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What's the Difference Between a Servovalve and a Proportional Valve?

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In nearly every industrial training program that I conduct, someone asks me to explain the differences between servo and proportional valves. I love to hear the question because it presents an opportunity to have some fun. The short answer to the question is, “I refuse to get sucked into the debate.” Technical Standard ISO 10770-1, and the way it deals with the question, only adds to the fun. Before explaining why, I need to provide a little background.

Fluid power technical standards
The increasing use of electronics to control all types of hydraulic systems has naturally led to greater in interest in servo and proportional valves. This fact has not escaped the eyes of the ISO technical committees that are responsible for developing our international standards, primarily ISO/TC-131/SC-8, the technical committee for standardized testing of hydraulic components.

Technical Committee 131 is the ISO designation for the body that is responsible for all fluid power standards. The official US representative body at the ISO level is the American National Standards Institute, but ANSI has the authority to delegate secretariat functions for any technology to other U.S. national bodies. In the case of fluid power, ANSI has delegated that authority to the National Fluid Power Association, Milwaukee.

Sub-committee 8, Secretariat in the British Standards Institute, London, concerns itself specifically with standard test methods for fluid power equipment, including valves. In 2000, ISO issued an International Standard, ISO 10770-1, titled Hydraulic fluid power — Electrically modulated hydraulic control valves — Part 1: Test methods for four-way directional flow control valves. It deals with the methods of test and, by implication, the methods for rating servo and proportional valves. ISO 10770-1 replaced ISO 6404, which addressed servovalves only, and 10770-1 is an improvement over ISO 6404.

A do-it-yourself approach
In 1988 and 1989, while at Milwaukee School of Engineering’s Fluid Power Institute, I led a research project that was to deal with the practices and uses involving proportional and servovalves. One of our goals was to offer definitions in the form of a glossary, or lexicon, as we called it.

We surveyed all known manufacturers of such products and summarized their most important characteristics. Our goal was to identify the significant discriminators. In an attempt to determine how common industrial usage separated the two products, we tallied such features as:
• actuation method (pilot vs direct),
• frequency response,
• use of internal or external feedback,
• spool overlap,
• intended use of the valve (whether used in open-loop or closed-loop control systems), and.
• “accuracy” (whatever that means).

When we finished, only one discriminator was used to separate the two devices: The amount of spool overlap. This led us to formulate the following definitions of servo and proportional valves:

Servovalve — Any continuously variable, electrically modulated, directional control valve with less than 3% center overlap.

Proportional valve — Any continuously variable, electrically modulated, directional control valve with more than 3% center overlap.

These definitions were incorporated into the glossary of terms that was published at the end of the research project. Bear in mind that we tried to reconcile the uses of the terms common to the industry. Since then, I have personally introduced them in all my classes and at every NFPA and ISO meeting in which the subject comes up. To my surprise, the opposition has been silent. Now and then, someone will ask “Well, what if the overlap is exactly 3%?”My response is, “Take your pick!”

I believe the problem started when people began using the terms servovalve and proportional valve before anyone defined them. As a result, each term conjures up an image of the exact device that comes to mind for any individual person. The distinctions may be perfectly clear within a given company. However, in business-to-business dealings, especially if they are international, attempts at distinguishing between a servovalve from a proportional valve will invariably lead only to bruised egos, if not bodies. At some point, the debate switches from technology to religion.

Back to the officials
The ISO Working Group that developed 10770-1 dodged the bullet in admirable fashion. This was because the convener knew the dispute could never be settled. And yet, the committee wanted a document that covered both servo and proportional valves. But how can you do it without defining them? More specifically, ISO wanted emphasis on the generally accepted use of the terms that proportional valves are used in situations with a lesser pressure drop than were servovalves. (I take vehement exception to this thinking.) Therefore, according to ISO, proportional valves need a lower pressure rating.

The ISO committee decided that servovalves (as has always been the case) would be flow rated at a “rounded off” 1000 psi, or 7 MPa (1015 psi), but proportional valves would have a new flow rating pressure of 1 MPa, or about 145 psi.

Part of the document reads, “Set the valve total pressure drop to 1 MPa or 7 MPa, as appropriate.” The “as appropriate” phrase got the committee off the hook. Who judges what is appropriate? Well, the testers do. They can test it at either servo or proportional levels. But, more importantly, they can call the valve anything they want! So now we have proportional valves that are flow rated and tested at 1000 psi. We also have “zero-lapped proportional valves,” so this war of words may never be won, only fought.

The reality of the situation
Only one question influences whether a servovalve should be selected over a proportional valve, or vice versa: Does the performance of the valve meet the requirements demanded of the application? Answering that question, unfortunately, usually requires more than just some definitions. It requires translating the application needs into the valve performance specifications or vice-versa.

This usually involves some calculations, which are not normally taught in basic hydraulics courses. Nonetheless, the methods are straightforward, if not tedious, but will lead to good, reliable, high performance system designs. More importantly, though, the calculations can be performed so that such questions as “How much overlap can I tolerate?” are answered quantitatively, without ambiguity.

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