Cork Institute of Technology

Bachelor of Engineering (Honours) in Mechanical Engineering -

Award

(NFQ – Level 8)

Autumn 2006

Systems Engineering and Control

(Time: 3 Hours)

Answer any FIVE Questions Examiners: Prof. M. Gilchrist

ALL questions carry equal marks. Mr. J.E. Hegarty

Dr. M. J. O’Mahony

1. (a) A process control system has the following open loop transfer function;

GsHs Ke

ss s

s

() () ()( )

=

++

−2

110

(i) Assuming initially K=5, plot the bode diagram and determine the gain margin

and phase margin for the system. Comment on its stability.

(10 marks)

(ii) What value of K will result in a phase margin of 60o and what would be the

corresponding gain margin?

(4 marks)

(b) Explain how dead-time compensation can be introduced to improve the

performance of control systems such as (a) above.

(6 marks)

2. A unit feedback position control system has the following open loop transfer

function:

GsHs s

ss

() () (. )

()

=

+

+

20 01 1

4

Determine the following closed loop frequency response parameters:

(i) Resonant Peak Mr

(ii) Resonant frequency ωr

(iii) System Bandwidth ωb (15 marks)

Comment on the values obtained and hence sketch the expected response to a unit

step input in the time domain? (5 marks)

3. (a) An automatic level control system for a effluent treatment plant is shown in

Fig. Q 3. Explain briefly the operation of the system and tune the controller to

give PI control of the tank level (ignore for the present the effects of the

disturbance flow Qd). The following parameters relate to the system block

diagram:

K

sp = Set point conversion factor = 4mA per m

K

v = Control valve coefficient = 0.028 m3/s per mA

τv = Control valve time constant = 20 s

A = Tank area = 10 m2

R = Outlet hydraulic resistance = 0.069 m3/s per m

τT = Transmitter time constant = 5 s

(12 marks)

(b) A disturbance flow stream Qd will on occasion enter the system. Suggest a

suitable control strategy that will minimise the effect of the disturbance flow on

the tank level Hact. The disturbance stream cannot be controlled but it can be

measured. Show the implementation of your proposed control strategy on a

modified block diagram of the system.

(8 marks)

4. Fig Q.4 shows the block diagram representation of the liquid level control system

in a steam boiler drum. Under certain conditions this system will exhibit an

“inverse response”. Explain what this means and determine the conditions under

which it will occur. (15 Marks)

How can the control system be modified to compensate for this effect?

(5 Marks)

5. Figure 5 shows a digital speed control system. Given that D(s)=KP transform

this diagram into the Z-domain and plot the root locus for the system. Hence

determine the limiting value of KP for stability and the value of KP that will

result in an underdamped response with a damping ratio of 0.5.

Data

K

m = Motor/Load gain constant = 10 rad/s per V

τm = Motor/Load time constant = 0.4 s

K

t = Tachometer gain = 2.4 V per rad/s

T = Sampling Time = 0.01 s

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University:
Acharya Nagarjuna University

Subject:
System Engineering

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10/04/2013