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About one year ago we replaced the MOOG IGV actuator on this machine which is controlled by a GE MARK Ve. Recently we began experiencing IGV fault trips (i.e., LVDT feedback too far from the demanded position). The LVDTs looked good with both of them producing nearly identical feedback in the R,S & T cores (or packs as they call them now) at the min position and the max position.
I replaced the servo and using the calibration program, checked the IGV's response to step increases in the manual position and the also the "POSITION" test routine that produces a full stroke ramp from min to max and back. The LVDTs tracked the demand signal very accurately.
However, I noticed that the servo current trend trace (which is a nice feature of the Ve cal program) appeared as a very thick line, unlike the premix and quat valve trend lines. It appeared to be the result of rapid oscillation of the servo current. I assumed it was dither, but it seemed unusually high. This "dither" disappears at the extreme ends of travel when the servo is saturated with pos or neg current.
I went out to the IGV and could hear the servo rapidly cycling like a machine gun in every position between min and max. The actuator shaft was rapidly moving up and down in short strokes, and I could feel the pressure pulsations. This seems quite excessive to me and could be the cause of such a short life span for the servo and possibly the actuator itself. Note that the servo spool oscillation disappeared with the IGV driven to the min or max positions.
There is a value for dither in the Regulator configuration (I don't recall it off hand), with the dither frequency set at zero, I believe. Maybe it's excessive, or maybe there's something else going on. I'm convinced that the equipment is going to have a short service life with this kind of wear and tear and I can't see the need to move actuator that much that often to prevent it from sticking. Ideas anybody?
I replaced the servo and using the calibration program, checked the IGV's response to step increases in the manual position and the also the "POSITION" test routine that produces a full stroke ramp from min to max and back. The LVDTs tracked the demand signal very accurately.
However, I noticed that the servo current trend trace (which is a nice feature of the Ve cal program) appeared as a very thick line, unlike the premix and quat valve trend lines. It appeared to be the result of rapid oscillation of the servo current. I assumed it was dither, but it seemed unusually high. This "dither" disappears at the extreme ends of travel when the servo is saturated with pos or neg current.
I went out to the IGV and could hear the servo rapidly cycling like a machine gun in every position between min and max. The actuator shaft was rapidly moving up and down in short strokes, and I could feel the pressure pulsations. This seems quite excessive to me and could be the cause of such a short life span for the servo and possibly the actuator itself. Note that the servo spool oscillation disappeared with the IGV driven to the min or max positions.
There is a value for dither in the Regulator configuration (I don't recall it off hand), with the dither frequency set at zero, I believe. Maybe it's excessive, or maybe there's something else going on. I'm convinced that the equipment is going to have a short service life with this kind of wear and tear and I can't see the need to move actuator that much that often to prevent it from sticking. Ideas anybody?