The combination of a flapper or baffle and a nozzle
And their relative positioning can cause variations in pressure of the control air varying from 1.2 bar to 2.0bar. With the flapper or baffle almost closing the nozzle the pressure attained is 2.0 bar .An on-off operation, or low-rate and high rate can be utilised with these two pressures using a flapper travel as small as .05mm or less.
The average flapper travel between two limits is often about is 0.1 mm
The relationship between flapper travel and and pressure p is non- linear , but over a fairly wide range of travel, say 30 to 70% the relation ship is fairly linear.
Hence proportional control can be established between 30percent to 70 percent of flapper travel.
Secondly the pneumatic relay is an amplifier and this gives considerable increase in control movement and its combination enables integral control.
The third advantage is the valve positioned . This unit functions to make the valve movement occur rapidly and accurately to position by supplying large forces to overcome inertia and friction
In pneumatic controls the movements are small , thereby reducing the errors of lost motion in loose pin joints, thereby increasing accuracy as well as speed of control.
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