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COSMOSMotion Problems

Question asked by ralph angell on Mar 10, 2009
Latest reply on Apr 8, 2009 by ralph angell

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hi, i am designing a linkage to carry data from the piston in an engine to the outside of the engine and have come into a bit of trouble. i have successfully designed my component and was trying to build a cosmosmotion model so i could predict the forces in the pivots. every time i run the model, it works fine untill around 1500 rpm where appears to change rotational direction (despite me setting it to motor clockwise) and then fail. i have included a model of the engine without the linkage and u can see, if u motor it to 2000rpm, that after around 8 seconds it changes direction. anyone have any ideas what im doing wrong, any help would be greatly apprechiated! thanks

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Ian Hogg Mar 11, 2009 11:36 AM
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Hi Ralph,

You need to include part files, not just the assembly. Use the pack and go tool from the file menu to capture everything referenced from the assembly.

Since you are talking about the model workign at lower RPM, but at higher RPM it seems to go backwards, I believe what you are most likely seeing is a stroboscopic effect. That is that the part "appears" to be going backward based on the position of the parts at the next visible frame. Usually what this means is that there are insufficient frames to clearly see the motion. For example, if you specifcy a speed of 360 deg/sec and run a simulation for 10 seconds and only have 1 frame per second, it would look like the part is stationary when you animate it.

If the model is actually failing 9solver error message), then the most likely issue since you're dealing with high speeds is the time step intervals.

For 8000 rpm, you have one rev every 0.0075 seconds. You need to ensure that the maxium time step is a coupld of orders less than that for accurate results. Go to the advanced options and ensure it is say .00005 or less for example.

Now there is currently a limitation in SW2008/2009 where you can only animate results at 1000 frame/second, which is a result every .001 seconds. So you will get around 7 result points per revolution. Not ideal as I would prefer a result every 5-10 degrees for smooth plots.

Note that you can change your time units to milliseconds, but it doesn't get around the 1000 frame persecond limit.

I hope this gives you some ideas for addressing your problem.

Cheers,

Ian
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- ralph angell @ Ian Hogg on Mar 13, 2009 3:24 PM
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  thanks very much for such a fast response ian. i think the problem was due to the stroboscopic effect and it appers to work much better now. even though i increased the frames/sec to the max, the model did look particularly slow however, whereas the thing appears to spin rapidly when the frames/sec are lower but then crashes? i have furthered the model by adding a basic linkage and am having more troubles when motion is applied. when the model goes above 1000ish rpm, it comes up with a message saying that it has locked? i have played with a few options and nothing seems to work so im sure i;ve just overseen something. could someone please take a look at it and see where im going wrong? i know the model is very basic, i have not designed the specific componentry yet, rather shapes to represent their dimensions roughly to analyse the motion and whether it will fit in our engine.
  
  thanks
  ralph
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  - Ian Hogg @ ralph angell on Mar 13, 2009 3:44 PM
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    Hi Ralph,
    I can't send a model back to you as you have the educational version.
    It ran fine on my version (2009sp2.1)
    I did clean up the mates a little first. You have pins connecting the parts, but they are free to rotate without any control. This complicates the solution and may be causing part of you issue. Normally I just lock the pins to one of the parts they connect to eliminate unrestricted rotation (or make subassemblies out of them).
    
    I would fix the liner if the position is not going to change.
    
    Redundancies can play a part in models failing to run afte a certain period of time, but you really need to go to SW 2009 to make this simple (they added a revolute joint to this version).
    
    One thing to think about is what benefit there is in running the mechanism for more than a few cycles. You are typically just making the results harder to look at (just a bunch of spikes) so I normally only run a system like this for a couple of revolutions unless looking at some dynamic response for a system that is not kinematic.
    
    Hoep this helps you out.
    
    Cheers,
    
    Ian
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    - ralph angell @ Ian Hogg on Mar 16, 2009 1:51 PM
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      cheers ian,
      u were spot on about the mates, i locked the pins to the componenetry attatched and fixed the liner and it all works perfectly up to the max required (6750 rpm)! last problem with this model is that i am trying to calculate the forces acting at the pivots. ideally, i would like to know the x, y and z directional forces/loads acting on each of the pivots (pins) at different stages of the crank rotation for the maximum engine speed. this will help me choose the correct bearings etc. im not quite sure how to do this and i keep getting a message telling me to "select on mate and one face of part attatched to mate or one simulation element only to create result" - not quite sure what it is asking me to do - any ideas?
      
      thanks
      
      ralph
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      - Ian Hogg @ ralph angell on Mar 16, 2009 5:53 PM
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        Hi Ralph,
        
        For defining a force component, the default results are always in the assembly coordinates. for entities that remain in a plane, this is usually fine since you'll be interested typically in only 2 components.
        
        When plotting against an angular result, the best option is to use the projection angle option. You then select the crankshaft you want the projection angle on and the relevant component (if the crank shaft is aligned with a global axis, just select that).
        
        Steps:
        Plot - Force - Reaction Force - Component -
        Then Select mate connecting pin to other body
        Change "Plot Result" From Time to New Result - Other quantiies - Projection Angle, Component
        Then select a location on a body where you want to measure it e.g Face or
        a mate to use for a result value.
        
        Hope this helps you out.
        
        Cheers,
        
        Ian
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        ralph angell @ Ian Hogg on Mar 17, 2009 1:55 PM
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        Thanks again ian - i am starting to get some forces but they seem suspisciously low. i was jsut wondering, when selecting the pivot, i chose the face of the pin and then one of the mates acting on it, is this right and does it matter which mate u choose as on each pin, i have 2 concentric, one lock and one coincident mate (as they all join 2 components)?
        
        also, i am having problems with the projection angle, i have selected the coincident mate where the crank sits in the part it rotates within but it doesnt let me select any face. is this correct or am i selecting the wrong things?
        
        cheers
        
        ralph
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        Ian Hogg @ ralph angell on Mar 17, 2009 5:04 PM
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        Hi Ralph,
        
        I captured a couple of movies to show the usage. The main use of a motion entity and a face is sign convention. When you have a spring or a mate, it connects 2 bodies, so it's a matter of the results wiht respect to which body. Note that for the projection angle, I just picked the side face of the crank on the 4bar.
        
        Hopefully these will clarify usage.
        
        Cheers,
        
        Ian
        Attachments
        reaction_force_plotting.zip5.0 MB
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        ralph angell @ Ian Hogg on Apr 5, 2009 12:25 PM
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        hi everyone, if ians not available, is there anyone else that can help me as i need to try and get this model working asap!
        im basically trying to motor the crank at 5000rpm and calculate the forces in the pins but the mates i have used keep causing it to break up when it runs, i have a attached the files to the model on the above post - if someone could look at my mates and let me no where i have gone wrong that would be great!
        
        cheers
        
        ralph
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        ralph angell @ ralph angell on Apr 8, 2009 8:49 AM
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        solved it!
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