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Thursday, February 4, 2010

Build your own humanoid - Part 1

Not long ago, I spoke about the advantages of humanoid robo-one style combat over battlebots/robot wars matches.

I started thinking about how difficult it might be to start up a competition at my university. Would people be interested? Would it be too expensive? Too complex? Perhaps I would be wrong about people appreciating the tactics and design over brute force destruction.

So I thought I'd walk myself through the process of designing and building a small (under 1kg) humanoid robot, to see how it might play out. Keep in mind that while I do know a lot about humanoid robots, and I also own one; I have never build one from scratch before. I do have experience building other robots, but not walking ones.

Design

First of all, lets have a look at how other people have gone about it. This is often a very good first step when you are uncertain about a project.






One of the first things I notice about these robots is that they are humanoid, not human. When it comes to designing robots, people often get very caught up in biomimicry, without stopping to think about why they are imitating nature's design. In the case of both these robots, the proportions have been modified to make it easier for the robot to balance and walk.

As a general rule, shorter and squatter will make your life easier later on down the track. Keeping the robot's center of mass low will also make him harder to knock over.

At this stage, you will also need to decide how many degrees of freedom you want your robot to have. To robotologists, a degree of freedom is a controllable aspect of the robot. In this case, it refers to the number of joints your robot will have and therefore how complex his actions can be.

Bottom line is, more degrees of freedom means more versatile actions, but at the additional cost of more servos/actuators.

Now, since we are trying to build a humanoid robot, the most attention needs to go to the legs. Robobob has 5 degrees of freedom in each leg. He has two ankle servos, one knee servo, one thigh servo and one hip servo.



I would consider this a pretty good number, but you could get away with removing either the hip or one of the ankle servos and manage.

So that's 10 controllable degrees of freedom just for the legs.

The torso doesn't need to move at all, but giving it the ability to rotate will help allow your robot to shift it's weight quickly to maintain it's balance. Adding one additional servo won't break the bank, and it will contribute a lot to the moves you can perform.

Arms can be as simple or as complex as you like. They do contribute to balance and walking a little, but far less than the torso or the legs. If you are building a robot to fight or wrestle, you will need arms to achieve this. However, if you have a torso which can rotate, all you will need to do with the arms is to raise and lower them in a flapping motion and rotate. If you take this approach, you can get away with as little as 13 servos.

Of course, the exact implementation is up to you. It will depend on your budget and your requirements.

Robobob has 16 degrees of freedom. 5 for each leg and 3 on each arm. This has worked out so far for me, and 16 is actually on the low end of kits available.

If you want to have more than this, I would definiately consider adding them to the legs. A lot of robo-one robots use the double-knee joint, which allows them to stand up and walk twice as fast. The ankles also have a very big impact on how your robot will be able to walk, so if you can get x, y and z rotation on the ankles you will appreciate the advantages.

The next stage is figuring out the approximate dimensions of your robot. I say approximate, because there is every chance that a design decision you make a little further on down the track will will require you to rethink his shape (eg, servo, battery size and weight).

Draw up a skeletal model of your robot and come up with some trial values for the distances between joints. Here are a few things to consider:
  • Taller Robots are easier to knock down because of their higher center of gravity
  • Longer legs mean faster movement.
  • Shorter distances between joints will require less powerful servos.
  • Longer distances between joints will move the limb much more quickly.
  • Leave room for your batteries and circuitry.
  • Make it look cool! It's not all about optimizing numbers.
At this stage, keep it simple. These dimensions will help you select appropriate servos for your needs. If you aren't sure, have a look at existing humanoids and see what works. I will go over a more detailed design in the future.

Next time, I'll talk about choosing the right servos. As you can tell, there is a bit of a catch-22 about design, since your choice of servos depends on your dimensions, which in turn will depend on your servo choice.

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