Control Systems Final - Problem 6

6.     In the first problem, you encountered a compensator with this transfer function.

Gc(s) = (.02s + 1)/(.002s + 1)

(20 pts) Assuming that the sampling period, T, is .0005 sec., determine the sampled equivalent (z-form), Gc,eq[z].  Use the simplest algorithm for conversion.

Solution

        We make a substitution for s in the compensator.  The substitution is:

s <=> (z - 1)/T

If you make that substitution and simplify, the first thing you get is:

Gc[z] = [.02(z- 1) + T]/[.002(z - 1) + T]
or
Gc[z] = [.02(z- 1) + .0005]/[.002(z - 1) + .0005]
Gc[z] = [40(z- 1) + 1]/[4(z - 1) + 1]
Gc[z] = [40z- 39]/[4z - 3]
   Gc[z] = 10[z- 0.975]/[z - 0.75]
or, one last acceptable form
Gc[z] = [.02z- .0195/[.002z - .0015]

Next time, need to make it a requirement that the answer be in the form of a ratio of polynomials.

If the4y used s<=>(z - 1)/(Tz), then the answer is:

Gc[z] = [41z- 40]/[5z - 4]