MATLAB: Basic math error

MATLABsubtraction

I'm working on writing my own orbital dynamics code; when I go to calculate the difference between two data points, I get different answers. Basically the logic is: change = new position – (old position + velocity*time); this yields a result of 0, which is not correct. I switched it to the following: change1 = new pos – old pos; change = change1 – velocity*time; which gives me a non-zero result with the correct magnitude. Its the same calculation, but I get different results!

I then try to calculate the same difference for a different planet, and while both answers are non-zero, they both differ from each other by a factor of 10^-6…

Should I change my different planet's code the same way I changed the first?

Best Answer

My guess is that you are seeing this.

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MATLAB: ODE45 – Modeling GHR Microspic Car following Model

This code runs, but still has a problem:

load mytrafficdata5;
basic_ghr(Distance_n,Time_t,Velocity_n)
function basic_ghr(Distance_n,Time_t,Velocity_n)
% y is velocity
% t is time
% Using interpolant for distance
close all;
time_vector = Time_t;
init_speed = 0;
dist_vector = Distance_n;
vel_vector = Velocity_n;
% dist_interp = interp1(time_vector, dist_vector,);
% vel_interp = interp1(time_vector, vel_vector,);
F_time = time_vector(end);
[T,Y] = ode45(@(t,y)some_system(t,y),[0,F_time],init_speed);
plot(T,Y(:,1),'b-',time_vector,vel_vector,'r--')
legend('By GHR','Real Velocity')
title('Comparisam of velocity with time bewteen Real Data Set VS. GHR model')
xlabel('Time')
ylabel('Velocity (m/sec)')
function dy = some_system(t,y)
c = 125;
m = 0.2;
l = 1.6;
%%%%%%%%%%%%%%%%%
% Using interpolant for distance and velocity 
dist_interp = interp1(time_vector, dist_vector, t);
vel_interp = interp1(time_vector, vel_vector, t);
dy = c *(y^m)/(vel_interp)^l*(dist_interp);
end
end

The result of Y(:,1) is a vector of one zero and then all NaN - so you will have to check your equation.

MATLAB: How to draw an animated line using the data

Hello,

In this scenario, I'd recommend just plotting the line every step as the line develops.

You can do this by recording your x and y positions with a counter variable and plotting, I'll paste your code with the changes below:

count = 1;                                 %%%%%%%%%%%%ADDED




%Declaring the balls initial conditions
R_Ball = 2;
initpos.x = 0;
initpos.y = 2.4;
initvel.x = 2;
initvel.y = 4;
gravity.x = 0;
gravity.y = 9.81;
restitution = 0.7;
GroundBall_friction = 0.2;
%Animation timestep
dt = 0.01;
%Executing the animation
pos.x = initpos.x; % initial position

pos.y = initpos.y; % initial position
vel.x = initvel.x; % initial velocity-x
vel.y = initvel.y; % initial velocity-y
t_arc = linspace(0,(2*vel.y)/gravity.y,5000);
for k =  1:5000
%Updating the ball's position
vel.x = vel.x;
vel.y = vel.y - gravity.y*t_arc(k);
pos.x = pos.x + vel.x*t_arc(k);
pos.y = pos.y + vel.y*t_arc(k) - (1/2)*gravity.y*(t_arc(k).^2);
if vel.y < 0 && pos.y < 0
vel.y = (restitution)*vel.y;
vel.y = -vel.y;
vel.x = vel.x + GroundBall_friction*(restitution - 1)*vel.x;
end
clf;
%Drawing the frame
subplot(2,1,1)
hold on
line([0 30],[0 0]);
rectangle('position', [pos.x pos.y R_Ball R_Ball],'Curvature',[1 1],'FaceColor', 'r');
axis([0 30 0 10]);
hold off
subplot(2,1,2)
posxx(count) = pos.x;                 %%%%%%%%%%%%ADDED
posyy(count) = pos.y;                 %%%%%%%%%%%%ADDED
plot(posxx, posyy,'r')                %%%%%%%%%%%%ADDED
axis([0 30 0 10]);
count = count+1;                      %%%%%%%%%%%%ADDED
%Refresh rate
pause(dt)
end

Note that I also removed your axis equal commands as they contradict your axis([0 30 0 10]) command (you won't notice a difference without them, but it will be buggy with them).

My last comment is that your x and y positions seem to be the left side of the ball, so you probably will want to figure out the variable that represents the center of the ball (or just shift these to the right by the ball radius by adding ball radius to posxx in the added line).

Hope this helps!

Best Answer

Related Solutions

MATLAB: ODE45 – Modeling GHR Microspic Car following Model

MATLAB: How to draw an animated line using the data

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