# A block diagram reduction

mech-eng
I would like to ask why cannot I think G5 in series with G3(G4+G6G7) and then after muliplying them why I cannot think of G5*G3(G4+G6G7) in parallel to G2 so sum G2 and G5*G3(G4+G6G7) ?

Thank you.

Last edited:

## Answers and Replies

Mentor
If you do that, what happens to G8 and C(s)?

G5 would be at the other side, by the way.

Gold Member
2022 Award
I would like to ask why cannot I think G5 in series with G3(G4+G6G7) and then after muliplying them why I cannot think of G5*G3(G4+G6G7) in parallel to G2 so sum G2 and G5*G3(G4+G6G7) ?
.
I suggest you review the definitions of serial and parallel and see how they apply in this case. It's completely clear from the definitions that what you want to be series isn't series.

mech-eng
If you do that, what happens to G8 and C(s)? G5 would be at the other side, by the way.

If you do that I would ignore the multiplication of G8 and G5 so the result would be incorrect. I cannot think of what happens to the C(s) ? Is that the same thing for G(8) i.e ignorance of multiplication? But problem is that is G5 also series with C(s) so when multiply G5 with G8 and (G4 +G6G7) should I also multiply G5 with C(s) ?

Thank you.

Last edited:
mech-eng
I suggest you review the definitions of serial and parallel and see how they apply in this case. It's completely clear from the definitions that what you want to be series isn't series.

I try to still understand. Because there is a common point between G5, G8 and (G4 + G6G7), G5 cannot be only series with (G4 + G6G7) and the signal G5 is going to both G8 and (G4 + G6G7) so G5 is both series with them. When I do this result is correct but I think simpler.

Thank you.

Mentor
You can make multiple G5 blocks, but I don't think that makes it simpler.

mech-eng
You can make multiple G5 blocks, but I don't think that makes it simpler.

But if I do that, I can get rid of rational expressions so I can write transfer function easily.

Thank you.