by Dinesh Thakur

Control is essential for monitoring the output of systems and is exercised by means of control loops. It is necessary for monitoring the desired output of a system with the actual output so that the performance of the system can be measured and corrective action taken if required. Schoderbek, 1985 mentions four elements required for effective control:

  1. A control variable is the variable whose value would determine the degree of performance of the system.
  2. A detector is to monitor the output of the system by measuring the control variable parameters.
  3. A comparator is to compare the actual and planned output of the system.
  4. An effecter is to make suitable changes.

To illustrate these in greater detail, let us visualize the cooling system of a refrigerator. The cooling coils cool the refrigerator to bring the temperature to a certain level and then the effecter is relied upon to change the system inputs so that the cooling process is stopped once the desired temperature is reached. The detector measures the temperature and compares it with the desired temperature and the effecter stops the cooling process once the desired temperature is reached. Again, if the temperature rises above the desired temperature, the effecter comes into play again by putting on the cooling system. This is called control and the process in which this was done in this case is called a control loop. In this case a closed loop. The open loop control systems have a structure in which the output of the system is not coupled with the input of the system.

Types of Control

Control mechanisms can be of two types: feedback control and feed forward control.

Feedback Control

When we have a control structure in which the output is used to directly alter the inputs we call that as a feedback control mechanism. Feedback control can itself be of two types, i.e. positive feedback and negative feedback. Positive feedback is when the output of a system is positively correlated with the input, i.e., more output prompts more input or less output prompts less input. For example, stock market sometimes exhibit positive feedback. Positive feedback generally indicates an instable system unless there is an outside mechanism to stop the process beyond a point. Negative feedback on the other hand is the opposite of positive feedback in the sense that in negative feedback the relationship between output and input is negative. The refrigerator example given earlier is an example of negative feedback.

Feedback control systems particularly the ones with negative feedback have a tendency to oscillate around the desired values of control variables. For example, take the example of a driver driving a vehicle and wanting to keep a speed of around sixty km per hour. He will apply brakes when he is beyond sixty km per hour and as a result the vehicle speed will come back, to let us say, fifty-five km per hour at which time he will again press the accelerator and push the speed to near sixty km per hour and this will again result in the speed crossing sixty km per hour alerting him to press brakes and slow down. Thus vehicle speed will oscillate around sixty km per hour, which happens as control mechanisms are not designed to work in a step wise manner, instead they have a steady effect on the system. This means that system oscillation happens when control mechanisms might take some time to react to an alert or may also take a finite time to take effect or both. This can also happen if the control mechanism overcompensates for the deviation from a stable state.

Feed Forward Control

This is a type of control mechanism to address the problem of system oscillation. In this type of control mechanism, the control is exercised after predicting the output and if the prediction about the output is that it will cross the stable limit or the target then control mechanisms are applied even before the target value or control value of the control variable is attained bringing down the system automatically below the danger limit. For example, if in our driver example, if the vehicle had an intelligent braking system controlled by computer aided automatic brake controls then whenever the vehicle would go over fifty-eight km per hour, automatic brakes would be applied irrespective of whether the driver applies brakes or not bringing down the speed to the desired level. This is called feed forward system and it works on a proactive philosophy rather that the reactive philosophies of a feed back control mechanism. However, to apply feed forward control mechanism we need to have complete understanding of the system.