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FINAL CONTROL ELEMENT

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  1. FINAL CONTROL ELEMENT

  2. FINAL CONTROL ELEMENT • The final control element adjust the amount of energy/mass goes into or out from process as commanded by the controller • The common energy source of final control elements are: • Electric • Pneumatic • Hydraulic

  3. ELECTRIC FINAL CONTROL ELEMENT • Electric current/voltage • Solenoid • Stepping Motor • DC Motor • AC Motor

  4. CHANGING CURRENT/VOLTAGE • Current or voltage can be easily changed to adjust the flow of energy goes into the process e.g. in heating process or in speed control • Heater elements are often used as device to keep the temperature above the ambient temperature. Energy supplied by the heater element is W = i2rt (i=current, r=resistance, t=time) • Motor is often used as device to control the speed

  5. CHANGING CURRENT/VOLTAGE • Using • Potentiometer • Amplifier • Ward Leonard system • Switch (on-off action)

  6. Changing Current/VoltageUsing Rheostat I = V/(R1+R2) Power at rheostat P1 =I2R1 Power at heater P2 =I2R2 Disadvantage loss of power at rheostat Rheostat Heater R1 V R2 I

  7. Example of Heating elements

  8. Changing Current/VoltageUsing Amplifier Potentiometer V+ Heater amplifier R1 V R2 V− Disadvantage loss of power at potentiometer (very small) and at Amplifier

  9. Changing Current/VoltageUsing Ward Leonard System • Introduced by Harry Ward Leonard in 1891 • Use a motor to rotate a generator at constant speed • The output of generator voltage is adjusted by changing the excitation voltage • Small change in excitation voltage cause large change in generator voltage • Able to produce wide range of voltage (0 to 3000V) • Ward Leonard system is popular system to control the speed of big DC motor until 1980’s • Now a days semi conductors switches replaces this system

  10. Changing Current/VoltageUsing Ward Leonard System excitation voltage MOTOR GENERATOR

  11. Changing Current/VoltageUsing Switch • The switch is closed and opened repeatedly • No power loss at switch Switch closed VL Switch V LOAD V VL t Switch opened

  12. DUTY CYCLE VL V • T is period time typical in millisecond order (fix) • Ton is switch on time (adjustable) • Toff is switch off time Duty Cycle is: (Ton/T) 100% t Ton Toff T • Of course we can not use mechanical switches to carry on this task, electronic switches to be used instead. • E.g. Transistor, Thyristor, or IGBT • This methods is often called as Pulse Width Modulation (PWM)

  13. SOLENOID • When the coil is energized the core will be pulled in core coil core coil SOLENOID

  14. SOLENOID • When the coil is energized the core will be pulled in V SIMULATE

  15. SOLENOID • When the coil is energized the core will be pulled in V SIMULATE

  16. SOLENOID Rotary solenoid Tubular solenoid Open frame solenoid

  17. Solenoid

  18. Solenoid Usage • pushing buttons, • hitting keys on a piano, • Open closed Valve, • Heavy duty contactor • jumping robots • etc

  19. STEPPING MOTOR The top electromagnet (1) is turned off, and the right electromagnet (2) is energized, pulling the nearest teeth slightly to the right. This results in a rotation of 3.6° in this example. The top electromagnet (1) is turned on, attracting the nearest teeth of a gear-shaped iron rotor. With the teeth aligned to electromagnet 1, they will be slightly offset from electromagnet

  20. STEPPING MOTOR The left electromagnet (4) is enabled, rotating again by 3.6°. The bottom electromagnet (3) is energized; another 3.6° rotation occurs. When the top electromagnet (1) is again enabled, the teeth in the sprocket will have rotated by one tooth position; since there are 25 teeth, it will take 100 steps to make a full rotation in this example.

  21. STEPPING MOTOR • Practical stepping motor can be controlled for full step and half step. • Common typical step size is 1.8o for full step and 0.90 for half step • Full step is accomplished by energizing 2 adjacent electromagnet simultaneously. • Half step is accomplished by energizing 1 electromagnet at a time.

  22. Stepping motor

  23. DC Motor The brush

  24. DC Motor

  25. Practical DC Motors Every DC motor has six basic parts – axle, rotor (a.k.a., armature), stator, commutator, field magnet(s), and brushes. For a small motor the magnets is made from permanent magnet

  26. 2 pole motor Animate

  27. 2 pole motor Animate

  28. 2 pole motor Animate

  29. 2 pole motor Animate

  30. 2 pole motor Animate

  31. 2 pole motor Animate

  32. 2 pole motor Animate

  33. 2 pole motor Animate

  34. 2 pole motor continue Animate

  35. 3 pole DC motors 1 The coil for each poles are connected serially. The commutator consist of 3 sector, consequently one coil will be fully energized and the others will be partially energized. 2 3 − +

  36. 3 pole DC motors The commutator and the coil is arranged in such a way that the polarity of each pole is as shown animate next

  37. 3 pole DC motors The commutator and the coil is arranged in such a way that the polarity of each pole is as shown animate next

  38. 3 pole DC motors The commutator and the coil is arranged in such a way that the polarity of each pole is as shown animate next

  39. 3 pole DC motors The commutator and the coil is arranged in such a way that the polarity of each pole is as shown animate next

  40. DC motors • As the rotor is rotating, back emf (Ea) will be produced, the faster the rotor turn the higher Ea and the smaller Ia. • The starting current of motors will be much higher then the rating current. motor Ia Ea V

  41. DC motors For big motors the magnet is made from coil and core. The current flowing in the coil is called If and the current flowing in the armature is called Ia. The armature winding and the field winding are connected to a common power supply The armature winding and the field winding are often connected in series, parallel, or compound. The torque characteristic will be different for each connection. The figure shows a parallel connection Field winding Armature winding

  42. SERIES DC MOTOR Field and armature winding are series connected, this type of motor is called series DC motor

  43. DC motors Field and armature winding are parallel connected, this type of motor is called shunt DC motor

  44. DC MOTOR Compound DC motor is DC motor having 2 field winding the first one is connected parallel to the armature winding and the other is connected series

  45. DC MOTOR Torque: T = KΦIa • K is a constant • Φ magnetic flux • Ia is armature current • Magnetic flux is constant if it is from permanent magnet • It is depend on the If if it is produced by current

  46. DC MOTOR TORQUE-SPEED CURVE Torque: T = KΦIa

  47. SERIES DC MOTOR TORQUE-SPEED CURVE Torque: T = KΦIa T= KIa2

  48. SHUNT DC MOTOR TORQUE-SPEED CURVE Torque: T = KΦIa

  49. COMPOUND DC MOTOR TORQUE-SPEED CURVE

  50. N S SYNCHRONOUS AC MOTOR The rotating field. When alternating current is applied to the field coil the magnetic field will also alternating. Therefore the permanent magnet will rotate o 311 -311 ~