Using MATLAB to create Position, Velocity, and Acceleration plots

Question

I am extremely new to MATLAB (as in just picked it up today) and trying to create basic Position, Velocity, and Acceleration plots.

I have this so far:

theta = (0:1:pi);
beta = asin((h+ b*cos(theta))/d);
x = b*cos(theta) + d*cos(beta);

plot(radtodeg(theta), x*1000);

b, d, and h are constants, and I'm trying to plot Position (x), Velocity (x_dot), and Acceleration (x_dd) versus the angle theta. This first part works fine and spits out a plot for x.

Is there a way to get the Velocity and Acceleration plots without me having to find the derivative of my function by hand and plotting that? I'm not sure what MATLAB's capabilities are. I don't need the actual derivatives, just the plot, so if I can avoid doing the derivations by hand I would like to. I looked into diff but it looked as though it required syms which I'd also like to try to avoid.

Any help is appreciated. Thanks.

There are multiple versions of diff. Base MATLAB's diff does not require symbolic math. — excaza
I used diff on x as such: xdot = diff(x), but when it comes to plotting I get the error "Vectors must be the same length." — Sunden

Nakini Nakini · Accepted Answer · 2018-05-08T18:21:21

You could use diff in another way, i.e., when you do diff(x) it will return a vector with length 1 less than the original one. Here is the modified version of your code:

h = 5; b = 2; d = 4;
theta = (0:0.1:pi);
beta = asin((h+ b*cos(theta))/d);

% Position 
x = b*cos(theta) + d*cos(beta);
plot(radtodeg(theta(1:end-2)), x(1:end-2));

% Velocity 
hold on;
v = diff(x);
plot(radtodeg(theta(1:end-2)), v(1:end-1));

% Acceleration
a = diff(v);
plot(radtodeg(theta(1:end-2)), a);

legend('Position', 'Velocity', 'Acceleration');
hold off;

Here, I assumed some random numbers for h, b, and d. Also, beta has imaginary numbers. You have to take care of that.

Using MATLAB to create Position, Velocity, and Acceleration plots

1 Answers