• Control structure: The control system can be classified into two broad structures, (1) Feedback (2) Feed forward. In feedback structure, the effect of disturbance is sensed by measuring the process output and the same information is passed on to the controller, which eventually compares it with the desired setpoint, calculates an appropriate action and dictates the actuator to manipulate the input accordingly. On the other hand, in feed forward structure disturbance is directly sensed by a measuring device and the same information is passed on to the controller, which eventually calculates an appropriate action and dictates the actuator to manipulate the input accordingly. Thus, a feed forward controller takes a control action even before the disturbance is felt by the process. It does not mean that effect of disturbance on the process would be nullified by feed forward controller, however, the effect would be greatly reduced especially when the process demonstrate large delay. More on this would be discussed in Module IV. At this point one should understand the concept of input/output pairing. In a simple control system, if there is only one output to be controlled, one needs only one manipulated input for that purpose and the system is called Single Input Single Output (SISO) process. However, there might be several control objectives for a particular process which would ideally need more than one output to be controlled using multiple manipulated inputs. Such processes are termed as Multi Input Multi Output (MIMO) processes. A crucial decision at this juncture would be to decide which output would be paired with which manipulated input for a MIMO process for the best performance of the control system.