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ptc-46314451-Visitor
1-Visitor
March 6, 2019
Question

Tips on shortening mechanism run time

  • March 6, 2019
  • 1 reply
  • 5548 views

Hi -- I am having issues with long run times in mechanism dynamics. I am driving my mechanism with a velocity servo motor at 1750 rpm. There are 15 bodies and about 15 connections. The dynamics analysis runs quickly when I drop the motor speed to 2880 deg/sec (480 rpm), but when I try to run at 960 rpm, the program runs fast at first, but then slows down to a crawl at about 55%, and finally stops. On the "Mechanism Settings" I have the "Graphical display during run" un-checked, I do have some parts in assemblies not fully constrained. These are round parts where I have not defined their rotation. Is that a problem? I am trying to make only 1 revolution, for the sake of the graphs, For instance, 1200 rpm would have a duration time of .05 seconds. I can live with the slower rpm's, but feel the real rpm would be more accurate.

Any suggestions would be appreciated.  Thanks

1 reply

C
ChrisKaswer16-Pearl
16-Pearl
March 6, 2019

You will need to define joints on all your parts (along with density/mass) AND you will need to make sure you constrain the motion of each body (lock them down if you don't need to consider them in the solution). If you leave it up to the solver to determine unconstrained bodies positions/velocities, etc., you'll chew-up compute time.

 

Hope this helps,

 

Chris

P
ptc-46314451-VisitorAuthor
1-Visitor
March 6, 2019

Thanks for the tips. What I can't understand is why the program runs great at low rpm's, but can't handle the high rpm's. If I had errors in my connections, etc.,wouldn't that show up at any rpm? Is the issue with the short duration time? When I attempt do reduce the frames, it automatically jumps to a higher number.

P
ptc-46314451-VisitorAuthor
1-Visitor
March 7, 2019

Thanks for the help. This obviously quite complicated. I am trying to do a balancing of this mechanism. I have attached a graph of a resulting moment around a 6DOF connection. A single revolution is .125 seconds, but I am getting what appears to be 2 cycles instead of 1. I am showing the length & rate. There seem to be more than 51 frames. Notice how the frames are closer together at the peaks. Is this what you refer to as a short time step? The run time at this speed is about 15 seconds, even with the graphics on to verify a single revolution.

 

I have done this balancing before, and had to settle for using the lower rpm's. A balance is a balance at any rpm, just more accurate at the real rpm.

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