Simulink - Algebraic Loops errors

Question

I'm trying to build a system in Simulink, but I get errors about Algebraic Loops. Could you please help?

The goal of this system is to observe behaviour of double pendulum with a spring attached to the lower part of it.

Here's my system: http://1drv.ms/1GPqeeQ

I can't post pictures, because i don't have enough points on StackOverflow.

Answer 1

Yep it's common problem. The problem is that simulink try to use variable value to calculate itself (at one step). You can solve this problem easily - you just need to add Unit Delay block for this variable. Like this:

You can see I use variable Vd to calculate itself again at every step. I added Unit Delay and simulink use the value of Vd from PREVIOUS STEP! It works perfectly!