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Gratitude to Arno K. and rombio for helping me with derivatives and CAS programs.

The following CAS functions calculates the curvature of:

functions, y(x)
polar functions, r(t) (t: Θ)
parametric functions, x(t), y(t)

Let Δα be the angle of rotation angle and Δs is the slight change of distance. Then the radius of curvature is:

K = abs(Δα ÷ Δs) as Δs → 0

And the radius of curvature is the reciprocal of K.

For circles, the radius of curvature is constant. Wankel engines and rotary engines have their pistons traveling in a circle.

Calculating the curvature depends on the form of the function.

Function: y(x)

K = abs( y''(x) ) ÷ (1 + (y'(x))^2) ^(3/2)

Polar: r(t) (t replaces Θ)

K = abs( r(t)^2 + 2 * (r'(t))^2 - r(t) * r''(t) ) ÷ ( r(t)^2 + r'(t)^2 )^(3/2)

Parametric: x(t), y(t)

K = abs( x'(t) * y''(t) - y'(t) * x''(t) ) ÷ ( x'(t)^2 + y'(t)^2 )^(3/2)

Radius of Curvature:

r = 1 ÷ K

HP Prime CAS Program: crvfunc

Code:

#cas

crvfunc(y,x):=

BEGIN

// curvature

// function

// radius = 1/curvature

LOCAL a,b;

a:=diff(y,x,2);

b:=diff(y,x,1);

RETURN ABS(a)/(1+b^2)^(3/2);

END;

#end

HP Prime CAS Program: crvpol

Code:

#cas

crvpol(r,t):=

BEGIN

// curvature

// polar (t: θ)

// radius = 1/curvature

LOCAL a,b,n,d;

a:=diff(r,t,2);

b:=diff(r,t,1);

n:=simplify(r^2+2*b^2-r*a);

d:=r^2+b^2;

RETURN ABS(n)/(d)^(3/2);

END;

#end

HP Prime CAS Program: crvpar

Code:

#cas

crvpar(y,x,t):=

BEGIN

// curvature

// parametric

// radius = 1/curvature

LOCAL y1,y2,x1,x2,n,d;

y2:=diff(y,t,2);

y1:=diff(y,t,1);

x2:=diff(x,t,2);

x1:=diff(x,t,1);

n:=simplify(x1*y2-y1*x2);

d:=simplify(x1^2+y1^2); 

RETURN ABS(n)/(d)^(3/2);

END;

#end

Source:

Svirin, Alex Ph.D. "Curvature and Radius of Curvature" Math24 https://math24.net/curvature-radius.html 2022. Last Updated September 12, 2022.

Nice to see we could help you.
Arno

Eddie W. Shore

Arno K