Positioner/actuator stiffness

Eressure difference—see Fig. 8-71). Dynamic stiffness is usually igher with piston designs than with spring and diaphragm actuators see Positioner/Actuator Stiffness. Failsafe action, if necessary, is achieved without a spring through the use of additional solenoid valves, trip valves, or relays. See Valve Control Devices. ... 

The stiffness characteristic of the positioner/actuator varies with frequency. Figure 8-75Z indicates the stiffness of the positioner/actu-ator is high at low frequencies and is directly related to the locked-stem pressure gain provided by the positioner. As frequency increases, a dip in the stiffness curve results Trom dynamic gain attenuation in the pneumatic amplifiers in the positioner. The value at the bottom of the dip is the sum of the mechanical stiffness of the spring in the actu-... 

FIG. 8-75 Frequency response curves for a pneumatic positioner/actuator (a) input signal to stem travel for a 69-inch spring and diaphragm actuator with a 1.5-inch total travel and. 3-15 psig input pressure (h ) dynamic stiffness for the same positioner/actuator. 

FIG. 8-82 Actuator stiffness as a function of frequency for a 69-in2 spring and diaphragm pneumatic actuator. Actuator with positioner exhibits higher stiffness over the lower frequency range compared to that of the pneumatic actuator without a positioner. 

The stiffness characteristic of the positioner/actuator varies with the... 

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