If a given system is complicated, it is very difficult to analyze it as a whole. With the help of transfer function approach, we can find transfer function of each and every element of the complicated system. And by showing connection between the elements, complete system can be splitted into different blocks and can be analyzed conveniently. This is the basic concept of block diagram representation.
Basically block diagram is a pictorial representation of the given system. It is very simple way of representing the given complicated practical system. In block diagram, the interconnection of system components to form a system can be conveniently shown by the blocks arranged in proper sequence. It explains the cause and effect relationship existing between input and output of the system, through the blocks.
To draw the block diagram of a practical system, each element of practical system is represented by a block. The block is called functional block. It means, block explains mathematical operation on the input by the element to produce the corresponding output. The actual mathematical function is indicated by inserting corresponding transfer function of the element inside the block.
Note : For a closed loop systems, the function of comparing the different signals is indicated by the summing point while a point from which signal is taken for the feedback purpose is indicated by take off point in block diagrams.
All these summing points, blocks and take off points then must be connected exactly as per actual elements connected in the practical system. The connection between the blocks is shown by lines called branches of the block diagram. An arrow is associated with each and every branch which indicates the direction of flow of signal along the branch.
Note: The signal can travel along the direction of an arrow only.
It cannot pass against the direction of an arrow. Hence block diagram is an unilateral property of the system.
In short any block diagram has following five basic elements associated with it:
1) Blocks 2) Transfer functions of elements shown inside the blocks.
3) Summing points 4) Take off points 5) Arrows.
1. Illustrating Concept of Block Diagram Representation
For example: Consider the liquid level system as shown in the Fig. 1. So to represent this by block diagram, identify the elements which are,
i) Controller ii) Pneumatic valve iii) Tank iv) Float.
In this system, the level of water is sensed by the float. Hence the float position acts as the feedback. According to the float position, with respect to desired level of water, the controller operates the pneumatic valve controlling the flow of water in the tank. When the required level is reached, controller operates the pneumatic valve in such a way that the flow of water in the tank, stops. If the output from the tank is taken i.e. the water from the tank is drained then the float position changes from the desired position and accordingly the controller operates the pneumatic valve to start the flow of water in the tank.
Hence indicating all the elements by blocks, the block diagram of the system can be developed as in the Fig. 2.
2. Advantages of Block Diagram
The various advantages of block diagram representation are,
1) Very simple to construct the block diagram for complicated systems.
2) The function of individual element can be visualised from block diagram.
3) Individual as well as overall performance of the system can be studied by using transfer functions shown in the block diagram.
4) Overall closed loop T.F. can be easily calculated by using block diagram reduction rules.
3. Disadvantages
The various disadvantages of block diagram representation are,
1) Block diagram does not include any information about the physical construction of the system.
2) Source of energy is generally not shown in the block diagram. So number of different block diagrams can be drawn depending upon the point of view of analysis. So block diagram for given system is not unique.