servo vs proportional valves

[Hansen66]

Hi all
I have a quick question about the use of servo- vs proportional valves. A hydraulic technician told me the other day that servo valves are used to control flow and proportional valves used to controle pressure. Does any one know if this is true and if why? Wont servo valves be just as good at controling pressure as flow and the same with proportional valves?

Best regards
Hans

 

[hydromech]

In my experience, it is more a question of control.

Proportional valves are adjustable in terms of flow or pressure dependent on the type of coil used, force control or stroke control.

Servo valves offer the same but the have better performance characteristics such as hysteresis control, response time and repeatability.

Proportional adjustment of a spool position can adjust pressure or flow.

Adrian

 

[Hansen66]

Thank you for your answer Adrian

Just to clarify, if you are using a material test stand (as i'm working with) you would use a proportional valve to control the supply pressure and a servo valve to control the flow to the actuator because of the better peformance characteristics?

//Hans

 

[OilShear]

heck out this site:

http://www.hydraulicspneumatics.com/200/Issue/Article/False/83026/Issue

 

[grockj]

Hansen,

In general, a servo valve and/or a proportional valve is a variable restriction. They will control both pressure and flow. The question is actually in regard to how your controls are set up. You can close the loop on pressure or you can close the loop on flow.

There are physical characteristics of each valve that will change the applications they are used in. What you are stating probably arises from the fact that the majority of proportional valves I have seen are smaller in size, and thus usually deal with lower flow applications. However, with such a small size, higher pressures can usually be present (think instrumentation level pressures in a lab gas bottle) Whereas a servo valve, based on its construction, can be larger in regards to price concerns. Usually such large valves are used in higher flowing applications (think process piping) where pressures are lower (water or oil line.)

What you were told is based on common practice for closed loop control - USUALLY applications that require a servo valve are concerned with flow, and check it to close loop on the servo valve position (effective restriction.) USUALLY applications that require a proportional valve are concerned with pressure, and check it to close loop on the proportional valve position.

In fact, I'm working on an application right now with a proportional valve that has a closed loop on flow - we measure the gas flow through a hotwire mass flow meter and adjust current to the valve as required to hit our spec.

Hope this helps

Jack G