I’ve written an article on the wiki on what I call controller fusion.  I refer to it as controller fusion becuase, like Sensor Fusion, I use filters to blend non-ideal outputs from more than 1 transfer function into 1 output which is closer to the desired output.

For a work proposal on a reaction cancellation mechanism I used a proportional controller for fast response and a PI-Lead controller to drive the steady-state error to 0.  In simple sensor fusion the sensor outputs are filtered and then added together to form a better single output.  In this form of controller fusion I use filters on the error signal to adjust the gain of the controller in real-time.  As a result, PI-Lead output is almost turned off for a step command and the proportional controller output is almost turned off when the system is holding a steady-state value.

I have not had the time to bring this idea to full maturity but I thought it was an interesting enough idea to share.