Created on：2023-02-28 11:46

# Test method for frequency characteristics of electrohydraulic servo valves test method for frequency

Test method for frequency characteristics of electrohydraulic servo valves test method for frequency characteristics of electrohydraulic servo valves the frequency characteristics of electrohydraulic flow servo valves are defined as: when the control current changes sinusoidally within a certain frequency range, the complex ratio of the no-load control flow of the valve to the control current. Because there are many nonlinear links inside the servo valve, and the performance of the servo valve is affected by the external environmental conditions, it is specified that the frequency characteristics of the servo valve should be measured under the standard test conditions and the peak to peak value of the control current is 50% of the rated input signal. Let the transfer function of the servo valve be g (s) and the frequency characteristic of the servo valve be g (J ω)= G(s)|s=j ω， G|(j ω)| Represents the amplitude of frequency characteristic, ∠ g (J ω) Represents the phase angle of the frequency characteristic. When the threshold is constant and the frequency changes within a certain range, G (J) can be obtained ω)| And ∠ g (J ω) The variation is called the amplitude frequency characteristic and phase frequency characteristic of the servo valve respectively. You can also input a step signal of a certain amplitude to the servo valve, measure the step response of the servo valve output accordingly, and then use the Fourier transform method to calculate the frequency response of the servo valve. At present, manufacturers generally use sinusoidal input method to measure the frequency characteristics of servo valves. Because the instantaneous flow of servo valve is difficult to measure directly, it is usually measured indirectly by dynamic hydraulic cylinder. Let the flow of the servo valve be q, the piston area of the dynamic hydraulic cylinder be AP, and the piston movement speed be y. obviously, there is the following relationship

The frequency characteristic of electrohydraulic flow servo valve is defined as the complex ratio of no-load control flow to control current when the control current changes sinusoidally within a certain frequency range. Because there are many nonlinear links inside the servo valve, and the performance of the servo valve is affected by the external environmental conditions, it is specified that the frequency characteristics of the servo valve should be measured under the standard test conditions and the peak to peak value of the control current is 50% of the rated input signal.

Let the transfer function of the servo valve be g (s) and the frequency characteristic of the servo valve be g (J ω)= G(s)|s=j ω， G|(j ω)| Represents the amplitude of frequency characteristic, ∠ g (J ω) Represents the phase angle of the frequency characteristic. When the threshold is constant and the frequency changes within a certain range, G (J) can be obtained ω)| And ∠ g (J ω) The variation is called the amplitude frequency characteristic and phase frequency characteristic of the servo valve respectively. You can also input a step signal of a certain amplitude to the servo valve, measure the step response of the servo valve output accordingly, and then use the Fourier transform method to calculate the frequency response of the servo valve. At present, manufacturers generally use sinusoidal input method to measure the frequency characteristics of servo valves. Because the instantaneous flow of servo valve is difficult to measure directly, it is usually measured indirectly by dynamic hydraulic cylinder. Let the flow of the servo valve be q, the piston area of the dynamic hydraulic cylinder be AP, and the piston movement speed be y. obviously, there is the following relationship

Q=Apy

The piston speed is proportional to the flow of the servo valve. The change of the piston speed can be used to represent the change of the valve output flow. It is easy to measure the piston speed in engineering. The block diagram of testing the frequency characteristics of the servo valve with the input sinusoidal signal is shown in Figure 5.

(1) Frequency characteristic test. The frequency characteristic tester has the following functions: ① send a sinusoidal signal with constant amplitude and increasing frequency according to a certain step to the electric controller; ② Receive the speed signal of dynamic hydraulic cylinder; ③ The sent and received signals are processed to show the amplitude response and phase response of the valve.

(2) Functions and requirements of electric controller. The functions and requirements of the electric controller are as follows: ① accept the sinusoidal signal sent by the frequency characteristic tester, send the corresponding constant amplitude current sinusoidal signal to the servo valve, and make the servo valve make the corresponding sinusoidal response; ② Receive the displacement Y signal of dynamic hydraulic cylinder, and make the servo valve and dynamic hydraulic cylinder form a closed-loop position control loop; ③ The speed Y signal of the dynamic hydraulic cylinder is received and directly transmitted to the dynamic tester after appropriate amplification.

The function of the position circuit of the electric controller is to make the average position of the piston movement always be in the middle position of the dynamic hydraulic cylinder or close to the middle position in the dynamic test process, so that the cylinder collision phenomenon will not occur in the test process, and the test accuracy is also improved. The position feedback channel in the electric controller shall be low-pass filter type, and the filter turning frequency shall be ≤ 0.5Hz. For signals in the range ≤ 0.2hz, the position loop realizes the closed-loop control function. If the frequency is greater than the above frequency, the feedback loop is essentially "open", and the position closed loop becomes the position open loop. Only under the open-loop condition can the real frequency response of the servo valve be obtained. The position closed-loop circuit should have appropriate loop gain, so that the average position of the piston can always be maintained at a certain position during the test process, so as to make the test results more accurate and reliable, which is very important. Generally, the consistency of dynamic test results of servo valve is not very good, one of the reasons is that the piston position is not well positioned during the test process. Because although the distribution of the magnetic field of the speed sensor in the axial direction is constant, it is not equal. In the test process, such as the positioning and floating of the piston, it is possible that the speed signals detected before and after the two tests are not unique and constant, resulting in poor consistency of dynamic test data. The power amplifier of the electric controller should be designed into a deep current feedback type, and have enough power, so that the requirements can be met in the test process in a wide frequency range: in the test range up to hundreds of Hertz, the dynamic characteristics of the servo valve will not be affected by the inductance of the valve coil, and the sinusoidal signal transmitted to the valve will not be distorted. The input voltage of the electric controller and the calibration value of the input current of the valve have nothing to do with the coil resistance of the valve. The function of the position circuit of the electric controller is to make the average position of the piston movement always be in the middle position of the dynamic hydraulic cylinder or close to the middle position in the dynamic test process, so that the cylinder collision phenomenon will not occur in the test process, and the test accuracy is also improved. The position feedback channel in the electric controller shall be low-pass filter type, and the filter turning frequency shall be ≤ 0.5Hz. For signals in the range ≤ 0.2hz, the position loop realizes the closed-loop control function. If the frequency is greater than the above frequency, the feedback loop is essentially "open", and the position closed loop becomes the position open loop. Only under the open-loop condition can the real frequency response of the servo valve be obtained. The position closed-loop circuit should have appropriate loop gain, so that the average position of the piston can always be maintained at a certain position during the test process, so as to make the test results more accurate and reliable, which is very important. Generally, the consistency of dynamic test results of servo valve is not very good, one of the reasons is that the piston position is not well positioned during the test process. Because although the distribution of the magnetic field of the speed sensor in the axial direction is constant, it is not equal. In the test process, such as the positioning and floating of the piston, it is possible that the speed signals detected before and after the two tests are not unique and constant, resulting in poor consistency of dynamic test data. The power amplifier of the electric controller should be designed into a deep current feedback type, and have enough power, so that the requirements can be met in the test process in a wide frequency range: in the test range up to hundreds of Hertz, the dynamic characteristics of the servo valve will not be affected by the inductance of the valve coil, and the sinusoidal signal transmitted to the valve will not be distorted. The input voltage of the electric controller and the calibration value of the input current of the valve have nothing to do with the coil resistance of the valve.

(3) Dynamic hydraulic cylinder. The piston motion friction, mass m and internal leakage of the dynamic hydraulic cylinder should be as small as possible, and the piston cavity VT should be as small as possible, which should be designed with reference to the structural principle of Figure 6. (3) Dynamic hydraulic cylinder. The piston motion friction, mass m and internal leakage of the dynamic hydraulic cylinder should be as small as possible, and the piston cavity VT should be as small as possible, which should be designed with reference to the structural principle of Figure 6. The seal between the piston head and the cylinder barrel can be sealed with high-precision clearance instead of seals. If seals are used for sealing, the compression of seals should not be large, and seals with reasonable structure and low friction coefficient should be used. The displacement sensor and speed sensor shall be designed as built-in type, immersed in the left and right cavities of the hydraulic cylinder. There is no extended piston rod at both ends of the piston, which will avoid the rod seal and rod end support of the usual structure. According to this structure, the friction force of the dynamic hydraulic cylinder and the mass of the moving parts should be very small.

The seal between the piston head and the cylinder barrel can be sealed with high-precision clearance instead of seals. If seals are used for sealing, the compression of seals should not be large, and seals with reasonable structure and low friction coefficient should be used. The displacement sensor and speed sensor shall be designed as built-in type, immersed in the left and right cavities of the hydraulic cylinder. There is no extended piston rod at both ends of the piston, which will avoid the rod seal and rod end support of the usual structure. According to this structure, the friction force of the dynamic hydraulic cylinder and the mass of the moving parts should be very small.

The main structural dimensions of the dynamic hydraulic cylinder are the hydraulic cylinder diameter D and the piston limit stroke L. the sizes of D and L are related to the rated flow QN of the servo valve to be tested. The following design and calculation formula is derived based on the principle that the piston cavity is the minimum. The main structural dimensions of the dynamic hydraulic cylinder are the hydraulic cylinder diameter D and the piston limit stroke L. the sizes of D and L are related to the rated flow QN of the servo valve to be tested. The following design and calculation formula is derived based on the principle that the piston cavity is the minimum. Suppose that the piston moves sinusoidally under the drive of the servo valve, the piston stroke is y, and the amplitude at low frequency is Y0, then there is y=y0sin2 π ω Where t: ω Is the sinusoidal motion frequency, Hz. Accordingly, the piston speed is y=2 π ω y0cos2π ω T set the rated flow of the servo valve as QN, the peak value of the current signal transmitted to the servo valve when detecting the frequency characteristics is 0.5in, and in is the rated current of the valve, then the peak value of the flow of the valve at the initial low frequency is 0.5qn, so equation (2) holds

Suppose that the piston moves sinusoidally under the drive of the servo valve, the piston stroke is y, and the amplitude is Y0 at low frequency, then there is

Y=y0sin2π ω t

Where: ω Is the sinusoidal motion frequency, Hz.

Accordingly, the piston movement speed is

y=2π ω y0cos2π ω t

Suppose the rated flow of the servo valve is QN, the peak value of the current signal transmitted to the servo valve when detecting the frequency characteristic is 0.5in, and in is the rated current of the valve, then the peak value of the flow of the valve at the initial low frequency is 0.5qn, so equation (2) holds

When detecting frequency characteristics, the initial frequency is usually 5hx. When the frequency is 5Hz, the position loop in Figure 5 is actually disconnected. In addition, the frequency response of the servo valve is generally very high, and the output amplitude is basically not attenuated when the frequency is 5Hz. Therefore, the starting frequency of 5Hz is reasonable. Replace the initial frequency 5Hz with equation (2), and when there is a detection frequency characteristic, the initial frequency is often 5hx. When the frequency is 5Hz, the position loop in Figure 5 is actually disconnected. In addition, the frequency response of the servo valve is generally very high, and the output amplitude is basically not attenuated when the frequency is 5Hz. Therefore, the starting frequency of 5Hz is reasonable. Replace the starting frequency 5Hz with equation (2), then y0=0.3377 × (qn/d2) this is the relationship between the rated flow QN, cylinder diameter D and initial amplitude Y0 of the valve. In the design and calculation, the initial amplitude Y0 of the initial frequency point of the piston can be obtained by substituting the rated flow QN of the known valve and the preliminarily selected cylinder diameter D into equation (3). The limit stroke l of the piston should meet L ≥ Y0, and it is recommended to take l=y0. The recommended parameters of dynamic hydraulic cylinders with different flow rates are shown in the table below.

Y0=0.3377 × (qn/D2)

This is the relationship between the rated flow QN, cylinder diameter D and initial amplitude Y0 of the valve. In the design and calculation, the initial amplitude Y0 of the initial frequency point of the piston can be obtained by substituting the rated flow QN of the known valve and the preliminarily selected cylinder diameter D into equation (3). The limit stroke l of the piston should meet L ≥ Y0, and it is recommended to take l=y0.

The recommended parameters of dynamic hydraulic cylinders with different flow rates are shown in the table below.

 Rated flow qn/ (l/min) ≤10 ≤100 ≤400 缸径D/cm 4 8 10 活塞限位行程L/cm 0.7 1 1.8

Carry out digital simulation according to the model shown in Figure 7. Taking different output state parameters, i.e. valve core displacement XV amplitude and dynamic hydraulic cylinder piston movement speed y amplitude, respectively, and comparing with the sinusoidal current signal with input amplitude of I0, it is concluded that servo valve core XV (output frequency response) and dynamic hydraulic cylinder piston movement speed y (output frequency response) are consistent, which shows that the quality of piston and other moving parts and piston cavity have little influence on the real frequency response of servo valve. The dynamic hydraulic cylinder designed according to the dimensions in Table 3 will not bring errors to the test results. Carry out digital simulation according to the model shown in Figure 7. Taking different output state parameters, i.e. valve core displacement XV amplitude and dynamic hydraulic cylinder piston movement speed y amplitude, respectively, and comparing with the sinusoidal current signal with input amplitude of I0, it is concluded that servo valve core XV (output frequency response) and dynamic hydraulic cylinder piston movement speed y (output frequency response) are consistent, which shows that the quality of piston and other moving parts and piston cavity have little influence on the real frequency response of servo valve. The dynamic hydraulic cylinder designed according to the dimensions in Table 3 will not bring errors to the test results. In order to ensure the reliability of the frequency characteristic test of the servo valve, it is necessary to calibrate the detection reliability of a newly designed dynamic hydraulic cylinder or outsourced dynamic hydraulic cylinder, or the dynamic hydraulic cylinder after a long time of use. The calibration method is as follows:

In order to ensure the reliability of the frequency characteristic test of the servo valve, it is necessary to calibrate the detection reliability of a newly designed dynamic hydraulic cylinder or outsourced dynamic hydraulic cylinder, or the dynamic hydraulic cylinder after a long time of use. The calibration method is as follows:

The electrohydraulic flow servo valve with electric feedback is installed on the dynamic hydraulic cylinder instead of the general electrohydraulic flow servo valve. The dynamic hydraulic cylinder with small diameter adopts two-stage electric feedback electro-hydraulic flow servo valve; For those with large cylinder diameter, three-stage electro-hydraulic flow servo valve is adopted. The signal transmission block diagram of calibrating the dynamic hydraulic cylinder is shown in Figure 8. The function of the electric controller in Figure 8 is the same as that in Figure 5, except that the output signal of the electric controller here is voltage, and figure 5 is current. The valve core displacement X in the figure is derived from the feedback channel of the built-in amplifier of the electric feedback servo valve. It can be seen that the frequency characteristics of the servo valve can be detected from two different output signals at the same time: ① from the output speed signal Y of the dynamic hydraulic cylinder; ② It is detected from the valve core displacement signal XV of the servo valve. If the frequency characteristics measured by the two are consistent, it shows that it is credible to use the dynamic hydraulic cylinder to detect the frequency characteristics; If the frequency characteristics detected by the two are inconsistent and the difference is large, it means that the frequency characteristics detected by the dynamic hydraulic cylinder is not credible, and the cause should be found out and eliminated. The electrohydraulic flow servo valve with electric feedback is installed on the dynamic hydraulic cylinder instead of the general electrohydraulic flow servo valve. The dynamic hydraulic cylinder with small diameter adopts two-stage electric feedback electro-hydraulic flow servo valve; For those with large cylinder diameter, three-stage electro-hydraulic flow servo valve is adopted. The signal transmission block diagram of calibrating the dynamic hydraulic cylinder is shown in Figure 8. The function of the electric controller in Figure 8 is the same as that in Figure 5, except that the output signal of the electric controller here is voltage, and figure 5 is current. The valve core displacement X in the figure is derived from the feedback channel of the built-in amplifier of the electric feedback servo valve. It can be seen that the frequency characteristics of the servo valve can be detected from two different output signals at the same time: ① from the output speed signal Y of the dynamic hydraulic cylinder; ② It is detected from the valve core displacement signal XV of the servo valve. If the frequency characteristics measured by the two are consistent, it shows that it is credible to use the dynamic hydraulic cylinder to detect the frequency characteristics; If the frequency characteristics detected by the two are inconsistent and the difference is large, it means that the frequency characteristics detected by the dynamic hydraulic cylinder is not credible, and the cause should be found out and eliminated.

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