Before going into control system, let's review some of basic terminology in block diagram. There are three basic elements in a block diagram: summing junction, pick up point, and multiplication block.
|Figure 1: Summing junction|
Figure 1 shows a summing junction where C is addition of A and B. A simple example of this block is addition of two encoder signal values.
|Figure 2: Pick up point|
Figure 2 shows a pick up point, where the A signal is duplicated into two places. An example of this diagram is the duplication of encoder reading to be used in two different place.
|Figure 3: Multiplication block|
Figure 3 shows a multiplication block, where C is multiplication of A and B. An example of this is; A is applied voltage, B is motor, and C is the motor speed. In other word, if voltage is applied at a motor, the motor will turn with a specific speed.
Open Loop versus Closed Loop System
The main function of control system is to bring actual output to desired output. In other word, we would like to have the output, Y to be exactly same with the reference, R. In short, Y = R.
|Figure 4: Open loop system|
Consider the block equation of the open loop system. Y will be equal to R if product of C and P is equal one and we have zero disturbance, D. However, the plant parameter value, P, is usually hard to obtain, so it is quite hard to obtain a suitable controller value, C.
Let's put that the controller value was obtained to produce C.P = 1. But somehow, the plant parameter changed (because of robot weight change, low battery, etc), a new C has to be obtained, which is tedious and hard. In addition, it is quite impossible to ignore disturbance, D (because of gravity, friction, and other non-linearity).
|Figure 5: Closed loop system|
It would be more advantageous to use a closed loop system rather than an open loop system although the implementation of closed system is more complicated. However, it is not as simple as "closing the loop". Consideration has to be put in: a) System transient response, b) System stability, and c) Steady state error.