The design of any feedback control system with even modest complexity requires simulation capability. The controlled plant and feedback law are simulated in order to assess the possible effects of discrepancies between the design model of the plant and the actual plant, and in order to verify the correctness of design approximations. In feedback system, the effects of such discrepancies can be particularly serious and may be manifested as classical instability in continuous-state systems, or as errors (trap states) and/or loss of synchronization in discrete-state systems. Simulation averts the most serious errors of these types because the control system design can be corrected without physical damage to the actual plant. Most simulations today are digital rather than analog.