# Re: [Scilab-users] Problems with Scilab routine "conv"

I can't figure out weither the problem resides in your model or in the
computations, but when I had (many years ago) to use discrete convolution to
approximate continuous convolution, I noticed that the best way to obtain
coherent results is to use the composed midpoint rule to approximate the
integral

h(t)=\int_0^t f(\tau) g(t-\tau) d\tau.

For two causal signals t->f(t)], t-> g(t) with respective support [0,N*δ] and
[0,M*δ], sampled in the middle of each [i*δ,(i+1)*δ] interval, define the discrete
sequences

f_i = f((i-1/2)*δ), i=1...N
g_i = g((i-1/2)*δ), i=1..M

then define h_i = h((i*δ) for i=0...N+M by

h_0 = 0
h_i  = δ*\sum_{k=1}^i \tilde f_{i-k+1}*g_k, i = 1...N+M-1
h_{N+M} = 0

h_i for i = 1...N+M-1 is the approximation of the integral by the composed
midpoint rule and is directly given in Scilab by δ*conv(f,g) where f and g are
defined above. Here is an example:

function y=f(t)
y=sin(2*%pi*t);
endfunction

function y=g(t)
y=sin(%pi*t);
endfunction

δ=0.01;
N=50;  // t->f(t) has support [0,0.5]
M=100; // t->g(t) has support [0,1]

h=[0 δ*conv(f((1/2:N)*δ), g((1/2:M)*δ)) 0];

th=(0:N+M)*δ; // t->h(t) has support [0,1.5]
tf=(0:N)*δ;
tg=(0:M)*δ;

clf
plot(tf,f(tf),tg,g(tg),th,h)
legend f g f*g

S.

Le 03/02/2023 à 11:52, Heinz Nabielek a écrit :

This is my latest code version: the 'convoluted secondary failure fraction' is
nicely below primary failure,  but seems too low.
Heinz

m=2;         // Weibull modulus in mechanism  #1
k=1E-7;      // corrosion rate(s-1) in mechanism  #1
n=500;       // number of time steps in hourly intervals
t=(1:n)';    // timesteps in hours
ts= 3600*t;  // timesteps in seconds
PHI= 1 - exp(-((k*ts)^m)); // failure fraction in mechanism #1
deriv=m*k*((k*ts)^(m-1)) .* exp(-((k*ts)^m)); //derivative of PHI
phi=3600*deriv;  //derivative of PHI integrated over an hour
f= 1 - exp(-ts/3E5);         // "pure" failure fraction in mechanism #2
F=(convol(phi,f)/n)(1:n);   //"convoluted" failure fraction mechanism #2
plot("nl",t, [PHI phi f F'],'-');xgrid;
legend('PHI primary failure','phi primary failure rate','pure secondary failure
fraction','convoluted secondary failure fraction',4);
xlabel('time (hours)');
ylabel('failure fraction/ failure rate');

On 03.02.2023, at 11:24, Heinz Nabielek
<heinznabie...@gmail.com><mailto:heinznabie...@gmail.com> wrote:

On 03.02.2023, at 11:13, Stéphane Mottelet
<stephane.motte...@utc.fr><mailto:stephane.motte...@utc.fr> wrote:

Thanks for the code.

Just a remark on the notations, you should write :

F(T)  = Int_{0}^{T}  PHI(t) . f(T-t) . dt

i.e. not F(t) since t is mute.

However, you should pay attention to the delay notion associated with
convolution and the relationships between discrete convolution and continuous
convolution. I am not sure that the output of conv matches with a given
discretization of the integral above. Maybe rectangle method, but I am not sure
at all. Anyway, you should have F(0)=0 which does not seem to be the case in

F(t), of course.

But the formula is fundamentally wrong and one should have seen it already from
a dimensional consideration.

Correct should be F(t)  = Int_{0}^{T}  (d PHI(t)/dt) . f(T-t) . dt

Hope you can help between conv and convol and possible something else.
Heinz

Dear SciLab Friends:

I have an object consisting of many (~10,000) small components that can fail in
a statistical way during long-term operation at extreme conditions.

My primary failure model is described by PHI(t) going monotonically from zero
to one at times from t=0 to T. In the computer, this is realized in n timesteps.

Mechanism 2: There is a secondary failure mechanism that starts only when a
component has previously failed by mechanism 1 and occurs after some delay. The
delay function is f(t) and final expression for the evolution of the mechanism
2 failure is given by:

F(t)  = Int_{0}^{T}  PHI(t) . f(T-t) . dt

a classical convolution Faltungsintegral.

In Scilab, I write

F= conv(PHI, f, "same")/ n;

and the resulting function F looks reasonable for most of the time. Under some
conditions, however, I can have

F>PHI

at early stages, but this is physically impossible. Because it comes later, F
must always be below PHI.
What do I do wrong?
Heinz

_________________________
PS: massively simplified code below......

k=1E-7;              // corrosion rate(s-1) in mechanism #1
n=100;               // number of timesteps
t=(1:n)';             // time in days
ts= 3600*t;          // time in seconds
PHI= 1 - exp(-((k*ts)^2));   // failure fraction in mechanism #1
plot("nl", t, PHI, 'r--');
f= 1 - exp(-ts/3E5);         // "pure" failure fraction in mechanism #2
F=conv(PHI,f,"same")/length(t); //"convoluted" failure fraction mechanism #2
plot(t,F,'b--');
legend('failure fraction #1','subsequent failure fraction #2',4);
xlabel('time (hours)');
ylabel('failure fraction');
title('Component failure evolution #1 is followed by #2');

--
Stéphane Mottelet
Ingénieur de recherche
EA 4297 Transformations Intégrées de la Matière Renouvelable
Département Génie des Procédés Industriels
Sorbonne Universités - Université de Technologie de Compiègne
CS 60319, 60203 Compiègne cedex
Tel : +33(0)344234688
http://www.utc.fr/~mottelet

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