APPS: SERVO VALVE NULL Bias Current

Sat Mar 21 13:56:37 EDT 2009

==> Automation List post by Mehul...
To view or reply to this thread: http://www.control.com/thread/1235491574#1237632643

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Re: APPS: SERVO VALVE NULL Bias Current
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Dear CSA,
Here I have concept of NULL Bias, You are requested to confirm the same. 

There is good talk on the Null Bias Current for the servo. We can use the drawing of Servo along with schematic of IGV or GCV-SRV to understand the concept.

For this talk reference, let us take IGV scheme, the IGV is stable at one position 57 deg. By viewing the scheme of IGV actuating Piston & Cylinder, both side of Piston must be equal pressured, as CSRGVOUT is CSRGVBAK(for Mark_IV) to keep IGV at 57 deg STABLE. 

Now look at the drawing of Servo, to maintain the both sides of piston equally pressurized, the spool should be at the middle of the bushing(such that it neither allows any port{connected to one side of piston} to drain nor gives extra pressure to Port{Other side of Piston}). And thus flapper (Feedback Sleeve) should be at centre and so the armature should be at the centre. And to keep the armature at physical centre position, amount of current applied, is null bias current. 

Consider what would happen, if the spool is not coming back to centre position. Definitely! it will drive the IGV in either extreme position.

Now consider, the IGV is required to be open to maintain the Exhaust temperature. The Controller will change the current to servo such that the armature will forces spool to move. The movement of the spool will be such that one port will be connected to pressure source and at the same time, opens the other port to the drain. This will create the pressure diff. on both side of piston. Ultimately Piston will move the IGV to open.      At the end, when CSRGVOUT=CSRGVBAK(command=feedback), once again armature is required to be at to null position i.e. centre position, which is done by providing null bias current by controllers

Similarly the mechanical null bias key has been provided to adjust the armature at centre positon.

Thanks in advance
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