% Retrieve or compute parameters for ducted-fan VTOL micro-UAV.
%
% Copyright (C) 2024, Naoki Sean Pross, ETH Zürich

function [params] = uav_params()

% Unit of measurements unless specified otherwise are
% Mass in kg
% Lenghts in m
% Time in s
% Frequencies in Hz
% Angular velocities in rad / s

params = struct();

% ------------------------------------------------------------------------
% Physical constants

params.physics = struct(...
  'Gravity', 9.81, ...
  'AirDensity', 1.204 ... % kg / m^3
);

% ------------------------------------------------------------------------
% Mechanical measurements

params.mechanical = struct(...
  'Mass', 4.0, ...
  'DuctRadius', 46e-3, ...
  'DuctHeight', 171e-3, ...
  'FlapZDistance', 98e-3, ... % flap distance along z from center of mass
  'InertiaTensor', 5e-3 * eye(3), ... % TODO true values, assume diagonal
  'GyroscopicInertiaZ', 0 ... % assume no gyroscopic effects for now
);

% ------------------------------------------------------------------------
% Actuator limits and measurements

% Cheap servos usually give a "speed" in seconds / 60 degrees without load
servo_speed = 0.19; % seconds per 60 deg
servo_angular_velocity = (pi / 3) / servo_speed;

params.actuators = struct(...
  'TurbineMaxSpeed', 620.7, ... % in rad / s
  'ServoAbsMaxAngle', 25 * pi / 180, ... % in radians
  'ServoNominalAngularVelocity', servo_angular_velocity ...
);

params.measurements = struct(...
  'SensorFusionBandwidth', 1e3, ...
  'SensorFusionDelay', 8e-3 ...
);

% ------------------------------------------------------------------------
% Aerodynamics modelling

% Compute thrust proportionality factor from actuator limits
% from thrust relation F = k * omega^2.
% FIXME: this is not ideal, need better measurements
omega_max = params.actuators.TurbineMaxSpeed;
F_max = 38.637; % in N (measured)
k_T = sqrt(F_max / omega_max);

params.aerodynamics = struct(...
  'ThrustOmegaProp', k_T, ... % in s^2 / (rad * N)
  'FlapArea', 23e-3 * 10e-3 * 1.5, ... % FIXME factor 150% was chosen at random
  'DragCoefficients', [1, 4] .* 1e-3, ... % TODO
  'LiftCoefficient', 1e-3 ... % TODO
);

% ------------------------------------------------------------------------
% Linearization point of non-linear dynamics.

% Compute theoretical thrust required to make the UAV hover
% from the relation mg = k * omega^2
% FIXME: This value should probably be replaced with a measurement 
g = params.physics.Gravity;
m = params.mechanical.Mass;
k = params.aerodynamics.ThrustOmegaProp;

omega_hover = sqrt(m * g / k);

params.linearization = struct(...
  'Position', [0; 0; -3], ... % in inertial frame, z points down
  'Velocity', [0; 0; 0], ... % in inertial frame
  'Angles', [0; 0; 0], ...   % in body frame
  'AngularVelocities', [0; 0; 0], ... % in body frame
  'Inputs', [0; 0; 0; 0; omega_hover] ... % Flaps at rest and turbine at X
);

% ------------------------------------------------------------------------
% Performance requirements

params.performance = struct(...
  'MaxHorizontalSpeed', 1e-2, ... % in m / s
  'MaxVerticalSpeed', 5e-2 ... % in m / s
);

end
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