# Control System | Questions & Answers

Q. What is a System?

Answer. When a number of elements or components are connected in a sequence to perform a specific function, the group of elements that all constitute a System.

Q. What is Control System?

Answer. 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 a controlled variable or response and the input quantity is called a command signal or excitation.

Q. Define Transfer Function?

Answer. Transfer Function of a control system is defined as:

1. the ratio of Laplace transform of Output to the Laplace transform of the Input with zero initial conditions
2. A transfer function is defined as the Laplace transform of an Impulse response of the system with zero initial conditions.

Q. What is Zero of the system?

Answer. Zero of a function F(s) is a value at which the function F(s) becomes zero, where F(s) is a function of complex variable s.

Q. What is Pole of the system?

Answer. Pole of a function F(s) is the value at which the function F(s) becomes infinite, where F(s) is a function of the complex variable s.

Q. What is Order of the system?

Answer. Order of the system is defined as the order of the differential equation governing the system. Order of the system can be determined from the transfer function of the system. Also, the order of the system helps in understanding the number of poles of the transfer function. For nth order system for a particular transfer function contains ‘n’ number of poles.

Q. What are the basic components of the feedback control system?

Answer. Basic components of the feedback control system are process system (open loop system), feedback path element, error detector, and controller.

Q. What are the Characteristics of Negative Feedback?

Answer. Negative Feedback in a Control System has following Characteristics

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

Q. Explain Mechanical Translational System?

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

1. Weight the mechanical system is represented by mass and is assumed to be concentrated at the center of the body.
2. The elastic deformation of the body can be represented by the spring.
3. Friction existing in a mechanical system can be represented by dash-pot.

Q. What is the mathematical model of a control system?

Answer. Control system is a collection of physical elements connected together to serve an objective. The output and input relations of a various physical system are governed by differential equations. The mathematical model of a control system constitutes a set of differential equations. The response of the output of the system can be studied by solving the differential equations for various input conditions.

Q. What is S-domain and its significance?

Answer. By taking Laplace transform for a 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.

Q. What is Signal Flow Graph?

Answer. A Signal Flow Graph is a diagram that represents a set of simultaneous linear algebraic equations. By taking Laplace transform the time domain differential equations governing a control system can be transferred to a set of algebraic equations in s-domain. The signal Flow graph of the system can be constructed using these equations.

Q. What are the basic properties of Signal Flow Graph?

Answer. The basic properties of the signal flow graph are

1. Signal Flow Graphs are applicable to linear systems.
2. 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.
3. A node adds the signals of all incoming branches and transmits this sum to all outgoing branches.
4. Signals travel along branches only in a marked direction and are multiplied by the gain of the branch.
5. The algebraic equations must be in the form of cause and effect relationship.

Q. What is the effect of positive feedback on the stability of the system?

Answer. Positive feedback is not used generally in the control system because it increases the error signal and drives the system to instability. But positive feedbacks are used in minor loop control systems to amplify certain internal signals and parameters.

Q. Why Negative Feedback is preferred in the Control System?

Answer. The role of Feedback in the control system is to take the sampled output back to the input and compare output signal with the 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.

Q. What is a feedback in Control System?

Answer. The Feedback in Control System in one in which the output is sampled and a proportional signal is fed back to the input for automatic correction of the error ( any change in desired output) for further processing to get back the desired output.

Q. What are different types of Control Systems?

Answer. Two major types of Control Systems are

1) Open loop Control System
2) Closed Loop Control Systems

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.

Q. Where Servomechanism is used?

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

A servomechanism is widely used in Governor value position control mechanism used in the power plants where the speed of the turbine is taken and processed using the transducers and final control element is brought as the mechanical movement of the value. Nowadays Governor value control is done with Electronic controls using power Thyristors. The servomechanism is also widely used in the robotic hand movements.

Q. What is the basic rule for Block Diagram Reduction Technique?

Answer. The rules of the Block Diagram reduction techniques are designed in such a manner that any modifications made in the diagram will not alter the input and output relation of the system.

Q. What is Time Invariant system?

Answer. Time-Invariant System is one in which the input and output characteristics of the system does not change with time.

Q. What are Test signals and their significance?

Answer. The knowledge of the input signal is required to predict the response of the system. In most of the systems, input signals are not known ahead of the time and it is also difficult to express the input signals mathematically by simple equations. In such cases determining the performance of the system is not possible.

Test signals help in predicting the performance of the system as the input signals which we give are known hence we can see the output response of the system for a given input and can understand the behaviour of the control system. The commonly used test signals are the impulse, ramp, step signals and sinusoidal signals.

Q. What is Time response of the control system?

Answer. Time response of the control system is defined as the output of the closed-loop system as a function of time. Time response of the system can be obtained by solving the differential equations governing the system or time response of the system can also be obtained by the transfer function of the system.

Q. How Time response of the system is divided?

Answer. Time response of the system consists of two parts:

1. Transient state response

Transient response of the system explains about the response of the system when the input changes from one state to the other. Steady state response of the system shows the response as the time t, approaches infinity.

Q. What is Phase Margin?

Answer. The phase margin is the amount of additional phase lag at the gain crossover frequency required to bring the system to the verge of instability.

Q. What is Gain Margin?

Answer. The Gain Margin is defined as the reciprocal of the magnitude of open loop transfer function at phase crossover frequency. The gain margin indicates the amount by which the gain of the system can be increased without affecting the stability of the system.

Q. What is Phase crossover frequency?

Answer. The frequency at which the phase of the open loop transfer function is 180° is called the phase crossover frequency.

Q. What is Cut-off rate?

Answer. The slope of the log-magnitude curve near the cut-off frequency is called the cut-off rate. The cut-off rate indicates the ability of the system to distinguish between the signal and the noise.

Q. What is Resonant Frequency?

Answer. The frequency at which resonant peak occurs is called the resonant frequency. Resonance frequency explains about the speed of the transient response.

Q. Enlist the applications of sampled data systems.

Answer. Sampled data system find applications in

1. High-speed tinplate rolling mills using quantized data for control.
2. Numerically controlled machine tool operations.
3. Pulse controlled or digital controlled electric drives.
4. Large complex systems using telemetry links based on pulse modulation (PM) translation of data.

Q. What is meant by overshoot?

Answer. Overshoot is the maximum difference between the transient and steady-state solutions for a unit step function input.

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