Tag Archives: Dynamixel

Second leg assembly and painting

Quadbot Forward Kinematics

The Forward Kinematics for the left leg of the Quadbot have been formalised, using modified Denavit-Hartenberg parameters and axes conventions.

I also made a simple Python applet to verify the maths and visualise the leg’s poses. I used Tkinter and three Canvas widgets to show orthogonal views.

The reason I am testing the maths in a quick Python program is that I want to be able to port them easily over to Arduino, as my latest aim is to drop the Raspberry Pi and A-Star 32U4 LV Pi expansion module (shown in some of the latest CAD models) in favour of trying out an ArbotiX controller. A benefit with the latter is that I wouldn’t need a Dynamixel-to-USB converter (e.g. USB2AX) or separate motor power supply.

Next up will be to work out the Inverse Kinematics.

	Link Twist	Link Length	Link Offset	Joint Angle
j	alpha_i-1	a_i-1	d_i	theta_i
1	0	0	0	th_1
2	pi/2	29.05	0	th_2 – 34
3	0	76.919	0	th_3 + 67.5
4	0	72.96	0	th_4
5	-pi/2	45.032	0	th_5

D-H Parameters

Quadbot kinematics applet, zeroed position

Quadbot kinematics applet, test position using sliders

Quad-legged robot ideas

CAD software

I have been using FreeCAD for the Bioloid project, which is a great free tool but somewhat hard to use and lacking many of the features found in modern CAD software.

As I need to design a large number of new parts, I’ve been looking around for a good CAD software with most or all of the following key features:

free, or at least licenced for hobbyist use
parametric modelling
export to STL
3D printing support tools a plus, but not necessary
nice renderer?

Autodesk’s line of tools seem to meet these requirements very well. Autodesk 123D seemed like an excellent choice, with more advanced features than its sibling Tinkercad, a CAD program which runs inside a web browser. However, Autodesk has recently been restructuring its suite of tool according to this announcement, so this lead me to check out Fusion 360.

It has a very flexible licensing model, which means it can be used for free as a student, educator, start-up and most importantly, as an enthusiast (non-profit). I have only been using it for a few days, but so far it seems impressive. It works very similar to SolidWorks, and has a number of useful features such as direct integration with various 3D printing tools and services, and an easy-to-use renderer. One thing that may be seen as a downside is the fact that everything is stored on the cloud, but local backups are possible.

Components & 3D printing

The main design of the robot will be centred around the use of Dynamixel’s AX range of servos, as they are the most competitively priced motors I’ve found for the power and features they offer. Most other high-torque servos are prohibitively expensive, considering I will need about 16 and each costs over £100!

The exact model will probably be the AX-12A, which is an improved version of the AX-12+ used on the Bioloid. I might be able to stretch to the faster, more expensive AX-18A, however as their external design is identical, any frames used will be compatible with both.

For the basic servo joints I will be using a combination of the plastic frames in the Robotis range, as well as possibly some metal frames by Trossen Robotics. The rest of the robot will be designed with 3D printed parts in mind. Whether I go for an expensive online 3D printing service or try and revive my 3D printer remains to be seen.