Inter-dependant Mates - am I expecting too much or is this a bug.

brett_sutton

I'm trying to model a fairly simple robot arm.


Basically I'm trying to move a segment of the arm using a linear actuator (a threaded rod between the two segments).

The two arm segments and the threaded rod form a triangle. Shortening or lengthening the rod changes the position of the segments relative to each other.

If you look at the assembly I have four inter related mates however the fifth mate (which 'closes' the triangle) causes an error.

Robot arm with four mates
https://cad.onshape.com/documents/95e94cab4009425aa536295c/v/4557c9ceedc34c36a89bf326

Robot arm with fifth mate and error
https://cad.onshape.com/documents/95e94cab4009425aa536295c/v/74f1713dfa684e56ad5130b0/e/d39db2ce3fcb45708f25fed9

The two arm segments are joined with a rotational mate.

Each segment than has an actuator end point 'holder'. The holders are mated to the arm segments using a rotational mate. 

The threaded rod is then inserted into each end point  using a sliding mate.

Now from a simple physical process the geometry of the arm works, however it appears  that onshape  is unable to handle the 'circular' nature of the full set of mates.

Am I expecting too much from onshape mates or is this a bug?
 

nav

Hi @brett_sutton  complementing @viru `s post I`ll add a screw relationship to the cylindrical mate to create the linear actuator type of motion, if this is the case as I saw in your model the actuator has a thread.

**Modified the axle so the motion could be visualised. You can also set limits to avoid collisions.

Here`s a copy of the model.

https://cad.onshape.com/documents/cf4239112a3042719c3c456a/w/34ef9eb9c3c340d186764a95/e/522d1aeecc4640f88fb4bfa4

viru

@brett_sutton , I think you haven't given proper constrain as per assembly requirement. I have seen your document and edited in your assembly by giving required mate-constrain. You can check below link for getting motion.
https://cad.onshape.com/documents/1f7fad849fe24fae9240e2be/w/1b38032fe84d4b3cbe59797d/e/6baa3211925f4c9b8c868cb9
If you required any more motion as shown in above link document and below video then share motion video for your requirement. I will give constrain as per your requirement.

nav

Hi @brett_sutton  complementing @viru `s post I`ll add a screw relationship to the cylindrical mate to create the linear actuator type of motion, if this is the case as I saw in your model the actuator has a thread.

**Modified the axle so the motion could be visualised. You can also set limits to avoid collisions.

Here`s a copy of the model.

https://cad.onshape.com/documents/cf4239112a3042719c3c456a/w/34ef9eb9c3c340d186764a95/e/522d1aeecc4640f88fb4bfa4