As we learn pneumatic type positioner, they use cam, lever and other physical components to achieve closed-loop control.
In electronic type positioner, it uses the sensor to detect valve stem positioner, an internal microprocessor is used to compare that sensed stem position against control signal by producing an error.
Then pneumatic signal and relay ti send air pressure to the valve actuator. Following the diagram of the electronic positioner.
As you can see in the above figure that the two control algorithm is used to control the valve position.
One is monitoring and controlling the pressure applied to actuator and another is controlling the valve stem position.
First, command signal is sent from controller which compare with actuator position and give an error based on calculation, which then sent to PI controller.
On actuator side pressure sensor used to monitor pressure, which gives a feedback of applied pressure on actuator.
Now position error coming from PI controller and actuator pressure compared, based on an error it sent it to PID controller which provide a signal to I/P converter and the corrected signal is sent to an actuator through a relay.
Unlike Pneumatic positioner, the electronic positioner is microprocessor-based SMART positioner provide handly diagnostic information.
You can do leak test, Can easily do loop check using HART protocol or using any source, can perform self-calibration, etc.
Nowadays such type of positioner is widely used because of its easy maintenance, calibration and display access.