Servo Valve

Look on the data sheet or ask the supplier.

Do your own work!

When you start asking questions like this, you should then also be asking a question of yourself.

Given your low level of understanding, are you qualified to use this equipment?

I say this, not to be mean. You must understand that using equipment like this can have very severe consequences if used incorrectly. This can include: Destroying hardware, injuring people, etc.

Act wisely.

When you use the term cylinder, are you talking about the cylinder (actuator) that the servo valve is providing fluid flow to and from? Or are you using that term (cylinder) as the servo valve itself?

The reason I ask is that in my experience the servo valve is connected to an actuator (hydraulic cylinder). The servo valve provides fluid to one side or another of the cylinder causing it to extend or retract or hold position. The servo valve does not cause the cylinder to open or close. Do you mean open and close as extend and retract?

I will assume by open and close you mean extend and retract in my answer below.

Ideally, most servo valves are flow devices thus the fluid flow will be proportional to the current provided. This is effected by the load against which the cylinder is acting and the available pressure the hydraulic system is able to provide and the type of cylinder that's being used and the type of servo valve.

In a perfect world 0 mA should correspond to 0 gallons/min of flow thus holding the cylinder stationary. Servo valves will have some bias in them so that the spool may not be truly centered cutting off flow. The valve may be able to be mechanically adjusted to correct such a bias, but only within a certain range of no flow (for instance it may be specified as 0 +/- 0.8 gpm).

If the cylinder has a hydrostatic bearing and piston then there will be fluid flow from the servo valve to the bearings even with a stationary cylinder as the fluid.

The positive (or negative0 current will give flow in one direction (or the other), whether or not that extends (opens) the cylinder or retracts (closes) it, will depend on how the ports of the servo valve are connected to the cylinder.

Thank u JB. Here,servo valve is connected to the cylinder and LVDT is attached to the cylinder (cylinder full stroke is 30 mm ) ,cylinder is fixed to the roll assembly .Purpose of this is to maintain uniform gap between two rolls. for eg :Now cylinder is fully open i.e 30 mm and I require 20 mm gap so I have changed the set point then how much current supplied to the servo valve . If the same setpoint is changed to 5 mm then how much input current is supplied to the servo valve. As it is a servo valve,I think fluid flow will happen proportionally according to the input current and input current vary as the difference between set valve of cylinder to actual value of cylinder varies. Pl clarify my doubt. Nithin

"...so I have changed the set point then how much current supplied to the servo valve . If the same set point is changed to 5 mm then how much input current is supplied to the servo valve. "

The current will essentially dictate how quickly the cylinder moves to the new set point. Once the cylinder is at the new set point, the servo valve spool should go to the closed position cutting off flow and causing the cylinder to stop and stay at the new set point location.

As I mentioned before, there may be more current depending on the type of cylinder you have (i.e. if there is any leakage across the piston or if there is a hydrostatic bearing). Also, if the servo valve spool is over lapped it will have a range of current where there is no flow as the spool is wider than lands (the orifice opening it ports fluid through)....or if the servo valve spool is under lapped there may be some small amount of flow through the valve even at zero current as the spool is narrower than the lands.

"As it is a servo valve,I think fluid flow will happen proportionally according to the input current and input current vary as the difference between set valve of cylinder to actual value of cylinder varies."

Yes, for most servo valves, fluid flow is proportional to the provided current.

And yes....depending on how the control system for you set up is designed it would make sense for the current to the valve to vary as a function of the difference between the set point value and the actual value of the cylinder.

I am sorry to say that before asking what you ask you should understand what a control loop in position is. I shall try yo explain in a few words.

The curent to the valve is supplied by a "controller" which makes the difference between th signal you give for the positon you expect and the signal from the LVDT.

If you cylinder is at 30 mm an you want 20 in the controler difference is build and converted in a curent send to the valve. If the valve was closed (or almost close because of possible leaks as was very well explained) the curent was small and now it jumps to the value proportional to the difference, an important flow is made available to the cylinder which moves but its movement modifies the LVDT signal and consequently the difference and the curent to the valve and the flow passing through.

I do not go into more details since you knowledge will not allow you to folllow but I hope now that you do not control directly the valve but only indicate which position you want achieve.

What does the rest of <...We...> have to say about it?

<...What is the fluid flow with respect to input current...>

It isn't a straight line, that's for certain. Please let the forum know how the telephone calls with the equipment's original supplier went?

"It isn't a straight line, ..."

For most of the servo valves I've worked with the the relationship is linear. The exception being what goes on around the centered spool position (i.e. under lapped or over lapped or zero lapped spool). And those that are lapped, the relationship is linear once you're away from that centered spool position.

Normally the flow line is straight provided that the pressure difference at the valve is constant.

A servo-valve is a 4/3 spool valve (in most cases) which displacement from the middle position is proportional to the curent as value and direction. In order to obtain it a displacement control closed loop connects the spool with the magnetic actuator (in most cases a torque motor).

It was already mentioned that around the zero position the behavior is no more "perfect". This is due to the edges which have minute radii and the clearance which cannot be zero. In order to reduce leaks in this position many have a positive overlap. A dither over the control curent reduces the overlap negative influence in control loops.

Seems to me from how you ask your question that you should not work at all with hydraulics or control systems.

The sad thing is that qualified hydraulics and control system engineers are out of work.