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Lecture 3 Elements of a Control System

Lecture 3 Elements of a Control System. Types of Control Systems. Open loop Process output has no effect on the system input Light switch to turn a light on or off Closed loop Process output determines how controlled input to the process changes Feedback control system: cruise control.

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Lecture 3 Elements of a Control System

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  1. Lecture 3Elements of a Control System

  2. Types of Control Systems • Open loop • Process output has no effect on the system input • Light switch to turn a light on or off • Closed loop • Process output determines how controlled input to the process changes • Feedback control system: cruise control

  3. Parts of a Control System • A process; • A measuring instrument or device; • An error-detecting mechanism; • A controller; • A final control element or actuator. 3 & 4 are often combined together

  4. Process Control Diagram

  5. Constant Level Head Tank

  6. Simplified Ball-Mill Control System

  7. Complete B/M Control Typically, a complete grinding circuit control system includes several interacting Control Systems such as: • Mill Feed Rate Control • using power draw or CF flowrate or COF density or BMD sump level) • Ball Mill Water Addition Control • ratio control with respect to Mill Feed Rate • Ball Mill Discharge Sump Level Control • using CF pump speed • COF pulp density Control • using Mill Feed Rate and Total Water Addition • Cyclone Pressure or Cyclone Feed flowrate Control • using CF pump speed or Mill Feed Rate or a combination

  8. Control System Responses • Systems Analysis involves instituting a deliberate change in one of the system inputs • Set-Point Changes (loads held constant) • Load Changes (set-point held constant) • Change can be • Impulse • Step • Ramp • Sine Wave Transient Responses Steady-State Responses

  9. Desired Response • To a Set-Point Step Change • Immediately rising or lowering to new set-point • To a Load Step Change • Zero response (no change at all) • To a Set-point Sine Change • In-phase cycling with stable amplitude • To a Load Sine Change • Stable response at reduced amplitude

  10. Responses to a Set-Point Step Change Servo Operation

  11. Responses to a Load Step Change Regulator Operation

  12. Common Types of Controllers • Two-position controller (ON-OFF) • may be time-varying as well; • Integral Controller • Often called Proportional Speed-Floating or Reset • Proportional Controller; • Derivative Control • Often called Rate control

  13. Two-Position Controller

  14. Two-Position Controller (shifted)

  15. Integral Controller

  16. Integral Controller – sine wave

  17. Proportional Controller

  18. Proportional Controller – sine wave

  19. Derivative Controller

  20. Derivative Controller – sine wave

  21. Relative Cost of Controllers

  22. General Guidelines • On-Off is cheapest • Proportional is simplest but gives off-set • Integral is slowest, but eliminates off-set (reset) • Derivative is fastest, but off-set still occurs • Never use Derivative with noisy signals • PID is ultimate (fast response, no offset)

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