Generate manual repeatable signal for matlab

Dear matlab forum,

At this moment I'm evaluating a 'sample entropy' algorithm, that is used in heart and gait data currently.

I want to show the effect of sample frequency and time scales on the algorithms outcomes.

For this I want to create a manual function that can be repeated, that resembles the gait flexion angle of the knee.

This function must look like the negative part of a sinusoid (1Hz), amplitude -5, followed by the positive part of a sinusoid (2Hz) amplitude +1.

I should be able to use this function in a matlab script. At this point I'm quite a novice on this topic, and I want to see what your recommendations for designing such a function for matlab would be. I added a sketch to give a better idea on the curve of the function.

I look forward to seeing your answer(s).

Greetings,

Jeroen

Answer 1

[Alan Stevens]

Like this?

dt = 0.01;
tf = 1.5;
n = tf/dt + 1;

for i = 1:n
    t(i) = (i-1)*dt;
    y(i) = wave(t(i));
end

plot(t,y),grid

function y = wave(t)
        
        w1 = 2*pi;  % frequency rads/sec
        w2 = 4*pi;  %  "
        A1 = -5;    % amplitude
        A2 = 1;     %    "
        
        if t<1
            y = A1*sin(w1*t);
        else
            y = A2*sin(w2*t);
        end

end

[Jeroen Vermeulen]

Thanks very much Alan! For the ones intrested, I found one other possible method (upper). The bottom method is your method, slightly adjusted to make the signal look like a better flexion-extension angle of the knee joint.

a1 = 5; %amplitude

t1 = 0:dt:T1; %timevector

f = @(x) [ -5*sin(2*pi*0.8*x).*(0.005<=x & x<=0.6) + -sin(2*pi*2.5*x).*(0.6<x & x<0.8) + 0.8*sin(2*pi*1.25*x).*(0.8<x & x<1.2) + 0.*(1.2<=x & x<1.3) ];

x = linspace(0,1.3);

intvl = [0 6];

repfx = repmat(f(x), 1, diff(intvl)/3);

px = linspace(intvl(1),intvl(2),length(repfx));

figure(1)

plot(px, repfx)

grid

%% Try 2

dt = 0.01;

tf = 1.2;

n = tf/dt + 1;

for i = 1:n

t(i) = (i-1)*dt;

y(i) = wave(t(i));

end

plot(t,y),grid

function y = wave(t)

w1 = 2*pi; % frequency rads/sec

w2 = 6*pi; % "

A1 = -5; % amplitude

A2 = 1; % "

if t<=0.5

y = A1*sin(w1*t);

elseif t>0.5 && t<0.67

y = -A2*sin(w2*t);

elseif t>0.67 && t<1

y = 0.8*sin(3*pi*t)

else

y = 0

end

end