Dublin Institute of Technology

(Code for Paper)

DUBLIN INSTITUTE OF TECHNOLOGY KEVIN STREET, DUBLIN 8 ______Bachelor of Engineering Technology in Control and Automation Systems

Bachelor of Engineering Technology in Electrical Energy Systems

Year 3 ______

WINTER EXAMINATIONS 2008/9 ______

MODULE

Mr. Gavin Duffy Dr. Michael Conlon Dr. Eugene Coyle Professor

DD MM 2008 Time : ?? to ??

Question 1 is compulsory [40 marks] Attempt any two other questions [30 marks per question]

The following should be available for this examination:

Mitsubishi FX 4AD User’s Manual Q1. (a) The block diagram for a control system with a proportional controller acting on a first order process with unity feedback is shown in Figure Q1.

 (s) +  (s) i K K o p s+1 -

Figure Q1

For the overall system determine the following the transfer function the static gain the time constant [8 marks]

(b) What is meant by the term integral control and why is it often used in control systems? What is meant by the term ‘integral windup’? [8 marks]

(c) Describe a procedure to tune a PID controller. [8 marks]

(d) Explain each of the four steps which make up the programme scan cycle of a PLC. [8 marks]

(e) Explain the terms open loop and closed loop as applied to a control system. Illustrate your answer by reference to a particular [8 marks] example of each system and sketch its relevant block diagram. Q.2 (a) Write a ladder logic program to configure a Mitsubishi FX-4AD analogue input card for the following situation:

The card is installed in the first slot next to the PLC, Channel 1: 0 – 100 ºC temperature sensor with a 4 to 20 mA output signal Channel 2: 0 - 5 kPa pressure sensor with a 0 to 10 V output signal Channel 3: Not used Channel 4: Not used The number of averages for each channel is set to 20, The averaged value from channel 1 is read into address D0, The averaged value from channel 2 is read into address D1.

Include comments and line statements. [12 marks]

(b) List and explain methods of minimising the effects of noise and interference in the transmission of instrument signals from the field to a Mitsubishi FX-4AD analogue input card. [6 marks]

(c) A process control valve is installed in a pipe through which water is flowing at a maximum flowrate of 160 gpm. The maximum allowable pressure drop across the valve is 15 psi and the specific gravity of water is 1.0. It has been decided to use a ball valve for this application and the control valve characteristics for this type of valve are given in Table Q1.

Choose an appropriate valve size for this application. [6 marks]

(d) Using a suitable sketch, explain three different valve characteristics. [6 marks] Cv at Degrees Open Size 10 20 30 40 50 60 70 80 90 100 (inches) ½ 0.52 1.17 2.0 2.8 3.9 5.7 9.1 13 19.8 26 ¾ 1.00 2.25 4.0 5.5 7.5 11.0 17.5 25 38 50 1 1.88 4.23 7.5 10.3 14.1 20.7 32.9 47 71.5 94 1 ½ 5.2 11.7 20.8 28.6 39 57.2 91 130 197.6 260 2 9.6 21.6 38.4 52.8 72 105.6 168 240 364.8 480 2 ½ 15.0 33.8 60 82.5 112 165 262.5 375 570 750

Table Q1

TIC-01 HS-01

TET-01

TK-01 Hot Water Supply Tank

Figure Q3 Q3. Hot water storage tank TK-01 is heated by an electric element at the base of the tank as shown in Figure Q4. A Pt100 temperature transmitter TET-01 is used to monitor the temperature of the water in the tank. The transmitter is wired to an analog input card on a Programmable Logic Controller (PLC) and the resulting signal is passed to the on-off controller TIC-01 which switches the output HS-01 to the electric heater.

(a) Draw a block diagram of the temperature control system. [8 marks]

(b) What are the advantages and disadvantages of On-Off control and when is it appropriate to use an On-Off controller? [8 marks]

(c) Compare the operation of an On-Off controller without a deadband to that of an On-Off controller with a deadband. [4 marks]

(d) Graph the following variables against time:

(i) water temperature, (ii) temperature error, (iii) output to the heater.

Assume a temperature setpoint of 65 ºC, a deadband of 2 ºC and display the temperature error changing sign at least twice. [10 marks] Q4. The contents of a hot water storage tank are heated by an electric heater at the base of the tank. The relationship between the tank temperature, TT, and the temperature of the heating element, TH, is described by the following first-order differential equation:

dT  T  T  T dt T H

where  is the time constant of the tank TT is the temperature of the tank TH is the heating element temperature

(i) Use Laplace transforms to convert this equation to the frequency domain and hence determine the transfer function for the system [6 marks]

(ii) What is the frequency domain equation for the output from the system if the temperature of the heating element is suddenly [6 marks] increased by 40 ºC?

(iii) Apply the inverse Laplace transform to provide the output from the system in the time domain in response to the step input. [6 marks]

(iv) A small laboratory tank similar to the one described above with a time constant of 30 seconds is heated from an initial steady state temperature of 20 ºC by suddenly increasing the element temperature to 80 ºC. Sketch the time response of the tank water temperature over a period of 3 minutes. [6 marks]

What is the temperature of the water 1 minute after the element temperature is increased? [6 marks]