The objective of the Nitrous Oxide control loop is to take data from instrumentation on the engine (i.e. pressure data) and use that data as a feedback parameter to control oxidizer mass flow in real time.
This image, also taken from the paper by Airborne Engineering, depicts a nitrous oxide throttle control loop. The demanded variable here is the chamber pressure, which was found to have a linear relationship with N2O throttle valve pulsewidth. First, the loop computes pc_err, the difference between the demanded and current pressure. Then, pc_err is passed into a controller, which converts pc_err into a pulsewidth. This pulsewidth is sent off to the servo (with an additional pulsewidth to offset deadband), which moves the servo slightly to try and get pc_err to zero. The pulsewidth is also sent to a plant, which calculates the gradient of chamber pressure with respect with pulsewidth. This gradient tells us how much this pulsewidth will affect the chamber pressure; the same graph that plots chamber pressure versus N2O valve pulsewidth is used to calculate the gradient. After the pressure gradient is calculated, it is passed into a plumbing delay. The plumbing delay just says that the pulsewidth sent to the valve will take a nonzero amount of time to affect the chamber pressure. For example, if the valve received a pulsewidth that made it open 10% more than it was before, the greater mass flow through the valve would have to travel to the injector and combust in the chamber before the chamber pressure changed. The exact response time is determined during open loop testing. Finally, the chamber pressure is fed back to the beginning of the loop and is used to calculate a new pc_err.
