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Published byDeddy Iskandar Modified over 6 years ago
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System type, steady state tracking, & Bode plot
C(s) Gp(s)
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As ω → 0 Therefore: gain plot slope = –20N dB/dec. phase plot value = –90N deg
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If Bode gain plot is flat at low freq, system is “type zero”
Confirmed by phase plot flat and 0° at low freq Then: Kv = 0, Ka = 0 Kp = Bode gain as ω→0 = DC gain (convert dB to values)
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Example
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Steady state tracking error
Suppose the closed-loop system is stable: If the input signal is a step, ess would be = If the input signal is a ramp, If the input signal is a unit acceleration,
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N = 1, type = 1 Bode mag. plot has –20 dB/dec slope at low freq. (ω→0) (straight line with slope = –20) Bode phase plot becomes flat at –90° when ω→0 Kp = DC gain → ∞ Kv = K = value of asymptotic straight line at ω = 1 =ws0dB =asymptotic straight line’s 0 dB crossing frequency Ka = 0
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Example Asymptotic straight line
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The matching phase plot at low freq. must be → –90°
type = 1 Kp = ∞ ← position error const. Kv = value of low freq. straight line at ω = 1 = 23 dB ≈ 14 ← velocity error const. Ka = 0 ← acc. error const.
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Steady state tracking error
Suppose the closed-loop system is stable: If the input signal is a step, ess would be = If the input signal is a ramp, If the input signal is a unit acceleration,
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N = 2, type = 2 Bode gain plot has –40 dB/dec slope at low freq. Bode phase plot becomes flat at –180° at low freq. Kp = DC gain → ∞ Kv = ∞ also Ka = value of straight line at ω = 1 = ws0dB^2
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Example Ka Sqrt(Ka) How should the phase plot look like?
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Steady state tracking error
Suppose the closed-loop system is stable: If the input signal is a step, ess would be = If the input signal is a ramp, If the input signal is a unit acceleration,
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System type, steady state tracking, & Nyquist plot
C(s) Gp(s) As ω → 0
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Type 0 system, N=0 Kp=lims0 G(s) =G(0)=K Kp w0+ G(jw)
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Type 1 system, N=1 Kv=lims0 sG(s) cannot be determined easily from Nyquist plot winfinity w0+ G(jw) -j∞
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Type 2 system, N=2 Ka=lims0 s2G(s) cannot be determined easily from Nyquist plot winfinity w0+ G(jw) -∞
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In most cases, stability of this closed-loop
Margins on Bode plots In most cases, stability of this closed-loop can be determined from the Bode plot of G: Phase margin > 0 Gain margin > 0 G(s)
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If never cross 0 dB line (always below 0 dB line), then PM = ∞.
If never cross –180° line (always above –180°), then GM = ∞. If cross –180° several times, then there are several GM’s. If cross 0 dB several times, then there are several PM’s.
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Example: Bode plot on next page.
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Example: Bode plot on next page.
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Where does cross the –180° line Answer: __________ at ωpc, how much is
Closed-loop stability: __________
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crosses 0 dB at __________ at this freq,
Does cross –180° line? ________ Closed-loop stability: __________
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Margins on Nyquist plot
Suppose: Draw Nyquist plot G(jω) & unit circle They intersect at point A Nyquist plot cross neg. real axis at –k
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