Questions about the principle of hydraulic servo system

JJbow

Hi,

I found that most documents describe the open-loop transfer function of servo hydraulic system as the relationship between the size of the servo valve opening (X) and the piston displacement of the hydraulic cylinder (Y), whcih is Y/X. After adding a closed-loop, the target position can be controlled.

My understanding of this system is that in the case of open-loop, if we give the servo valve a constant drive current, servo valve opening size is fixed, and then the hydraulic cylinder piston will eventually stop at a specific position. But in actual use, it is found that the piston will continue to move until the end of the stroke. Why is this?

Thanks!

CSA

What current values are you providing the servo-valve in question?

There are 4-20 mA servos, +/-10 mA servos (bipolar), 0-100 mA servos, 0-200 mA servos, etc.
And, if the feedback loop is open, depending on the design of the system the device might go to one end or the other of the total travel. For example, consider a fuel control valve for a 250 MW combustion (gas) turbine. On open loop (feedback), what would you want the servo to make the valve do? Open fully or close fully?

You really haven’t defined the process or the servo-valve very well.

pnachtwey

What documents? Many are wrong when it comes to hydraulic servo control.
You are definitely confused.
if we give the servo valve a constant drive current, servo valve opening size is fixed
Yes!

, and then the hydraulic cylinder piston will eventually stop at a specific position.
No!

But in actual use, it is found that the piston will continue to move until the end of the stroke. Why is this?
If the valve is still open then oil is still flowing so the piston will keep moving to the end of the cylinder or until the piston rod is obstructed.

The above is an simple explanation.

When talking about the open loop transfer function you need to specify whether you are talking about the one for velocity or the one for position. There are almost the same except the one for position is the integral of the one for velocity.
You need to learn the difference between integrating and non-integrating systems. Temperature and velocity are non-integrating systems. If you take away the control the system will go back to an ambient temperature or 0 velocity. An position and tank level control are integrating systems. The position is the integral of velocity and the tank level is the integral of the net flow into or out of a tank. If you take away the control signal to a velocity control system it will coast to 0 or stop but it will not go back to the original position.

Most books are wrong. They do not calculate the open loop gain correctly.

CSA