valve positioners

Question 1

What are positioners?

Answer 1

are motion contorl devices to actively compare stem position against the control signal adjusting pressure to the actuator diaphragm or piston until the correct stem position is reached,

Question 2

advantage of having a positioner as opposed to having no positioner at all

Answer 2

superior valve seating (tight shutoff)

Question 3

Are positioners essential for spring less types of actuators?

Answer 3

yes
A positioner must alternately apply air pressure to both surfaces of the piston to raise and lower the valve stem.