Introduction to Control Systems
- A system, whose output can be managed, controlled or regulated by varying its input is called Control System.
- If we look around, we will find many control systems in our surroundings i.e. Refrigerator, Air Conditions, Washing Machines etc.
- A control system can also be a combination of smaller control systems and are normally used to get desired/required output.
- It's block diagram is shown in below figure:
- The above figure represents a simple control system and we can think of this control system as a mathematical equation i.e.
X + 5 = Y
where, X is input, Y is output and Constant 5 is acting as a Control System.
- So, by changing the value of input parameter ( X ), we can change our output value (Y).
- Similarly, if we want a particular output value, we can achieve it by fixing input value.
- A pre-installed Temperature sensor reads the temperature, which is then fed into Comparator.
- Comparator compares current temperature with user-inputted temperature value and then regulates the Fan speed.
Types of Control SystemsControl Systems are classified into two main categories, which are:
- Open Loop.
- Closed Loop.
Open Loop Control System
- In Open Loop Control Systems, we have three main components i.e. Input, Controller & Output.
- Input signal is directly fed to the controller, which utilizes it and generates the required output.
- In Open Loop systems, generated output has no affect on the Input signal i.e. no feedback provided.
ExampleHere we will be discussing two examples of open loop control systems, to get a better understanding of the process.Clothes Dryer is a very simple example of an open loop system. When damp clothes are put in the dryer machine, the operator / user sets the time for drying the clothes. This time acts as the input signal for the dryer. Correspondingly at the end of that time, the machine stops and clothes can be taken out. Now the thing to be noted here is that no matter if the clothes are dry enough or not, the machine will stop because of the time (input signal) fed to it. So the output of the system does not affect the input in this case. For a better understanding the block diagram of a cloth dryer control system is shown below:Traffic Light system is another easy to understand example of an open loop system. Certain input signals are fed to the controller, which then displays one of the three lights at the output turn by turn. The direct input signals can be altered to change the output light but the output has no affect on the input. As we are not passing any feedback i.e. which light turned ON or OFF.
Closed Loop Control System
- Closed Loop Control System (Feedback control system) is an advanced automated system, which generates the desired output by using inputs, Controllers and feedback elements.
- These systems use feedback element to fed the Output back to the controller. By doing that, we can compare the current output with input to get errors.
- Here's the block diagram of a closed Loop Control System:
ExampleAir conditioner is a very typical example of a closed loop control system. The input signal in the form of required room temperature is fed into the controller of the air conditioner. The compressor along with its various electrical and mechanical components help in achieving the required temperature. Now whenever the room temperature changes, the temperature sensor at the output senses the change in the room temperature and the signal from the sensor is calculated by the error detector and fed back to the controller through the feedback loop to maintain the required room temperature. In this way the required output is always maintained automatically without any manual interference. The block diagram illustration of this process is shown below:
Pros and Cons of Open and Closed loop
- It must have been cleared by now that wherever an accurate and precise system is required, a feedback closed loop control system is the answer, as they provide the feedback and thus automatically reduces the error while open loop control systems are not reliable enough to provide an accurate output.
- However another thing to be kept in mind here is the cost of the system. More precision and complexity means a greater cost. So closed loop control systems are clearly more expensive than open loop control systems. Open loop control systems are cheaper and cost effective.
- Last but not the least, Open loop control systems are also simpler to design as compared to closed loop control systems.
SISO and MIMO Control SystemsControl systems are also classified into theese two categories, named as:
- SIngle Input Single Output systems. (SISO)
- Multiple Input Multiple Output systems. (MISO)
Continuous and Discrete Time Control SystemBefore discussing the continuous and discrete time control system, it is important to understand the concept of a continuous time signal and a discrete time signal:
- A continuous time signal refers to a signal, which is continuous in the time domain.
- On the other hand, when a signal is discrete in the time domain, i.e. it can be obtained at discrete time intervals, it is called a discrete time signal.
- As we know the difference, the only thing needed to understand is that in a continuous time control system, all the signals (including the input & output signals) are continuous in time.
- Whereas in a discrete time control system, all the signals of the control system (including the input and output signals) are discrete time signals.