Control system error based on differential process value
You're trying to think of this as a "reference tracking control problem". The situation you describe is still related to control systems, but it attempts to solve a different problem: an optimization problem.

If you wish to locate a "peak strength", this means you wish to solve the problem of finding a local maximum. The same mathematical approaches used to numeric optimization problems can be applied. The most intuitive one is the gradient descent method. I will attempt to describe how to find a local maximum in generic terms, so that you see how it is applicable to your problem.


For the currrent state of the system, determine the direction of greatest increase (the direction of the gradient vector).
Take your system to a new state along such direction, in a step that is proportional to the magnitude of the increase.
Iterate until the increase in any direction is lower in magnitude than a given tolerance (the derivatives at the point are arbitrarily close to zero, optimization point found).


Another way to put it is, as you've described in your question, the error signal is a derivative. Well, to solve an optimization problem essentially means to find a point with derivatives close to zero.