%SerialLink.friction Friction force
%
% TAU = R.friction(QD) is the vector of joint friction forces/torques for the 
% robot moving with joint velocities QD.  
%
% The friction model includes:
% - viscous friction which is linear with velocity;
% - Coulomb friction which is proportional to sign(QD).
%
% See also Link.friction.



% Copyright (C) 1993-2011, by Peter I. Corke
%
% This file is part of The Robotics Toolbox for Matlab (RTB).
% 
% RTB is free software: you can redistribute it and/or modify
% it under the terms of the GNU Lesser General Public License as published by
% the Free Software Foundation, either version 3 of the License, or
% (at your option) any later version.
% 
% RTB is distributed in the hope that it will be useful,
% but WITHOUT ANY WARRANTY; without even the implied warranty of
% MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
% GNU Lesser General Public License for more details.
% 
% You should have received a copy of the GNU Leser General Public License
% along with RTB.  If not, see <http://www.gnu.org/licenses/>.
%
% http://www.petercorke.com

function  tau = friction(robot, qd)

	L = robot.links;

    tau = zeros(1,robot.n);
    if robot.issym
        tau = sym(tau);
    end
    
    
	for j=1:robot.n
		tau(j) = L(j).friction(qd(j));
	end