(a) What are the limitations of (i) Proportional (P), (ii) Integral (I), (iii) Derivative (D), and (iv) PID Controllers? What is the application of positive feedback control system? (10 marks)

(b) Explain the operation performed by 8085 microprocess

Question

(a) What are the limitations of (i) Proportional (P), (ii) Integral (I), (iii) Derivative (D), and (iv) PID Controllers? What is the application of positive feedback control system? (10 marks)

(b) Explain the operation performed by 8085 microprocessor when the following arithmetic instructions are executed: (i) ADD M (ii) ADC M (iii) DAD rp (iv) SBI d8 (v) DCR reg (5×2=10 marks)

(c) The ohmmeter circuit has V_B = 1·5 V, R_1 = 15 kΩ, R_m = 50 Ω, R_2 = 50 Ω and meter FSD = 50 μA. Determine the ohmmeter scale reading at 0·5 FSD.

(d) Calculate the power loss in a cable insulation having capacitance 9 μF, loss angle 0.05 degree and operating at 11 kV, 50 Hz. Draw the phasor diagram and equivalent circuit also.

(e) Explain the concept of a constellation diagram. Draw the PSK signal constellations for the value of M = 2, 4 and 8, if all have same transmitted signal energy E_s.

UPSC Answer Check · Accepted Answer

This multi-part question requires explaining theoretical concepts for (a), (b), and (e) while solving numerical problems for (c) and (d). Allocate approximately 25% time to part (a) covering controller limitations and positive feedback applications, 20% to part (b) detailing 8085 instruction operations with register-level explanations, 15% to part (c) showing ohmmeter circuit analysis, 20% to part (d) with dielectric loss calculations and phasor diagrams, and 20% to part (e) explaining constellation diagrams with PSK signal space representations. Begin each part with clear identification, use bullet points for controller limitations, step-by-step execution flow for microprocessor instructions, and labeled diagrams for constellation and phasor representations.
- Part (a): P controller causes steady-state error and offset; I controller causes slow response, integral windup, and instability; D controller amplifies noise, causes saturation, and is sensitive to disturbances; PID requires careful tuning; positive feedback used in oscillators (Barkhausen criterion), regenerative amplifiers, and Schmitt triggers
- Part (b): ADD M performs (A) ← (A) + [(HL)], affects flags; ADC M includes carry addition (A) ← (A) + [(HL)] + CY; DAD rp adds 16-bit register pair to HL (HL) ← (HL) + rp, affects only CY flag; SBI d8 subtracts immediate with borrow (A) ← (A) - d8 - CY; DCR reg decrements register without affecting carry flag
- Part (c): Calculate half-scale deflection current (25 μA), determine total series resistance (R₁ + R₂ + Rₘ = 15.1 kΩ), find unknown resistance using Rₓ = (V_B/I) - R_total giving approximately 45 kΩ at 0.5 FSD
- Part (d): Calculate dielectric loss using P = V²ωC tan δ = (11×10³)² × 2π×50 × 9×10⁻⁶ × tan(0.05°) ≈ 16.7 W; draw parallel RC equivalent circuit and phasor diagram showing δ angle between total current and capacitive current
- Part (e): Constellation diagram represents signal points in I-Q plane with distance from origin indicating amplitude and angle indicating phase; draw M=2 (BPSK: 2 points at 180°), M=4 (QPSK: 4 points at 90° intervals on circle), M=8 (8-PSK: 8 points at 45° intervals), all with same radius √(E_s)

Dimension	Weight	Max marks	Excellent	Average	Poor
Concept correctness	25%	2.5	Accurately identifies all controller limitations in (a) with specific phenomena (offset, windup, noise amplification); correctly describes flag effects and memory addressing for all 8085 instructions in (b); properly defines constellation diagram as signal space representation in (e); no conceptual errors across theoretical parts	Identifies most limitations but misses critical ones like integral windup or derivative kick; describes instruction operations but confuses flag effects (e.g., claims DAD affects Z flag); basic understanding of constellation concept but imprecise definition	Major conceptual errors such as confusing positive/negative feedback applications, wrong flag descriptions for multiple instructions, or fundamental misunderstanding of constellation diagrams as time-domain plots
Numerical accuracy	20%	2	Correctly calculates ohmmeter reading of ~45 kΩ or 44.85 kΩ in (c) with proper unit conversion; computes dielectric loss ~16.7 W in (d) using correct formula P = V²ωC tan δ with angle in radians; handles significant figures appropriately	Correct method but arithmetic errors leading to wrong final values; uses tan(0.05°) ≈ 0.05° in radians correctly but makes calculation mistakes; correct formula substitution but wrong unit conversions (kΩ vs Ω)	Wrong formulas (uses series RC instead of parallel for dielectric loss), ignores tan δ completely, or makes order-of-magnitude errors (10³ vs 10⁶); no numerical work shown for (c) and (d)
Diagram quality	20%	2	Clear labeled phasor diagram for (d) showing V, I_C, I_R, I_total with δ angle marked; neat constellation diagrams for (e) with axes labeled, M values identified, equal energy circles shown, and phase angles marked; ohmmeter circuit diagram in (c) with all components labeled	Diagrams present but poorly labeled or missing critical elements (e.g., constellation points without angle markings); phasor diagram lacks δ angle or shows wrong reference; circuit diagram missing some component values	Missing required diagrams for (d) and (e); unrecognizable sketches with no labels; completely wrong diagrams (e.g., time-domain waveforms instead of constellation)
Step-by-step derivation	20%	2	Shows complete derivation: for (c) calculates I_FSD, finds I_half, applies KVL to find Rₓ; for (d) derives tan δ = ωCR for parallel equivalent, substitutes values stepwise; for (b) shows register contents before/after each instruction with memory addressing	Some steps shown but skips critical intermediate steps (e.g., jumps from formula to answer); shows final calculations without setup; instruction descriptions lack register transfer notation	No derivations shown—only final answers stated; incorrect or irrelevant mathematical steps; no explanation of how instruction results are obtained in (b)
Practical interpretation	15%	1.5	Relates controller limitations to real-world tuning challenges (Ziegler-Nichols); cites Indian context for 8085 (still used in legacy industrial controllers); explains dielectric loss relevance to XLPE cable insulation in Indian power grids; notes PSK used in ISRO satellite communication systems	Generic practical mentions without specific context; basic awareness that PID needs tuning or that dielectric loss causes heating; no Indian examples or real-world system references	No practical interpretation provided; purely theoretical treatment; incorrect practical claims (e.g., positive feedback used for stability)

Q1

Directive word: Explain

How this answer will be evaluated

Approach

Key points expected

Evaluation rubric

Practice this exact question

More from Electrical Engineering 2024 Paper II