Different PID response from normal feedback loop and unity feedback loop

Maybe this can be a banal question, but i've two equivalent schemes with same input and same PID controller, but the unity feedback response differs from the normal feedback scheme. Can someone explain the reason for this behaviour? Thanks

Full image here: http://i51.tinypic.com/2enubf5.png

response:

Answer 1

[Guy Rouleau]

You are not comparing the same thing.

In the upper case, you take a measurement between process and sensor/transmitter. In the bottom case, you take a measurement at the end, after the sensor/transmitter.

To compare similar things, you can either

- In the upper case, take the measurement after the sensor

OR

- In the bottom case, compute a transfer function that does not include the sensor