Diego – the humanoid biped

After adding Diego’s arms and head the robot became too heavy for the servos to support it. I had four HS645 and six HS475 analog servos in the legs. I decided to upgrade to digital ones, because of their higher holding torque. Since, I’m still on a budget I browsed around for cheaper solutions. Finally I decided to give some Chinese servos a try that have received some acceptable comments: the Towardpro MG996R. They can be found on Ebay for under 15$ a piece which is an unbeatable price for digital servos with metal gears and around 10 kg.cm torque.  I gradually replaced all ten leg actuators with these. The horns that ship with the MG996 are not compatible with the Lynxmotion brackets. Therefore I had to use Hitec horns. Here a small problem arose, since the MG’s are marginally compatible with HS horns, so I had to heat up the horns to fit them on the MG996R’s. After installing the motors, they proved to be very responsive and fairly precise with acceptable backlash. On the other hand they are not protected from overheating which allowed me to burn one of them down while it was trying to work against a mechanical barrier. All in all, I would recommend these servos to people on a budget who do not want to spend 50-100$ on higher quality actuators.

After extensive mechanical, electrical and programming work, I finally got Diego to stand on his own feet with motors powered. This was a very proud moment for me.

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.