Diego – the humanoid biped

I gave Diego a color rehaul. It is now totally clad in matte black. It looks really sleek.

Let me know what you think about it.

In order to drive the legs autonomously, the controlling electronics and batteries have to be attached to the robot. The torso of the robot can be used to hold these parts. I decided to build the torso out of regular aluminum L profiles, which are available in any home improvement store.

The dimensions of the torso were chosen so that it would have similar ratios as the human body. I wanted to build a robot that would be as much antropomorphic as possible. The front side of the torso holds the batteries. It will also hold additional servos for the arms and head. To power the biped I used 4 AA sized and 2 AAA sized batteries connected in series. Only 4 AAs did not provide enough voltage for the LM7805 stabilizers to work properly, that is why I added 2 AAAs more. The back of the torso holds the electronics board:

This way the electronics is pretty woulnerable. I will have to change the design to protect the servo motor connections (seen at the bottom) and the whole board from possible falls. The following article will discuss the electronics design for Diego.

Before ordering the motors and brackets, I needed to conceptualize my robot. The most important thing, is to determine how many degrees of freedom will it have and where to put them. A degree of freedom is the ability of the robot to rotate one of its joints around one of the three orthogonal Cartesian axes (x,y,z). Most of the human joints have three degrees of freedom, but for robots each servo can provide one degree of freedom (rotation around one axis). The more DOFs one chooses, the more servos are needed and the heavier the robot gets. On the other hand more degrees of freedom allow more natural movement. Usually, 5-6 DOFs per leg should be adequate for proper motions. Here’s my initial concept with 5 DOFs per leg.

In the image above, each red line represents one DOF, i.e. one axis of rotation. As it can be seen the knee joint can easily be approximated with one axis, while the ankle needs at least two.

Once the concept was finished, I proceeded with selecting and ordering the needed servos and brackets. I started with designing only the legs, and left the rest for later.

After the parts arrived, building the frame and installing the motors was really easy. I only needed a Philips screwdriver and sometimes a pair of pliers. I was so thrilled when I received the package, that I build a leg in less than an hour. The other one was soon to follow.

Comparing this image to the concept, it can be seen that each servo motor is responsible for one DOF.

As said before, I wanted to have a cost effective but highly customizable modular joint system for my walking robot. For building the joints and skeleton I chose Lynxmotion’s Servo Erector Set parts like these servo holding brackets and C brackets :

The servo motor is fixed to the holder bracket. Then, a C bracket is attached at one end to the horn of the servo and at the other end to the back of the holder bracket using a ball bearing adapter. This way, the C bracket is fixed at both its ends but allows rotational movement:

This smart solution allows to build joints using simple parts. other C brackets and holder brackets can easily be attached to the basic joint to form a whole frame (skeleton) of a biped.

There For actuators I chose to start with a cost aware solution. I ordered two Hitec HS422. Soon after I got them I figured out the they do not provide enough torque (57 oz-in) for the robot to be able to walk. So my next order was for six HS475 (76 oz-in torque).

Since then I realized that these servos are good for supporting a basic two legged robot, but they hardly can support  more than the legs, torso, electronics and batteries. For a heavier robot, with arms and head, stronger servos are suggested. For robotic applications digital servos are highly recommended. They have higher positioning resolution and often times they have higher torque. On the other hand they are more expensive.

Hello,

This blog is documenting the progress of my first humanoid walking biped robot project. The robot is named Diego Alejandro Luis David, or for short: Diego.

When I decided to build a robot, I looked into what kits there were already on the market for building robots. First I was pondering if I should buy a Lego MindStorms NXT robotics kit.

It proved to be less customizable than what I was looking for. Therefore I continued on the search for something that I could design my own electronics for and add custom sensors to it, since I really like working with microcontrollers and embedded computing. Also I wanted to find what kind of solutions there were for building kinematic joints using motors. I stumbled on a few all-included biped robot kits like Robonova-1 by Hitec, Kondo KHR-1HV and Robovie-X.

These robots and kits seemed to be a bit expensive at the time. Also they seemed as not too much fun, since everything is designed for you, in the best case it just has to be assembled. I was looking for some building blocks to build joints using positional servo motors, using which I could conceptualize my own biped. Then I luckily found Lynxmotion. I fell in love with their Servo Erector Set bracket/joint designs. These can be integrated with low cost servos to build robot joints. Because the initial cost of building two legs and a torso seemed not too high and because of high customizability I ordered some of their brackets together with Hitec HS422 servos. I will talk about this in my next post.