What effect does the positioner cam have on the valve characteristic?

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The feedback cam in the positioner controls the relationship between the control signal and valve position. With a linear cam at 50% signal the valve will be 50% open.

It is possible to alter the appearance characteristic of a valve by changing the shape of the cam e.g. for a ball valve that has an inherent equal percent character it is possible to make it appear linear so that the flow rate through the valve at 50% signal is half of the maximum flow – the valve will however only be 25% open to achieve this result.From the control point of view there are advantages in doing this, but changing the valve characteristic and keeping the linear cam in the positioner is a better technical solution if it is possible.