# Thread: Issues with simulating using laws of a geneva gear on CATIA DMU kinematic

1. Junior Member
Join Date
May 2019
Posts
4

## Issues with simulating using laws of a geneva gear on CATIA DMU kinematic

Hi everyone,

This is my first post and I am very excited to be a part of this community! please bare with me for any mistakes.

I am trying to simulate geneva gear mechanism on CATIA DMU Kinematics, I am using this tutorial as a guide

I have managed to simulate the kinematics using laws as demonstrated in the video (the bigger disk is the geneva wheel and it is moved by the geneva crank)

However, the issues I am having is based the movement of the geneva wheel (the bigger wheel) where it kept resetting its position. The motion is given by the equation in this document in page 8 "the position of the geneva wheel is given by" - http://gpusupercomputing.com/adarsh/Geneva.pdf

I have written the law as below

if Geneva_drive\KINTime <=90s

Geneva_drive\Commands\Command.2\Angle =-atan(sin(Geneva_drive\Commands\Command.1\Angle )/(c_distance /`radius _crank`-cos(Geneva_drive\Commands\Command.1\Angle ) ) )

else

{

if Geneva_drive\KINTime>=260s

Geneva_drive\Commands\Command.2\Angle =-atan(sin(Geneva_drive\Commands\Command.1\Angle )/(c_distance /`radius _crank`-cos(Geneva_drive\Commands\Command.1\Angle ) ) )

}

One thing I have realised is that, when the Geneva wheel starts moving again after the "else" function at 260s, the position of the Geneva wheel resets itself to the original position instead of continuing on from where it stopped. see video on daily motion for reference and you can see towards the end when it resetted. You can also view the reset on this time series image.

https://imgur.com/a/4NMgepZ.

My question is: Is it possible to create law so that the Geneva wheel continues from the last known position at 41s rather than re-setting at 319s and provide an continuous motion?

If not, do you know any other way?

Thanks a lot for reading this! Looking forward to hearing your inputs.

•

2. Moderator
Join Date
Jul 2006
Posts
1,682
A Geneva Mechanism is a very unique machine. And it is difficult (if not impossible) to simulate with CATIA Kinematics (and many other mechanism software packages). As with other mechanisms, CATIA Kinematics requires using techniques that simulate, but do not truly represent the physical motion. For instance, instead of having the driving pin be in contact with the sides of the slotted wheel, the centerpoint of the pin is driven along the centerline of the slot with a Point-Curve joint. Another CATIA technique is to use Laws to drive the motion with respect to the timing intervals.

•

3. Junior Member
Join Date
May 2019
Posts
4
Before I begin, I realised that the link to the video was not working: Please refer to this link to watch the animation - https://www.dailymotion.com/video/x7925vb

Also, what I meant to say on the first post - Is it possible to create law so that the Geneva wheel continues from the last known position at 90s rather than re-setting at 260s and provide an continuous motion?

My question to MrCATIA - will it be possible to create two different video one using the current simulation and second video starting using the last known position at 90s as the starting position? I am more than happy to share the files if it helps.

Also, do you have any guide that I could look at for using the laws to drive the motion with respect to timing interval?

•

4. Junior Member
Join Date
May 2019
Posts
4
I have realised that the video was not working, so please refer to this link https://www.dailymotion.com/video/x7925vb .

Also, I meant to say on the first post was, "Is it possible to create law so that the Geneva wheel continues from the last known position at 90s rather than re-setting at 260s and provide a continuous motion?"

My question to MrCATIA- Is it possible to create two mechanisms and combine them using laws? First using the current simulation and the second mechanism starts with the starting position using the last known position at 90s? I am more than happy to share the files if it helps?

By any chance, you could share a guide on how to use laws to drive the motion with respect to the timing intervals?

5. Moderator
Join Date
Jul 2006
Posts
1,682
Originally Posted by akman88
"Is it possible to create law so that the Geneva wheel continues from the last known position at 90s rather than re-setting at 260s and provide a continuous motion?"
Yes, you can make the Law start and stop wherever.

(I'm confused about the 260 seconds. 90 seconds X 4 = 360 seconds)

Originally Posted by akman88
" Is it possible to create two mechanisms and combine them using laws? First using the current simulation and the second mechanism starts with the starting position using the last known position at 90s?
I see several options; depends on what you feel most comfortable with.
2. change the timing in the Law to provide the continuous motion
3. create 2 mechanisms. then run 2 simulations and capture the 2 videos. then use video editing tools to combine the videos.
4. create 1 mechanism, but have two input commands. Use Law to drive inputs during various time intervals.

6. Junior Member
Join Date
May 2019
Posts
4
Originally Posted by MrCATIA
Yes, you can make the Law start and stop wherever.

(I'm confused about the 260 seconds. 90 seconds X 4 = 360 seconds)

I see several options; depends on what you feel most comfortable with.
2. change the timing in the Law to provide the continuous motion
3. create 2 mechanisms. then run 2 simulations and capture the 2 videos. then use video editing tools to combine the videos.
4. create 1 mechanism, but have two input commands. Use Law to drive inputs during various time intervals.
Thank you for the suggestions, it certianly give me some ideas to move forward!

With the 90 seconds and 260 confusion. The way the law is written is the position of the geneva wheel = angle displacement of geneva crank (see below)

Geneva_drive\Commands\Command.2\Angle =-atan(sin(Geneva_drive\Commands\Command.1\Angle )/(c_distance /`radius _crank`-cos(Geneva_drive\Commands\Command.1\Angle ) ) )

Referring to the fully written law in the first post - the geneva wheel will follow the law up to 90 seconds (1 second = 1 degree). It will then stop till 260 seconds before following the law again. Which is the reason behind the reset because it is not a continues motion. I am struggling to write the law for continuous motion.

Hope that clears it up.