Getting an error of 'Reference to non-existent field'

The function when invoked gives the error 'Reference to non-existent field 'vel'.'

function [ desired_state ] = traj_generator(t, state, waypoints)
    % This function is called with variable number of input arguments.
    % During initialization, it is called with arguments
    % trajectory_generator([], [], waypoints) and later, while testing, it will be
    % called with only t and state as arguments.
    
    % t,state: time and current state that is used for computing desired_state
    
    % waypoints: The 3xP matrix listing all the points needed in order
    % to generate the trajectory
    
    % desired_state: Contains all the information that is passed as output
    
    
    function[t_m] =t_mat1(t)
        t_m=[1 t^1 t^2 t^3 t^4 t^5 t^6 t^7]';
    end
    function[t_m_dot] =t_mat2(t)
        t_m_dot = [0 1 2*t 3*t^2 4*t^3 5*t^4 6*t^5 7*t^6]';
    end
    function[t_m_ddot] =t_mat3(t)
        t_m_ddot = [0 0 2 6*t 12*t^2 20*t^3 30*t^4 42*t^5]';
    end
    function[C]=pols_coef_retrn(n,wayp)
        A = zeros(8*n,8*n);
        B = zeros(1,8*n);
        
        p=1;
        l=0;
        for i=1:n
            k=1;
            for j=1:8
                if j==1
                    
                    A(p,p)=1;
                    B(p)=wayp(i);
                    A(p+1,p:p+7)=[1 1 1 1 1 1 1 1];
                    B(p+1)=wayp(i+1);
                    
                    
                end
                p=p+1;
                if j>=3
                    for theta = l+k:l+6
                        if i==1 && k<=4
                           
                            A(j,k+1)=1;
                           
                        elseif i==n
                            if k<=4
                                
                                A(l+j,theta)= factorial(theta-l)/(factorial(k)^2);
                                
                            elseif k==5
                                
                                A(l+j,l+7)=1;
                                
                                A(l+j,8)=-1;
                                
                            elseif k==6
                                
                                A(l+j,l+6)=1;
                                
                                A(l+j,l-3)=-1;
                                
                                A(l+j,l+j)=-1;
                                
                            end
                        
                        else
                            
                            
                            A(l+j,theta)=-(factorial(theta-l)/(factorial(k)^2));
                            
                            if theta==(l+6)
                                A(l+j,theta+2+k)=1;
                            end
                        end
                    end
                    k=k+1;
                end
            
            end
        l=l+8;
        end
        
        C=inv(A)*B';
    end
   
    persistent waypoints0 traj_time d0
    if nargin > 2
        d = waypoints(:,2:end) - waypoints(:,1:end-1);
        d0 = 2 * sqrt(d(1,:).^2 + d(2,:).^2 + d(3,:).^2);
        traj_time = [0, cumsum(d0)];
        waypoints0 = waypoints;
        
        
    else
        if (t >= traj_time(end))
            t = traj_time(end)-0.0001;
        end
        t_index = find(traj_time>t,1) - 1;
        t_index = max(t_index,1);
        scale = (t-traj_time(t_index)) / d0(t_index);
        if(t == 0)
            desired_state.pos = waypoints0(:,1);
        else
            
            index = ((t_index-1)*8+1:t_index*8);
            
            nn=size(waypoints0(1,:));
            n=nn(2)-1;
            coeff_x = pols_coef_retrn(n,waypoints0(1,:)');
            coeff_y = pols_coef_retrn(n,waypoints0(2,:)');
            coeff_z = pols_coef_retrn(n,waypoints0(3,:)');
            
            desired_state.pos = [coeff_x(index)'*t_mat1(scale);coeff_y(index)'*t_mat1(scale);coeff_z(index)'*t_mat1(scale);];
            desired_state.vel = [coeff_x(index)'*t_mat2(scale);coeff_y(index)'*t_mat2(scale);coeff_z(index)'*t_mat2(scale);].*(1/d0(t_index));
            desired_state.acc = [coeff_x(index)'*t_mat3(scale);coeff_y(index)'*t_mat3(scale);coeff_z(index)'*t_mat3(scale);].*((1/d0(t_index))^2);
        end
        
        desired_state.yaw = 0;
        desired_state.yawdot = 0;
    end
end

the error message is as follows

Reference to non-existent field 'vel'.

Error in simulation_3d (line 70)
        QP.UpdateQuadPlot(x, [desired_state.pos; desired_state.vel], time);

Error in runsim (line 28)
[t, state] = simulation_3d(trajhandle, controlhandle);

where runsim and in simulation_3d invoke the function

[Guillaume]

See also getting an array indices must be positive integers or logical values error for previous question on this topic.

[Guillaume]

The error you show occurs in a completely different function than the one you've posted.

[Orpheus]

The function in simulation_3d works fine when any other function is passed as traj_generator. It shows an error with this hence i assumed it would be an error in this function.

The function in simulation_3d is as follows

function [t_out, s_out] = simulation_3d(trajhandle, controlhandle)

addpath('utils');

% real-time
real_time = true;

% max time
max_time = 20;

% parameters for simulation
params = sys_params;

%% **************************** FIGURES *****************************
disp('Initializing figures...');
h_fig = figure;
h_3d = gca;
axis equal
grid on
view(3);
xlabel('x [m]'); ylabel('y [m]'); zlabel('z [m]')
quadcolors = lines(1);

set(gcf,'Renderer','OpenGL')

%% *********************** INITIAL CONDITIONS ***********************
disp('Setting initial conditions...');
tstep    = 0.01; % this determines the time step at which the solution is given
cstep    = 0.05; % image capture time interval
max_iter = max_time/cstep; % max iteration
nstep    = cstep/tstep;
time     = 0; % current time
err = []; % runtime errors

% Get start and stop position
des_start = trajhandle(0, []);
des_stop  = trajhandle(inf, []);
stop_pos  = des_stop.pos;
x0    = init_state(des_start.pos, 0);
xtraj = zeros(max_iter*nstep, length(x0));
ttraj = zeros(max_iter*nstep, 1);

x       = x0;        % state

pos_tol = 0.01;
vel_tol = 0.01;

%% ************************* RUN SIMULATION *************************
disp('Simulation Running....');
% Main loop
for iter = 1:max_iter

    timeint = time:tstep:time+cstep;

    tic;

    % Initialize quad plot
    if iter == 1
        QP = QuadPlot(1, x0, 0.1, 0.04, quadcolors(1,:), max_iter, h_3d);
        current_state = stateToQd(x);
        desired_state = trajhandle(time, current_state);
        QP.UpdateQuadPlot(x, [desired_state.pos; desired_state.vel], time);
        h_title = title(sprintf('iteration: %d, time: %4.2f', iter, time));
    end

    % Run simulation
    [tsave, xsave] = ode45(@(t,s) quadEOM(t, s, controlhandle, trajhandle, params), timeint, x);
    x    = xsave(end, :)';

    % Save to traj
    xtraj((iter-1)*nstep+1:iter*nstep,:) = xsave(1:end-1,:);
    ttraj((iter-1)*nstep+1:iter*nstep) = tsave(1:end-1);

    % Update quad plot
    current_state = stateToQd(x);
    desired_state = trajhandle(time + cstep, current_state);
    QP.UpdateQuadPlot(x, [desired_state.pos; desired_state.vel], time + cstep);
    set(h_title, 'String', sprintf('iteration: %d, time: %4.2f', iter, time + cstep))

    time = time + cstep; % Update simulation time
    t = toc;
    % Check to make sure ode45 is not timing out
    if(t> cstep*50)
        err = 'Ode45 Unstable';
        break;
    end

    % Pause to make real-time
    if real_time && (t < cstep)
        pause(cstep - t);
    end

    % Check termination criteria
    if terminate_check(x, time, stop_pos, pos_tol, vel_tol, max_time)
        break
    end
end

%% ************************* POST PROCESSING *************************
% Truncate xtraj and ttraj
xtraj = xtraj(1:iter*nstep,:);
ttraj = ttraj(1:iter*nstep);

% Truncate saved variables
QP.TruncateHist();

% Plot position
h_pos = figure('Name', ['Quad position']);
plot_state(h_pos, QP.state_hist(1:3,:), QP.time_hist, 'pos', 'vic');
plot_state(h_pos, QP.state_des_hist(1:3,:), QP.time_hist, 'pos', 'des');
% Plot velocity
h_vel = figure('Name', ['Quad velocity']);
plot_state(h_vel, QP.state_hist(4:6,:), QP.time_hist, 'vel', 'vic');
plot_state(h_vel, QP.state_des_hist(4:6,:), QP.time_hist, 'vel', 'des');

if(~isempty(err))
    error(err);
end

disp('finished.')

t_out = ttraj;
s_out = xtraj;

end

[Guillaume]

Is trajhandle equal to @traj_generator ?

[Orpheus]

Yes

Answer 1

[Guillaume]

The way traj_generator is coded, when t is 0, it returns a structure with only 3 fields: pos, yaw and yawdot (these last two are always 0). It's only when t is not 0 that it creates the fields vel and acc.

So, indeed if t is 0 and you try to use the vel field you'll get your error. Either you mustn't use vel or acc if t is 0, or you must modify traj_generator so that it creates these fields even when t is 0:

%original code in traj_generator
        if(t == 0)
            desired_state.pos = waypoints0(:,1);
        else

replace with

        if t == 0   %brackets were completely unnecessary
            desired_state.pos = waypoints0(:,1);
            desired_state.vel = ????;  %no idea what you want there
            desired_state.acc = ????;  %no idea what you want there
        else
            %...