Instrumentation: Measurement and Control Systems Analysis

Posted on Nov 16, 2024 in Other subjects

Basic Concepts and Definitions

Understanding measurement and instrumentation is crucial for effective control systems. The goal of accurate measurement is to achieve precise control.

Key Definitions

  1. Variable: Anything with dynamic, static, or physicochemical characteristics that can be measured continuously.
  2. Controlled Variable: The variable directly regulated within the control loop, captured by the transmitter for feedback.
  3. Manipulated Variable: Adjusted to directly influence the controlled variable; the tool for modification within the process.
  4. Process: A series of components with specific functions that progressively yield a final result.
  5. System: Components working together to achieve a goal. Example: Valves manipulating flow to control tank levels.
  6. Disturbance: A signal affecting system response, causing measurement errors (e.g., magnetic fields, inductance).
  7. Feedback Control: Self-correction against disturbances to achieve the ideal output. Example: A thermostat regulating temperature.
  8. Process Control Systems: Automatic regulation systems determining variable response based on programmed parameters.
  9. Closed-Loop Control System: A feedback control system where the signal is compared and forwarded to set the desired setpoint.
  10. Open-Loop Control System: Output doesn’t affect control action; no feedback. Example: Timed Christmas lights.
  11. Adaptive Control System: Self-tuning parameters in response to random and systematic errors.
  12. Learning Control System: Operator experience drives parameter adjustments.
  13. Instrument: A device interpreting signals proportional to variable magnitude.
  14. Blind Instruments: No visible indication; manipulative devices like switches, thermostats, transmitters.
  15. Indicator Instruments: Display measurements on a scale (e.g., manometers). Can be concentric, eccentric, or digital.
  16. Recorder Instruments: Express signal points with continuous strokes.
  17. Final Control Element: Receives signals from the controller and acts directly on the controlled variable.
  18. Primary Element Action: Directly contacts the variable and transmits energy transformations.
Instrument Examples
  • Blind Instruments: Flow meters (primary sensor element)
  • Optical Sensors, Gauges, Recorders: Allow monitoring
  • Transmitters: Blind and indicator types
  • Monitoring and Control Panel: Virtual instrumentation simulated on a PC

Measurement Terminology

  1. Range: Values between the upper and lower limits specified by the instrument.
  2. Span: Difference between upper and lower range limits; the operating area.
  3. Zero Elevation: Amount the zero value exceeds the lower range limit.
  4. Zero Suppression: Amount below the lower range limit.
  5. Sensitivity: Minimum detectable change the instrument can express.
  6. Dead Zone: Range where the instrument’s sensitivity is null.
  7. Error: Difference between instrument reading and the true variable value.
  8. Accuracy: Degree of measurement reproducibility.
  9. Precision: Closeness of the reading to the true variable value.
  10. Hysteresis: Decompensation between ascending and descending measurements. Example Calculation: (18.2 – 17.7 / 100 – 0) * 100 = +/- 0.5%
  11. Reliability: Probability the instrument stays within error limits.
  12. Traceability: Ability to relate measurements to established standards.
  13. Noise: Impure signals affecting the measuring system.
  14. Resolution: The instrument’s display precision.
  15. Linearity: Direct proportionality and error-free calibration equivalence.
  16. Stability: Instrument’s ability to maintain performance over time.
  17. Service Temperature: Instrument’s operating temperature range.
  18. Reproducibility: Obtaining consistent measurements for a constant variable.
  19. Repeatability: Instrument’s ability to give the same reading for repeated applications of the same variable value.
  20. Transmitter: Captures the signal from the primary element and transmits it over a distance (electric, pneumatic, etc.).
  21. Transducer: Receives signals from one or more measured variables and may change them into another signal.
  22. Converter: Modifies the input signal and delivers a standard output signal.
  23. Signal: Output from the instrument representing a quantified value.
  24. Analog Signal: Continuous function of the measured variable.
  25. Digital Signal: Represents the variable’s magnitude as a series of discrete coded values.
  26. Set Point: Desired value for the variable.

Note: All signals have standard minimum and maximum values.