Thursday 6 August 2015

Biped Robot Construction (V0.50) (3)

Finally completed the debugging and writing the drivers routines for the various systems on the robot.  Figure 1 shows the PCBs on the robot, these consist of the dsPIC33E core, the G15 motor interface board (with on-board buck converter to regulate the 11.1V battery voltage to 5V for the dsPIC33E core), and a Freescale 9-axis sensor toolbox development board (FRDM-STBC-AGM01).  The Freescale board contains a 3-axis gyroscope, a 3-axis accelerometer and a 3-axis e-compass (magnetometer).  Figure 2 is another view from the top, with the dsPIC33E core flipped over, exposing the Freescale sensor board.  At the moment I am only using the accelerometer and magnetometer from the Freescale sensor board.  I am also adding a pair of arms, as evident by the normal servo motors on both sides of the robot.

     Figure 1 - Top view


     Figure 2 - Another view from the top, showing the Freescale 9-axis sensor board.

Tuesday 14 July 2015

Biped Robot Construction (V0.50) (2)

I have also designed my own foot plate for the robot.  As the foot plate that comes with the Rero platform does not allow one to adjust the lateral position of the plate with respect to the "Wide U-joint".  The picture below shows the 3D printed foot plate attached to the Wide U-join, which form the ankle of the bipedal robot. The stl file can be obtained from Thingiverse:

Thursday 18 June 2015

Biped Robot Construction (Ver 0.50)

Currently working on bipedal robot.  Here I am using the G15 smart servo motor from Cytron Technologies.  I have also evaluated Robotis Dynamixel MX28 smart servo motors (Have bought two of these to try out).  These are great, exceptional performance but unfortunately quite expensive for hobbyists.  Ultimately I would like to built a small humanoid robot similar to the Darwin OP Robot.  The G15 is cheaper than the MX28, cost about USD30-35 a piece at present, it has lower holding torque at 12kg/cm at 12V supply and plastic gear and bearing.  I believe standard brushed DC motor is used as opposed to the rare-earth permanent magnet brushed DC motor used in MX28.  The Dynamixel MX28 in contrast uses titanium gear and metal bearing.  Apart from the mechanical differences both the G15 and Dynamixel actually share fairly similar instructions and protocols.  Also Arduino libraries for both smart servos are readily available.  In terms of documentation, the G15 stood out as detailed user manual is available from Cytron's website.  This is useful if the user wishes to develop their own custom controller instead of using the controller provided by the company.

In view of the lower cost and documentation, I decided to use the G15 smart servo for my project.  G15 is actually part of Cytron's Rero robotic platform, which apart from the controller and smart servo, also contains items such as various types of bracket, clips, adapters, wheels etc.  At this stage I have managed to built the legs of the robot, these are 10 degrees of freedom with 5 motors for each leg.  I also built my own custom controller using dsPIC33E micro-controller, this is hooked to a HC-05 Bluetooth module so that I can control each motor from my computer via wireless link.  Below is a video of the first prototype machine walking.  The robot is powered by a 11.1V 2200mAH LiPo battery pack.

I am using the standard U-joints and Inter-Connects from the Rero platform as can be seen in the video.  Since the joints and interconnects use snap-in mechanism to hold to each other, the main issue with this is the assembly is not as robust as using bolt and nuts method of joining.  There is noticeable movement or free-play in the joints among the brackets.  This makes it difficult to come up with the correct angle for each joint during movement.  To 'strengthen' the assembly, I make some U-shape plastic clips from 3D printers.  As in the figure below, the long and short U-joints are hold in place by the clips in addition to the snap-in Interconnect.  Epoxy glue is also applied to the plastic clip.

I am sharing the stl files for the plastic clip here: