Control Systems, Instrumentation and Mechatronics
Core content
Open-loop control acts on a setpoint without feedback; closed-loop compares measured output to setpoint and acts on the error. Learn first-order response (time constant τ: 63.2 % of final value in one τ, about 95 % in three) and second-order response (damping ratio, overshoot, settling time). PID terms: proportional responds to present error but leaves offset; integral removes offset but adds lag and can wind up; derivative anticipates and damps but amplifies noise. Stability follows from pole location, Routh–Hurwitz or Bode margins, and gain and phase margins must both be positive. Instrumentation: thermocouples (Seebeck effect, wide range, cold-junction compensation), RTDs (accurate, stable, slower), thermistors, strain-gauge bridges, LVDTs and flow meters. The 4–20 mA current loop has a live zero (4 mA = 0 %, 20 mA = 100 %) and is immune to voltage drop. Keep calibration, accuracy, precision, resolution, hysteresis and linearity as distinct concepts, and know PLC scan cycles and the integration of sensors, actuators and controllers.
Traps: confusing accuracy with precision; treating 4 mA as zero percent when the question really asks about a 0 mA fault; expecting proportional-only control to eliminate offset; ignoring integrator windup during start-up; and calling the time constant the settling time.
Worked example. A transmitter spans 0–600 kPa over 4–20 mA. At 12 mA the fraction is (12 − 4)/16 = 0.5, giving 300 kPa. A reading of 8 mA corresponds to (8 − 4)/16 × 600 = 150 kPa.
Rule of thumb: percentage of span = (mA − 4)/16 × 100.