and the higher the voltage, the lower the lower limit. In addition, the experimental results are compared with the output characteristic results obtained from simulation, which verifies that the established model can reflect the real physical laws of the intelligent pump and is correct and credible.

Helian et al. [60] established an experimental setup, as shown in Figure 26, in order to validate an adaptive robust backstepping control strategy designed for pressure tracking control of variable displacement axial piston pumps (VDAPPs). The experimental setup mainly consists of a variable displacement axial piston pump (Bosch Rexroth A10VS/45) shown in (a) and a control valve (Bosch Rexroth 4WRPEH6) shown in (b). Also included are swashplate inclination sensors, pressure sensors, a real-time controller, and a motor that runs at a constant speed. In order to verify the advantages of the proposed control strategy (C1) in dealing with dynamic nonlinearities and parameter uncertainties, comparative experiments with a nonlinear backstepping controller (C2) are conducted. The pressure tracking results and the reference signal for both controllers are shown in Figure 27. The results show that the proposed adaptive robust backstepping control strategy has higher tracking accuracy and better error convergence, which is due to the fact that its adaptivity compensates well for the deviation that exists between the actual system and the model compensation. The pressure tracking error accuracies of the two controllers are further quantitatively evaluated in both simulation and experimental results. The mean square error (MSE) and root mean square error (RMSE) of the pressure tracking errors are given in

Table 4. Quantitative analysis of pressure tracking errors. Reprinted/adapted with permission from Ref. [60]. 2022, Elsevier.

Sets	Controller	MSE/(bar2)	RMSE/(bar)
Experiment	C1	6.94	2.63
	C2	503	22.4
Simulation	C1	0.29	0.54
	C2	8.16	2.86

. These results fully validate the superior performance of the proposed control strategy in dealing with dynamic nonlinearities and parameter uncertainties.

Kemmetmüller et al. [61] developed an experimental platform, as shown in Figure 28a, to validate the pressure control strategy of a self-supplied variable displacement axial piston pump. The platform mainly includes an induction motor, a control valve, a load throttling orifice, and a variable displacement axial piston pump to be tested. The principle of its hydraulic system is shown in Figure 28b. Experiments were conducted to follow the outlet pressure of the piston pump when the load throttling orifice was adjusted rapidly. Figure 29 shows the system behavior when the load coefficient varies rapidly. The results show that the load regulation introduces a large tracking error, but this error is quickly compensated. The results of the dynamic estimation of the load coefficient show that the estimated value can follow the rapidly changing load coefficient in a favorable way. The tracking behavior of the load pressure for slowly varying load factors is demonstrated in Figure 30. The load coefficient increases slowly while the load pressure varies along a rectangular-like reference trajectory. In this case, the system still achieves excellent tracking performance while obtaining good estimates of the load coefficients. Thus, the experimental results show the excellent resistance of the control strategy to load perturbations, thus demonstrating the practical feasibility of the proposed control strategy consisting of feedforward control, feedback control, and an extended estimator.

5. Conclusions and Prospects

This paper conducted an in-depth investigation of the related research on aviation intelligent pumps. This paper summarized the current research status of intelligent pumps from the aspects of system scheme, working mode, structure form, and control method. Based on the above research status, the following main conclusions can be summarized:

(1)

An intelligent pump is an effective way to solve the increase in ineffective power consumption under the development of high pressure and high power in the aircraft hydraulic system. The self-supplied intelligent pump has become a highly competitive structural solution due to its high power-to-weight ratio and high reliability.

(2)

In the research of system schemes and working modes, scholars at home and abroad have constructed information interaction and command signal acquisition schemes between the intelligent pump system and flight control system and have established a relatively complete working mode-switching guideline.

(3)

In order to meet the demand for fast and accurate regulation of the output of the intelligent pump in the pressure and flow regulation modes with overshooting within a certain range and to effectively suppress the system nonlinearities, parameter perturbation, and external load disturbance, the research on the control method for the intelligent pump system in the pressure and flow modes is crucial to meet the aeronautical standards and specifications and to promote the practical applications.

Based on the above analysis and summary of the relevant research on aviation intelligent pumps, it can be found that there are still some research directions that need to be further explored. The subsequent research can be centered on the following six aspects:

(1)

The current research on the output characteristics control strategy of the highly competitive self-supplied intelligent pump does not pay attention to the dynamic perturbation of the output performance caused by the forward channel variable gain problem. Therefore, for the self-supplied intelligent pump forward channel variable gain problem, carrying out targeted control method research to improve its pressure servo dynamic performance is of great significance.

(2)

In the current research on the flow control of intelligent pumps, most of the traditional PID or fuzzy control strategies that do not depend on the accurate model of the system are used, resulting in limited performance of flow regulation. In addition, most of the current research directly equates flow control to swashplate inclination adjustment while ignoring pump leakage, oil compression, and other flow losses. Therefore, research on the precise regulation of flow and the application of advanced control strategies for flow control is very helpful in improving its dynamic response performance and robustness.

(3)

For aviation intelligent pumps, in addition to pressure and flow regulation during flight, there will also be power regulation conditions. Such as when the aviation pump from the engine extracts power close to the power limit, it needs to be maintained at a certain power value. In the power regulation, the pump outlet pressure and output flow are both in the process of change, which brings great challenges to the power regulation. Therefore, the high-precision and fast-response power regulation method of aviation intelligent pumps is a research direction that is worth exploring in the future.

(4)

As the core component of the intelligent hydraulic system, the intelligent pump needs to work in the corresponding mode according to the requirements of the working conditions during the continuous flight. Timely and effective mode switching is an important guarantee for the smooth operation of the intelligent hydraulic system. Although the current research has paid attention to the conditions for mode switching of intelligent pumps, how to ensure the smooth transition of system pressure and flow during mode switching is still an important research direction.

(5)

The reliable operation of intelligent pumps is of great significance to flight safety. Real-time and accurate fault localization and degradation status monitoring are important means of ensuring its reliability. Currently, scholars have conducted research on various fault diagnoses [68,69,70,71,72,73] and health management methods [74,75,76,77] for aviation pumps. It is worthwhile to study how to integrate the function of health management in intelligent pumps to meet the dual demand of high performance and high reliability of aviation intelligent pumps.

(6)

A new type of electrohydrodynamic (EHD) pump that generates kinetic energy by inducing the movement of a dielectric fluid through an electric field has been used in applications such as soft robots and artificial muscles [78,79,80,81,82,83]. The novel working principle of this pump determines that it has the advantages of no noise and vibration, simple structure, and so on. Although this pump is currently mainly applied to small power occasions, its working principle is still worth borrowing in the intelligent pump and subsequently can be considered in the airborne hydraulic system for small power actuation or heat dissipation cooling and other occasions.