Control.com - Nerds in Control

I have an actuator with a temposonic positioner in it, which controls a servo valve (Moog). When a scanner tells the tempo where to position itself, the tempo tells the servo to respond in either direction until the desired position is reached. The problem I have is that, when the control cable is disconnected from the servo, there is fluid flow, and the servo will move. Now I have to null the valve. It is my understanding that if there is fluid flow, I have to null the valve. But I have been told that I am not supposed to do this. Please help me understand these servo valves better.

dan
iownthebruins at yahoo. ca

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?

Dan,

My experience with servo valves was on a non-critical failure process where there was a Moog servo valve, a feedback device which was an LVDT, an error operational amplifier, an output amplifier, and a PLC that issued position commands to the error amplifier.

It was about the simplest servo system there was. It was a closed loop system, and it drove everybody nuts.

A lot of times, what had caused a shift in position was a change in the mechanical null adjustment of the servo valve. This would cause an offset in the final attained position of the system. You told it to go to 1.000 inch and it went to say 0.950 inch.

In our system, there were two ways to compensate for this: one was to move the feedback device, the other was to adjust the valve.

Our system was fail safe. No mechanical damage would occur if the actuator moved one end or the other. In our system, a properly adjusted valve with a cable disconnected would cause the actuator to move to its end of stroke, one way or the other, at a very slow speed.

So, good luck, please describe your system, and include the numbers of your Moog valve.

John Ross

john1542chupa at aol. com