ENGR202 2024 Q B1Past paperOld spec ENGR2022:125 marks30 min

ENGR202 Summer 2024 Q B1[VALID]

Figure B1-1 consists of some control actions and a model. Using standard terminology, X(s)X(s) is the output, U(s)U(s) is the control input and V(s)V(s) is the set point. Also, KIK_I and KpK_p are control gains, whilst A(s)A(s) and B(s)B(s) are polynomials representing the model.

Figure B1-1: Block diagram of a control system.
Figure B1-1: Block diagram of a control system.
Formulas you may need
  • Negative feedback rule G11+G1G2\dfrac{G_1}{1 + G_1 G_2}; series rule G1G2G_1 G_2 (learn this)
  • Steady state gain: set s=0s = 0 (learn this)
  • Pole placement: desired poles p1,2p_{1,2} give the characteristic polynomial (s−p1)(s−p2)(s - p_1)(s - p_2); match coefficients (learn this)
  • s=−ζωn±jωn1−ζ2s = -\zeta\omega_n \pm j\omega_n\sqrt{1 - \zeta^2}; ωn=∣s∣\omega_n = |s| (learn this)
  1. (a)
    Giving your reason, state if Figure B1-1 is an example of open-loop or closed-loop control. What are the advantages and disadvantages open-loop and closed-loop control?
    [8]
  2. (b)
    Develop the Transfer Function for the relationship between X(s)X(s) and V(s)V(s) in Figure B1-1. Determine the steady state gain of this Transfer Function and comment on your answer.
    [8]
  3. (c)
    In the case that B(s)=3s+1B(s) = 3s + 1 and A(s)=(s+α)(s+β)A(s) = (s + \alpha)(s + \beta), where α\alpha and β\beta are coefficients, determine the pole(s) and zero(s) of the model (without control).
    [2]
  4. (d)
    Using a different model, one that represents a DC motor in a laboratory experiment, an engineer has determined the following Transfer Function for the control system in Figure B1-1. Here, X(s)X(s) is the motor speed. X(s)=2.3KIs2+(4.6+2.3Kp)s+2.3KI V(s)X(s) = \frac{2.3K_I}{s^2 + (4.6 + 2.3K_p)s + 2.3K_I}\,V(s) Design a controller such that that the poles of the characteristic equation are a complex conjugate pair −3±2j-3 \pm 2j.
    [3]
  5. (e)
    In terms of the response mode and stability, what is the significance of the pole positions stated in part (d) above? When the controller from part (d) is implemented in the lab, the control system starts to yield an unstable oscillatory response to a change in the set point. Speculate as to what might have happened. Hint: even if you were unsure how to answer part (d), you can still attempt this question part.
    [4]