⒌Fig.3 shows a unity-feedback control system. By sketching the Nyquist diagram of the system, determine the maximum value of K consistent with stability, and check the result using Routh’s criterion. Sketch the root-locus for the system（20%）
R C KR S ( S 2 ? 4 S ? 5 ) E Fig.3
⒍Sketch root-locus diagram.（18% ） Im Im Im Re Re Re Im Im Im Re Re Re
⒎ Determine the transfer function. Assume a minimum-phase transfer function.（10% ）
L(dB) 0 20 0 –20 –40 -60
40
30
20
5
ω
0.1 ω1 ω2 ω3 ω4
⒈ ⒉
V2(S)1? 2V1(S)R1C1R2C2S?(R1C1?R2C2?R1C2)S?1G1G2G3G4C(S)? R(S)1?G2G3G6?G3G4G5?G1G2G3G4(G7?G8)
⒊ There are 4 roots in the left-half S plane, 2 roots on the imaginary axes, 0 root in the RSP. The system is unstable.
⒋ 8?K?20
⒌ K=20 ⒍
S31.62(?1)0.1⒎ GH(S)?
SSSS(?1)(?1)(?1)(?1)0.3163.48134.8182.54
AUTOMATIC CONTROL THEOREM (3)
⒈List the major advantages and disadvantages of open-loop control systems. (12% )
⒉Derive the transfer function and the differential equation of the electric network shown in Fig.1.（16% ） C1
R1 R2 U2 U1 Fig.1 C2
⒊ Consider the system shown in Fig.2. Obtain the closed-loop transfer function
C(S)E(S)C(S), , . （12%） R(S)R(S)P(S)G5 E R P C G1 G2 G3 G4 H2 Fig.2 H1 H3
⒋ The characteristic equation is given 1?GH(S)?S3?3S2?2S?20?0. Discuss the distribution of the closed-loop poles. (16%)
5. Sketch the root-locus plot for the system GH(S)?K. (The gain K is
S(S?1)assumed to be positive.)
④ Determine the breakaway point and K value.
⑤ Determine the value of K at which root loci cross the imaginary axis. ⑥ Discuss the stability. (14%)
6. The system block diagram is shown Fig.3. G1?4K, G2?. Suppose
S(S?3)S?2r?(2?t), n?1. Determine the value of K to ensure eSS?1. (15%)
N
R E C G1 G2
Fig.3
7. Consider the system with the following open-loop transfer function:
GH(S)?K. ① Draw Nyquist diagrams. ② Determine the
S(T1S?1)(T2S?1)stability of the system for two cases, ⑴ the gain K is small, ⑵ K is large. (15%)
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