Looking for advice on valves, my Servo driven ball valves have too much slop/hysteresis.
Liquid oxygen compatible.CV varies linearly from .13 to 3.0
It needs to shut off bubble tight , but can be very non-linear from off to a CV of .13.Max pressure will be 300PSI,Safety factor of 2
It sounds as if the seals are not the ones intended for Cryo, or if they are automating a valve that was originally for manual service. 
Worcester cryo seats are configured somewhat differently than seats for standard service, to allow for the not-insignificant dimensional change. There are other mods to the seat as well.   Also Worcester valves for modulating have a high-cycle packing stack with extra seal rings and 4 belleville washers (instead of just 2) to compensate for wear and thermal changes. 
I was involved with some test-stand globe valves once where NASA stipulated that they wanted 20 hz response(!). The testing procedure assumed that the 20 Hz was continuous.  It was possible to do this with an electrohydraulic actuator, but the valve packing melted. With LO2 in the valve, it's a good thing the packing did not ignite!.

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