Two major types of Control Systems are

**Open loop Control Systems:**The Open loop Control System is one in which the Output Quantity has no effect on the Input Quantity. No feedback is present from the output quantity to the input quantity for correction.**Closed Loop Control System:**The Closed loop Control System is one in which the feedback is provided from the Output quantity to the input quantity for the correction so as to maintain the desired output of the system.

The role of Feedback in control system is to take the sampled output back to the input and compare output signal with input signal for error ( deviation from the desired result).

Negative Feedback results in the better stability of the system and rejects any disturbance signals and is less sensitive to the parameter variations. Hence in control systems negative feedback is considered.

Model of mechanical trlational system can be obtained by using three basic elements Mass, Spring and Dash-pot.

Weight the mechanical system is represented by mass and is assumed to be concentrated at the center of body.

The elastic deformation of the body can be represented by the spring

Friction existing in a mechanical system can be represented by dash-pot.

Negative Feedback in a Control System has following Characteristics

- Reduction in the gain at the expense of better stability of the system
- Rejection of disturbance signals in the system
- Low Sensitivity to parameter variations
- Accuracy in tracking the steady state value

In a System the output and inputs are interrelated in such a manner that the output quantity or variable is controlled by input quantity, then such a system is called Control System.

The output quantity is called controlled variable or response and the input quantity is called command signal or excitation.

By taking Laplace trform for differential equation in the time domain equations in S-domain can be obtained. L{F(t)}=F(s)

S domain is used for solving the time domain differential equations easily by applying the Laplace for the differential equations.

Servomechanism is used in control system where the output is pertained to vary the mechanical position of a device.

Servo Mechanism is widely used in Governor value position control mechanism used in the power plants where speed of the turbine is taken and processed using the trducers and final control element is brought as mechanical movement of the value. Now a days Governor value control is done with Electronic controls using power Thyristors. Servomechanism is also widely used in the robotic hand movements.

Trfer Function of a control system is defined as:

- Ratio of Laplace trform of Output to the Laplace trform of the Input with zero initial conditions
- Trfer function is defined as the Laplace trform of Impulse response of the system with zero initial conditions

The basic properties of the signal flow graph are:

- Signal Flow Graphs are applicable to linear systems
- It consists of nodes and branches. A node is a point representing a variable or signal. A branch indicates the functional dependence of one signal on another
- A node adds the signals of all incoming branches and trmits this sum to all outgoing branches
- Signals travel along branches only in a marked direction and is multiplied by the gain of the branch
- The algebraic equations must be in the form of cause and effect relationship