Using 3D Servo Erector Models w/ Alibre

[kaliatech]

(There’s a long thread explaining the models and some issues here: lynxmotion.net/phpbb/viewtopic.php?t=273, but that thread is more generic and is getting very long).

This post is to summarize some of the more subtle issues from thread above, some issues I’ve run in to, present some ideas, and ask some questions. Possibly of interest to anyone using SolidWorks or Alibre and wanting to be able to fully constrain the parts to allow for easier assembly, constrained motion in assemblies, etc.

1. Servo mount alignment. One of the more subtle issues is that, in the real world, the servo erector pieces’ servo bracket holes (e.g. ASB-04) are spaced slightly larger than than the bracket holes of the servo cases. (Example from other thread). In the real world, this makes them easier to fasten and easier to use with different servo brands. However, for 3D modeling, this presents a problem (at least for solids/parametric modelers like SolidWorks and Alibre) because it’s not possible to use absolute constraints to align the servos with the erector set piece holes. There have been two workarounds proposed for this (I think):
   [list]
   1a) Modify the generic hitec servo model so that it’s holes align exactly with the servo erector piece holes. The servo wouldn’t be real-world accurate, but probably wouldn’t matter for the majority of cases.

1b) The other option would be to add reference sketches to the servo model. (I.m not sure what equivalent term is in SolidWorks. Construction lines maybe?) This reference geometery would reflect the hole position of the erector set overlayed on the servo and could be used for applying positioning constraints.

2. Servo horn holes. It looks to me like the servo horn holes don’t match exactly to the servo erector piece servo horn mounting holes. I’m guessing this is also the case in the real world. It’s hard to describe, and perhaps I should post a picture to help explain for those that haven’t run in to this, but the servo horn holes are are slightly smaller and are generally placed to the outside of the erector piece holes. Again, maybe this is actually the case in the real world to assist in assembly, but it makes it difficult to constrain. I think I was able to constrain two holes in Alibre without errors by using two Tangent-Outside constraints though, so *maybe *this doesn’t matter. I was surprised when it worked though, and not positive it’s repeatable or fully accurate.

3)Cylindrical Surfaces. Another issue I’ve been running in to, is that most of the cylindrical surfaces/faces on the modeled parts seem to be in two pieces rather than one uniform cylindrical surface (which would’ve been the case if the piece was modeled in Alibre directly). I don’t know if this is specific to the Alibre models only, is related to the export/import process being used to convert the formats, or what? However, because of this, it seems to me that Alibre is having problems applying necessary align constraints for things like aligning the servo shaft to the servo horn. It seems to align (sometimes) using one of the half-cylindrical surfaces, but still doesn’t seem correct because Alibre will not allow rotation around the aligned axis. Anyone else notice this?
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If anyone has other ideas, see mistakes in my thinking above, etc., please post up. If I’m correct on the above tough, then I’ll eventually remodel some things (cylinder surfaces) and create reference geometry (for hole alignment) before being able to use the models for their full potential. Just wanted to try double checking before going through the extra effort.

[linuxguy]

@kaliatech

1a) Modify the generic hitec servo model so that it’s holes align exactly with the servo erector piece holes. The servo wouldn’t be real-world accurate, but probably wouldn’t matter for the majority of cases.

This is the approach I’ve taken in creating the new servo model. We know the parts fit together in the real world, so I just adjusted the servo model to fit the ASB-4 and other brackets properly. This does not affect the model other than to make sure all the 3D part models fit together properly as they do in real life.

@kaliatech

2. Servo horn holes. It looks to me like the servo horn holes don’t match exactly to the servo erector piece servo horn mounting holes. I’m guessing this is also the case in the real world. It’s hard to describe, and perhaps I should post a picture to help explain for those that haven’t run in to this, but the servo horn holes are are slightly smaller and are generally placed to the outside of the erector piece holes.

As far as I know, the holes on the SES servo horns fit all the SES brackets properly. We don’t align the horns using the holes, but use the larger center holes to align with the large center holes in the brackets and spline of the servo.

@kaliatech

Again, maybe this is actually the case in the real world to assist in assembly, but it makes it difficult to constrain. I think I was able to constrain two holes in Alibre without errors by using two Tangent-Outside constraints though, so *maybe *this doesn’t matter. I was surprised when it worked though, and not positive it’s repeatable or fully accurate.

You have to use the right alignments constraints. With Alibre at least, you need at least two alignments and a mate to lock parts together. The 3D standard servo model will align all four holes and one surface to the ASB-04 bracket. For flexible joints, such as servos, you align just the large center hole of a bracket to the raised part of the servo horn and then mate the surface of the bracket to the surface of the servo horn to lock them together and allow the joint to move.

@kaliatech

3)Cylindrical Surfaces. Another issue I’ve been running in to, is that most of the cylindrical surfaces/faces on the modeled parts seem to be in two pieces rather than one uniform cylindrical surface (which would’ve been the case if the piece was modeled in Alibre directly). I don’t know if this is specific to the Alibre models only, is related to the export/import process being used to convert the formats, or what?

I’m not sure what you mean here. A picture or two showing what you mean would be helpful.

@kaliatech

However, because of this, it seems to me that Alibre is having problems applying necessary align constraints for things like aligning the servo shaft to the servo horn. It seems to align (sometimes) using one of the half-cylindrical surfaces, but still doesn’t seem correct because Alibre will not allow rotation around the aligned axis. Anyone else notice this?

How are you trying to align the parts and what types of parts do you see this with? Only cylindrical alignments? For hole alignments, you have to align the right parts of the holes in each part to get a proper connection between the parts. Alibre can be a bit finicky, to say the least, about this so some trial and error is needed occasionally to get things locked into place, especially with movable joints.

8-Dale

[kaliatech]

Thanks a lot for responding Dale. It’s helped me out a lot.

We don’t align the horns using the holes, but use the larger center holes to align with the large center holes in the brackets and spline of the servo.

If you align only the center holes though, there will be nothing to constrain the bracket/whatever to the horn rotation. e.g. The bracket/whatever would rotate independently of the horn and so the screw holes would not align. Of course, right after I wrote that, I realized for most modeling purposes it probably doesn’t really matter if the part rotates and the servo horn does not (unless someone was trying to visualize screw locations, etc.)

I really just now realized that after spending a few hours looking in to it more. So… IF someone was interested in constraining the servo horn to rotate with the attached part for some reason (and in hopes that my time on this wasn’t completely wasted ), these images might be of interest:

This image shows how the servo horn screw holes don’t align exactly to the erector piece pattern holes:

With that in mind, it’s easy to see why it’s not possible to use align-center constraints on the servo screw holes. However, it is possible to use tangent-outside constraints as this image shows:

Note that a part can be “locked-in” to the horn rotation in two different ways. First option is use an align-center constraint on the center holes with a tangent-outside constraint on one of the screw holes. The other option is to use two tangent-outside constraints on two different screw holes. Either one of those ends up with a part locked to the servo horn so that they rotate together.

@linuxguy

@kaliatech

3)Cylindrical Surfaces. Another issue I’ve been running in to, is that most of the cylindrical surfaces/faces on the modeled parts seem to be in two pieces rather than one uniform cylindrical surface .

I’m not sure what you mean here. A picture or two showing what you mean would be helpful.

I modeled a new servo horn from scratch using the original model for dimensions. This image shows my model on the left, and the original model on the right. I’ve selected four different surfaces (two pairs) to show what I meant:

I think it’s interesting still, but after going through all of this though, I don’t believe it makes as much difference as I originally thought. I believe, as you said, that Alibre is just very particular about how constraints are setup. In my test assembly, I created two setups side-by-side, one using my horn, and other using the original horn. They both are working exactly the same now. (Both can be rotated as expected.) I’ll have to blame my original problems on inexperience with Alibre unless I’m able to replicate again.

Fwiw, this is the test assembly I’ve been working with. The left servo is using my horn model, the right servo is using the original horn model. They both rotate exactly the same now.

Thanks again for your help.

[robotdude]

The brackets have the holes on a 0.656" circle. The servo horn was modeled with a 17mm circle. I recommend just changing the servo horn to be the same as the brackets.