mason_rule - X1“) .2. cm :1; 02mm (5)]: (A) G30)Glawmsmu)...

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Unformatted text preview: X1“) .2. cm :1; 02mm (5)]: (A) G30)Glawmsmu) Comroller Plant Feedback Output transducer (a) Plan: and Controllsr Acmaxing signal (error) Output Mason’s Rule Earlier in this chapter, we showed how to reduce block diagrams to single transfer functions. Now we are ready to show a technique for reducing signal~flow graphs to single transfer functions that relate the output of a system to its input. The block diagram reduction technique we studied in Section 5.2 requires suc— cessive application of fundamental relationships in order to arrive at the system transfer function. On the other hand, Mason’s rule for reducing a signal—flow graph to a single transfer function requires the application of one formula. The formula was derived by S. J. Mason when he related the signal—flow graph to the simulta— neous equations that can be written from the graph (Mason, 1953). In general, it can be complicated to implement the formula without making mistakes. Specifically, the existence of what we will later call nontouching loops increases the complexity of the formula. However, many systems do not have non— touching loops. For these systems, you may find Mason’s rule easier to use than block diagram reduction. Mason’s formula has several components that must be evaluated. First, we must be sure that the definitions of the components are well understood. Then we must exert care in evaluating the components. To that end, we discuss some basic definitions applicable to signal-flow graphs; then we state Mason’s rule and do an example. Definitions Loop gain: The product of branch gains found by traversing a path that starts at a node and ends at the same node without passing through any other node more than once and following the direction of the signal flow. For examples of loop gains, see Figure 5.20. There are four loop gains: 1. G2(s)H1(s) (5.25a) 2. G4(s)H2(s) (5.25b) 3. G4(s)Gs(s)H3(s) (5.25c) 4. G4(S)Gé(S)H3(S) (5.25d) F orward—path gain: The product of gains found by traversing a path from the input node to the output node of the signal—flow graph in the direction of signal Figure 5.20 SignaI-flow graph for demonstrating Gl(s) Mason’s rule R“) . flow. Examples of forward—path gains are also shown in Figure 5.20. There are forward—path gains: ‘ 1. Gi(S)G2(S)G3(S)G4(S)Gs(S)G7(S) (5.2; 2. G1(S)G2(S)G3(S)G4(S)G6(S)G7(S) (52 Nontouching loops: Loops that do not have any nodes in common. In '3 ure 5.20, loop Gz(s)H1(s) does not touch loops G4(s)H2(s), G4(s)G5(s)H3(s), G4<s)06<s)H3<s). Nontouching-loop gain: The product of loop gains from nontouching is: taken two, three, four, etc., at a time. In Figure 5.20, the product of 1' 1. [Gz(s)Hi (s)] [G4(S)H2(S)] (52 : 2- [G2(S)H1(s)][G4(s)Gs(s)H3(s)] (52 3. [Gz(s)Hi(s)][G4(s)G6(S)H3(S)] (5.27 The product of loop gains [G4 (s)Gs (s)H3 (s)] [G4 (s)G6 (s)H3 (s)] is not a nontouchi loop gain since these two loops have nodes in common. In our example there .: no nontouching-loop gains taken three at a time since three nontouching loops Ii not exist in the example. ‘ We are now ready to state Mason’s rule. Mason's Rule i The transfer function, C(s)/R(s), of a system represented by a signal-flow graph I (52* where k = number of forward paths Tk = the kth forward-path gain A = 1 — 2 loop gains + Z nontouching—loop gains taken two at atime — I nontouching-loop gains taken three at a time + Z nontouching-loo gains taken four at a time — ~- Ak = A — 2 loop gain terms in A that touch the kth forward path. In other words, Ak is formed by eliminating from A those loop gains that touch the kth forward path. Notice the alternating signs for the components of A. The following example will help clarify Mason’s rule. Transfer function via Mason’s rule Problem Find the transfer function, C(s)/R(s), for the signal—flow graph in Figure 5.21 . Gl(S) 02(5) G3(S) 04(5) G5(3) MS) 0 O C(s) H 4(3) Solution First, identify the forward-path gains. In this example there is only one: Gi(S)Gz(S)G3(s)G4(S)Gs(s) (529) Second, identify the loop gains. There are four, as follows: 1. Gz(s)H1(s) (5.30a) 2. G4(S)H2(s) (5.30b) 3. G7(s)H4(s) (5.30c) 4- G2(S)G3(S)G4(S)Gs(S)G6(S)G7(S)Gs(s) (530d) Third, identify the nontouching loops taken two at a time. From Eqs. (5.30) and Figure 5.21, we can see that loop 1 does not touch loop 2, loop 1 does not touch loop 3, and loop 2 does not touch loop 3. Notice that loops 1, 2, and 3 all touch loop 4. Thus, the combinations of nontouching loops taken two at a time are as follows: Loop 1 and loop 2: G2(s)H1(s)G4(s)H2(s) (5.313) Loop 1 and loop 3: Gz(s)H1(s)G7(s)H4(s) (5.31b) Loop 2 and loop 3: G4(s)H2(s)G7(s)H4(s) (5.31c) Finally, the nontouching loops taken three at a time are as follows: Loops 1, 2, and 3: G2(S)H1(s)G4(s)H2(s)G7(s)H4(s) (5.32) i ._,. 74W” 7, 7 Now, from Eq. (5.28) and its definitions, we form A and Ak. Hence A = 1 — [02(S)H1(S) + G4(S)H2(S) + G7(S)H4(S) , + 020003 (S)G4(S)Gs (S)G6(S)G7 (9% 6;. 't [(?2(S)111(S)(34(S)I¥é(s) 4' (72(5)111(S)(37(S)114(S) i “t (34(3)172(S)(;7(S)114(S)] _ "— [(32(S)1¥i(S)(?4(S)I¥2(S)(77(S)IY4(S)] l We form Ak by eliminating from A the loop gains that touch the kth forward A1 = 1 — 67(s)H4(s) <5; Expressions (5.29), (5.33), and (5.34), are now substituted into Eq. yielding the transfer function: _ T1A1 _ [G1(S)Gz(S)G3(S)G4(S)Gs(S)l[1 — G7(S)H4(S)] Since there is only one forward path, G(s) consists of only one term rather thy sum of terms, each coming from a forward path. Figure 5.19 Signal-flow graph development: a. signal nodes; b. signal-flow graph; 0. simplified signal- flow graph Converting a block diagram to a signal-flowgraph Problem Convert the block diagram of Figure 5.11 to a signal—flow graph. R(S) O O V1(S) O V6(S) O V2(S) H1(S) O V3 (5) (a) (b) O V7(S) O V4(S) O V5(S) Problem Reduce the system shown in Figure 5.11 to a‘single transfer function. Vg(S) O C(s) F orward-path gains are Gle G3 and G103. Loop gains are —G1G2H], —-G2H2, and —G3H3. Nontouching loops are [—GIGZHJ ][—G3H,] = GszG3HlH3 and [—GzH2][—G3H3] = G2G3H2H3. Also, A = l + GleHl + 0sz + G3H3 + (;1G2G3I1f115I3 + G2G3H2H3. Finally, Al :1 and A2 = 1. C (S) 2 TkAk Substituting these values into T (s) 2 ~— = " yields R(s) A ‘ T(S)_ G1(s)Gg(s)[1+Gz<s)] _ [1+ G2 (S)Hz(S) + G (SW2 (S)Hl (3)][1 + G3 (5)113 (5)] ...
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This note was uploaded on 01/14/2012 for the course EEL 3657 taught by Professor Staff during the Spring '08 term at University of Central Florida.

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mason_rule - X1“) .2. cm :1; 02mm (5)]: (A) G30)Glawmsmu)...

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