INFO: Servo valves

Wed Nov 5 00:57:42 EST 2008

==> Automation List post by CSA...
To view or reply to this thread: http://www.control.com/thread/1225784442#1225860613

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Re: INFO: Servo valves
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When the cable is disconnected from the servo valve, does the actuator move to a "fail-safe" condition?

That's what's generally supposed to happen with one of these valves when the cable is disconnected or severed or the control system fails with no current output: the device is moved to a position that protects the process, such as shutting off the flow of fuel to an engine or turbine, for example.

Some Moog servo-valves have an internal "null bias" spring which serves, in the event of loss of current, to move the actuator which it controls to  prevent damage or personnel injury, the "fail-safe" condition.

I'm sure that there are applications that don't require this action, but if this is the type of servo valve which was supplied for this application then it's a safe bet that it was properly chosen to provide safety in the event of loss of current to the servo-valve. It's really important to understand the application and what it requires.

I have seen people try to adjust servo-valve null bias springs with disastrous results, meaning that in every case the servo-valve had to be replaced.

If you haven't already done so, please try to understand the application and what it requires for safety. Can you refer to any documentation about the process and the actuator/servo-valve to see what the intent of the application is? Have you contacted Moog for any information on the servo-valve?
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