Plotting an Archimedean Spiral

Asked by Rajbir Singh on 15 Oct 2019

Latest activity Commented on by Rajbir Singh on 17 Oct 2019

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r = 12.5; %outer radius
a = 0;    %inner radius
b = 0.01; %incerement per rev
n = (r - a)./(b); %number  of revolutions
th = 2*n*pi;      %angle
Th = linspace(0,th,1250*720);  
x = (a + b.*Th).*cos(Th);
y = (a + b.*Th).*sin(Th);
plot(x,y)

The code executes well r, a, n and b are correct. Th and th both are also correct, but the problem which arises is in the values of x and y.

outer value or last value (desired) should be 12.5, but after execution it gives 78.53 and same corresponds to y.

what can be the solutions of this problem?

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KALYAN ACHARJYA on 15 Oct 2019

Then what is the problem?

Rajbir Singh on 15 Oct 2019

Sir,

The output which i am getting is an Archimedean Spiral, thats fine. But the problem arises with the output values x and y.

According to the software that i am using r, a, n, b, th and Th values are correct.

My desired outer radius for archemedean spiral is 12.5 but it gives 78.53

Rajbir Singh on 16 Oct 2019

How can i change the rotation (clockwise or anti-cloclwise) of Archimedean Spiral?

1 Answer

Link

Answer by Jos (10584) on 15 Oct 2019

Accepted Answer

In the computation of x and y you wrongly multiply b with Th. You should multipy by Th / (2*pi):

r = 12.5; %outer radius
a = 0;    %inner radius
b = 0.5; %incerement per rev % Jos: changed to see the spiral!!
n = (r - a)./(b); %number  of revolutions
th = 2*n*pi;      %angle  
Th = linspace(0,th,1250*720);  
x = (a + b.*Th/(2*pi)).*cos(Th);
y = (a + b.*Th/(2*pi)).*sin(Th);
% better:
% i = linspace(0,n,1250*720)
% x = (a+b*i).* cos(2*pi*i)
plot(x,y)
[x(end) y(end)]

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Rajbir Singh on 16 Oct 2019

How can i change the rotation (clockwise or anti-cloclwise) of Archimedean Spiral?

Jos (10584) on 16 Oct 2019

hint: use a minus sign :-)

Rajbir Singh on 17 Oct 2019

It works, thanks once again. :)

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