Having problems building custom block

Hi, I'm making a custom block to mimic the spring accumulator block provided in simscape (r2010b). The block has three states that define the flowrate into the block : one when the pressure is below the preload pressure, an intermediate state when the pressure is above preload and below maximum pressure, and finally a maximum pressure condition.

When I try to use ssc_build I get the following error message " Error using ==> nonlinear_accumulator>equations at 38 Provided units '1/s' and 'in^3/s' are not commensurate.". I don't seem to understand where in the code the units are not consistent. Line 38 is where q is set equal to vf.der in when vf=0.

Also, when testing the intermediate condition I get this error message " Error using ==> nonlinear_accumulator>equations at 40. Conditional equation has inconsistent differentiability for true and false branches. If the time derivative of a variable appears on a true branch, the same time derivative must appear on the false branch in the same order." Line 40 is the intermediate pressure condition ppr<p<pmax.

Have I made some sort of syntax error? Any help would be appreciated. My block code is as appears below. Thanks for all the help!

component nonlinear_accumulator
% Nonlinear Accumulator
% This block models damping behavior of voided silicone
% liners for use as accumulators in systems affected by water hammer 
  outputs
    Q = { 0, 'in^3/s' }; % Q:top
    VF = {0, 'in^3'};% VF: top
  end
  nodes
    a = foundation.hydraulic.hydraulic; % a:bottom  
  end

    parameters
        vmax = { 1.4 'in^3'}; % accumulator capacity
        ppr = { 4 'psi'}; % preload pressure
        pmax = { 80 'psi'}; % max pressure 
        vpr = { 0, 'in^3'} % inital fluid volume
        c   = { 1, 'psi*s/in^3'}%damping coefficient

    end

    variables
      q = { 0 , 'in^3/s' }; %flowrate
      p = { 0 , 'psi' };%pressure
     vf = { 0, 'in^3'} %instantaneous fluid volume
    end

    function setup
      through( q, a.q, [] );
      across( p, a.p, [] ); 
    end

    equations
        if p<= ppr
            let
                vf = 0;
            in
                q == vf.der;
            end
        if p > ppr && p < pmax
            let
                vf =((vmax-vpr)*((p-ppr)-(c*q)))/(pmax-ppr);
            in
                q == vf.der;
            end
        else
            let
                vf = vmax;
            in
                q == vf.der;
            end
        end
       Q == q
       VF == vf
    end
  end

Answer 1

[Ryan G]

On line 36 you set

vf = 0;

With no units. I believe that is why vf.der is 1/s instead of in^3/s. To resolve this you can construct a new variable with units of in^3 and value 0 and set vf equal to that .