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Robotics and FPGAs (3) - Designing the 17 DOF Robot's "Shoes" Using AutoCAD - Rev. 1

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In my last blog post, on the 17 Degrees-of-Freedom (DOF) Biped Robot in Robotics using FPGAs, I expressed my slight concern about the stability of the robot due to the four screws protruding out of the Robot's footbase. The screws are used to attach each footbase to a multi-functional servo bracket. This led me to take a break from the assembly and see what I could do about it.

 What I discovered while designing the "shoes" is that it could be possible to redesign all of the robot parts in a Computer Aided Design (CAD) package and create a 3D printed robot like Intel's Jimmy the 21st Century Robot. However before I got too carried away with designing a 3D printed robot I firstly designed the "shoes" and the method I used to design this covering, using AutoCAD, is the subject of this blog post.

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The Figure above shoes the bottom of the footbase when the multi-functional servo bracket unit is attached to it and hence the problem I am attempting to solve. This is not untypical of any engineering project where you set out to do one thing only to end up spending time on something else that has not been budgeted for. 

This current divergence was born out of the idea, as mentioned previously, of creating a covering or "shoes" to improve the stability of the robot. This thought was conceived with the knowledge that the arrival of an UP! Mini 3D Printer (which you can read about in my 3D Printing blog category) to aid in this task, was imminent. I intend to use the printer to experiment with different footbase cover designs and designs of other parts of the robots anatomy.

Firstly, to begin the modelling, by measuring the footbase, I detached the footbase from the part of the design that I had already assembled. Then, before using a digital Vernier Caliper to actually measure the footbase, I had a brainwave and knowing that Google is my friend went for it. The idea I had was to scan the footbase and import the scan into AutoCAD. Now for seasoned CAD designers this might be common practice, but for someone who does not use AutoCAD regularly this was revolutionary thinking, or so I thought.

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 To begin this mini project I firstly checked the bookcase for a book on AutoCAD, which I knew I had. Eventually, I found a book titled "Introduction to AutoCAD 2004 by Alf Yarwood", which looked useful. Surely it could not have been that long ago since I last fired-up AutoCAD, was it? Probably not, but I wasn't too concerned about this ancient material, given the wealth of information available online. Some of the YouTube videos I watched to complete this exercise were extremely helpful too.

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It's quite interesting to know that no matter how simple YouTubbers demonstrate things to be, it always takes one twice as long to do for ones' self. In some cases, like in mine, it probably took ten times as long! Anyway, I kind of worked out that all that is required is to type "ImageAttatch" into the command line, which rewards you with the Attach Image dialog, shown in the Figure above. When I started the AutoCAD application  I used the acadiso template so I didn't actually know how the imported scan would scale to the current drawing. However, when I imported the footbase image into AutoCAD, using the settings in the dialog above, I had a reasonable sized image on my display.

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I imported the design onto the default layer and created a new layer called "Tracing" to be used to create an outline of the footbase. This can be seen in the Figure above. I subsequently went on to create the three following layers. The Screw layer was used to design the screws, while the Shoe layer was used for the final design of the footbase's covering. The 3D layer was used to reconstruct the footbase from the imported 2D drawing. My next task was tracing over the scanned image.

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To construct the outline of the footbase I used a combination of the "LINE" command, to create the straight segments, along with the "ARC" command for the corners. Then it was a matter of using the circle command for the rest of the design, as can be seen, in the images, above. The next thing I attempted to do was to correct the scale of the design in AutoCAD. The outline that I had drawn above measured 730.015mm, while the actual footbase measures 123.5mm. This meant that I had to use the "SCALE" command to reduce the design by a ratio of approximately 0.169 units. After I had scaled the design I printed it out and compared it for size to the actual footbase. Their sizes were more or less identical. A perfect start.

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 Now for the fun part, designing the rest of the footbase in Three Dimensions (3D)! Viewing the design in a SW Isometric parallel view produced the view point seen in the Figure above. Before actually working in 3D I used the "JOIN" command to connect the parts of the outer perimeter of the traced image. I did this to make it easier to create a solid image of the footbase.

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Its amazing how fantastically rewarding 3D design using AutoCAD actually is. This is partly due to AutoCAD itself, which is incredibly easy to use. I knocked up the design shown in the Figure above in a much longer time, a few hours even, than it would have taken a more accomplished designer. Although, admittedly, now that I know how to use the "EXTRUDE" command, effectively, it would take a fraction of my original design time. Anyway, as mentioned previously this has been an immensely rewarding learning process. The work carried out so far resulted in a 3D design of the footbase, as can be seen in the images directly above and below this paragraph.

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My next task was to create the four screws on the Screw Layer. This part of the assignment was soon done, with aplomb, as can be seen in the Figure above. The insert at the top-right hand corner is the top view of the footbase. This view shows that the footbase should encroach no more than 2mm onto the top of the footbase, to avoid obstructing the multi-functional servo bracket unit. I had overlooked this aspect of the design to begin with and it meant that I had to rethink my covering strategy, as my initial plan was to slide the "shoes" over the footbase. In the end I settled on the design shown in the image below.

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For now I have settled on a design, hence Revision 1, where the footbase covering does not actually cover the footbase, but instead is attached underneath it. This can be seen in the image, above, where the screws used to attach the multi-functional servo bracket at the top of the footbase are also used to attach the covering to the bottom of it. I think this design should work. However, I might revise the design further to reduce the weight of the "shoes". This depends on the contribution  the 1.75mm ABS filament makes to the overall weight, when the design is printed using our keenly awaited UP! Mini 3D Printer.

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The measurements that I  have used to provide a flat surface for the footbase, seen in the Figure on the left, are the following a = 8mm,  b = 6.3mm, c = 5mm, d= 3.6mm, e = 5mm and f=2.3mm. Although, these measurements seem adequate for now I am thinking of revising the design, even more, as they seem over generous. I could reduce the overall thickness of the covering so that e = 4mm and c = 1mm or 2mm. Hence, in the second revision, I am considering reducing the thickness of the covering by half.

Well, that's it for now. I'll continue with the assembly of the biped robot until the 3D printer arrives. I have caught the 3D design bug now and quite fancy designing the whole of the robot, using AutoCAD. TBC

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