Being Inventive: Dynamic simulation

Degrees of freedom and redundancies calculation in Dynamic Simulation

You have constrained the components of your assembly, but you have left some degrees of freedom to be used in the Dynamic Simulation environment.

By the way, converting the constraints to joints, you get the warning that the mechanism is over-constrained for the simulation.

Let’s try to explain this behavior with the simple data set you can download from here.

Open the assembly 4_Bar_Linkage.iam
Un-suppress the constraint Insert:4

As you can see, at this point you can drag one of the ungrounded components to verify the correct kinematic of the assembly.

By the way, if we take a closer look to the assembly we can see that, by virtue of the face-on-face types of constraints, the 3rd link face is already co-planar with the Base face.

If we add a 4th insert constraint, actually we are over-constraining the system.

As you know the insert will do both an axial align and planar mate, but the planar mate is not needed.

So, we can conclude that in assemblies we allow the constraints to be “over-constrained but consistent”.

At this point we go in the Dynamic Simulation environment.

For demo purposes we have disabled the option “Automatically Convert Constraints to Standard Joints” in the Simulation Settings.

Click on the Convert Constraint command

Select Link:2 and the Base
Click OK for creating the Revolution joint and get the warning that the mechanism is over-constrained

What the message in the warning says is that the current configuration allows kinematic movement, but the reaction forces will not be unique.

It is the same sort of thing you learn in Statics engineering course, where a system can be statically indeterminate.

That is, there are more unknowns than equations, so there is no unique solution.

So, in order to avoid the indetermination you have two options.

The first one is applicable when the option “Automatically Convert Constraints to Standard Joints” and consists in clicking on the command Mechanism Status.

The following dialog will propose you to replace the problematic joint with another one that doesn’t create the over-constrained mechanism. In this case, it is the Point – Line Joint.

By the way, with more complex assemblies, the Mechanism Status and Redundancies dialog, in some cases, cannot provide the alternative Joint that solves the issue.

In such cases, it is necessary to edit the assembly constraints and avoid the “over-constrained but consistent” situation allowed in the assembly, but that generates the warning in the Dynamic Simulation environment.

So, in this case, the second option is the following.

Delete the Revolution Joint
Exit the Dynamic Simulation environment
Replace the Insert:4 constrain with, for instance, a Mate constraint between the axis and the center point of the circular edges of the components Link:2 and Base

The kinematics will be equivalent, and the system is no longer the allowed in assemblies over-constrained system that creates the over-constrained mechanism in Dynamic Simulation.

Posted at 07:12 AM in Dynamic simulation | Permalink | Comments (0)

Converting assembly constraints of two independent parts displays the error "Both parts are the same!"

You have opened your assembly that contains components and constraints.

Going into the Dynamic Simulation environment some of the constraints get automatically converted in Standard Joints, but you don’t need them for the simulation you want to run.

So, you go in the menu Manage, Simulation Settings and uncheck the option “Automatically Convert Constraints to Standard Joints“.

In the following dialog you click “No”, for not maintaining the Standard Joints automatically created.

Then, for converting different constraints in the joints you need, you click on the Convert Constraints command in the Joint menu and select two components.

But you get the error “Both parts are the same!".

Our development is working for fixing the problem.

In the meantime, you can use the following workaround for converting the assembly constraints in the joints you need for the simulation.

Menu Manage, Simulation Settings, check again the option “Automatically Convert Constraints to Standard Joints“ and click Apply for recreating automatically the Standard Joints.
Then, uncheck the option again, but, in the following dialog click “Yes”, for maintaining the Standard Joints automatically created.

Select the Standard Joints automatically created in the browser and suppress them.

Click on the command Convert Constraints and select the components.
Once you have converted all the component constraints you can delete the suppressed joints.