Lecture 29: Root Locus Summary, MATLAB

Massachusetts Institute of Technology Department of Mechanical Engineering 2.004 Dynamics and Control II Spring Term 2008 Lecture 291 Reading: • Nise: Chapter 8 1 Root Locus Refinement The complete set of sketching rules contains additional methods to make a sketched plot more accurate. While these were useful in the days before ubiquitous computation, today with the existence of tools such as MATLAB makes these graphical refinements somewhat unnecessary. We therefore just mention them here and refer you to Nise, Section 8.5, for more detail. 1.1 Real-Axis Breakaway and Break-In Points jw a " b r e a k a w a y " p o in t X K = 0 K in c r e a s in g a " b r e a k - in " p o in t o X K = ¥ K = 0 K in c r e a s in g o K = ¥ s A breakaway point is the point on a real axis segment of the root locus between two real poles where the two real closed-loop poles meet and diverge to become complex conjugates. Similarly, a break-in point is the point on a real axis segment of the root locus between two real zeros where two real closed-loop complex conjugate zeros meet and diverge to become real. Because the closed-loop poles originate from open-loop poles (when K = 0), a breakaway point will correspond to the point of maximum K along the real-axis segment. Similarly, a break-in point will correspond to the point of minimum K on the real axis segment between the two zeros. The closed-loop characteristic equations is 1 + KG(s) = 0, so that along the root locus segments on the real axis (s = σ) K=− 1 1 D(σ) =− N (σ) G(σ) c D.Rowell 2008 copyright � 29–1 K K 0 0 s X K = 0 s o X K = ¥ K = 0 b s s o s K = ¥ b The breakaway/break-in points (maximum/minimum points) will therefore occur where � � dK d D(σ) =− =0 dK N (σ) dσ or when N (σ)D� (σ) − N � (σ)D(σ) = 0. Example 1 Find the real axis breakaway/break-in points for the closed-loop system with s2 + 10s + 24 (s + 6)(s + 4) G(s) = 2 = . s + 3s + 2 (s + 1)(s + 2) The root locus has two real-axis segments, between the pole pair and between the zero pair. There will therefore be a breakaway point and a break-in point. jw " b r e a k a w a y " p o in t a " b r e a k - in " p o in t o -6 o X -2 -4 X -1 s The breakaway/break-n points will be contained in the roots of N (σ)D� (σ) − N � (σ)D(σ) = 0. or (σ 2 + 10σ + 24)(2σ + 3) − (σ 2 + 3σ + 2)(2σ + 10) = 7σ 2 + 44σ + 52 = 0 giving σ1,2 = −4.708, −1.578, as shown below: 29–2 jw o s = -4 .7 0 8 o -6 1.2 s = -1 .5 7 8 X -2 -4 X -1 s Angle of Arrival and Departure from Zeros and Poles Further refinement of the Root Locus may be made by computing the angle at which the branches of the locus depart from the open-loop poles, and arrive at the open-loop zeros. jw a n g le o f a r r iv a l f o q X a n g le o f d e p a r tu r e s Consider a point a small distance � from a pole: jw a n g le o f d e p a r tu r e e o f 1 o f X q d 2 X q 1 s The angle condition at the point requires � � angles from the zeros − angles from the poles = (2k + 1)π or φ1 + φ2 − θ1 − θd = (2k + 1)π 29–3 Let k = 0 and let � → 0, then θd = φ1 + φ2 − θ1 − π where the angles are measured to the pole itself. A similar argument defines the angle of arrival at a complex zero. Example 2 Find the angle of departure at the pole p = −1 + j2 for the closed-loop system where s+4 G(s) = 2 . s + 2s + 5 a n g le o f d e p a r tu r e q X o f j2 s -1 -4 X In the above figure θ = 90◦ , is therefore d jw q - j2 φ = arctan(2/3) = 33.7◦ . The angle of departure θd = φ − θ − π = 33.7◦ − 90◦ − 180◦ = −236.31◦ = 123.69◦ jw 4 1 2 3 .7 o 2 -7 .6 1 -1 0 -8 -6 -4 -2 s 0 -2 -4 29–4 1.3 Summary of Root Locus Sketching Rules Definitions Symmetry Number of branches Start and end points Locus on real axis Asymptotes as |s| → ∞ • The open-loop transfer function is KGc (s)Gp H(s) which can be rewritten as KN (s)/D(s). • N (s), the numerator, is an mth order polynomial; D(s) is nth order. • G(s) has zeros at zi , (i = 1 . . . m); and poles at pi (i = 1 . . . n). The locus is symmetric about real axis (i.e., complex poles ap­ pear as conjugate pairs). There are n branches of the locus, one for each pole of the closedloop transfer function. The locus starts (when K = 0) at poles of the open-loop transfer function, and ends (when K = ∞) at the zeros. Note: there are n − m zeros of the open-loop transfer function as |s| → ∞. The locus exists on the real axis to the left of an odd number of poles and zeros. If n > m there are n − m asymptotes � � of the root locus that intersect the real axis at σa = ( pi − zi )/(n−m), and radiate out with angles θk = (2k +1)π/(n−m), for k = 0 . . . (n−m−1). Refinement of the Root Locus: Breakaway and break-in points on the real axis Angle of departure from a complex pole There are breakaway or in points of the locus on the real axis wherever N (s)D� (s) − N � (s)D(s) = 0. The angle of departure from pole pj is ◦ θd,pj = ±180 + m � � (pj − zi ) − i=1 Angle of arrival at com­ plex zero n � (pj − pi ) � i=1,i�=j The angle of arrival at zero zj is ◦ φa,zj = ±180 + m � � i=1,i�=j Imaginary axis crossings (stability limits) (zj − zi ) − n � � (zj − pi ) i=1 Use Routh-Hurwitz to determine where the locus crosses the imaginary axis, or assume a form for the closed-loop char. eqn. and solve for the coefficients Determine the poles for Substitute the value of K into D(s) + KN (s) = 0 and find roots a given gain K of characteristic equation. (This may require a computer) Determine K for a given Use the magnitude condition with s = σ + jω, ie K = pole location −D(s)/N (s). (If s is not exactly on the locus, K may be com­ plex, but the imaginary part should be small. Take the real part of K for your answer.) 29–5 2 2.1 MATLAB Root Locus Functions MATLAB Language Functions rlocus() The function rlocus(sys) produces a root locus plot for the system object sys. For example sys = zpk([ -1.5 -4.5],[0 -1 -4],1) rlocus(sys) Root Locus 2 1.5 1 Imaginary Axis 0.5 0 −0.5 −1 −1.5 −2 −12 −10 −8 −6 −4 −2 0 2 Real Axis produces the plot sgrid() The function sgrid without any arguments generates a grid over an existing con­ tinuous s-plane root locus or pole-zero plot. Lines of constant damping ratio (ζ) and undamped natural frequency (ωn ) are drawn. If invoked with a pair of arguments, sgrid(zeta, wn), lines of constant damping ratio for the values given in the vectors zeta and wn will be plotted. For example sgrid([0.92 0.95 0.98], [3 4 5 6 7]) superimposed on the above example pro­ duces the plot Root Locus 2 0.95 1.5 0.92 0.98 Imaginary Axis 1 0.5 7 0 6 5 4 3 −0.5 −1 0.98 −1.5 0.95 −2 −12 −10 −8 0.92 −6 −4 −2 Real Axis 29–6 0 2 Example 3 Use MATLAB to find the gain K to achieve a closed-loop damping ratio 0f ζ = 0.707 for the open-loop system G(s) (s + 1)2 s(s2 + 1) The commands sys = tf([1 2 1],[1 0 1 0]) sgrid(0.707, 0) produce the plot Root Locus 2.5 0.707 2 System: sys Gain: 4.52 Pole: −1.97 + 1.97i Damping: 0.707 Overshoot (%): 4.31 Frequency (rad/sec): 2.78 1.5 Imaginary Axis 1 0.5 0 −0.5 −1 −1.5 −2 0.707 −2.5 −7 −6 −5 −4 −3 Real Axis −2 −1 0 1 With the cursor, the gain is found to be K = 4.52. The step response of the closed-loop system is found with the following commands: closed loop = feedback (4.52*sys, 1) step(closed loop) 29–7 Step Response 1.4 1.2 Amplitude 1 0.8 0.6 0.4 0.2 0 0 1 2 3 Time (sec) 4 5 6 We note that the overshoot is greater than the 5% predicted by the poles with a damping ratio of ζ = 0.707. This is because the closed-loop system retains the open-loop zeros, and these zeros accentuate the overshoot. Example 4 Plot the root locus for (s + 1.5)(s + 5.5) . s(s + 1)(s + 5) sys = zpk([-1.5 -5.5],[0 -1 -5],1) rlocus(sys) G(s) Root Locus 2 1.5 Imaginary Axis 1 0.5 0 −0.5 −1 −1.5 −2 −14 −12 −10 −8 −6 Real Axis 29–8 −4 −2 0 2 2.2 RLTOOL, an Interactive Root Locus Design Tool RLTOOL is a variant of SISOTOOL, which is an interactive GUI based control system design tool in MATLAB. It will be introduced through a classroom demo. • The plant Gp (s) may be imported as a system object from the MATLAB workspace. • The compensator/controller may be defined interactively by placing poles and zeros directly on the root locus plot. • These poles may be moved around by dragging on the plot. • Response curves (step, impulse, etc) may be displayed as the gain is changed. 29–9