Possible Alteration: Multiply u in the code with k1 where k1 = Km*Kg/(N*Jm), Km = 0.955. This can allow for smaller values of b and c (but technically, the control signal will remain large).
For those who are new to the MATLAB simulation of integrator based control, please watch
• Simulating State Space based Integral Cont...
And those who are new to the sliding mode control may refer to
• Sliding Mode Control Part I
and
• Sliding Mode Control Part II