AXON FBL Notes Simplified-ish
I prefer to think in very simplified terms. If someting can be explained in a clear and concise fashion I tend to retain the info better.
In an effort to help me understand better the inner workings and objectives of the different AXON parameters I've tried to reduce them down and simplify the termonology to explain each of those parameters and how they affect the flight of the helicopter if and when I modify them.
The following is my set of notes. While I believe they are reasonbly accurate I certainly welcome any input that would increase the accuracy or correct any misinformation.
AXON and RX Parameters
HEAD
Direct Control Portion: (aka Cyclic Feed Forward) The ratio of FBL input to Pilot direct input
- More = Crisper to Twitchy (more TX direct control, less FBL stability input)
- Less = Softer to Sluggish (less TX direct control, more FBL stability input)
Agility: Cyclic Rates/speed - a higher number gives faster cyclic movement
- Minimum setting best for ‘Scale’ ops.
Decay Rate: (Washout) Swash-plate Return Rate/Speed when stick returned to neutral.
- Slow = stays longer in a cyclic Pitch/Bank after cyclic input released
- Fast = quicker positioning back to Neutral cyclic
- Slower rate can/will reduce nose up tendencies in FFF
- Too slow can make Heli feel less agile or slow to respond.
An example would be the Heli continues to bank after a bank input requiring a counter input to keep or return to the desired bank. This relation ship could result in unecassary counter controlling and/or overcontrolling
Attitude Hold Range: Resistance to moving away from a given attitude. A.k.a. resistance to outside forces/disturbances.
- Higher setting makes hovering more stable but other flight realms less agile
Elevator Filter: Dampens pitch oscillations aka 'Anti Dolphin' effect.
- Helps moderate porpoising with abrupt/moderate collective inputs in FFF or climb (more particular to 3D flying)
TAIL
Gyro Gain - adjusts all preset PID parameters in unison
- Too low, tail slow wags or drifts from heading
- Too high, tail shakes/oscillates quickly
P Gain - Primary gain setting. Modifies built in setting independently
D Gain - Dampens P-Gain overshoot oscillations. Modifies built in setting independently.
Agility - Pirouette speed. Higher is faster. Slow better for Scale
Lock In Delay - Tail Stopping authority. Helps prevent overshoots on rapid direction changes or stops
Revo Mix - Adds tail input relative to collective input(more for gas or nitro not for scale electric or turbine)
PID Systems
Gain: degree of correction per sensed error size.
P Gain: Proportional
- P Gain is the major adjustment component
- It is the amount of ‘aggression’ used to acquire target
- The amount of correction relative to size of error. Ie. for a given distance away from target setting, using a higher or lower correction gain input which then proportionally reduces as target is approached.
- Only error ‘distance’ is considered not rate(time taken)of change
- Too Low Proportional Gain (under-correction)causes slow movement to/acquiring of target.
- Too low may never reach target.
- Too High Proportional Gain (over-correction) causes overshoots- rapid wags/oscillations
- Best gain minimizes 'bracketing' to target
D Gain: Derivative
- Best P gain may still bracket somewhat
- D gain dampens P efforts as related to time
- Reduces P as target is approached relative to the rate of approach
- Too Low a D gain: P continues to bracket to target.
- Too High a D gain: slows and/or inhibits ability of P to acquire target
- Best gain: minimizes or eliminates P bracketing. I.e. tail will stop without overshoots or wags.
I Gain: Integral
- Corrects for external disturbances that move P and D off target
- Dampens proportional oscillations. Fine tunes P and D Gains.
- If too high it can cause overshoot of a good P Gain value setting (causing appearance of incorrect P gain).
Mechanical setup will have effects on PID abilities.
- A frictionless system would be ideal but unrealistic, so effort should be made to make the system as smooth as possible.
Even seemingly insignificant items like a ball or ball-link with a nick can cause an issue. Lubricants on tail box shaft can affect performance as well. Dry shafts are better and less prone to accumulations of dust and dirt.
- Rapid application or removal of torque to tail drive can cause PID to overcorrect. Ie. high head-speed gains(aggressive power increases or decreases) which impart rapid torque changes can cause tail to wag.
- Too slow tail rotor speed requiring greater pitch changes can inhibit PID performance as well.
- Adequate servo speed and power are essential.