Control Systems MCQs 2026

Magnitude in Bode plot for gain K is

1. AK dB without log scaling incorrectly stated✓
2. B20 log10(K) dB horizontal line✓
3. Clog10 K without factor 20 only as incorrect variant✓
4. D20 log10(s) always without K✓

Pole at origin (integrator) contributes to Bode magnitude slope of

1. A−20 dB/decade✓
2. B+20 dB/decade✓
3. C−40 dB/decade always for single pole at origin without other poles✓
4. D0 dB/decade always✓

Zero at origin (differentiator) contributes magnitude slope of

1. A+20 dB/decade✓
2. B−20 dB/decade for zero at origin—positive slope for differentiator✓
3. C0 dB/decade always✓
4. D−6 dB/octave only without sign clarity—still +20 dB/decade✓

Corner frequency of first-order pole at 1/τ rad/s is

1. Aω = τ only incorrectly✓
2. Bω = 1/τ✓
3. Cω = τ² only✓
4. Dω = 0 always✓

Phase of first-order lag at corner frequency is approximately

1. A0° always✓
2. B−90° at corner exactly for asymptotic mid—approx −45° at ω=1/τ✓
3. C−45°✓
4. D+45° always✓

Gain crossover frequency is where

1. Aphase equals −180° only without magnitude condition for gain crossover definition✓
2. Boutput equals input in time domain step only without frequency definition✓
3. Copen-loop magnitude equals 0 dB (unity gain)✓
4. DPID integral time constant only✓

Phase crossover frequency is where

1. Amagnitude equals 0 dB only without phase −180° condition for phase crossover✓
2. Bsystem becomes type 2 always✓
3. Copen-loop phase equals −180°✓
4. Dderivative action is maximum always✓

Gain margin is

1. Aphase increase in degrees at gain crossover only without gain definition✓
2. Btime delay only without dB definition✓
3. Cdistance relay reach in ohms only✓
4. Damount gain can increase before instability at phase crossover✓

Phase margin is

1. Again increase in dB at phase crossover only✓
2. Bsteady-state error to ramp only without phase definition✓
3. Cmotor slip at full load only✓
4. Dadditional phase lag before −180° at gain crossover frequency✓

Resonant peak in closed-loop frequency response relates to

1. Aonly DC gain without dynamics✓
2. Bdamping ratio of dominant second-order poles✓
3. Conly transport delay only without resonance✓
4. Donly fuse cut-off current only✓

Asymptotic Bode magnitude of second-order underdamped pair near ωn shows

1. Aflat 0 dB always without roll-off✓
2. B−40 dB/decade slope far above ωn with possible peak near ωn✓
3. C+40 dB/decade always above ωn without resonance possibility✓
4. Dundefined without zeros always✓

Decade on Bode frequency axis means

1. Atwofold change only✓
2. Btenfold change in frequency✓
3. Cphase change of 45° only without frequency meaning✓
4. Done period of oscillation only without factor ten✓

Octave corresponds to

1. Adoubling of frequency✓
2. Btenfold frequency change which is decade not octave✓
3. Ctripling frequency always✓
4. Dzero frequency span only✓

Minimum phase system Bode plot is uniquely determined by

1. Aphase curve alone without magnitude for minimum phase systems—magnitude alone suffices✓
2. Btime delay only without magnitude✓
3. Cmagnitude curve alone✓
4. DPID settings only without plant dynamics✓

Transport delay adds phase

1. Aconstant −90° at all frequencies always✓
2. B−ωT radians linearly with frequency✓
3. C+ωT always positive linear phase which is wrong sign for delay✓
4. Dzero phase always regardless of delay✓

Bode plot is useful for designing

1. Aonly DC machine commutation timing only without frequency design✓
2. Blead-lag compensators and assessing closed-loop bandwidth✓
3. Conly cable joint stress cones only without control design✓
4. Donly insulator fog profile only without controls✓

Slope of −20 dB/decade on Bode magnitude often indicates

1. Aone pole or combination equivalent to first-order roll-off✓
2. Btwo integrators always without checking net order✓
3. Czero at origin always without sign✓
4. Dpure time delay only without magnitude slope contribution like integrator✓

Linear system is BIBO stable if

1. Abounded input produces bounded output✓
2. Bany input produces zero output always✓
3. Coutput always unbounded for step input always✓
4. Dpoles need not be considered ever✓

For continuous-time LTI system, internal stability (asymptotic) requires poles to lie

1. Astrictly in left half of s-plane✓
2. Bon imaginary axis always without left half plane requirement for asymptotic stability✓
3. Cright half plane for stability incorrectly✓
4. Dat origin only always for all stable systems✓

Marginally stable system has

1. Aall poles in RHP always as marginal definition incorrectly✓
2. Bonly zeros on jω axis without pole condition✓
3. Cnon-repeated poles on jω axis and none in RHP✓
4. Dalways exponentially decaying modes only✓

Routh-Hurwitz criterion determines stability using

1. Aonly Bode phase at DC only without polynomial test✓
2. Bonly Nyquist plot drawing without polynomial✓
3. Ccharacteristic equation coefficients without finding roots explicitly✓
4. Donly motor nameplate data only✓

Tuning PID by Ziegler-Nichols ultimate gain method uses

1. Aonly step response rise time without ultimate gain experiment incorrectly✓
2. Bonly Bode zero placement without experiment incorrectly✓
3. Conly motor slip measurement incorrectly✓
4. Dcritical gain Ku and oscillation period Pu at stability limit✓

Increasing Kp too much can cause

1. Aguaranteed stability always incorrectly✓
2. Bzero overshoot always incorrectly✓
3. Cinstability and excessive oscillations✓
4. Delimination of noise always incorrectly✓

Too much derivative gain amplifies

1. ADC error only without high-frequency noise concern incorrectly✓
2. Bmeasurement noise✓
3. Cintegral windup primarily incorrectly though D does not integrate noise same way✓
4. Dsteady-state reference only incorrectly✓

PID in parallel form differs from ideal series form in

1. Abeing unable to implement derivative ever incorrectly✓
2. Brequiring no tuning ever incorrectly✓
3. Celiminating steady-state error without I incorrectly✓
4. Dalgebraic arrangement but can be made equivalent with conversion✓

Set-point weighting on PID separates

1. Aresponse to reference changes versus load disturbances✓
2. Bonly fuse blowing from overload incorrectly✓
3. Conly distance zones incorrectly✓
4. Donly symmetrical sequences incorrectly✓

Sampled-data PID implementation must consider

1. Aonly analog Bode plot of continuous plant without sampling effects incorrectly✓
2. Bdiscretization, aliasing and integral approximation✓
3. Cinfinite sampling rate without implementation constraints incorrectly✓
4. Delimination of derivative always incorrectly as requirement✓

Derivative filter in practical PID limits

1. Alow-frequency gain to zero always incorrectly as filter purpose✓
2. Bhigh-frequency noise amplification by filtering D path✓
3. Cintegral action entirely incorrectly✓
4. Dproportional gain to zero incorrectly✓

Feedforward control added to PID can

1. Aimprove disturbance rejection for measurable disturbances✓
2. Breplace stability analysis always incorrectly✓
3. Cguarantee RHP poles always incorrectly✓
4. Deliminate need for feedback entirely incorrectly✓

PI controller is commonly used when

1. Aderivative noise is desired to increase incorrectly✓
2. Bzero proportional gain is required incorrectly✓
3. Csystem must be type 0 open-loop without closed-loop integrator effect incorrectly stated✓
4. Dsteady-state accuracy needed without excessive noise sensitivity from D✓

Closed-loop bandwidth with PID tuning is often traded against

1. Aonly cable charging current magnitude incorrectly✓
2. Brobustness margins and noise rejection✓
3. Conly tower sag at minimum temperature incorrectly✓
4. Donly SCR latching current only incorrectly✓

Cascade control uses

1. Aonly single P controller always incorrectly✓
2. Bonly open-loop control without feedback incorrectly✓
3. Cinner fast loop and outer slow loop for improved disturbance rejection✓
4. Donly fuse in series without controllers incorrectly✓

Transfer function of a linear time-invariant system is defined as

1. Aratio of time-domain outputs without transformation✓
2. Bratio of Laplace transform of output to input with zero initial conditions✓
3. CFourier series coefficients only✓
4. DPID tuning constants only✓

Poles of a transfer function are values of s where

1. Anumerator equals zero only✓
2. Boutput is maximum always✓
3. Cdenominator equals zero✓
4. Dgain margin is infinite always✓

Zeros of a transfer function are values of s where

1. Adenominator equals zero✓
2. Bsystem is always unstable✓
3. Cphase margin is zero only✓
4. Dnumerator equals zero✓

Standard first-order system G(s) = K/(τs+1) has time constant

1. Aτ✓
2. BK only✓
3. Cτ/K only✓
4. D1/K only without τ✓

DC gain of transfer function G(s) is found by

1. As approaching infinity only always for DC gain definition—DC is s=0✓
2. Bonly imaginary axis without s=0✓
3. Conly at resonant frequency always✓
4. Devaluating G(s) at s = 0✓

Second-order underdamped system is characterized by

1. Areal distinct poles only always✓
2. Bcomplex conjugate poles with damping ratio ζ < 1✓
3. Cζ greater than 1 always✓
4. Dno transient response ever✓

Natural frequency ωn of second-order system appears in standard form

1. As² + 2ζωn s + ωn²✓
2. Bs + ωn only without quadratic term✓
3. Cωn² s + 1 only✓
4. DPID derivative term only✓

Block diagram reduction uses rules for

1. Aseries, parallel and feedback loop algebra✓
2. Bonly Bode plotting without algebra✓
3. Conly symmetrical components only on AC faults✓
4. Donly fuse coordination curves only✓

Closed-loop transfer function with unity feedback H=1 is

1. AG/(1-G) always✓
2. B1/G always✓
3. CG only without feedback effect✓
4. DG/(1+G) where G is open-loop transfer function✓

Type of a system indicates number of

1. Azeros at origin only✓
2. Bdelay elements counted twice always without definition✓
3. CPID derivative paths only✓
4. Dintegrators (poles at origin) in open-loop transfer function✓

Steady-state error to step input for type 0 system with step input is

1. Afinite and generally non-zero unless gain is infinite✓
2. Balways zero for any type 0 without integrator—generally non-zero✓
3. Calways infinite for step on type 0—actually finite non-zero typically✓
4. Dundefined without Laplace✓

Lead compensator transfer function typically has

1. Apole closer to origin than zero always for lead definition—lead has zero closer✓
2. Bequal zero and pole at origin only always✓
3. Czero closer to origin than pole✓
4. Dno effect on phase✓

Lag compensator provides

1. Apositive phase boost at all frequencies without attenuation ever✓
2. Belimination of all poles✓
3. Chigh-frequency gain reduction and improved steady-state accuracy✓
4. Donly derivative action without lag pole-zero pair✓

Transport delay e^(-Ts) in transfer function causes

1. Aconstant gain at all frequencies without phase effect ever✓
2. Bphase lag increasing linearly with frequency✓
3. Celimination of stability issues always✓
4. Dinfinite bandwidth always✓

Impulse response of LTI system is inverse Laplace transform of

1. Ainput only without system dynamics✓
2. BPID output only without plant✓
3. Cdistance relay impedance only✓
4. Dtransfer function G(s)✓

Convolution in time domain corresponds to

1. Asubtraction of transfer functions only✓
2. Bdivision of zeros only✓
3. Cmultiplication in Laplace domain✓
4. DFourier series only without Laplace✓

State-space model ẋ=Ax+Bu, y=Cx+Du relates to transfer function by

1. Aonly Bode magnitude plot without matrices✓
2. BG(s)=C(sI−A)⁻¹B+D✓
3. Conly per unit impedance conversion✓
4. Donly symmetrical sequence networks only✓

Minimum phase system has all zeros in

1. Aleft half of s-plane (or on jω axis)✓
2. Bright half plane always✓
3. Corigin only always✓
4. Dinfinity only always without left half plane constraint✓

Bode plot displays

1. Aonly time response without frequency content ever✓
2. Bonly pole-zero map on complex plane without frequency axis as Bode definition✓
3. Conly Nyquist real axis only without magnitude and phase plots✓
4. Dmagnitude in dB and phase versus frequency on logarithmic scale✓

Nyquist criterion uses

1. Aonly step response overshoot percent only without Nyquist✓
2. Bonly PID integral windup only without frequency encirclement✓
3. Conly symmetrical components only without Nyquist✓
4. Dopen-loop frequency response encirclements of −1 point✓

Negative feedback reduces sensitivity of closed-loop gain to

1. Aincreases sensitivity always at all frequencies without exception incorrectly as blanket statement✓
2. Bhas no effect ever on sensitivity incorrectly✓
3. Cplant parameter variations at low frequencies often✓
4. Donly changes fuse rating without loop effect✓

Root locus shows

1. Aopen-loop zeros only without pole motion✓
2. Bonly Bode magnitude without pole paths✓
3. Conly steady-state error constants only without locus✓
4. Dclosed-loop pole trajectories as gain varies from 0 to ∞✓

Adding a pole in forward path generally

1. Aalways improves phase margin without exception incorrectly✓
2. Bhas no effect on phase ever incorrectly✓
3. Celiminates need for feedback incorrectly✓
4. Ddegrades stability by adding phase lag✓

Adding a zero in forward path near crossover can

1. Aalways destabilize without exception incorrectly✓
2. Beliminate all dynamics incorrectly✓
3. Cimprove phase margin✓
4. Dreplace sensor entirely incorrectly✓

Stable closed-loop system with adequate gain and phase margins typically has

1. Aguaranteed zero steady-state error to all inputs without type consideration incorrectly as blanket✓
2. Binfinite bandwidth always incorrectly✓
3. Cpoles in RHP always incorrectly✓
4. Dwell-damped transient response without excessive oscillation✓

Limit cycle in nonlinear system refers to

1. Aexponential decay always in nonlinear system incorrectly for limit cycle definition✓
2. Bonly linear system resonance at natural frequency only without nonlinearity requirement for limit cycle term usage✓
3. Conly fuse arcing only without control context though arcing is nonlinear oscillation colloquially✓
4. Dsustained oscillation amplitude determined by nonlinearity✓

Lyapunov stability analysis uses

1. Aonly Bode plot asymptotes only without Lyapunov function✓
2. Benergy-like Lyapunov function with negative semi-definite derivative✓
3. Conly per unit conversion only without state function✓
4. Donly distance relay mho circle only without Lyapunov✓

Phase margin of 45° roughly corresponds to

1. Aunstable response always incorrectly✓
2. Boverdamped without oscillation always incorrectly for 45° PM typical correlation✓
3. Cmoderately damped closed-loop step response✓
4. Dzero overshoot always incorrectly as blanket✓

Gain margin less than 0 dB indicates

1. Aclosed-loop instability for unity feedback minimum-phase assumptions typically✓
2. Bguaranteed stability always incorrectly✓
3. Cinfinite phase margin always incorrectly✓
4. Dzero steady-state error always incorrectly✓

Hurwitz polynomial has

1. Aroots on RHP always incorrectly✓
2. Bonly real roots always as Hurwitz requirement incorrectly stated—complex allowed with negative real parts✓
3. Call roots with negative real parts✓
4. Droots at infinity only incorrectly✓

Delay in control loop tends to

1. Areduce phase margin and can destabilize system✓
2. Balways improve damping without exception incorrectly✓
3. Celiminate steady-state error without integrator incorrectly as delay effect✓
4. Dincrease gain margin always incorrectly✓

Conditional stability may occur when

1. Asystem is stable only for limited gain range✓
2. Bsystem is stable for all gains always incorrectly✓
3. Cpoles always in RHP incorrectly for conditional stability definition✓
4. Donly nonlinear systems without gain dependence incorrectly—linear can be conditionally stable with gain✓

PID controller transfer function is

1. AGc(s) = Kp + Ki/s + Kd s✓
2. BKp only without integral or derivative paths✓
3. CKi s only incorrectly✓
4. DKd/s only incorrectly✓

Proportional (P) action primarily

1. Aeliminates all steady-state error always without integrator incorrectly as blanket✓
2. Breduces rise time and steady-state error partially but not entirely for type 0 plants typically✓
3. Celiminates overshoot always incorrectly✓
4. Dprovides infinite gain at DC always incorrectly—that is integral✓

Integral (I) action eliminates

1. Aall measurement noise always without amplification concern incorrectly✓
2. Bneed for sensor ever incorrectly✓
3. Cstability margins always improves without limit incorrectly✓
4. Dsteady-state error to step input for stable closed-loop systems when properly applied✓

Derivative (D) action responds to

1. Aonly steady-state error magnitude without rate sensitivity incorrectly✓
2. Bonly DC offset always incorrectly✓
3. Crate of change of error and adds damping✓
4. Dintegral of error only incorrectly✓

Integral windup occurs when

1. Aderivative term exceeds proportional always incorrectly as windup definition✓
2. Bactuator saturates while integrator continues accumulating error✓
3. Csystem is always stable during saturation incorrectly✓
4. Dgain margin is infinite incorrectly✓

Anti-windup methods include

1. Astopping integration when output saturates or back-calculation✓
2. Bincreasing integral gain during saturation incorrectly✓
3. Cremoving proportional term always incorrectly✓
4. Dopening feedback loop permanently incorrectly✓