linear bounds in linear problem

Hello,

I created the following matrix G, which is multiplied with F and results in F_soll.

With the sign function I wanted to tell Matlab, that z1 has to be positive at all time.

With the last lines I am including slackvariables to set bounds for my forces.

But Matlab doesnt accept the sign*z1.

Does anyone has an idea, how I can solve that? Is there maybe an other command or another way to do this?

G = [1,0,1,0,1,0,1,0,0,0;
    0,1,0,1,0,1,0,1, 0,0,0;
    -w_vl,l_v,w_vr,l_v,-w_hl,-l_h,w_hr,-l_h, 0,0;
    1,1,0,0,0,0,0,0,1,0;
    0,0,0,0,0,0,0,0,sign,0;
    0,0,1,1,0,0,0,0,0,0,1;
    0,0,0,0,0,0,0,0,0,0,sign];

F = [Fx1; Fy2; Fx2; Fy2; Fx3; Fy3; Fx4; Fy4; z1; z2];
F_soll = [Fx_V; Fy_V; Mz_V; Fmax_1;  1; Fmax_2; 1];

That you very much!