Help needed - Strength of Joints /Animation export
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I suspect the answers to my queries are present somewhere on the these forums, I have looked but so far have failed to find them, so I'll apologise now if I'm duplicating previous questions.
I'm a dummy compared to the majority of posters on this forum, but I had managed to get my head round most of funtionality of SP2, and produced a functioning model of an engineering project I'm working on. SP3 obviously has new features, some are obvious, I'm hoping there are others that I've failed find information on that will assist me.
The problem I have is with the strength of joints. Someone please correct me if I'm wrong here, but the joint strengths appear to be directly related to the size (and hence mass) of the objects to which they are connected. I would like to ensure that the joints work how a rigid joint would behave in the real world, i.e. no flex or deviation from the intended axis of rotation/ direction of slider etc. The work round I have employed so far has been to use large invisible blocks within the connected groups to increase their mass. The problem with this approach is that I'm running out space, and no amount of playing with their underlying 'SP shape' can prevent them colliding and fouling each other.
I note that on SP3 there is a 'properties' window at the bottom of the UI, I can't find any posts that expalin what (or not) can be achieved with assigning a property. Obviously a density setting would do the job, but I did find a post on that topic which rules that out.
There are some new 'states' in the UI, I'm not clear on 'static-mesh' as a state (i understand what the shape static-mesh is), and what is 'noautofreeze' and 'showcollision'.
My other query concerns exporting an SP animation. Is there an easy way to export the frames as .jpg's rather than full SU models? The size of my project will make the only export method I can find unusably slow, and I'd quickly run out of HD space.
I'm hoping someone can direct me to a guide/tutorial?
Anyway thanks for reading and my thanks to C Phillips for all his time and effort.
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I think you mean like static, freeze and ignore. I will go with that. a body with the status Ignore, when in a group with another body(not ignore)will just be registered as mass. the ignored entity will not collide with any other entity during simulation. you can change the size of the ignored body to increase mass. It can be used for bullets to help maintain momentum after collision with it's target. if you mean the states like convex hull that wouldn't really affect anything that you need to achieve your goal. hope this helps! tell me if i didn't get what you needed, i'll repost.
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I think he means more that in the real world, if you had for example a hinged joint capable of supporting 1 tonne of loading and still continuing to pivot on its axis, it would do that up to 1 tonne. In SP joints tend to pull apart and have an artificial elasticity which allows them to slip out of axis or break completely.
I can't find a way to get around this yet either; what may be needed is an "absolute strength" type setting for joints and perhaps also a power/force rating for motors, pistons etc. Basically put, a "don't allow joints to shift out of their axis or past min/max bounds, no matter what". In addition a power control to ramp up the load that a powered joint can shift.
I don't know much about the underlying physics libraries used in SP but such a thing must be at least possible, if not implemented within the libraries.
SP is great though, please don't misunderstand my post anyone - big respect to CPhillips for this amazing software!
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Sorry if I'm wrong but I THINK that this is a problem with the physics engine, not the way it's been implemented into SketchUp. It's probably something to do with the way joints work, and Chris can't change the engine; we just have to live with it for now.
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the only thing you can really do is either make the machine on a small scale, or place a series of identical joints. The joint technique does have a large effect. I noticed, when building a claw for a robot, that the claws would bend at the location of the joint. By placing two more joints at the opposite end,(pistons) the problem vanished. Try a large amount of joints.
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Its true that the physics engine doesn't allow joints to be perfectly ridged. Actually its a big part of how it works.
But you can still get them to be very strong if you do it right. There are a lot of good suggestions in the above posts. But if you want to post a model I can take a look at your specific case.
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Thanks for the responses
There may be a solution amongst them, I hadn't tried making the invisible blocks into sub-groups and then giving those groups the 'ignore' state. I hade assumed doing this would take their 'mass' out of the equations and I'd be back with flexi joints.
Another thing that occured me, but I hadn't thought to try, is the size of the joint itself. Does the joint itself become proportionately stronger as you increase its size by scaling?, I'll try both ideas and post back.
Not sure if I should have posted two threads, but has anyone got an answer for the animation export query?
Thanks
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Just checked the suggestion of giving my invisible ballast blocks a state of 'ignore'. Whilst their mass still seems to be taken into account, their presence is not ignored, because the overall collision geometry of the complete moving part is based on the size and shape of the complete group (of which the ballast is only a sub-group)and I can't give the whole group a state of 'ignore' because I want it to move.
To explain further my problem, it's an ergnomics project I'm working on. I've got a ball-jointed manikin supported on a device. My model is an attempt to show that the mankind who has effectively fainted is safely supported and won't fall off the device under the influence of gravity. The problem is that the manikin has a much larger volume than the device, compare your own volume to the chair you're probably sat on. The device has lots of moving parts, some very small in relation to the manikin, as a result the device joints get distorted under the weight of manikin. The ballast blocks hidden within the device components solve the problem, but its a difficult job to stop them from fouling on one another.
There was a 'no-collision' state listed on an earlier SP tutorial I came across, this would resolve my particular problem but I guess was never implemented?.
I'll try more and bigger joints next, any other suggestions welcome
As an illustration the attached file shows what I'm on about. If you unhide the layer called 'physics joints' you can see the mesh segments have a slab of ballast beneath them. Without the ballast when the ball is dropped it rips through the mesh! Ballast in place and the joints have enough strength to catch the ball.
Anyway regardless of what can or can't be done with SP, I think its absolutely brilliant, and its creative potential is freely available to anyone, as stated above full respect to Mr Phillips.
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The ignore suggestion will only work if the overall group's shape is set to 'default'. The first group in this model contains two grouped boxes; one is normal, the other is set to ignore... you can apply this to your model by having a hidden box set to ignore surrounding the actual geometry, as shown with the second group.
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This is also a good way to change the center of gravity, like when building a thruster airplane.
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Thanks Wacov, your suggestion has helped me resolve my problem. My earlier post is incorrect, setting ballast blocks to the 'ignore' state does eliminate them from the mass of the overall group, so the joints do go flexi. However using the overall group shape as default seems to get around the unwanted collisions provided any colliding sub-groups are set to 'ignore'. I can now place my ballast anywhere because although the overall group can be huge its not getting used in collision calculations.
Another property of note is that the largest ballast needs to be concealed within the connected group, NOT the group containing the joint itself.Many thanks, I don't think I would have worked that out, definately not in the short term anyway.
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