Saving values from a loop - Error, not real integer...

I am trying to run a code where it will estimate the revolutions per minute in order for an arm to hold a certain angle.

I want it to do this for different values of masses, from 0.05 to 0.4kg, and have therefore used a while function.

Within this function, I have used a loop, which adds on a small increment to the RPM until the requirmements have been met. I have then saved the final answer for each mass into a matrix which can then be plotted.

This works fine.

However I want to increase the accuracy, and have decided to make the increases in RPM smaller for every iteration, from 1, to 0.1 (or smaller), but when I do this I get an error;

'Subscript indices must either be real positive integers or logicals.

Error in EstimateSpeed (line 90)

RPM_all(RPM,:)=RPM;

I wondered if anyone would be able to help

Cheers Tom

 while M1<=0.4

        for M1=M1+0.05                          %Add on to mass in 0.05kg increments
        omega=(RPM./60).*2.*pi;              %(rad/s)   

        a=g*(-M1-m1+M2+m2);                 %Set Equations
        b1=((M1.*omega.^2.*L1)+(sum(m1_sec.*L1_sec.*omega.^2)));
        b2=((M2.*omega.^2.*L2)+(sum(m2_sec.*L2_sec.*omega.^2)));

        theta=asind(a./(-b1-b2));           %Find value of theta for particular mass and RPM

                %Loop to continue interations
                while theta>10                  %Theta should approx equal 10 degrees
                    RPM =RPM+0.1;                   %add on to RPM
                    omega=(RPM./60).*2.*pi;                  

                    a=g*(-M1-m1+M2+m2);             %Set equations
                    b1=((M1.*omega.^2.*L1)+(sum(m1_sec.*L1_sec.*omega.^2)));
                    b2=((M2.*omega.^2.*L2)+(sum(m2_sec.*L2_sec.*omega.^2)));

                    theta =asind(a./(-b1-b2));      %Find value of theta for particular mass and RPM
                    RPM;
                end

        %Show final RPM values for each mass
        RPM_all(RPM,:)=RPM;
        RPM_all(RPM_all==0)=[];
        RPMs=reshape(RPM_all,1,[])

Answer 1

[Walter Roberson]

You have

   while M1<=0.4

        for M1=M1+0.05

You need to replace that with

M1_vals = initial_mass : 0.05 : 0.4;
for M1_idx = 1 : length(M1_vals)
  M1 = M1_vals(M1_idx);

    ...
    RPM_all(M1_idx,:) = RPM;
  end

%the below two lines are suspicious
RPM_all(RPM_all==0) = [];
RPMs = reshape(RPM_all,1,[]);

For the rest of your code, please clarify whether RPM is a scalar or a vector? If it is a scalar then there is an analytic solution. The expression of it that I give below depends upon all variables involved being real valued (not complex):

theta = 10;               %the desired angle in degrees
sin_theta = sind(theta);

m1_sec_L1_sec = sum(m1_sec .* L1_sec);
m2_sec_L2_sec = sum(m2_sec .* L2_sec);

t = L1*M1 + L2*M2 + m1_sec_L1_sec + m2_sec_L2_sec;
RPM = 30/pi * sqrt(g * (M1+m1-M2-m2)) ./ sqrt(sin_theta*t);

You should be able to vectorize this over M1. Unless, that is, L1 or L2 is unexpectedly a vector, or you omitted showing code that is in the loop.

[Tom Weir]

Hi Walter thankyou for your help.

M and L refer to mass and length, so both scalars

The two lines which you had said were suspicious I have used only to change the shape of the matrix that had been formed and to remove the zeros in it, so really they dont do too much!

I have not used a lot of the stuff you have used in the code above, but I shall give it a go and see what happens.

Cheers, Thomas