Faculty of Engineering Digital Control
Electrical Power Dept. Transfer function and
Level four stability
▪ Transfer function and stability
1) Consider the following difference equations
a) y[k] = y[k-1]+u[k]
b) y[k] = 0.6y[k-1]+u[k]
c) y[k] = 0.3y[k-1]+u[k]
d) y[k] = u[k-1]
e) y[k] = 0.707y[k-1]-0.25y[k-2]+u[k-1]
f) y[k] = -0.25y[k-2]+u[k-1]
g) y[k] = -0 .707y[k-1]-0.25y[k-2]-u[k-1]
i. Find the transfer function for each system
ii. Find the poles of each system, identify which system is stable
iii. Assuming zero initial conditions, find the unit pulse response for each
system analytically.
iv. Assuming zero initial conditions, find the unit pulse response for each
system by successive substitution.
2) Use the z-transform to determine the output of the difference equation
𝑦(𝑘 + 2) − 1.5𝑦(𝑘 + 1) + 0.5𝑦(𝑘) = 𝑢(𝑘 + 1)
where u is a unit step applied at 𝑘 = 0 and 𝑦(0) = 0.5, 𝑦(−1) = 1
3) The transfer function of a discrete-time system is
0.1 𝑧 + 0.2 𝑧 2 − 0.4𝑧 3 + 0.3 𝑧 4 + 𝑧 5
𝐺(𝑧) =
𝑧6
Find and plot the unit step response of the above system.