# Four bar linkage odd behaviour? (Simscape Multibody)

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1. Jul 31, 2016

### Rodders

Dear community,

I posted this thread in Mathworks Simscape Community two days ago or so. And I have decided to record a video and post all the information in this forum. This is my first time, so hello and thanks for reading!

Last month I was playing around with Simscape Multibody and I realised something. They have a "four bar linkage" preset to simulate its physical behaviour, so I changed a bit the initial positions and I found this:

I have simulated the same mechanism in Autodesk Inventor and It behaves as I expected (as a pendulum). I supposed that the simmetric position in this case should be the equilibrium position, the attractor.

Then, I decided to create a new simulation in Matlab and this very weird situation popped up. But I do not see why, how, where... I mean, It goes far beyond my understanding.

Some data are:

-Density: 1000 kg/m^3
-Volume: 10x2x0.5 cm^3

-Density: 100 kg/m^3
-Volume: 25x2x0.5 cm^3

-Density: 100 kg/m^3
-Volume: 25x2x0.5 cm^3

Conditions:
-No friction.
-No external foces.
-Just gravitation acting.
-State targets (initial positions): specified in the video. Sorry about the size. Please watch full screen in 1080.

Thanks again!

2. Aug 1, 2016

### Mech_Engineer

So, what exactly is the issue? Looks to me like the simulation is not reflecting reality due to some minuscule unbalance in forces.

3. Aug 1, 2016

### Rodders

Thanks for your feedback Mech_Engineer, really appreciate it.

I have refused the possibility you mention because, when initial positions are eliminated (see 1:28 in the embedded video) the linkage behaves as expected. It oscillates around the equilibrium position (the attractor), the simmetric position. I cannot believe that just by changing its initial position, the physical behaviour could vary in such a different way...

I have simulated dynamically the linkage in Autodesk Inventor, and it behaves as shown in the video below:

It definitely looks better... I refuse to believe that a change in initial position might affect (in this way) the dynamics of the mechanism.

To double-check the dynamics of this mechanism, I have simulated in Matlab the mechanism and I have come across the following result:

My results are quite similar to Simscape Multibody (similarly wrong?).
As you can see (adjusting the resolution to 1080p), it seems there is an attractor when the angle on the upper-left is set to 0.7 Rad, the mechanism shows a minimal displacement. It seems that this position is its equilibrium position, but this is definitely wrong, as it is supposed to be the simmetric position.

I feel quite lost in this regard... Sometimes physical problems are quite frustrating and mind blowers. This one is blowing my self-esteem! XD

Thanks a lot for your time, hope anyone can help me...

4. Aug 1, 2016