In, 2019Electrohydraulic servo valve mechanisms of missiles and rockets are subjected to various complicated spatial curve motions during flight. How to model electrohydraulic servo valves and their characteristics in centrifugal environments are important issues that need to be studied and solved. During flight, in order to fly steadily in a certain orbit, missiles, rockets and spacecraft, will perform pitching, yawing and rolling operations; electrohydraulic servo valves within them can be regarded as being in a similar state of circular motion in a centrifugal environment. In this case, influence of centrifugal environment should be considered when studying actions of components in electrohydraulic servo valves. Therefore, based on the study of the effects of vibration and impact environments on the motion of components in electrohydraulic servo valves, discussed in the last chapter, this chapter studies the movement characteristics of components in electrohydraulic servo valves when transport motion is centrifugal. Taking a force feedback type electrohydraulic servo valve as its object, characteristics of an electrohydraulic servo valve in a uniform circumferential centrifugal environment, and a uniformly accelerated circular motion centrifugal environment are analysed, respectively. Two-stage servo valveThe right-hand end of the main spool is permanently connected to the pilot pressure line, but because of the linkage rod its area is reduced to an annulus of area A.
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Pressure at the left-hand end of the spool is controlled by the pilot valve. Figure 4.41 illustrates the construction of a different type of servo valve, called a jet pipe servo. Pilot pressure is applied to a jet pipe which, with a 50% control signal, directs an equal flow into two pilot lines. A change of control signal diverts the jet flow, giving unequal flows and hence unequal pressures at ends of the main spool. The main spool is linked mechanically to the jet pipe, causing it to move to counteract the applied electrical signal. Spool movement ceases when the jet pipe is again centrally located over the two pilot pipes.
Servo valves are part of the Honeywell family of Electro-Mechanical Interface Devices (EMIDs). The servo valves used in these applications control the blade pitch thus controlling the speed at which it moves. Earthquake Simulation: There are many companies who manufacture earthquake simulation equipment. They lower a plate onto the ground and produce vibrations through the plate.
This occurs when the main valve spool movement exactly balances the electrical control signal. A feedback control systemIt can be appreciated that, with small movements of the pilot spool (in Figure 4.40) and the fine jets (in Figures 4.41 and 4.42), servo valves are particularly vulnerable to dirt. Cleanliness is important in all aspects of pneumatics and hydraulics, but is overwhelmingly important with servo valves. A filtration level of 10 μm is normally recommended (compared with a normal filtration of 25 μm for finite-position valve systems).Servo valves which are stationary for the majority of time can stick in position due to build-up of scum around the spool. This is known, aptly, as stiction. A side effect of stiction can be a deadband where a large change of control signal is needed before the valve responds at all. 1.The electrohydraulic servo valve with compensating throttle can realize a reasonable ratio of orifice diameter and length of compensating throttle by optimizing the design of parameters of the throttling device and realizing better dynamic performance.
2.According to the transfer function model of the electrohydraulic servo valve with compensating throttle, a model of the electrohydraulic servo valve can be analysed by adjustable parameter liquid resistance. Simulation results show that the size of the chamber formed by the new structure has little influence on the performance of the valve. 3.The theory, method and results of this paper can be used as a theoretical basis for the design and analysis of new electrohydraulic servo valve. The jet-pipe servo valve has strong pollution resistance, and in particular has the unique ability of ‘failure → return to zero’ and ‘fault → safe’, which is widely used in actuator hydraulic power control of various aircraft. Almost all passenger planes and fighters use the jet-pipe servo valve.
The flight control actuator distribution of US fighter F-15 S/MTD is shown in Fig. 3.33. The application example of the stable actuator cylinder jet-pipe servo valve in fighter F-18 is shown in Fig. 3.34. Parallel connection of two single-stage jet-pipe servo valves forming a double redundancy single-stage jet-pipe servo valve was used to control the fighter’s stable actuator cylinder, and a fault sensor was also installed. The system can still work reliably when any one jet-pipe servo valve has a fault.
THE automatic brake pressure control unit system of a Boeing 737–300/400/500 is shown in Fig. 3.35. Pressure control is used in the double-stage jet-pipe servo valve.
When there is an automatic brake input signal, the first stage of the jet-pipe works, the oil of its nozzle jet is transported to two receiving holes and connects to the two end faces of the spool of the second stage valve. Therefore, the first stage valve controls the working condition of the second stage valve, and thus controls the pressure of the brake. The output pressure difference of the first stage valve also feeds back to the jet-pipe nozzle by a mechanical spring. Fig. 3.36 is an assembly drawing of the automatic brake pressure control module of a Boeing 737–300/400/500. Fig. 3.37 shows the automatic brake pressure control module of a Boeing 737–300/400/500.
Fig. 3.38 illustrates the electrohydraulic servo control unit of an in-and-out cabin of an Airbus A340. It employed the jet-pipe servo valve control actuator, using mode selector valve position sensor 3, mode selector valve 5 and solenoid valve 10 to control the condition of the oil circuit.
The displacement of the jet-pipe servo valve slide valve is fed back to the jet-pipe via a feedback bar. Fig. 3.39 shows the outside cabin servo control actuator of Airbus A340 ailerons. Jet-pipe servo valve 16, solenoid valve 10, mode selector valve 9 and position sensor for mode selector valve 10 are used to control the action of the actuator cylinder.
Fig. 3.40 illustrates the inside cabin servo control actuator of Airbus A340 ailerons. It controls the work condition of the actuator by jet-pipe servo valve 16, and feeds back the position of actuator piston by feedback rod 18 to the jet-pipe of the jet-pipe servo valve to form a closed loop. Fig. 3.41 shows the layout of a turbulator jet-pipe servo valve in a Boeing 777. Fig. 3.42 illustrates the layout of the jet-pipe servo valve of a turbulator electrohydraulic servo control unit in a Boeing 777. Fig. 3.43 shows the layout of a turbulator jet-pipe servo valve in a Boeing 777. Electrohydraulic servo control unit of in and out cabin of Airbus A340.
Notes: 1 – Accumulator; 2 – Accumulator observation window; 3 – Mode selector valve position sensor; 4 – Differential pressure sensor; 5 – Mode selector valve; 6 – Damping hole; 7 – Exhaust valve; 8 – Feedback sensor; 9 – Regulating device of feedback sensor; 10 – Solenoid valve; 11 – Jet-pipe servo valve; 12 – Oil filter; 13 – Built-in integrated valve; 14 – Test contact switch (0 Zero position, 1 Leakage detection, 2 Relieve); 15 – Return oil cut-off valve; 16 – Return oil overflow valve. Outside cabin servo control actuator of Airbus A340 ailerons. Notes: 1 – Solenoid valve; 2 – Test check valve; 3 – Oil filter; 4 – Built-in integrated valve; 5 – Test contact switch (0 Zero position, 1 Leakage detection, 2 Relieve); 6 – Total return oil check valve; 7 – Return oil overflow valve; 8 – Accumulator; 9 – Mode selector valve; 10 – Mode selector valve position sensor; 11 – Damping hole; 12 – Differential pressure sensor; 13 – Exhaust valve; 14 – Feedback sensor; 15 – Regulating device of feedback sensor; 16 – Jet-pipe servo valve. Inside cabin servo control actuator of Airbus A340 ailerons. Notes: 1 – Solenoid valve; 2 – Test check valve; 3 – Oil filter; 4 – Built-in integrated valve; 5 – Test contact switch (0 Zero position, 1 Leakage detection, 2 Relieve); 6 – Total return oil check valve; 7 – Return oil overflow valve; 8 – Accumulator; 9 – Mode selector valve; 10 – Mode selector valve position sensor; 11 – Damping hole; 12 – Differential pressure sensor; 13 – Calibration valve; 14 – Feedback sensor; 15 – Regulating device of feedback sensor; 16 – Feedback link rod; 17 – Mechanical input; 18 – Jet-pipe servo valve. In, 2019 14.3 Dynamic pressure damper technologyThe electrohydraulic servo valve system has a low frequency and relatively small damping coefficient of hydraulic machinery integrated resonance; this seriously affects the stability of a system.
To improve stability, a method of improving a system’s structure resonance frequency and hydraulic natural frequency can be adopted. However, the former involves significant change of the mechanical structure, which causes the volume and mass of the structure to increase greatly. It is bulky and uneconomical, and the increase of structure resonance frequency is very limited. Although the latter is easier to improve, the resonant frequency of the system is often lower than that of the natural frequencies of heavy equipment in a ship. This means that the structural resonance becomes the main factor affecting synthetic resonance of hydraulic machinery.
At this point, even if the natural frequency of a system is increased, synthetic resonance frequency cannot be significantly improved, so as to improve the stability of a system.Because a hydraulic servo system has low damping problems, the simplest and most economical way is to improve system damping. For example, in a valve-controlled hydraulic cylinder system, the relative damping coefficient is below 0.2, while a pump-controlled hydraulic cylinder is less damped and sometimes less than 0.005, so the relative damping coefficient is tried to improve the stability of a system. Although there are many ways to increase dynamic pressure damping of a system, the most direct method is mechanical dynamic pressure feedback.
Its advantage is that it has nothing to do with the dynamic performance of a pump control device and electrical control system. It can effectively improve the hydraulic damping ratio of system, improving the stability and bandwidth of the system. 14.3.1 Structure and working principle of dynamic pressure damper. A dynamic pressure damper consists of a shell, end cap, throttling damper, spring and isolating plunger ( Fig. 14.6). The working principle of dynamic pressure damper is shown in Fig. 14.7.
When the pump control device is connected to an input signal, the flow enters the dynamic pressure damping chamber to form pressure, while the other chamber pressure decreases (or does not change, because oil is supplied by the supply pump). If the two chamber pressures change rapidly, the hydraulic cylinder piston and its transmission will be impacted. The hydraulic oil in the two chambers of hydraulic cylinder becomes oil liquid spring, and a mechanical spring is formed from the transmission device, great mass of inertia formed by pistons, transmission, etc. When the damping of the two cavities and transmission are not enough, the system becomes a spring and an oscillating part. Under the action of impact force, the system will produce resonance or vibration, which will affect the system’s performance and reliability.
When a dynamic pressure damper is added, two cavity pressure difference feedback is added, so that part of the oil in the pump input cavity enters the dynamic pressure damping chamber through a throttling hole, so that the isolation plunger moves to the right and absorbs the peak value of the pressure. Working principle of dynamic pressure damper.When the pressure difference is small, the flow through the dynamic pressure damper is smaller, and equilibrium of pressure difference is realized by the spring. Thus the dynamic pressure damper has the characteristics of high frequency conduction and low frequency cut-off.
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It satisfies the dynamic damping ratio of the system and ensures that the system has steady-state accuracy and static stiffness. In addition, this kind of dynamic pressure damper has the characteristics of complete functions, compact structure, convenient maintenance and modification. Therefore, from the point of view of reliability and other performance aspects of a control system and development of technology, it is appropriate to choose a dynamic pressure damper to increase dynamic damping of the control system. 14.3.2 Influence of structural parametersChanging the aperture of the throttling damper has an obvious influence on the amplitude frequency characteristic and phase frequency characteristic of a dynamic pressure damper.
The smaller the aperture is, the lower the cut-off frequency is, but the damping coefficient is relatively small. The influence of spring stiffness on amplitude frequency characteristics and phase frequency of a dynamic pressure damper is not obvious. The greater the spring stiffness is, the lower the cut-off frequency is. The influence of an isolated plunger area on amplitude frequency characteristics and phase frequency characteristics of a dynamic pressure damper is more obvious than that of spring stiffness. The larger the isolation plunger area is, the lower the frequency cut-off frequency is.
The area of isolated plunger is related to the peak value of pressure absorption system of a dynamic pressure damper. In the case of a determination area of an isolated plunger, increasing deformation of the spring can increase the capacity of the dynamic pressure damper to absorb the peak value of pressure. Taking Moog31 electrohydraulic servo valve data as an example, the effects of an impact environment on the performance of a electrohydraulic servo valve is mainly shown in the following two aspects: 1.An acceleration environment mainly affects the opening of the main spool.
In a unit step acceleration environment, the main valve spool will eventually open a displacement about 0.97 μm; the baffle will show a displacement of about 0.0028 μm, and the armature will show a displacement of about 0.005 μm. In order to improve the impact resistance of the electrohydraulic servo valve, the main measures are as follows: 1.Reduce the mass of the armature baffle assembly – for example, for the armature, compared with a plain arm structure, an inclined arm structure can effectively reduce armature mass.
2.Reduce the distance between the centre of mass of the armature baffle assembly and the centre of rotation of the spring tube. Because the mass of the armature baffle assembly is mainly concentrated on the armature, this plus the baffle plate and spring tube are rigidly connected at the top of the spring pipe, and the centre of mass of the armature baffle assembly is at the end of the spring pipe. The spring tube is equivalent to a hollow cantilever beam whose rotation centre is its geometric centre. The distance between the centre of mass of the armature baffle assembly and the centre of rotation of the spring tube can be effectively reduced by reducing the length of the spring tube. 3.Increasing the stiffness of the spring tube. This can be achieved by increasing the outer diameter of the spring tube, reducing its inner diameter, or by other measures.
11.5.2 Influencing factors of the electrohydraulic servo valve in vibration environment and vibration control measures. The factors influencing the performance of the electrohydraulic servo valve in a vibration environment and measures of vibration resistance are as follows: 1.Displacement of the main valve is sensitive to vibration signals at low frequency (. In, 2019 10.2.1 Influence of temperature on fit clearance of electrohydraulic servo valvesTemperature changes cause electrohydraulic servo valve components and their matching dimensions to change, which can affect the performance of the valve. The most direct influence on hydraulic components is expansion and contraction caused by the change of ambient temperature. Deformation of parts and clearance between them are changed: excessive clearance increases valve internal leakage; too little clearance and the spool and valve sleeve can easily become stuck. Electrohydraulic servo valves are of small size, and components of servo valve are smaller in size, and many of them have special shapes.
For example, the feedback bar is a thin and long elastic rod with a maximum diameter of only 1.5 mm and a length greater than 30 mm; one end of the feedback rod is a cylinder, the other end is a sphere with a millimetre scale, and the middle part is a cone; the central wall of the spring tube is 0.05–0.07 mm thick and needs to bear the pressure of 30 MPa; the baffle is also a slender component, the large end diameter is only 3 mm, and there is a 1.5 mm precision hole, the small end has two symmetrical small planes. Other parts, such as nozzles and throttle holes, are also small and special parts.
10.6 and 10.7 are structure sketch maps of the valve sleeve and the spool of an electrohydraulic servo valve of an aircraft, respectively. Among them, A, B and C are axial fit distances between throttle windows of the valve sleeve. The inner diameter of the valve sleeve is 6 mm, the outer diameter is 16 mm, and the clearance between the valve core and valve sleeve is 4 μm at normal temperature; when the temperature increases, the clearance between them decreases. When the temperature is too high, the relative movement between valve core and valve sleeve is affected, which may lead to sticking. At an environmental temperature of 25°C, when the hydraulic oil temperature has increased to 120 °C, the coefficient of linear expansion of materials of the valve core and valve sleeve is α 1=11.4×10 −6/°C, according to Eq. (10.12) and valve sleeve clearance variation can be calculated to be 3.8 μm. At this point, it is theoretically known that the matching state between valve core and valve sleeve changes from clearance fit into the critical condition of interference fit.
When temperature is higher than 120°C, it often leads to the phenomenon of sticking. Structure of the spool of an electrohydraulic servo valve.Armature and upper and lower guide magnets are subject to thermal expansion and contraction, and the air gap is changed. Under normal temperature, the initial air gap length is 0.25 mm; when temperature is 150 °C, air gap length is 0.244 mm; when temperature is -40°C, the length is 0.253 mm.
The air gap change indirectly affects the flux of the control coil. When the material is heated and expanded, the gap between nozzle and baffle will easily become smaller as temperature increases, thus affecting flow rate and pressure gain. In, 2019The working characteristics of the hydraulic servo valve depend on the throttling working side of the valve core and the valve sleeve and the manufacturing accuracy of the overlapping amount of the core and the sleeve. The width of spool shoulder of the underlap slide valve is narrower than the width of the groove on the valve sleeve. When the four throttling edges have unequal openings, they are called underlap valves with unequal overlapping amount. This chapter analyzes the pressure characteristics, pressure gain characteristics and zero leakage of the symmetrical uneven hydraulic servo valve, and introduces application examples.
A common example from industrial applications is servo-valve shown in Fig. Its spool occludes the orifice with some overlap so that for a range of spool positions v there is no fluid flow u. This overlap prevents leakage losses which increase with wear and tear. Considering the spool position as the input v, and the load position y as the output, the hydraulic system in Fig. 7.5 is represented in Fig. 7.6 as a cascaded system consisting of a dead-zone block and a linear transfer function G ( s ) = K M s 2 + B s, where K = A k x k p, B = f + A 2 k p, k x = ∂ g ∂ x, k p = ∂ g ∂ P, g = g ( x, P ) = flow, A = area of piston, P = pressure, and f = viscous friction 11. Schematic diagram of a two stage force feedback electrohydraulic servo valve.When the input control electric current Δ i = 0, the armature, supported by a spring, is evenly spaced between the upper and lower permanent magnets, and the flapper is evenly spaced between the two nozzles, the main spool is in the zero position and the electrohydraulic servo valve has no output.
When the input control electric current has a value Δ i, the armature assembly deflects from the middle position, as does the flapper, the main spool deviates from the zero position, and the electrohydraulic servo valve opens and outputs corresponding pressure and flow. Changing the magnitude and direction of the control electric current changes the magnitude and direction of flow pressure correspondingly, as the magnitude of valve output and the deflection angle of the armature is in proportion to the control electric current. In Fig. 2.1, i 1, i 2 is the input control electric current; p 1, p 2 is the pressure of the main spool’s two ends; p s is the pressure of oil supply; p A, p B is the pressure of loads input and output; p 0 is the pressure of oil return.
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