Hi,
Don't really know how the spring return will work here...Can't really see why it wouldn't work....

The 0V at 0 error isn't really true.... When the error is 0 output from the P- and D-term is indeed 0 (the D-term might not be 0 but for this discussion let's say it is). But the output from the I-term is whatever was needed to bring the error to 0. (It might be 0 but not necessarily).

Think of the car analogy, you're trying to keep the speed at 50mph and you're starting to climb a steep hill, if you simply keep the gas-pedal at the exact same position the speed will start to drop. The P-term adds a little effort based on the error but not enough since the load now is larger (we're climbing a hill). The I-term starts adding a little effort (giving some gas) and soon the velocity is back at 50mph. Now the P term is 0 (since there is no error) the D-term is 0 since there is no error but the I-term is not zero - if it were the speed would start to drop again.

With that said, simply add a little to the output of the PID-filter rto overcome the force of the spring - if that's what you want. This is often called feed-forward and means that for a particular speed, torque, position or whatever you have like a "starting point" for the "drive" - the PID filter then takes it from there.

/Henrik.