HW3 – Nichols plots and frequency domain specifications FORMULAE FOR SECOND ORDER SPECIFICATIONS: If the higher order closed-loop system has two dominant poles (and no dominant zeros) it can be approximated by a second order system. Here are the formulae for transient specifications for second order systems or approximate second order systems: 1. Natural frequency: 𝜔𝑛 (distance of pole to origin) 2. Damping ratio: 𝜁 (related to angle of the pole) 3. The closed-loop dominant poles: 𝑝1, 𝑝2 = −𝜎 ± 𝑗𝜔𝑑 = −𝜔𝑛𝜁 ± 𝑗𝜔𝑛√1 − 𝜁 2 4. Angle of the poles with negative real axis: 𝛽 = arccos 𝜁 = cos−1 𝜁 5. The damped frequency of oscillations is the imaginary part of the roots: 𝜔𝑑 6. The exponential time constant is the reciprocal of the real part of the roots: 𝜏 = 1 𝜎 = 1 𝜔𝑛𝜁 7. Distance of the pole from origin is √𝜎2 + 𝜔𝑑 2 = 𝜔𝑛 = natural frequency. 8. Settling time: 𝑇𝑠 = 4 𝜎 = 4 𝜔𝑛𝜁 9. Peak time: 𝑇𝑝 = 𝜋 𝜔𝑑 10. Maximum overshoot: 𝑀𝑝 = 𝑒 − 𝜋𝜁 √1− 𝜁2 = 𝑒−𝜋 cot 𝛽 = 𝑒 −𝜋 𝜎 𝜔𝑑 11. To get percent overshoot multiply 𝑀𝑝 with 100. Similar characteristics in the frequency domain are: 1. Resonant frequency of the closed loop system: 𝜔𝑟 = 𝜔𝑛√1 − 2𝜁 2 2. Resonant peak amplitude the closed loop system: 𝑀𝑟 = 1 2𝜁√1−𝜁2 Note that there is no resonant frequency or peak when the damping ratio is greater than 1 √2 = 0.707 3. Cutoff frequency the closed loop system: 𝜔𝑐 = 𝜔𝑛√1 − 2𝜁 2 + √1 + (1 − 2𝜁2)2 Cutoff frequency tells about the bandwidth and is related to speed of the closed loop system. Higher cut-off frequency → higher bandwidth → lower settling time, rise-time, peak time. 4. Damping ratio of the closed loop system 𝜁 is related to the phase margin of the open loop system: 𝜙 = tan−1 2 √√4+ 1 𝜁4 −2 (𝑖𝑛 𝑟𝑎𝑑𝑖𝑎𝑛𝑠) ≈ 100𝜁 (𝑖𝑛 𝑑𝑒𝑔𝑟𝑒𝑒𝑠) The approximate formula for phase margin, 100𝜁, works only for phase margins up to about 60 degrees. Errors specifications: 1. For a typical system with a plant G and controller K. The error is 𝐸(𝑠) = 1 1+𝐺𝐾 𝑅 The steady state error is 𝑒(∞) = lim 𝑠→0 𝐸(𝑠)𝑠 = lim 𝑠→0 𝑠𝑅 1+𝐺𝐾 2. Steady state error for a step input R(s) = 1/s is: 𝑒(∞) = lim 𝑠→0 1 1 + 𝐺𝐾 = 1 1 + lim 𝑠→0 𝐺𝐾 = 1 1 + 𝑑𝑐 𝑔𝑎𝑖𝑛 𝑜𝑓 𝐺𝐾 = 1 1 + 𝐾𝑝 3. Steady state error for a ramp input R(s) = 1/s2 is: 𝑒(∞) = lim 𝑠→0 1 𝑠(1 + 𝐺𝐾) = 1 lim 𝑠→0 𝑠𝐺𝐾 = 1 𝐾𝑣 If GK has no integrator 𝐾𝑣 is 0 (ramp error is infinite). If GK has 1 integrator 𝐾𝑣 is finite, it is the x-intercept (frequency intercept) of the asymptote line from the left half of the bode(GK). If GK has more integrators 𝐾𝑣 is infinite. 4. To reduce the error due to reference command (and disturbance dy) of frequency 𝜔 by a factor of F: |𝐺(𝑗𝜔)| > 𝐹 + 1 . 5. To reduce the error due to a sinusoidal noise command of frequency 𝜔 by a factor of F: |𝐺(