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CORK INSTITUTE OF TECHNOLOGY
INSTITIÚID TEICNEOLAÍOCHTA CHORCAÍ
Semester 2 Examinations 2011/
Module Title: Control Engineering & Automation Systems
Module Code: MECH 8001
School: Mechanical & Process Engineering
Programme Title: Bachelor of Engineering (Hons) in Mechanical Engineering
Programme Code: EMECH_8_Y
External Examiner(s): Prof. Sean Leen, Mr John J. Hayes Internal Examiner(s): Mr Matt Cotterell, Mr Gordon R. Petrie
Instructions: Answer ANY THREE questions. Each question is worth 25 marks and the overall mark “ex 75” will be scaled “ex 100”, i.e. to a percentage.
Duration: 2 Hours
Sitting: Summer 2012
Requirements for this examination: None
Note to Candidates: Please check the Programme Title and the Module Title to ensure that you have received the correct examination paper. If in doubt please contact an Invigilator.
- A temperature control system for a heat exchanger is shown below:
Steam ATO TT 1
T ( t )
T
RA
TIC 1
m S ( t )
Ti ( t )
m P ( t )
HX - 104
Tm ( t )
C ( t ) TSP ( t )
The transfer functions for the valve, process &measuring element are provided below:
GV (^) s KV 0. 016 (kg/s)/(%CO)
s GP e^ s KP 60 (oC)/(kg/s)
GM (^) s KM 1. 0 (%TO)/( oC)
Where, the time constants and dead-time each have units of “seconds” – the static gains associated with each transfer function, together with units, are also listed.
(a) Using the general substitution rule “ s i ”, derive expressions for the amplitude ratio “ AR ” and the phase angle “ ” for a first-order system. (5 marks) (b) Use frequency response analysis and Ziegler-Nichols (Z-N) tuning correlations to estimate the “best” parameters for a P-I-D controller (see Table Q1 at end of this examination paper for Z-N correlations). (20 Marks)
- A furnace, HX-104, is used to preheat a hydrocarbon “process fluid” – a cascade control structures to be used to control liquid outlet temperature:
FT 1 FIC 1 TIC 1 S.P.
Fuel Air Oil
Process Fluid
Exhaust Gases
HX – (^104) T
TT 1
S.P.
Given that the fuel-oil is subject to significant pressure fluctuations (fed in parallel to several utilities)determine the following: (a) The failure mode of the fuel control valve and the actions of both the primary and secondary controllers – justify your selections. (6 Marks) (b) If TIC-1 setpoint were to be increased explain in detail how the signals around the loop would respond. If fuel-oil supply pressure were to decrease explain in detail how the above system would respond (take each change in isolation). (12 Marks) (c) Repeat the last scenario, but with the output from TIC-1 controller connected directly to the control valve (ignore the inner secondary flow loop) – thus, demonstrate the advantages that cascade control can offer. (7 Marks)
(a) A digital speed control system is shown in Figure 5. Transform this diagram to the z- domain and obtain the transfer function R(z)/C(z). (15 marks) (b) Determine the closed loop response to a step input R(z) = and plot the response for the first six sampling instants. Comment on the system stability. (10 marks)
(a) Outline the assumptions upon which the describing function method for analysis of non-linear control systems is based. Show how it can be used to predict the occurrence of limit cycles and how stable and unstable limit cycles can be distinguished. (12 marks) (b) Consider the system shown above in which the non-linearity is an ideal relay. Investigate the possibility of a limit cycle in this system. If a limit cycle is predicted determine its amplitude and frequency and investigate its stability. (13 marks)
