# Additive disturbance in block diagram representation

Drawing the open loop block diagram between valve and pump is straightforward (please see illustration). But why should $$v$$ be an additional disturbance in this system (please see solution)? $$v$$ is going out of the tank?

• Did you notice the minus sign on top of the adder? Feb 15, 2021 at 13:10
• No I did not, thank you! Feb 15, 2021 at 13:45
• Why is it not after G_t(s)? Feb 15, 2021 at 13:47
• @Dovendyr Because the tank's transfer function input signal is the net flow, the tank will start to drain if the outflow is larger than the inflow, i.e. $v- x\gt 0$, whereas it will start filling up if $v-x \lt 0$. Feb 16, 2021 at 9:22

The block $$G_t(s)$$ describes the tank's response therefore the inflow-outflow difference is its input. The system $$G_v(s)$$ is the valve's response to the user input.