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  NATIONAL UNIVERSITY OF SINGAPORE EXAMINATION FOR (Semester I: 2010/2011) EE3302/EE3302E – INDUSTRIAL CONTROL SYSTEMS   November/December 2010 - Time Allowed: 2  Hours INSTRUCTIONS TO CANDIDATES:   1. This paper contains FOUR (4)  questions and comprises FIVE (5)   pages. 2.   Answer all questions. 3. All questions carry equal marks. 3.   This is a CLOSED BOOK examination. However, each student may bring ONE (1) A4 size crib sheet into the examination hall  EE3302/EE3302E Industrial Control Systems/ Page 2Q.1   In a production factory, a simple elevator is to be used to transfer materials from a lower to the upper floor as shown in Fig 1. The elevator will travel between two extreme positions corresponding to the lower floor and upper floor. S1 and S2 are  proximity sensors which will turn on when the elevator is at the lower and upper level respectively. There is a control panel containing a pushbutton (G) at the lower floor to command the elevator to proceed to move up after the loading of materials is done. The elevator will then move to the upper floor for the materials to be unloaded. There is a control  panel containing a pushbutton (R) to signal that the unloading has been completed and to command the elevator to return to the home position at the lower floor. The elevator is actuated by a motor to move in the up or down direction via the UP and DOWN inputs to the motor. Fig. 1 (a)   Provide a diagram to show the physical connections of the PLC. (5 marks) (b) Provide a Sequential Function Chart for the sequence control. (8 marks) (c) Provide a ladder program to implement the logic. (8 marks) Finally, include a non-repeat locking feature to disallow commands due to the “R” and “G” push buttons continuously actuated (i.e., the buttons have to be released and actuated to issue the next command). (4 marks)  EE3302/EE3302E Industrial Control Systems/ Page 3Q2 (a)   Explain the main characteristics which can be used to identify a cascade control system. (3 marks) Provide two block diagrams to show the differences between a series and a parallel cascade control system. (4 marks) (b) Fig. 2 shows the block diagram of a process with input u , primary output y and a secondary output v  (both v  and  y ). If v  is known to be affected by a disturbance w  which cannot be measured, draw a  block diagram of an advanced control scheme for the process. (4 marks) Explain why it can be effective. (5 marks) Explain how this controller should be designed/tuned. If a plain proportional gain is to  be used for any controller needed, design any control gain to give closed-loop stability. (9 marks) 2.01)( 2 += ssG v y u 231)( 21 ++= sssG  Fig. 2  EE3302/EE3302E Industrial Control Systems/ Page 4 Q.3 Consider the unit step response of a process in Fig. 3a. Time (s)          y             (            t             ) 0 1 2 3 4 5 6 700.  Fig. 3a (a) Using the Ziegler-Nichols step response method, find c K   and i T   of the PI controller ⎟⎟ ⎠ ⎞⎜⎜⎝ ⎛ +== icc sT K sGs E sU  11)()()( (5 marks) (b) Using the area method, find   p K  , i T   and  L  for the first-order plus dead-time model given as 1)()()( +== − sT eK sGsU sY   sL p p  (10 marks) (c) The controller)( sG c and the process )( sG  p  found in Parts (a) and (b) respectively are connected as shown in Fig. 3b. For a unit step change in )( t r  , give the values of the control signal,)( t u , at (i) + = 0 t  and (ii) ∞= t  . Assume that all initial conditions are at zeros and there is no need to show working. (10 marks) Fig. 3b )( sG c )( sG  p )( sY    )( sU  )( s E    )( s R  +  _
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