Add summation symbols and input polarities to transfer-function diagram

AERO3220_HW4_block_diagram.md

@@ -83,22 +83,22 @@ This version shows the same coupled dynamics using gain blocks and integrator tr
 ```mermaid
 flowchart LR
     Z1[State z₁] --> G11[Gain -k₁/m₁]
-    G11 --> S1[Sum z̈₁]
+    G11 -->|"-"| S1[Σ]
     Z2[State z₂] --> G12[Gain +k₁/m₁]
-    G12 --> S1
+    G12 -->|"+"| S1
     S1 --> I11[Transfer block 1/s]
     I11 --> Z1D[State ż₁]
     Z1D --> I12[Transfer block 1/s]
     I12 --> Z1
 
     U[Input force u] --> G2U[Gain 1/m₂]
-    G2U --> S2[Sum z̈₂]
+    G2U -->|"+"| S2[Σ]
     Z1 --> G21[Gain k₁/m₂]
-    G21 --> S2
+    G21 -->|"+"| S2
     Z2 --> G22["Gain -(k₁ + k₂)/m₂"]
-    G22 --> S2
+    G22 -->|"-"| S2
     Z2D[State ż₂] --> G23[Gain -b/m₂]
-    G23 --> S2
+    G23 -->|"-"| S2
     S2 --> I21[Transfer block 1/s]
     I21 --> Z2D
     Z2D --> I22[Transfer block 1/s]