Often when I'm designing interactive installations of some kind, I run into this problem, which I still haven't found a solution for, while I'm sure it's pretty simple.

Imagine the user having a control, for example the slider in the attached patch (could be a accelerometer that is shaken, for example). Whenever the user puts energy into this system, I want the signal to cumulatively build up. The problem is that, when one moves the slider back, one always gets a decrease in energy (which is logical). Every movement should make the system build up energy cumulatively, and only doing nothing should enable it to slowly go back to zero "energy".

Any ideas?