Just trying to get my head around something ...correct me if i am wrong on any of this. Modern excavators use engine speed sensor, pressure sensors,angle sensors etc to vary a pressure signal to the pump controller making it adjust the swash plate angle correct? On older machines with no electronic control like that how was the pump swash controlled? Dont have a problem with a particular machine just trying to educate myself a little better!
Speaking for Cat excavators, some were designed to run at flat out RPM and simply sensed pump output pressure and destroked the pumps as the pressure rose. Others did sense engine RPM by measuring pilot pump output flow. They called the sensing valve the underspeed valve. See some of my posts on hydrostatic track loaders for more info on underspeed valves. Either way it was all done hydraulicaly.
Excavator pump controls are different between manufacturers and a lot of times different between models and sub models. Old systems built in the sixties and seventies were positive displacement. That means the displacement only changed by running the engine faster or slower. As I recall horsepower summation came next. This is where a relief valve was in the system that vented off a variable amount of oil as the engine started to loose RPM. The next systems were partial load sensing and then full load sensing. Today the pump and control valves are a set. You will find open center and closed center load sensing systems. Some use the speed sensor to back off the pump through computer control when the engine looses RPM. Some do the whole thing hydraulically with a computer system only handling the most severe parts of the operation. The computers handle a whole lot more than just the pump also. I don't think I know anyone who is an expert on all manufactures but I know plenty who are good on two are three. In order to learn the systems you basically have to get your hands dirty on each and figure most of it out yourself. If you have a question on any one type of system I'm sure someone here will be able to provide some info. Good Luck!