In the design examples presented in this chapter, wehave been primarily concerned only with the transfer functions of compensators. In actual design problems, we must choose the hardware. Thus, we must satisfy additionaldesign constraints such as cost, size, weight, and reliability.The system designed may meet the specifications under normal operating conditions but may deviate considerably from the specifications when environmental changesare considerable. Since the changes in the environment affect the gain and time constants of the system, it is necessary to provide automatic or manual means to adjust thegain to compensate for such environmental changes, for nonlinear effects that were nottaken into account in the design, and also to compensate for manufacturing tolerancesfrom unit to unit in the production of system components. (The effects of manufacturing tolerances are suppressed in a closed-loop system; therefore, the effects may not becritical in closed-loop operation but critical in open-loop operation.) In addition to this,the designer must remember that any system is subject to small variations due mainlyto the normal deterioration of the system.