Double derivative of a vector of variables in Simscape components.

I'm constructing a Simscape component (.ssc file) and I'm confused by the mechanics of declaring the double derivative of a variable, specifically the double derivative of a vector of variables.

Say I declare a variable as follows:

variables
    eta     = {zeros(4,1),'m'};
end

Now I wish to use the double derivative in my equations can I use a variation of:

equations
    eta.der.der == %...
    %or 
    der(eta.der) == %...
end

Or must I also declare a 'derivative variable' say d_eta and use that as a intermediate value?

variables
    eta     = {zeros(4,1),'m'};
    d_eta   = {zeros(4,1),'m/s'};
end
equations
    eta.der == d_eta;
    d_eta.der == %...
end

Thanks in advanced!

PS.

Is there documentation on the working of der([...]) or [...].der?

Answer 1

[J Chen]

I would say you must use the second approach:

variables
    eta     = {zeros(4,1),'m'};
    d_eta   = {zeros(4,1),'m/s'};
end
equations
    eta.der == d_eta;
    d_eta.der == %...
end