幅频特性曲线如何辅助选型？

mars_haha

在液压伺服系统中，幅频特性曲线如何辅助选型
有一种说法是幅值选择-3db或者是频值选择滞后90°。这种选择在伺服系统中，标准是不是有点低。
看到有种说法是在伺服系统中，需要选择45°滞后是比较理想的。
这个标准到底怎么选择？专家给诊断讲解一下吧

mars_haha

我们公司做了一个疲劳试验机。
要求频次是20hz，这个对液压系统的幅频特性有什么和要求？

rk9004

以幅频和相频最低的频率为准，个人觉得怎么也要不低于30HZ.

PEN

在图片中的阀仅有3赫兹阀。看看那里的幅度比开始在0 dB下降。

制造商喜欢率90度相位滞后阀门，但由于任何超过45度是不能使用这不是一个良好的评价。

阀门频率响应应该比液压缸和负载的固有频率更高。

完整性检查。
在3赫兹气门相位延迟约为30度。
在3赫兹与10毫秒的扫描时间PLC增加了10.8度滞后。
液压缸和负载20 Hz的固有频率为8.8度的相位滞后。
位置控制系统开始与90度滞后。
90+ 30+10.8+8.8=139.6度的相位滞后。
相位滞后不应该是稳定的闭环系统超过135度。在180度的相位滞后，系统会振荡。
显然，这系统不能加速或减速很快。
在一个运动控制器微分增益可以添加相位超前减小相位滞后，使系统更加稳定。

The valve in the picture is only a 3 Hz valve.   Look at where the amplitude ratio starts drop from 0 db.

Manufacturers like to rate valves at 90 degrees phase lag but this is not a good rating because anything over 45 degrees isn't usable.

The valve frequency response should be greater than the natural frequency of the hydraulic cylinder and load.

Sanity check.
At 3 Hz the valve phase delay is about 30 degree.
At 3 Hz a PLC with 10 millisecond scan time adds 10.8 degree of lag.
A hydraulic cylinder and load with a natural frequency of 20 Hz has a phase lag of 8.8 degrees.
Position control systems start with 90 degrees of lag.
90+30+10.8+8.8=139.6 degrees of phase lag.
Phase lag should not be more than 135 degrees for stable closed loop system.   At 180 degrees of phase lag the system will oscillate.
Obviously this system cannot accelerate or decelerate quickly.
The derivative gain in a motion controller can add phase lead to reduce the phase lag and make the system more stable.

mars_haha

At 3 Hz a PLC with 10 millisecond scan time adds 10.8 degree of lag.
A hydraulic cylinder and load with a natural frequency of 20 Hz has a phase lag of 8.8 degrees.//////////////////////////////////////////////
Thanks

1:Can you tell me how calculate the red part(10.8 and 8.8)?
2:You think 45 degrees lag is the Lowest Standard in Closed-loop control，Right？
3：In open loop control degrees lag is not important，or Is there a lowest standard
4：In Frequency response characteristic curves，mainly reference Phase lag，Is the Amplitude ratio not important？
Some person said that -3db Amplitude ratio almost equivalent 90 degree lag，Is it right？

PEN

10.8度的相位滞后死区时间由于控制器扫描循环。
在3赫兹，以0.01秒的扫描时间PLC拥有3*0.01* 360=10.8度的滞后。

从气缸相位滞后应5.9度不8.8度。开始与假定开环传递函数。
ω=2π*Hz
ζ= 1/3
T(s)=1 /（（s /ω）^ 2+ 2 *ζ*（s/ω）+1）
替代
S =j2*PI*Hz
在找到答案的实部和虚部之间的角度。认为是由于在汽缸和负载的相位滞后。

The 10.8 degrees phase lag is dead time due to the controller scan cycle.
At 3 Hz a PLC with a scan time of 0.01 seconds has a lag of 3*0.01*360=10.8 degrees.

The phase lag from the cylinder should be 5.9 degrees not 8.8 degrees.   Start with the assumed open loop transfer function.
ω=2π*Hz
ζ=1/3
T(s)=1/((s/ω)^2+2*ζ*(s/ω)+1)
The substitute
s= j2*PI*Hz
The find the angle between the real and imaginary part of the answer.  That is the phase lag due to the cylinder and load.