Created on:2022-03-09 13:13

Fault analysis and troubleshooting of electro-hydraulic servo valve

Fault analysis and troubleshooting of electro-hydraulic servo valve

1. The servo valve does not act, resulting in the actuator does not act

(1) Cause of failure. More than 90% of the failure of servo valve is caused by dust particles. The specific reasons are as follows (mainly the introduction of nozzle baffle).

1) Some parts of the servo valve are damaged.

2) Failure of force motor and torque motor.

3) The slide valve core is stuck in the valve body (valve sleeve) due to dirt. As shown in Figure 171, the main valve core 9 is stuck in the valve body 10.

4) Nozzle 6 is blocked by dirt.

5) The dirt adheres to the baffle (feedback rod) 5 and pushes the baffle to death.

6) The filter element of 8 is blocked by dirt, and the pressure difference is too large, which breaks and degrades the filter element, resulting in blockage of orifice, nozzle or other pipelines.

7) The coil 13 of torque motor is disconnected, and the terminal of plug and socket is desoldered, loose and short circuited.

8) Wrong or poor wiring with servo amplifier.

9) The oil supply pressure is not adjusted above 1MPa, and the control pressure is too low.

10) The oil inlet and outlet of the valve mounting surface are installed reversely.

11) The flatness of the valve installation is poor, and the valve is deformed due to poor installation.

(2) Exclusion method.

1) Disassemble and repair the servo valve, and replace or properly handle the damaged parts.

2) Disassemble the left and right end covers on the wide body 10 (drawn as a whole in the figure), take out the filter element of the oil filter 8, and check the dirt blockage and damage: if the dirt is blocked, block one end of the filter element with your fingers, inject clean kerosene into the other end with a syringe for cleaning, and it is better to let the oil flow out evenly and slowly; If the filter element is damaged, replace it with a new one.

3) Clean both ends of orifice 7 (the orifice is set on the filter element plug) and the oil through holes on the left and right end caps.

4) Clean the nozzle, baffle, valve core, valve sleeve (not shown in the figure) and all oil circuit holes on valve body 10. Note that the valve core has direction requirements and cannot be assembled in a U-turn. When installing the main valve core 9, put the valve core in the middle position, and then install the nozzle baffle 5. The small ball at the lower end is inserted into the middle groove of the main valve core 9. The small ball must be inserted into the valve core while rotating the direction of the valve core. Do not insert the hardtop. After installation, push the valve core slightly by hand to see whether it is free to center.

5) Check the coil resistance of torque motor, connect the broken wire, and weld the loose plug and socket firmly to avoid poor contact, broken wire and short circuit.

6) Check the oil pressure of the hydraulic station, and the oil supply pressure shall not be lower than 1MPa.

7) If the wiring with the servo amplifier is incorrect, it shall be corrected, and if the wiring is not firm, it shall be welded firmly again.

8) Level the valve mounting surface to ensure the flatness error requirements.

9) If the oil inlet and outlet of the servo valve are installed reversely, correct it and check the condition of the connected pipeline.

2. Frequent zero offset (zero offset)

The zero position is also called the intermediate position. For the servo valve shown in Figure 171, the zero position of the valve refers to the geometric position of the valve core and armature baffle assembly when the load pressure drop is zero and the output flow of the valve is zero. Although the input current of the valve is expected to be zero when it is in the zero position, the input current is usually not zero at this time. The percentage between the input current required to make the servo valve in the zero position and the rated current value is called the zero position offset of the servo valve.

This fault refers to that the zero position changes frequently and the zero position offset is large. The causes and troubleshooting methods of the fault are as follows.

(1) The zero deviation is out of range and unstable due to the servo valve itself. For example, the armature assembly at the pressed or welded part is loose, the built-in oil filter is blocked by dirt, the pressed nozzle is loose, and the nozzle is blocked by dirt. General civil servo valves have external zero adjustment devices, and the zero deviation is required to be no more than 3%. The following methods can be used to solve the excess of zero deviation.

1) For the servo valve with valve sleeve, loosen the end cap screw and adjust the position of valve sleeve to adjust the zero deviation.

2) For the servo valve without valve sleeve or the servo valve that cannot be corrected by the above method, the positions on both sides of the orifice can be exchanged or another group of orifices can be changed to adjust the zero deviation.

3) It is also an effective method to correct the zero deviation by repairing the air gap of the torque motor. The tightening degree of the four screws of the torque motor shown in Figure 171 directly affects the size of the four air gaps of the torque motor. The four air gaps form a bridge and work in a differential state. Generally, the thickness of each air gap is 0.25mm, and the moving working distance of the armature of the torque motor is the air gap change value △ with zero deviation of 1%= δ/ three × 1% × 1/2=0.25/3 × 1/100 × 1/2=0.0004mm=0.4µm。 Where 1 / 2 is the differential t operation considering the four air gaps of the torque motor. From the above calculation, it can be seen that the change of air gap of torque motor by 0.4 µ m will cause 1% zero deviation. Therefore, the tightening torque of four screws of torque motor shall be consistent. During adjustment, pay special attention to cleaning and do not dirty the torque motor.

4) For the loosening of armature components, laser spot welding can be used at the edge of the mating surface between support and armature, support and baffle.

(2) The change of oil temperature causes zero offset. The change of oil temperature will lead to the change of oil viscosity, leakage and zero offset. In order to ensure normal operation, oil cooler and heater can be installed in the oil circuit to control the temperature within the required range. For general requirements, the input current of the valve shall be limited to 2% for every 40 ℃ change in oil temperature.

(3) The great change of oil source pressure leads to the great change of zero position. The oil supply pressure Ps of the servo valve can generally be selected according to PS = 1.5 (P1 + PT) (P1 is the load pressure drop of the servo valve and Pt is the back pressure value of the servo valve). Generally, a certain value can be taken in the range of 1 ~ 7MPa, and an accumulator is installed in front of the valve to stabilize the pressure.

(4) The oil pollution is serious, and there are many dirt particles in the oil. The hydraulic circuit can be cleaned and replaced with clean oil. A high-pressure oil filter is installed in front of the valve to improve its filtering accuracy.

(5) If the zero position cannot be adjusted (the zero position cannot be adjusted within the rotation range of the zero position adjusting screw), adjust the oil supply pressure PS to a pressure value greater than 1MPa. In addition, clean the built-in oil filter and the orifices at both ends.

(6) Effects of servo valve blockage and nozzle loosening. It can be cleaned regularly. When the nozzle is loosened, it shall be re debugged to make it return to normal operation.

(7) Ensure that the electrical zero position coincides with the mechanical zero position. Ensure that the electrical zero position coincides with the mechanism zero position, so that the elastic element will not be stressed when it is in the zero position, so that the stiffness of the elastic element will not affect the zero position when it changes with temperature. The specific methods are as follows: during assembly, pay attention to make the slide valve in the zero position when the nozzle does not work and the feedback rod is not stressed, which is the mechanical zero position; When the nozzle is put into operation, the slide valve is still in the zero position when the spring tube is not stressed, which is that the hydraulic zero position coincides with the mechanical zero position; After installing the coil and magnetic steel of the torque motor, the slide valve is still in the zero position, that is, the electrical zero position coincides with the mechanical zero position. In addition, the selection of materials with small elastic modulus and temperature coefficient (such as materials with constant elastic modulus) can also directly reduce the temperature bias caused by elastic elements.

(8) Improve mechanical symmetry. Improving the processing symmetry and assembly symmetry of parts and selecting materials with the same material and the same or similar linear expansion coefficient to manufacture relevant parts can reduce the zero deviation caused by thermal expansion. The parts with symmetry requirements are mainly valve body, valve sleeve, valve core and armature assembly.

(9) Improve hydraulic symmetry. In order to reduce the zero bias caused by the change of viscosity with temperature, the hydraulic symmetry of the hydraulic amplifier should be ensured as much as possible, and the influence of viscosity on flow characteristics should be reduced as much as possible. Therefore, it is required that the orifice should have a good shape, no burr, the throttling length should be as small as possible, the nozzle should have a good shape, the end ring should be as narrow as possible and no burr. The orifice and nozzle matrix shall have high enough hardness, and the hole shape and ring belt shall be ground.

(10) Increase the displacement of valve core. The purpose of increasing the displacement of the valve core is to minimize the relative effect of zero deviation when zero deviation occurs. According to the analysis, all kinds of temperature zero deviation are inversely proportional to the maximum displacement of the slide valve. Therefore, appropriately increasing the displacement is an effective measure to reduce the influence of temperature zero deviation. The variation of flow and torque motor load caused by this can be compensated by adjusting the square hole width and magnetic steel magnetization level.

(11) Increase the open-loop gain. The force feedback electro-hydraulic servo valve is a closed-loop automatic regulation system, and the temperature zero deviation is the steady-state error caused by temperature interference. Therefore, increasing the open-loop gain of the system can reduce the temperature bias. The specific method is to improve the magnetization level of magnetic steel, so as to improve the electromagnetic coupling stiffness; At the same time, increase the air gap length of the torque motor to keep the current torque constant unchanged. In this way, the value of 1 / Ke (Ke is the comprehensive stiffness of torque motor) can be increased, so as to improve the open-loop gain and reduce the temperature zero bias.

(12) Make all levels of servo valve at zero position at the same time.

1) The four air gaps must be equal during assembly. The geometric dimensions of the four air gaps are equal by replacing the adjusting gasket and the upper and lower guide magnets. In order to facilitate adjustment, the magnetic steel shall not be magnetized first.

2) Zero the servo valve step by step on the test bench. The specific method is; Remove the control pressure acting on the end face of the valve core, and first adjust the zero position of the feedback rod and the valve sleeve of the valve core. Replace the orifice with a plug. First, adjust the valve sleeve to the middle of the valve body, install the armature assembly, and fix the transverse position of the armature assembly. Then start the test bench and adjust the longitudinal position of the armature assembly to make the pressure of the two load chambers of the servo valve equal. Fix the armature assembly. Then replace the non orifice plug with an orifice, adjust the nozzle position on the test bench to make the pressure of the nozzle cavity P1 = P2 = PS / 2, and make the pressure of the two load cavities equal at this time. Finally, install the magnetic steel and magnet with air gap adjusted. If the zero position changes slightly, replace the adjusting gasket again until the valve core returns to the zero position. After completing the above adjustment, tighten the motor screws and valve core end cap screws evenly.

(13) The pressure difference between the two nozzles shall not be greater than 0.3MPa. For high-precision servo valve, the geometric asymmetry of the two nozzle cavities directly affects the symmetry of hydraulic parameters. The parts with asymmetric geometry include fixed orifice diameter, variable orifice diameter, 90 ° cone angle at the inlet and nozzle orifice shape, such as 90 ° at the inlet of variable orifice. The cone angle affects the flow coefficient G1. In addition, if there is edge collapse at the nozzle orifice, the liquid flow will not all shoot at the baffle, so the zero position is misaligned. When the zero deviation caused by geometric asymmetry is very regular, there are two ways to improve the accuracy of geometric tolerance: first, replace the left and right orifices, and use the difference of two fixed orifices to make up for the difference of nozzle holes; Second, replace the nozzle body and readjust it.

(14) The machining accuracy of slide valve and sleeve must be improved, the geometric tolerance and friction force must be reduced. For high-precision servo valves, the fit clearance between slide valve and sleeve mating parts is generally controlled at 0.002 ~ 0.003mm; At the same time, the geometric tolerance requirements of the matching pair of slide valve and sleeve are also quite high. Therefore, as long as there is a high point, the geometric tolerance of this valve can not meet the requirements. The only way to reduce the geometric tolerance is to grind the sleeve, expand the gap and reduce the friction. However, it must be noted that this method can only be used when the static consumption is qualified.

(15) For the zero deviation of servo valve caused by the change of electrical zero position, the tightness of four screws can not be used to adjust, but first, the assembly technology level should be improved, and second, the gasket should be reasonably selected for the adjustment of electrical zero position.

(16) When the servo valve works, the pressure drop at the front and back of the control edge of the valve core is large. If the four side slide valves work at the maximum power point, the pressure drop at the control edge is 1 / 3 of the oil source pressure. As long as the valve core has a small opening, the pressure oil will sweep over the control edge at high speed, and the particles in the oil will erode the sharp angle of the control edge and produce erosion wear, as shown in Figure 182. This erosion wear seriously affects the zero position characteristics and control accuracy of the servo valve. Especially when the pollution is serious, this erosion wear develops rapidly, and many servo valves fail, resulting in shortened service life.

3. The output flow of servo valve is small

(1) When the oil supply pressure is low, the oil supply pressure (PS) can be appropriately increased.

(2) When the flow of the input servo valve is insufficient, the oil supply can be increased, eliminated and reduced

Internal and external leakage of other parts of the system.

(3) If the output power of the servo amplifier is insufficient, first check whether the input signal of the input servo amplifier is normal, and check whether there are other faults of the servo amplifier and eliminate them.

(4) If the built-in oil filter is blocked by dirt, clean the hydraulic circuit and servo valve and change the oil, especially pay attention to the blockage caused by colloidal foreign matters in the working oil.

4. The dynamic characteristics of the servo castration are poor, the frequency response is sluggish, the response speed of the system transition process decreases, and the overshoot increases

For the electro-hydraulic servo valve, it means that the amplitude and phase angle of the output flow of the servo valve can not follow the change of the input current signal in a large frequency range, but the decrease of the flow amplitude and the lag of the phase angle are large. The dynamic characteristics of servo valve are affected by external factors such as oil supply pressure, amplitude of input signal (mechanical displacement or signal current), oil property, oil temperature, structural parameters of valve itself, pure stiffness and natural frequency of torque motor, stiffness of feedback rod, flow gain of front stage and other internal factors. The specific measures for improvement are as follows.

(1) Increasing the oil supply pressure can improve the speed amplification factor, improve the sensitivity of servo valve and system, and accelerate the response speed. However, generally speaking, dynamic and static stability are a pair of contradictions. When the oil supply pressure exceeds a certain limit value, the system will vibrate and become unstable (the same is true for the following situations).

(2) The changes of oil temperature and oil viscosity affect the dynamic performance of the system. The oil viscosity is large and the response speed of the system is reduced. In order to make the dynamic characteristics better or not change much in use, the oil with appropriate viscosity should be selected, and the oil temperature should be controlled within a certain range to eliminate the influence of oil temperature change on viscosity.

(3) Proper increase of the back pressure of the system can improve the stability, but the dynamic characteristics become worse, so the system should have appropriate back pressure requirements.

(4) The amplitude of input signal (such as current value) shall be controlled within a certain range.

(5) Increasing the width around the opening of the servo valve can increase the flow amplification factor, improve the sensitivity of the system and speed up the response speed. Similarly, pay attention to the impact on the stability of the system.

(6) The slide valve with positive pre opening, as shown in Figure 183 (c), has large flow amplification coefficient, high sensitivity to flow control, good dynamic characteristics, but also poor stability.

183

(7) The existence of mechanical feedback gap increases the adjustment time of overshoot, so the feedback gap should be reduced as much as possible.

(8) The increase of servo hydraulic cylinder area can improve the rigidity of the system and increase the stability of the system, but the dynamic characteristics usually become worse.

(9) The total volume of oil contained in servo hydraulic cylinder and connecting pipeline increases, which is not good for the dynamic characteristics of the system, and even the stability of the system will deteriorate.

(10) The hysteresis of the torque motor and the large static friction of the components of the servo valve lead to the deterioration of the dynamic performance. We must try our best to reduce the hysteresis of the torque motor and the friction of the components in the servo valve and improve the precision of parts.

(11) When the flatness error of the mounting surface of the servo valve is large, its influence on the dynamic characteristics of the valve and system must be eliminated.

(12) When the valve core is in a static state, the pulsating oil source pressure will continuously squeeze the particles equal to or slightly larger than the radial fit gap size into the radial gap between the valve core and the sleeve, causing the pollution and blockage of the valve core, so as to increase the friction of the valve core movement, reduce the response of the servo valve and increase the hysteresis.

5. The servo valve has poor stability and large steady-state error, resulting in vibration

The purpose of servo valve used in white motion control is to make the regulated quantity change according to the required law and keep it at a constant value, which puts forward a problem of accuracy. The stability of the system can be expressed by steady-state error. One reason for the change of system output (accuracy) is the control effect, the other is the external disturbance.

For the constant value regulation system, the control effect of the input is constant, and the change of the regulated quantity is caused by the trigram boundary disturbance, so the error is generally in terms of the external boundary disturbance; For the servo system, the control function of the input is constantly changing, and the external disturbance is often not the main, so its error is generally for the control function of the input.

The steady-state error analysis of hydraulic servo mechanism depends on the specific application. For example, for profiling tool holder, its error is caused by both input action and external load force; If only one mass load is controlled as a follow-up mechanism, its error is only caused by the control effect of the input. The reasons for the instability and even vibration of the system are as follows.

(1)       Discontinuous vibration creeping is caused by the friction characteristics of the actuator and the adjusted object, low stiffness and low speed.

(2)       There is a lot of air in the oil, the oil pressure pulsation of the hydraulic pump and the compressibility of the oil increase, resulting in unstable continuous vibration - crawling.

(3) The pipeline connecting the control valve and the servo cylinder is elastically deformed.

(4) Gap in feedback mechanism.

(5) In case of external interference, such as sudden change of load, speed, oil pressure, etc., natural vibration will be generated.

(6) The jet pipe servo valve is easy to vibrate when the oil supply pressure is high.

(7) The larger the follow-up speed, the larger the steady-state error of the system.

(8) The greater the load force acting on the servo cylinder, the greater the steady-state error of the system.

(9) Influence of system insensitive zone (influence of dead zone). The causes of dead zone are clearance and mechanical deformation in mechanical signal transmission mechanism. When there are gaps in the signal transmission mechanism, the output mechanism can produce output motion only after overcoming these gaps. Similarly, strong transmission in the signal transmission mechanism will cause mechanical deformation. Only after overcoming these changes can the output mechanism produce motion. In addition, as shown in figure 6-183 (a), because it is positive covering, there is a dead zone, so the leakage of each part of the system also increases the dead zone. The existence of dead zone or the change of dead zone width will lead to system instability.

In order to improve the stability of the system, the following measures can be taken.

(1)       The structural parameters of the follow-up valve should be selected correctly. For example, increasing the diameter of the servo valve will deteriorate the stability of the system; The piston area of the servo cylinder increases and the stability of the system improves; The volume of the oil chamber of the hydraulic cylinder increases and the stability of the system decreases. Therefore, it is necessary to correctly select the structural parameters of the follow-up valve.

(2)       Properly reducing the mass of the actuator and the moving parts of the adjusted object can improve the stability of the system.

(3)       When the stiffness of the elastic link in the system is increased, the stability can be increased.

(4) The speed of the object can be adjusted to improve the stability of the object.

(5) The servo valve with small flow gain can increase the stability of the system.

(6) Setting additional damping between the valve and the cylinder or between the two cavities of the hydraulic cylinder can improve the stability.

6. There is no signal input, but the actuator moves to one side

At this time, check whether the distance between the nozzle and the baffle is equal; Whether the working air gap is unequal; Whether one nozzle is blocked; The throttling passage to the nozzle is blocked; The main slide valve is stuck in a certain position, forming a unilateral opening, etc. Eliminate one by one according to the situation.

7. The static consumption increases and the pressure gain decreases

(1) Due to wear, the fit clearance between the slide valve and the sleeve increases, resulting in insufficient oil supply and performance degradation of the system. The fit clearance, geometric tolerance and surface roughness between the slide valve and the sleeve shall be strictly controlled, and the wear resistance of the slide valve and sleeve shall be improved.

(2) When the window collapses, the flow grinding method can be used to strictly control the overlapping amount of the window and keep the sharp edge of the window.

 

 

 

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