Spaghetti

Layperson "how it works"

The central idea to spaghetti is an linear compensator that computes a control effort, given the history of measured altitudes. You don't need to know how it works. This control effort is in kg/s. It this then scaled by a gain, called k. Since ballasting and venting are discrete on/off, and the commanded effort is continuous, this needs to be converted into a series of discrete actions, separated by a time interval. This is much like PWM, but instead of changing duty cycle directly, you are changing the time interval between events such that the action time is fixed to it's minimum acceptable value, and produce the least discretization (in the future this may want to be changed to conserve power and minimize the number of valve actions).

Constants

freq: The frequency that the controller is called out, which will be a constant 20Hz

k: Scallar gain of the controller. High gains mean more actions, tighter control. Low gain means less actions, looser control

b_dldt: Magnitude of change in lift vs time for ballast actions IN kg/s. This can be directly measured on vb, and us usually around 0.001kg/s

v_dldt: Magnitude of Change in lift vs time for vent actions IN kg/s. This is always an estimate as can not be measured directly. It's a magnitude, and therefor always positive, even though venting decreases net lift

rate_min: minimum threshold for commanding a dl/dt

rate_max: maximum threshold for commanding a dl/dt

b_tmin: the minimum time that a discrete ballast event can be called for. This sets the time that ballast event will be for.

v_tmin: the minimum time that a vent ballast event can be called for. This sets the time that ballast event will be for.

h_cmd: the command altitude for the controller to aim for

Flight Controller Guide

In the dev version of spaghetti, you can't really set bounds on your altitude as you can with the legacy controller. Instead, you set a target, and a gain, and the controller attempts to hit the target, using an effort proportional to the gain. What gain will give you the altitude range you want? Who knows.

You also have other things to tune. You want a positive effort to have an equal but opposite effect on the system as a negative effort. Thing is, you don't exactly know you dl/dt for vent actions. So, you have to guess. If it looks like venting isn't having much effect, and the controller is spending much more time above the commanded altitude than below, decrease the set dl/dt for the venting, as your estimate of it is too high.

Also, you don't want to act when your effort is really low and you are close to the target. There needs to be a minimum threshold before you start acting. This is rate_min. The higher you make rate_min, the more the controller acts like a bang-bang controller, where it waits till it's pretty late and then acts a bunch. about 1 order of magnitude below your max rate is probably a decent place to start for your rate min.