This blog is based on the reading public number "Robot study notes," the study notes from ~
Knock blackboard, this blog is a priority -
Rigid body translational phenomena and error performance
On one study, the trellis modulation accuracy vise is 5mm, deformed into a plane vise.
(Often not the learners can not literal translation error, but even translated into Chinese Chinese do not know the meaning of what is meant, even if they know can not find the wrong solution.)
Rigid body translation errors collection may occur:
1.This is usually a result of an ill conditioned matrix possibly due to unreasonable material properities, an under constrained model, or contact related issues. 由病态矩阵引起的
2.A solver pivot warning or error has been detected in the UY defree of freedom of node 9255 located in flat. Freedom in the Y direction in question
3.One or more bodies may be underconstrained and experiencing rigid body motion. Weak springs have been added to attain a solution. Entity has not been restrained, or rigid body motion. A weak spring can solve this problem
Applied enough Constrains appear to 4.Not to Prevent the Rigid body Motion. This May Lead to Solution Represents warnings or errors. The Check Results
Carefully. Let us confirm the calculation results
The above error, are likely to be rigid translation error occurred.
For apertured rectangular plate, if you want two side limiting surfaces located in the Y, Z axis direction is fixed , the selection displacement, then Y, Z to fill the Component 0, X can remain free.
When selecting the X-direction directional deformation, we will
choose to show undeformed wireframe, can be seen, the specific location prior to comparison under the force of the position of the rectangular plates after post rectangular plate.
Found deformation direction X are positive, indicating that the entire board is moved toward the positive X direction.
The grid resolution was changed to 4,3mm, the results reversed.
The reason is that there have been rigid body motion.
Rigid body motion : the overall object offset occurred.
Why does this happen?
First, let's look at the computer in the end is how to deal with these data .
Suppose both sides of the perforated plate are rectangular Videos nodes 90 and 88 nodes, five after the decimal point, then the load is assigned to each node are:
10000/10000 111.11111N = 90/88 = 113.63636N
when the actual loading force on both sides were: 111.11111 90 9999.99990N = 113.64 88 = 9999.99968N
. (PC force must be given a limited number of digits after a decimal point, it can not be infinite bit may assume a decimal retains the computer five)
so how 0.00022N difference on both sides of the load, the system offset?
Even as both sides draw a node, the final result will be different. Because this is the position caused by multiple factors, node Yeah, and so on.
So, anyway, as long as the constraint is not enough, it will vary.
So, way to eliminate small force of what is it?
Generally, in reality, there must be generated to offset constraints or contacts, but we did not set on, so carefully check the model to find out the cause of a slight force. At present, most finite element analysis software to provide functionality of a weak spring, in principle, can offset this load, but often there will be some newcomers can not find the problem, so although then I want to explain the weak spring function , but the main purpose is to we understand the cause of the load, and individuals and is not recommended for novice to use this feature .
Next, we add the following features on a weak spring 5mm grid model. In the Static Structual-Analysis Settings there, click, there is a weak Springs in the Details, which was transferred from the off to on.
And then it was solved, as we think, and the deformation where the positive and negative. Left deformed left, right deformed right.
Then we click Solution, and then Probe top menu bar, select reaction force reaction, and then select a weak spring in the Boundary Condition, and then update the look, you can see how much offset the weak spring force in the X direction .
Change the grid precision displacement results not vary, but a weak spring force is canceled different.
In fact, with a weak spring, above the displacement can not have them. Until then solving the results look almost the amount of deformation, a little bit changed.
To summarize:
the weak spring can only be offset by a slight force, this force is small system in the process of intercepting significant digits in the resulting general very, very small.
