Stability Analysis in Control Systems

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The root locus technique is used to adjust the location of closed loop ______ to achieve desired system performance.

poles

Root Locus is the plot of the roots of the characteristics ______ of the closed loop system.

equation

The condition of the root locus can be defined using the ______ Condition and Magnitude Condition.

Angle

The angle condition states that G(S)H(S) = ±(2q + 1)180°, where q = 0, 1, 2, and represents ______ multiples of 180°.

<p>odd</p> Signup and view all the answers

The magnitude condition can be stated as |G(S)H(S)| = |-1 + j0| = ______.

<p>1</p> Signup and view all the answers

G(S)H(S) cannot be found if it contains unknown variable ______.

<p>K</p> Signup and view all the answers

If any value of 's' satisfies the angle condition, then that point is considered to be on the root ______.

<p>locus</p> Signup and view all the answers

To determine whether S = -0.5 lies on the root locus, one must use the ______ condition.

<p>angle</p> Signup and view all the answers

The study of whether a system is stable or unstable is known as __________ analysis.

<p>stability</p> Signup and view all the answers

A necessary condition for a feedback system to be stable is that all the poles of the system transfer function have __________ real parts.

<p>negative</p> Signup and view all the answers

A system is said to be __________ stable if it produces a bounded output for a given bounded input.

<p>BIBO</p> Signup and view all the answers

In the absence of input, a system is said to have __________ stability if its output tends to zero irrespective of initial conditions.

<p>asymptotic</p> Signup and view all the answers

A system is called __________ stable if it is stable for all values of the parameters.

<p>absolute</p> Signup and view all the answers

A system is said to be __________ stable if it exhibits sustained oscillation with constant magnitude.

<p>marginally</p> Signup and view all the answers

A system is __________ if the output is unbounded for a bounded input.

<p>unstable</p> Signup and view all the answers

If the system output is stable for a limited range of parameter variations, it is termed __________ stable.

<p>conditionally</p> Signup and view all the answers

The characteristic equation related to root locus technique is given by 1 + G(s)H(s) = ______

<p>0</p> Signup and view all the answers

Break away points occur where multiple roots of the characteristic equation 1 + G(s)H(s) = ______ occur.

<p>0</p> Signup and view all the answers

The center of gravity or centroid is denoted by ______ and is calculated using poles and zeros in the transfer function.

<p>σA</p> Signup and view all the answers

The angle of asymptotes can be calculated using the formula where p = 0, 1, 2, ..., (N - M - ______).

<p>1</p> Signup and view all the answers

Asymptotes provide direction to the root locus when they depart from break away ______.

<p>points</p> Signup and view all the answers

Routh Hurwitz criterion is used to find out the point of intersection of root locus with the ______ axis.

<p>imaginary</p> Signup and view all the answers

Break in points occur when root locus is present between two adjacent ______ on the real axis.

<p>zeros</p> Signup and view all the answers

K is the open loop system ______ in the transfer function G(s).

<p>gain</p> Signup and view all the answers

The point of intersection of the asymptotes on the real axis is called the ______.

<p>centroid</p> Signup and view all the answers

The formula for the centroid σ is given by the sum of real parts of poles minus the sum of real parts of zeros divided by the ______.

<p>number of poles</p> Signup and view all the answers

For K = -S^3 - 4S^2 - 3S, the characteristic equation is S(S^2 + 4S + 3) + K = ______.

<p>0</p> Signup and view all the answers

When S = -0.451, the value of K is calculated to be ______.

<p>0.6327</p> Signup and view all the answers

The formula for the closed loop transfer function is C(S) / R(S) = K / ______.

<p>1 + S(S + 1)(S + 3)</p> Signup and view all the answers

To find the breakaway point, we differentiate K with respect to ______.

<p>S</p> Signup and view all the answers

The closed loop transfer function is equal to K divided by S(S + 1)(S + 3) + ______.

<p>K</p> Signup and view all the answers

The formula for the centroid σ is the sum of real parts of poles minus the sum of real parts of ______.

<p>zeros</p> Signup and view all the answers

The characteristic equation is represented as S(S+2)(S+4) + ______ = 0.

<p>K</p> Signup and view all the answers

Differentiating K with respect to S gives us the equation dK/dS = ______.

<p>-3S^2 - 12S - 8</p> Signup and view all the answers

For K to be positive and real, the corresponding S value is ______.

<p>-0.845</p> Signup and view all the answers

When S = -3.154, K is calculated to be ______.

<p>-3.08</p> Signup and view all the answers

The term used for when K is positive and real in relation to S is known as the ______ point.

<p>breakaway</p> Signup and view all the answers

To find breakaway points, we set dK/dS equal to ______.

<p>0</p> Signup and view all the answers

The characteristic equation is S(S+2)(S+4) + K = 0, which expands to S3 + 6S2 + 8S + ______ = 0.

<p>K</p> Signup and view all the answers

To find the crossing points on the root-locus, we substitute s = ______.

<p>jw</p> Signup and view all the answers

The equation −w^3 + 8w = ______ shows the condition for the imaginary part of the characteristic equation.

<p>0</p> Signup and view all the answers

The crossing points of the root locus are ______ with the value of K at this point being 48.

<p>±2.8</p> Signup and view all the answers

For δ = 0.5, α can be calculated as cos⁻¹______ which equals to 60°.

<p>0.5</p> Signup and view all the answers

The product of vector length from open loop poles to the point s gives Ksd, which is the ______ of the corresponding point.

<p>value</p> Signup and view all the answers

The Root Locus Plot technique is particularly useful in investigating the stability ______ of the system.

<p>characteristics</p> Signup and view all the answers

One application of the Root Locus method is to determine the ______ boundaries of the system.

<p>stability</p> Signup and view all the answers

Flashcards

Stability in Control Systems

A property of a system where the output remains predictable, finite, and stable for a given input.

BIBO Stability

A system is BIBO stable if a bounded input results in a bounded output.

Asymptotic Stability

In the absence of input, the system output approaches zero regardless of initial conditions.

Absolute Stability

A system is stable for all possible parameter values.

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Marginally Stable

The output oscillates with a constant magnitude.

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Unstable System

Produces unbounded output for a bounded input.

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Conditional Stability

Stable only within a specific range of parameter values.

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Stable System Poles

All poles of the system transfer function must have negative real parts for a feedback system to be stable.

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Root Locus

A plot showing the locations of the roots of the characteristic equation of a closed-loop system as a system parameter (like gain) changes.

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Angle Condition (Root Locus)

The requirement that the angle of the open-loop transfer function (G(s)H(s)) at any point 's' must be an odd multiple of 180° for that point to be on the root locus.

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Magnitude Condition (Root Locus)

The requirement that the magnitude of the open-loop transfer function (|G(s)H(s)|) at any point 's' on the root locus must be equal to 1.

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Characteristic Equation

The equation that determines the stability and response of a closed-loop control system.

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Open-loop Transfer Function (G(s)H(s))

The transfer function that describes the relationship between input and output without the feedback loop.

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Closed-loop system

A system with feedback that constantly monitors and adjusts the output.

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System parameter

A variable in a system's equations that changes the system's behavior.

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Stability

A property of a system that describes how it reacts to disturbances or initial conditions; does the system output not diverge to infinity?

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Breakaway Point

Points where root loci intersect or change direction, representing maximum gain (K).

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Break-in Point

Point where root loci enter the complex plane, occurring between adjacent zeros on the real axis.

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Centre of Gravity (Centroid)

The point from which asymptotes originate. Calculated by (sum of poles - sum of zeros)/(number of poles - number of zeros).

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Asymptotes

Lines that guide root locus behavior near infinity.

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Angle of Asymptotes

Angles formed by asymptotes with the real axis, calculated by (2p+1) * 180 / (N-M), where p = 0 to N-M-1.

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Imaginary axis intersection

Point where the root locus crosses the imaginary axis. Found using Routh-Hurwitz criterion.

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Centroid (σ)

The point of intersection of asymptotes on the real axis in a function.

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Centroid Calculation

Centroid = (Sum of real parts of poles - Sum of real parts of zeros) / (Number of poles - Number of zeros)

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Root Locus Angle

The angle formed by the asymptotes with the real axis.

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Breakaway Point Calculation

Find the breakaway point(s) by setting the derivative of 'K' with respect to 's' equal to zero in the characteristic equation.

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Closed-Loop Transfer Function

The transfer function of a control system after feedback is applied. It specifies the system's response to an input.

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Breakaway/Break-in Point

A point on the root locus where the root locus branches break away from the real axis or break into the complex plane.

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Gain (K)

A parameter often representing a system's amplification or control factor.

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Root Locus Crossing Imaginary Axis

The point where the root locus crosses the imaginary axis, indicating the transition between stable and unstable system behavior.

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How to find the crossing point?

Substitute s = jw (where 'j' is the imaginary unit and 'w' is the frequency) into the characteristic equation and separate the real and imaginary parts. Solve for 'w' to find the crossing point on the imaginary axis.

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What is 'K'?

A gain factor that influences the system's stability. A higher 'K' value can destabilize the system.

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How to determine 'K' at the crossing point?

Equate the real and imaginary parts of the equation to zero and solve for 'K' using the 'w' value found from step 3.

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Damping Ratio (δ)

A measure of how quickly a system's oscillations dampen out. A higher δ means faster damping.

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Dominant Pole (sd)

The pole that has the greatest influence on the system's response.

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Vector Lengths and Open Loop Poles/Zeros

The vector lengths from the open-loop poles and zeros to the point 'sd' on the root locus are used to calculate the gain 'Ksd' corresponding to the dominant pole.

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Study Notes

Stability Analysis in Control Systems

  • Stability is a crucial characteristic of a control system's transient performance
  • A stable system's output is predictable, finite, and stable for a given input
  • System stability is determined by analyzing the location of its poles in the complex plane.

Concepts of Stability

  • BIBO Stability: A system is BIBO stable if it produces a bounded output for any bounded input
  • Asymptotic Stability: In the absence of input, the output of the system goes to zero, regardless of initial conditions.
  • Stable System: A stable system produces a bounded output for a bounded input. Four cases of system stability are possible
  • Absolute Stable: Stable for all parameter values
  • Marginally Stable: Output has sustained oscillations at a constant magnitude.
  • Unstable: Output is unbounded for bounded input
  • Conditionally Stable: Stable only within a limited range of parameter variations

Effect of Pole Location on Stability

  • Real and Negative Poles: Exponentially decaying time response; stable
  • Complex Conjugate Poles with Negative Real Parts: Damped oscillations; stable
  • Real and Positive Poles: Exponentially increasing response; unstable
  • Complex Conjugate Poles with Positive Real Parts: Growing oscillations; unstable

Routh-Hurwitz Criterion

  • An algebraic method for determining the stability of a linear time-invariant (LTI) system
  • Examines the coefficients of the characteristic equation to determine the number of roots with positive real parts (unstable).
  • By examining the signs in the first column of the Routh array, the stability of the system is determined.
  • The number of sign changes in the first column of the Routh array equals the number of roots with positive real parts in the characteristic equation, indicating the number of unstable poles.

Root Locus Technique

  • A graphical method used to analyze the stability and transient response of a closed-loop control system as controller gain (K) changes
  • Plots the location of closed-loop poles in the complex s-plane as K varies from zero to infinity.
  • Poles and zeros of the open-loop system determine the root locus plot's characteristics.
  • Useful for determining stability boundaries and designing suitable system parameters
  • Utilizes angle and magnitude conditions to plot the root locus.

Terms in Root Locus

  • Characteristic Equation: The equation relating the stability of a closed loop system to its controller gain (K)
  • Breakaway Points: Points where multiple roots of the characteristic equation occur. Breakaway points on the real axis are where the root locus branches depart.
  • Break-In Points: Points where multiple roots of the characteristic equation occur. Points where root locus branches intersect the real axis and extend back into the complex plane.
  • Centroid (σa): The center of gravity of the poles and zeros of the open-loop system
  • Asymptotes: Lines originating from the centroid (σa), dictating how the root locus branches approach infinity. The angles of asymptotes are calculated from the number of poles and zeros
  • Intersection points with the Imaginary Axis: The auxiliary equation and Routh-Hurwitz stability criterion are used
  • Gain Margin: The amount by which the gain can be increased before the system becomes unstable
  • Phase Margin: The amount of additional phase shift required before the system becomes unstable

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