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Servo valve versus solenoid valve

Servo valve versus solenoid valve

Servo valve versus solenoid valve

I am having an issue with the difference between servo valves and solenoid valves. For example, if I needed one 4 way 3 position solenoid valve for one double acting cylinder, how would using a servo compare?

RE: Servo valve versus solenoid valve

The solenoid you are referring to will only position the cylinder in two positions:
1) fully extended
2) fully retracted

A servo valve can position the same cylinder in many positions between fully extended and fully retracted.

David Baird
Sr Controls Designer
EET degree.

RE: Servo valve versus solenoid valve

A 4 way, 3 position valve would normally be direct operated by a standard on/off solenoid. However, this would only be used on small valves with low flow rates as high flow rates make the spool hard to control and thus require huge spools.

For larger flow, a smaller valve is used as a pilot valve. Again, the simplest method of doing this is to use an on/off solenoid to make the main spool go one way, or the other.

Proportional and servo valves would be used to control a 4 port, 3 position valve. They wont be the main stage as it would be impossible to get the necessary level of control

To be able to use the spool to control speed or position with any sort of control or repeatability, then the solenoids would be proportional types. Depending on what money you have available to spend, proportional valves can give good performance in terms of response and accuracy. if you add close loop control, with a good algorithm, you can control flow and pressure to within about 3% in terms of accuracy and repeatability. There is also a limit to the frequency that proportional valves can maintain. The proportional or servo valve is used to control the position of a main spool, generally controlling the pressure acting on the end of the spool as it moves against a spring.

So as you move along the curve in terms of performance requirements, you will move into the world of servo valves. Servo valves are really just high performance proportional valves. Servo valves, when used with closed loop control, will allow a very high level of control of the actuator. Certainly to within 1% of accuracy and repeatability...if the correct amplifiers etc are used.

If you are familiar with PID control, you will understand how the gains affect the system response and performance. The ramp up, overshoot, settle time, steady state error...etc are all excellent with servo valves. However, it all comes at a cost and servo valves can be fragile. Equally, it is important to understand the transient conditions of the system, servo valve can be impossibly difficult to get right if they are not matched to the system correctly.

Of course, this is a very simplistic description of servo valves. It can be a very complicated subject, depending on what level you want to go down to.

I hope this helps a little.

RE: Servo valve versus solenoid valve

Thank you guys, I appreciate! I am going to use the valves to control a small mechanical ROV arm. It will have four double acting cylinders and I was trying to determine the best cost efficient method of control. The pressure is regulated at 150 pai and will put all control system components in a pressure hull onboard the ROV.

RE: Servo valve versus solenoid valve

A four way, three position valve would normally be operated by two solenoids, not one.
If the center position of the valve is 'closed', the valve may be used to position a cylinder anywhere along its stroke.

The arbitrary 150 psi system pressure limitation will require use of relatively large cylinders for a given force. On the other hand, low pressure rated cylinders are relatively cheap.

Putting part of the hydraulic system inside the pressure hull could get interesting, by affecting the force available based on the ROV depth. ... not to mention increasing the number of holes to be sealed in the pressure hull. You might consider a fully external system, using a bladder for a reservoir, and having only electrical feedthrus. ... but we don't know if electricity will be available, or whether a person or a computer will be running the arm, or if the arm's motion will be visible to the operator, or how much force will be required on any axis, or how much speed will be required on any axis, or much of anything else.

Frankly, TeamHydra..., I'd have fired you already if I were paying you for this project.

We've been discussing this same system for months, in what must be hundreds of messages, and the system design has clearly not progressed beyond generalizations, responses that are sometimes grossly inappropriate or just plain wrong

Mike Halloran
Pembroke Pines, FL, USA

RE: Servo valve versus solenoid valve

Forget the servo valves.
Control the positioning directly with small motors and pumps.
I agree with Mike Halloran, I would have fired your a$$ for ignoring all the good suggestions.
I suggested using a mobile like solution and closed hydraulics that maintain a pressure equal or above the pressure at your crush depth.
The speed of the arm would be controlled by the motor speeds not by valve openings.
You need to look at the latest mobile technology and closed hydraulic systems.
Purdue University's Maha hydraulic center does a lot of research on mobile hydraulics.
e-mail this person about mobile/off shore technology
I really think industry should look at what the best university hydraulic programs are doing and use them as a resource.

BTW, there is little difference between servo valves and solenoid valves as long as the spools are servo quality. Bosch-Rexroth calls their valves servo-solenoid valves. The difference is how the valve spool is moved. Servo valves move the spool using oil pressure. Servo solenoid valves move the spool using a solenoid. Duh. I have adopted that term servo-solenoid valves to differentiate between servo quality solenoid valves and those valves that are called proportional valves. The issue I have always had with people that use the term proportional valves is that proportional means proportional to what. In most cases the spool position is proportional to the control voltage but that doesn't mean much if the spool is cut so it has overlap or is non-linear.

Peter Nachtwey
Delta Computer Systems

RE: Servo valve versus solenoid valve

Mike and P y'all are absolutely right, you guys did have great suggestions and know that you guys have been in the industry and appreciate giving me an insight. As a student having no prior experience with hydraulics, I respect and take in all your feedback. I wouldve fired me too. I mistakenly (dumbly) didn't put much thought into how to control the actuators at such low pressure. Just wanted to be sure before making any purchasing decisions. Needed that push and sense put into me, thanks.

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