%*** CHAPTER 8: DYNAMICS OF OPEN CHAINS ***

function taulist = InverseDynamics(thetalist, dthetalist, ddthetalist, ...
                                   g, Ftip, Mlist, Glist, Slist)
% Takes thetalist: n-vector of joint variables,
%       dthetalist: n-vector of joint rates,
%       ddthetalist: n-vector of joint accelerations,
%       g: Gravity vector g,
%       Ftip: Spatial force applied by the end-effector expressed in frame 
%             {n+1},
%       Mlist: List of link frames {i} relative to {i-1} at the home 
%              position,
%       Glist: Spatial inertia matrices Gi of the links,
%       Slist: Screw axes Si of the joints in a space frame, in the format
%              of a matrix with the screw axes as the columns.
% Returns taulist: The n-vector of required joint forces/torques.
% This function uses forward-backward Newton-Euler iterations to solve the 
% equation:
% taulist = Mlist(thetalist) * ddthetalist + c(thetalist, dthetalist) ...
%           + g(thetalist) + Jtr(thetalist) * Ftip
% Example Input (3 Link Robot):
%{ 
  clear; clc;
  thetalist = [0.1; 0.1; 0.1];
  dthetalist = [0.1; 0.2; 0.3];
  ddthetalist = [2; 1.5; 1];
  g = [0; 0; -9.8];
  Ftip = [1; 1; 1; 1; 1; 1];
  M01 = [[1, 0, 0, 0]; [0, 1, 0, 0]; [0, 0, 1, 0.089159]; [0, 0, 0, 1]];
  M12 = [[0, 0, 1, 0.28]; [0, 1, 0, 0.13585]; [-1, 0 ,0, 0]; [0, 0, 0, 1]];
  M23 = [[1, 0, 0, 0]; [0, 1, 0, -0.1197]; [0, 0, 1, 0.395]; [0, 0, 0, 1]];
  M34 = [[1, 0, 0, 0]; [0, 1, 0, 0]; [0, 0, 1, 0.14225]; [0, 0, 0, 1]];
  G1 = diag([0.010267, 0.010267, 0.00666, 3.7, 3.7, 3.7]);
  G2 = diag([0.22689, 0.22689, 0.0151074, 8.393, 8.393, 8.393]);
  G3 = diag([0.0494433, 0.0494433, 0.004095, 2.275, 2.275, 2.275]);
  Glist = cat(3, G1, G2, G3);
  Mlist = cat(3, M01, M12, M23, M34); 
  Slist = [[1; 0; 1;      0; 1;     0], ...
           [0; 1; 0; -0.089; 0;     0], ...
           [0; 1; 0; -0.089; 0; 0.425]];
  taulist = InverseDynamics(thetalist, dthetalist, ddthetalist, g, ...
                            Ftip, Mlist, Glist, Slist)
%}
% Output:
% taulist =
%   74.6962
%  -33.0677
%   -3.2306

n = size(thetalist, 1);
Mi = eye(4);
Ai = zeros(6, n);
AdTi = zeros(6, 6, n + 1);
Vi = zeros(6, n + 1);
Vdi = zeros(6, n + 1);
Vdi(4: 6, 1) = -g;
AdTi(:, :, n + 1) = Adjoint(TransInv(Mlist(:, :, n + 1)));
Fi = Ftip;
taulist = zeros(n, 1);
for i=1: n    
    Mi = Mi * Mlist(:, :, i);
    Ai(:, i) = Adjoint(TransInv(Mi)) * Slist(:, i);    
    AdTi(:, :, i) = Adjoint(MatrixExp6(VecTose3(Ai(:, i) ...
                    * -thetalist(i))) * TransInv(Mlist(:, :, i)));    
    Vi(:, i + 1) = AdTi(:, :, i) * Vi(:, i) + Ai(:, i) * dthetalist(i);
    Vdi(:, i + 1) = AdTi(:, :, i) * Vdi(:, i) ...
                    + Ai(:, i) * ddthetalist(i) ...
                    + ad(Vi(:, i + 1)) * Ai(:, i) * dthetalist(i);    
end
for i = n: -1: 1
    Fi = AdTi(:, :, i + 1)' * Fi + Glist(:, :, i) * Vdi(:, i + 1) ...
         - ad(Vi(:, i + 1))' * (Glist(:, :, i) * Vi(:, i + 1));
    taulist(i) = Fi' * Ai(:, i);
end
end