Error "Error using + Matrix dimensions must agree."

This is the programme:

function [ u ] = controller(~, s, s_des, params)
%PD_CONTROLLER  PD controller for the height
%
%   s: 2x1 vector containing the current state [z; v_z]
%   s_des: 2x1 vector containing desired state [z; v_z]
%   params: robot parameters

kv = 100;
kp = 100;
Z2=diff(s_des,2);
e = (s_des - s);
Y=diff(e);
u = params.mass*(Z2+kp*e+kv*Y+params.gravity);

% FILL IN YOUR CODE HERE

end

function [ sdot ] = sys_eom(t, s, controlhandle, trajhandle, params)
% sys_eom Differential equation for the height control system

s_des = trajhandle(t);
u_des = controlhandle(t, s, s_des, params);

u_clamped = min(max(params.u_min, u_des), params.u_max);

sdot = [s(2);
        u_clamped/params.mass - params.gravity];

end

This is the run command

close all;
clear;

% Hover
z_des =1 ;

% Step
% z_des = 1;

% Given trajectory generator
trajhandle = @(t) fixed_set_point(t, z_des);

% This is your controller
controlhandle = @controller;

% Run simulation with given trajectory generator and controller
[t, z] = height_control(trajhandle, controlhandle);

% % Sample code to get more info on the response
% sim_info = lsiminfo(z, t, z_des);
% disp(['Settling time [s]: ', num2str(sim_info.SettlingTime)]);
% disp(['Overshoot [%]: ', num2str(max(0,(sim_info.Max-z_des)*100))]);

Answer 1

[James Tursa]

Type the following at the MATLAB command line:

dbstop if error

Then run your code. It will pause at the line with the error. Examine all of the variables in that line for size to determine the cause of the error.

Answer 2

[Walter Roberson]

You have

Z2=diff(s_des,2);

where s_des has been documented to be 2 x 1. diff(s_des,2) is diff(diff(s_des)) and with s_des being length 2, the result is going to be empty. When you later use the result, Z2, in

u = params.mass*(Z2+kp*e+kv*Y+params.gravity)

that is going to lead to an error because you are trying to add an empty array and a non-empty item.