What is a closed loop system ?
You would need to understand the difference between open and closed loop system, so you would need to get down to the details. In case of open loop system, you can avail a set point of a system, and whatever is bound to come out will do so. No system in the world is stiff, which means that the output is bound to vary which depends upon the load. Closed loop depicts a situation where someone is watching the output and then the set point is adjusted to whatever form so that the desired output is achieved.
Take an example of the gas pedal in your own car. From it to a car speed is an open loop system. You can go on to push the pedal to a fixed point, and the car will accelerate its speed higher or lower, depending on what it has recently done or whether it is going up a hill or not. Let us take another example of a cruise control. This tends to put feedback around the open loop system, to the car speed system and then regulate the speed. In fact, the speed is watched and then effectively stepped on the gas when the car is noticed to be slowing down. It is let up on the gas where it tends to go faster than what is desired.
Advantages of Closed Loop System
By now the advantage of a closed loop system would be quite obvious. You are in control of the parameter that you really want it to be. In this example it is car speed and it goes on to provide automatically the system the right form of input. This is the gas pedal system in order to achieve the desired result. But a point to consider is that this may happen at the cost of extra flexibility or convenience.
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When it comes to closed loop systems they need to be designed in such a manner so that they are stable. Say if the cruise control system always went on to slam the gas pedal to a maximum range, whenever the car is below the set speed and it is let up when it is a little above you are bound to have a bumpy ride. The net result is that it is bound to put severe stress on various parts of the car. In the midst of all this there are some form of trade-offs where the designer can make a lot of error.
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Another major issue with closed loop systems is behavior measurements. They are not that accurate all the time. If the speed of the motor system is sensed with the help of a rotary encoder, where the encoder drops pulses in the higher speed range, it is quite possible that the dropped pulses may force to underestimate the actual speed of the motor. It would speed up more than what is required. The net result is that more pulses is bound to be missed and if the motor proceeds in such a manner where no pulses arrive, it is obvious that the controller may detect a problem and eventually shut down. The closed loop control systems would control speed in a better manner in comparison to open loop systems though they may not have that failure nodes.
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