Hello,

any controls guy can please help me to know why we give -10ma to +10 ma instead of voltages to servo coils for controlling hydraulic flow in servo valve for actuator? as per my understandings there is coil inside servo that operates armature and then fkapoer and spool for hydraulic flow. here I m confused that why can't we give voltage to coil instead of current?

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yaseeeen,

If we're talking about an electro-hydraulic servo-valve such as the type used on GE-design heavy duty gas turbines, the sevo-valve coils are generally 1000 ohms (approximately), so 10 mA through 1000 ohms equals 10 volts. You can measure the voltage "being applied" to the coil with a voltmeter.

Torque is required to move the flapper, and torque is developed using an electro-magnet with current flowing through the coils turns--a parameter called ampere-turns, or amp-turns.

The current has to be bipolar (positive or negative) to move the flapper in one direction or the other. The magnitude of the current determines how fast the flapper moves, which determines how fast oil is sent to, or drained from, a hydraulic actuator.

When the feedback from the device being positioned by the hydraulic actuator and servo-valve is equal to the reference, the servo-valve requires 0 mA to keep the flapper from directing current to one end or the other of the servo-valve spool piece--which means no hydraulic oil is being sent to or drained from the hydraulic actuator. In other words, it's being held in a steady-state position--because the feedback is equal to the reference, which means the device is in the proper position and no flow to or from the actuator is required.

Applying current of one polarity or the other (positive or negative) will move the flapper in one direction or the other which will port oil to or from the hydraulic actuator to change the position or flow-rate through the control valve or IGVs. Again, increasing the magnitude of the current will just result in more oil flowing more quickly to the actuator, which will make it move faster.

But, when the feedback from the device equals the reference the servo-valve requires zero current.

The electro-hydraulic servo-valves used on GE-design heavy duty gas turbines have a spring which is always trying to push the spool piece in the direction to shut off the flow of fuel or air through the control valve or IGVs. It requires a little extra current to overcome this spring's tension and to keep the flapper in the mid position which would hold the control valve or the IGVs in the desired position or maintain the desired flow-rate. The spring is call the fail-safe spring, and it's purpose is to ensure that if there is a loss of current to the servo-valve the flow of fuel or air to the turbine will be shut off.

The extra current which is added to the servo-valve output of the turbine control system is called the "null bias current." Null can mean to negate the effect of something else--in this case, it means to negate the effect of the fail-safe spring. That's why when monitoring the servo-valve output--either with the Speedtronic operator interface computer, or with a volt-meter, or with an ammeter inserted into the servo-valve coil current circuit (loop), a small negative current will always be seen even when the device (the control valve or IGVs) is in the steady-state position (when the device feedback equals the device reference). Because a small amount of negative current is required to overcome the fail-safe spring tension to keep the flapper valve in the position to put a small amount of oil on the opposite end of the spool piece from the fail-safe spring, which will keep the device in the desired position.

Why negative? It had to either positive or negative, and long ago and far away someone somewhere decided applying negative current to the servo-valve would increase the flow of air or fuel to the turbine, and positive current would decrease the flow of air or fuel to the turbine. (There may be another reason--but I've never been able to learn what it was. And, still--a decision would likely have had to been made about which polarity would be used to increase or decrease air/fuel to the turbine.)

Servo-valves are NOT magical, mythical devices--they're just not proportional control devices like most other positioners used for control. BUT, the same control mode has to be implemented inside the positioner even when the signal is 4-20 mA, or 0-5 VDC or 0-10 VDC, or 0-50 mA. Inside the positioner (pneumatic or hydraulic) the force being applied to the actuator has to be controlled to hold the device in the desired position, or to move the device open or closed. It's just that that's not "visible" to the technician.

They're essentially the same device, just one works more "silently" in the background than the other. Because there's so little documentation written about servo-valves and because so many other control systems use 4-20 mA positioners servo-valves are perceived to be magical and mysterious and "old-shool" and many other derogatory references.

If you have a question about some other servo-valve application, you will need to be more specific.

 

Dear CSA,

Thanks a lot for your detailed explanation to clear my confusions. it has been always a lot of knowledge enhancement by reading your post, thanks for that. at present I am working on mark Five GE gas turbine frame 9e machine. I had a query about servo valves as I read on the tag plate of servo valve model -78k6100, serial-103 that signal to be given is negative 7.20 ma to +8.80 ma, so is -10 to +10 ma not standard? Or can we set it as pet our requirement.

Thanks again.

 

yaseeeen,

+/-10 mA is a maximum range. The Mark V never really puts out more than approximately +/-8 mA, and that's called saturation current. If the Mark V is putting out saturation current then something is REALLY wrong because the device isn't moving and the feedback isn't anywhere close to the reference.

You can sort of see this when the unit isn't running and the fuel control valve reference is -25% (or -40%; it seems to vary--but it's a negative number to ensure the valve won't open, even if there's no hydraulic pressure). The servo-valve output current may be +3 or +4 mA (30-40% in Mark V units).

The normal running value of servo-valve output current (per processor) should be approximately -1.33% to -4.00% under normal circumstances. So, +/-8 mA (80% in Mark V units), again, would only indicate serious problems.

I believe the reason +/-10 mA was chosen for Speedtronic control systems when the servo-valves have a slightly smaller range is to make conversion to the PCT Engineering Units (percentage) simpler. It's a simple 1:10 conversion. 1 mA equals 10%; 3 mA equals 30%. If the range was +/-8 mA equals +/-100%, then 1 mA would equal 12.5%, and that's not as simple a conversion. But, that's just a guess on my part.

Hope this helps!

 

Thanks CSA,

is above explanation true for mark ive system also? can u plz inform me that when I measured voltage across servo coils using voltmeter, I found 4v. this equals to 4ma, when coil resistance is nearly 1000ohms as per my understandings. how can it be as the valve is fully open, so I should get about 0 ma or some negative current (fail safe spring current)? please correct me if I have misunderstood.

 

yaseeeen,

There is no such product as a Mark IVe; there is Mark IV and Mark IV Migration, but no Mark IVe.

If a valve position reference is, say, 95%, and the valve can travel to 95% without hitting a mechanical stop or -limit, and the position feedback is approximately 95%, AND all of the servo-valve coils are in working order, AND the current being applied to the coils is of the proper polarity, then on a TMR Speedtronic turbine control system the individual voltages for each coil should be between -1.33 VDC and -4.00 VDC. Ideally they would each be -2.67 VDC. (These voltages correspond to -1.33 mA to -4.00 ma, and -2.67 mA, respectively.)

If a valve reference was 100% and the valve was on a mechanical stop or -limit right at or slightly below 100% it's possible that the voltage could be slightly higher (negative magnitude) than expected--again presuming proper current polarities, etc.

If a valve reference was 125%, but the valve could only travel to approximately 100%, the voltages would probably be much higher (negative magnitude) than expected.

<i>If the servo-valves DID NOT have fail-safe springs, then the current/voltage would be zero when the reference and feedback were equal</i>. But because the servo-valves have fail-safe springs a small amount of current (called a bias) is added to the outputs when the reference and feedback are equal to overcome the fail-safe spring tension to keep the device in the position that keeps the reference and feedback equal. And that bias is ALWAYS added to the servo-valve output current signal.

 

CSA

as i'm checking the regulator diagram in 6195D GE document, i can see that the output of the regulator (type 43) is as follows:
"(REF-FDBK)*gain+bias current"

here i can't understand where the minus sign take place exactly. because as you mentioned, if we have a negative output mean, the valve gonna open means REF is greater than FDBK; and if it is positive the valve gonna close; and of REF is equal to FDBK the output would be -0.267ma.

correct me if i'm wrong.

 

Isulamu,

The ((REF - FDBK) * gain + bias current) is all done in software, and if the feedback is less than the reference then the term will be positive (to increase the position), and if the feedback is more than the reference then the term will be negative (to decrease the position).

<b><i>HOWEVER,</b></i> when the software signal is sent to the D/A (Digital-to-Analog) converter, the output of the D/A converter is inverted (made opposite to the input)--and that's how the actual mA current is made negative to increase the flow of fuel or air, and negative to decrease the flow of fuel or air.

I don't have access to GEH-6195D (The Speedtronic Mark V Application Manual), and I don't recall if the inversion is shown on the regulator diagrams (I believe they are in Section 7--but I may be wrong about that). The electro-hydraulic servo-valves used on GE-design heavy duty gas turbines REQUIRE negative servo current to increase the flow of fuel or air, and positive servo current to decrease the flow of fuel or air. So, while the software part of the regulator is straight-forward and as described, it's the hardware part of the regulator that changes a positive input (calling for an increase in flow) to a negative servo-valve output current, and vice versa for a negative input to the hardware circuit (the D/A converter).

Hope this helps!

 

CSA

This is what i was doubting my self and i wanted to confirm it.

thank you very much indeed for your help

peace

 

It is ma that controls your magnetizing current, thereby controlling
valve plug opening. Good luck!