Groups, Components and Joints

jdc

I could really use some help regarding the relationship of groups to joints.

If I have a link ( a bar that will be connected to two others bars) with joints at each end Do I have to group them to the link all at once or can I group one joint to the link then group the other joint to the already grouped joint and link.
Do joints work with components the same way they work with groups.
Does it matter what order you joint connect a joint and group it?
Is there an advantage to doing one first?
I could really use a set of detailed rules about grouping joints. I have read as much as I can find but still do not have it all put together in my little brain.

Thanks for the help.

DanYHKim

I did a few experiments, because using joints is confusing.
So, I wanted to have a bar with a hinge at each end.
The bar would be joined to two other bars by the hinges (kind of like a semaphore arm).
I did not want the central bar to be static. I also wanted all three bars to be "solid" to each other. That is, they could not occupy the same space, but would block each other like solids.

Make the three bars
"Edit Group" the central bar
add hinges to the ends of the central bar
Close central bar group
Join the hinges each to one other bar
Secure the central bar to a static object, to hold it up.

Run simulation.
Result is that the two bars joined to the central one by the hinges will rotate as expected. The central bar acts like a solid with regard to its static holder.

Strangely, the central bar does not act like a solid WRT the joined bars. Instead, the two joined bars pass freely through the central bar.

--- Second experiment ---

Make three bars as before
Make two small squares.
Select one square and make it into a group.
repeat with the second square
"Edit group" one square
Add a hinge to this group, then close it.
Repeat with the other square.
Place the grouped hinges (hinge plus a square, all in a group) next to the two ends of the central bar and the bars making the two "arms"
Join one hinge to the central bar and one arm. Repeat with the other hinge.
Secure the central bar to a static object as a support

Run simulation

This works as I had hoped. The two arms rotate freely around their hinges, but do not pass through the central bar. Other objects physically interact with all three bars, acting as though they were "solid".

I will see if I can make a video of this, since my wordy description is probably not clear. Does this address some of your questions?

mptak

What an excellent post!

DanYHKim

I made a video of my experiments, and posted to YouTube.

http://www.youtube.com/watch?v=74ji-NXsdDI
[flash=425,344:21jt231s]http://www.youtube.com/v/74ji-NXsdDI[/flash:21jt231s]

For a simple articulated joint between two blocks, where the blocks must interact with each other and with other objects, and where the entire articulation is able to move about, the joint should be added to a third grouped object during an "edit group" action, making it into a moveable object itself, then joint-connected to the "arms" of the articulation.

Yeah, watch the video. My description confuses even me, and I wrote it!

tspco

DanYHKim, I am glad someone did a new video recently. It seems that there aren't many using SP right now. Thanks for your post!.

mitcorb

Could be that many of those who use SP are not so talkative.
Mr. Kim, I also appreciate your very eloquent and informative video.

tspco

I realize that SP is not easy to use, but if you are persistent, you can make it work using whatever resources are available. As I learn stuff for basic operation of the plugin, I will post some videos also.

x11joex11

I agree the video is very helpful, i've been having a very frustrating time understanding the physics engine and it was not for you to test all the different ways to do things and what errors could occur because of it.

Don East

Excellent video tutorial.
Would like to see more.
Don

superoudy

I am not so sure this is an oddity DanYHKim... I think this is an intended behavior which can be useful sometimes... You can always disable the default behavior and obtain the same result as shown in your video by checking the ConnectedCollide checkbox in the joint property window.

d.o. durant

@danyhkim said:

I made a video of my experiments, and posted to YouTube.

http://www.youtube.com/watch?v=74ji-NXsdDI
[flash=425,344:3uy85jfv]http://www.youtube.com/v/74ji-NXsdDI[/flash:3uy85jfv]

For a simple articulated joint between two blocks, where the blocks must interact with each other and with other objects, and where the entire articulation is able to move about, the joint should be added to a third grouped object during an "edit group" action, making it into a moveable object itself, then joint-connected to the "arms" of the articulation.

Yeah, watch the video. My description confuses even me, and I wrote it!
Thank you for the clarification of a few things.

cholme

I am pretty good at most programs and can figure out stuff pretty easily. I have had Sketchy Physics for quite a while now and rarely use it because I find it very unpredictable. Case and point, when I tried to do the exact same thing as in DanYHKim's video (i.e making the flat square and connecting the hinge to that etc.) it doesn't behave the same as in the video.

I am a furniture maker and frequently start SP but it is easier and faster for me to build a small, practical, prototype than to figure out how to get SP working with complex joints.

Any ideas for how to get a handle on this plug in? Manual? Video tutorials? Anything?

I LOVE the idea of it, and it seems like it is amazing but I just don't get it.

Thanks,

Chris

Tunkmountainman

Ok you sketchies....I have a Challenge for you...been working on this for a while and I give up. I want to make a model of a hydraulic cylinder...or an object with a hinge on one end and a shaft that moves in and out of the other end... that I can connect to other objects to move them in some fashion.
If I can learn how to use these joints then I can build my Physics test bench. I am always trying to design an arm or something and rather than doing it all on paper....I think Sketchyphysics would be Great.

DanYHKim

OK. Here is a machine I simulated in SketchUp/Sketchyphysics.
It is a mechanism to hold a set of infrared sensors above a cotton field. The IR reflectance of plants indicates some aspect of plant health, and so the sensors report plant reflectance to a small palmtop computer, which uses the data to apply fertilizer to the specific part of the field that is nutrient-poor.

Our machine is mounted on a Landini Mythos tractor. I slightly modified a similar-looking tractor that i downloaded from the 3D Warehouse, and then mounted the sensor mechanism. The elevation control on the model, as in the actual equipment, is controlled using a single piston that pushes on a pantograph, so the sensors remain level as they move up and down.

One surprise for me was that when the piston is extended from its lowest position, the swinging linker arm rotates forward and then suddenly rotates back. On later examination of video of the real machine, I found that the mechanism does the same thing in the field. This was rather satisfying to see.

Here is an embedded video of the machine and its model:

[flash=640,320:1py3cvuq]http://www.youtube.com/v/ZUL6S5JMxls[/flash:1py3cvuq]

mitcorb

Congratulations!!! Excellent concept. Excellent video. Question- what is the function of the U bar suspended down below?

scrollcrafters

@danyhkim said:

I made a video of my experiments, and posted to YouTube.

http://www.youtube.com/watch?v=74ji-NXsdDI
[flash=425,344:18h7jbs0]http://www.youtube.com/v/74ji-NXsdDI[/flash:18h7jbs0]

For a simple articulated joint between two blocks, where the blocks must interact with each other and with other objects, and where the entire articulation is able to move about, the joint should be added to a third grouped object during an "edit group" action, making it into a moveable object itself, then joint-connected to the "arms" of the articulation.

Yeah, watch the video. My description confuses even me, and I wrote it!

I watched the video (well done, BTW) but still cannot get the desired interaction. All I'm trying to replicate is the hinge and box which I create as you did. I use the joint connector, first selecting the hinge and then the box. When I run the simulation,the box just drops downward. I simply cannot figure out what I'm doing wrong!

I'm using SketchyPhysics 3.2 and Sketchup 8, free version on a Windows XP system.

Mike

pilou

@unknownuser said:

what I'm doing wrong!

Have you pressed and holdCTRL when you click on the Join Connector and the group ?

scrollcrafters

@unknownuser said:

@unknownuser said:

what I'm doing wrong!

Have you pressed and holdCTRL when you click on the Join Connector and the group ?

NO! I watched a video and did not pick up on that. I just tried it and get the proper simulation.

Thanks for the tip.

Mike

DanYHKim

@mitcorb said:

Question- what is the function of the U bar suspended down below?

The U-bar is used by the tractor driver to gauge the height of the plants he is running over. The IR sensor rack is raised so the bottom of that bar brushes over the tops of the plants. I think that determines the area that the sensors can "see".