The autotune function has rarely given me production-ready, usable values, but it gets you in the ball park. Fine tuning is usually required to get optimal results.
I don't see how it could but if it gets close is still is a big help. Before we had auto tuning one of the most common questions was what gains do I put in the PID or where do I start.
Most of these controllers do not have the processing power or the memory to auto tuning like we did in the Advanced Control thread. They just make a few estimates similar to the visual inspection techniques used on the
www.controlguru.com site. If one looks at auto tuning done with the Scilab scripts in the Advanced Control thread you can see that the second order model fits the process much better.
Keith, you are almost right, but what if the first order system is a type 1 system ( position ) verses a type 0 ( velocity ) system.
First order systems
Type 0, Gp(s)=K/(tc*s+1) // This requires a PI
Type 1, Gp(s)=K/(s*(tc*s+1)) // This requires a PID
Second order systems
Type 0, Gp(s)=K/(tc1*tc1*s^2+(tc1+tc2)*s+1) // This requires a PID
Type 1, Gp(s)=K/(s*(tc1*tc1*s^2+(tc1+tc2)*s+1)) // This requires a PID // This requires a PID+2ndD
This requiures a PID with a second derivative term. I have seen PLC PIDs with 2nd derivative terms so hope you never see one of these unless you have a real controller.
Most just do the best they can with the PID they have good and hope it is good enough.
http://www.engin.umich.edu/group/ctm/examples/motor2/PID2.html
They didn't do a very good job of tuning in this example.
Notice the type 1 systems have an extra integrator. Type 0 systems are those like velocity or temperature control system where PV will settle back to 0 or ******t when control is no longer applied. Type 1 systems are like position or tank level controllers where the velocity or flow is integrated to get position or level. If control power is removed the position stops but remains where it is unless someone distrubs it. Likewise for the tank level as long as there is no inflow. ( a distrubance ).