Can a PID Regulator have two or more feedback loops? For example, if we are measuring process variable which has negative values and positive.
My understanding is no. PID only has a single control loop.
There are 2 ways to go about PID.
1. Manually tune the PI parameters to get best response then tweek the D.
2. Characterise the open loop transfer equation of the system, so that you can use for the closed loop equation.
With the 2 option, you can have as many inputs as you like. They will just boil down to a simple multiplier or an offset constants. This sounds like the 'negative values and positive'.
There is however only a single feedback loop.