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Robotics and FPGAs (4) - The 17 DOF Biped Robot Assembly, Part 2

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In the last installment,  in this category of posts, I produced a 3D CAD drawing of a covering for the screws that protrude out from the footbase. In the blog post before that I introduced the parts of the kit and discussed the assembly of the lower part of the legs. In this installment I will discuss the parts that I have used to assemble the upper parts of the legs.

Firstly, to confirm the CAD drawing produced previously was proceeding in the right way I printed out the CAD drawing and placed the lower assembly build next to it, as can be seen in the image on the left. The result is quite pleasing to see, as it confirms that the dimensions of the CAD model that will eventually be printed on a 3D printer are as expected. This has given me enough confidence to know that the 3D model design is progressing along the right path.

Right, to quickly recap how and why I started this category on "Robotics Using FPGAs". After impulse buying a reasonably priced 17 DOF robot kit I was not too unsurprised to learn that the un-assembled parts of the robot were not accompanied by any build instructions. Hence, since acquiring the parts of the robot I have been blogging my progress in assembling it and eventually I will be blogging about controlling it too.

I didn't really impulse buy the robot, because although I have had my eye on 17 DOF robots for a while I've never considered buying one, as I've always considered them out of my price range. For example, the more luxurious models of Intel's 21st Century Robot are allegedly priced in the thousands of pounds, which is definitely way outside of my price budget.

Now, when I saw this reasonably priced robot kit for sale I thought it could be the right time to step into the robotics arena. A relatively cheap robot chassis combined with a FPGA make perfect companions for a website like ours. As an added bonus FPGAs are not only within my price budget, but programming them is within my skill set too! However, before I can begin controlling the robot I should finish building it hence, the focus of this blog post is in the assembly of the upper part of the legs.



The parts I used for this second part of the build, seen in the picture above, are the following:

  1. Long U Servo Brackets - There are 2 of these brackets in the kit, which get their name from their shape, presumably. One of these brackets in conjunction with a multi-functional servo bracket (described next) a servo, a servo horn and a ball bearing form a complete joint. This joint forms the "thigh" part of the upper leg. The assembly of this joint is identical to the short U servo bracket joint (described previously) and is described in more detail below.
  2. Multi-Functional Servo Brackets - The primary purpose of this unit is to perform the function of a joint by hosting the servos, which in my case are the MG966R metal gear servos. The servo slots into this bracket and is securely attached to it using four of the 3.5mm by 10mm screws and nuts. 
  3. Oblique U-type Servo Bracket - I am not quite sure how to describe this unit except to say it almost serves the purpose as providing the robot with the equivalent of "soft" knees, as physical trainers like to say. These can be seen installed in the diagram above.
  4. One-type Servo Bracket - This bracket has been used to join two multi-functional brackets, as can be seen in one of the diagrams below. It may be used in a different way, than used here, in other projects.

a1sx2_Original1_002-000015_20140831-181627_1.png The layout of this stage of the assembly can be seen in this image, above. The oblique U-type bracket (on the right-hand side) is connected to the multi-functional servo bracket to start this part of the build. Next the two mult-functional servo brackets are connected together using the one-type servo bracket (see the image below). Finally, the long U servo bracket is connected to the other mulit-functional servo bracket. The servos are added to the multi-functional servo brackets after the mutli-functional brackets have been joined by the one type bracket.

The image above shows the front side and the back side of the connection between the two multi-functional servo brackets and the one-type servo bracket. This combination effectively joins the upper thigh to the knee joint. Ideally all four screws should be used, rather than the two each shown in the image above.


Putting it all together. When the parts described in this build are added to the assembly process described previously the result can be seen in the image(s) above. From my experience I would recommend not tightening the screws too much, until the orientation of the parts has been ascertained. For example, in the image on the left-hand side it can be observed that we could have aligned all of the servos such that all their connection wires protrude from the back of the robot. I haven't done this, which means that in order to do so I will need to disassemble some parts of the robot, which is not unexpected when building anything this complicated from scratch.


Finally, I have manged to get my hands on a retort stand. I've always intended to use a stand of this type to support the robot, while it undergoes tests or as I develop new kinematic algorithms. The problem I have had in obtaining one is that I couldn't quite remember what it was called. Well, coincidentally, while browsing through a scientific journal, I came across an article that mentioned the stand by name. I typed the name into the usual online suspects and low and behold about two to three pages of them appeared for sell.

This particular one, that I bought, comes with two types of clamps, a ring clamp and thumbscrew clamp, as can be seen in insert two in the image above. I'm not quite sure yet which of the two clamps I'm going to use to secure the robot to the stand or how I'm going to secure the robot to the clamp. This is what I love about science and engineering there is always a problem to solve.

To add to all this excitement our UP! Mini 3D printer has just turned up too! This project is about to get really serious, more soon!

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Tagged in: 17 DOF Robot FPGA