Tag Archives: CAD

From paper to plastic

… or more correctly, from CAD to reality, as it is time for 3D printing!

I’ve recently got a new 3D printer in the form of a FlashForge Creator Pro 2017, which means I can start printing some of the structural components for the quadruped now, leaving the decorative pieces for later. In fact, some of them have already been printed, as you can see in the images below.


Chassis parts

All the parts were recently updated from their previous iteration slightly, by adding fillets around the edges, and decreasing nut hole diameters by 0.2 mm in order to provide some material for self-tapping threads. On the other hand, I increased the tolerance of some slots by the same amount, to allow a tolerance for their connection to interlocking plastic tabs.

The rear section has also been modified: the underside aluminium base will have a tab at 90° that connects to the rear, to provide more rigidity to the central connection with the spine servo bracket.

Here are the CAD models of the chassis parts:

Foot base

Front body assembly

Rear body assembly

 


Printing

All parts were printed in PLA plastic.

The first part I started with was the foot base. I printed it with a 20% honeycomb infill. I didn’t add any intermediate solid layers, but might do so in other parts. I have so far printed two out of the four bases.

Each leg will connect to a leg base bracket, which is the same design for all legs. The part was printed “upside-down” because of the orientation of the interlocking tabs. This meant that some support structure was needed for the holes. For the first print attempt I also added supports around the overhang of the filleted edge, along with a brim, but for the subsequent prints I didn’t bother with these, as the fillet overhang held fine without supports, and saved from extra filing/sanding down. These parts also used 20% infill.

For the front and rear “bumpers”, I reduced the infill to 10%.

For the larger part comprising of the central section of the front, the spine front bracket, I also used an infill of 10%. Due to the more complicated design that would have included many overhangs, I found it easier to cut the part lengthwise and print it as two separate pieces. These will be super-glued together after sanding.

Time-lapse GIFs and images of the printing process:

 


The parts so far

In terms of printing times, the foot bases and leg base brackets took about 3 hours each, the bumpers took around 4 hours each, and the two spine front bracket halves took about 7 hours combined, so total printing time is going to be fairly large!

The 0.2 mm clearance seems to work fine for self-threading the plastic with M2 size metal nuts, but was too large for some of the plastic-to-plastic interlocking tabs, possibly since this tolerance is close to the resolution limits of the printer (theoretically a 0.4 mm nozzle and 0.18 mm layer height). However after some filing and sanding down, all the plastic parts fit together nicely.

The resulting 3D prints before and after sanding:


The assembly so far

Finally, here are some images of how the chassis assembly is shaping up, as well as the foot bases shown attached to the foot metal brackets. These fitted snug without any sanding, and all the holes aligned perfectly with the metal brackets, which was reassuring!


The next step is to glue the front bracket halves together, and maybe spray paint all the parts, as they lose all their original shine and end up looking very scratched after sanding.

 

Four legs built – New videos and images

With the hardware for all four legs gathered, I have assembled the first standalone version of the quadruped. The MakerBeam XL aluminium profiles were adopted as before, to create a temporary chassis.

The fact that the robot can now stand on its four feet meant I could quickly give the walking gaits a test on the real hardware: The Python test software reads the up/down and forward/back position of each leg for a number of frames that make up a walking gait, the IK is solved, and the resulting joint values are streamed via serial over to the Arbotix-M, which simply updates the servo goal positions. No balancing or tweaking has been done whatsoever yet, which is why in the video I have to hold on to the back of the robot to prevent it from tipping backwards.

I took some time to make a video of the progress so far on this robot project:


A chance for some new photos:


Finally, here is an older video showing the Xbox One controller controlling the IK’s foot target position, and a simple serial program on the ArbotiX-M which receives the resulting position commands for each motor (try to overlook the bad video quality):


In the next stage I will start building the robot’s body, as per the CAD design, which is for the most part 3D printed parts and aluminium sheets, combined with the 2 DoF “spine” joints.

Fitting the foot force sensors

I finally got round to installing the force sensing resistors to the underside of the Bioloid’s feet. I am using the FSR 400 Short model, with male end connectors, which are small and flat and didn’t fit in the square female header pins which often come at the end of breadboard wires. I didn’t want to solder directly as I’ve damaged these sensors in the past, so I followed a suggestion on the Adafruit website and ended up using these PCB terminal blocks.

The force sensing element of the FSR fits nicely in the pre-existing cut-out on the underside of the Bioloid foot plate, and the pins are fed through a hole in the plate so they can be connected on the topside. However, in the standard Bioloid humanoid configuration, the foot plates are attached at an offset from the ankle angle brackets, which means there is not enough space between the footplate and the bracket to connect the pins. To fix this, I moved the footplates so that the ankle brackets sit along their centres. There is still some free space between the footplates when the Bioloid sits in its resting position, so hopefully the feet will not collide with each other after this change.

For completeness, I also made changes to the CAD model and also added FSR CAD models to the feet, although that is just a cosmetic addition to the model (on a related side-note, I also found a CAD model of the USB2AX which adds to the realism)! The robot’s URDF files and CAD models have been updated on GitHub.

Here are some pictures of the results:

Backpack printed

The backpack print from Shapeways has arrived!

Overall I am very pleased with the quality of the print. The walls are only 1.22 mm thick which makes the design somewhat delicate with the standard plastic material, so if I had to print it again I would thicken them. The dimensions perfectly match the screws, battery and back of robot chest, so the backpack fits in without any problems. I should have also added a few more holes around the sides, as the wiring is fairly cramped. The battery cabling is fairly awkward, maybe I should have gone with an even smaller one than originally planned!

Along with the battery, the backpack also houses the SMPS2Dynamixel servo power adaptor and a bunch of wiring. The power adaptor has a tricky shape with servo connectors at a right angle to the DC jack, but I was able to use a smaller plug with its plastic casing removed in order to fit it inside the limited space.

Hopefully the battery is close enough to the centre of mass, and not too high, so it remains to be seen if the servos are powerful enough to keep the robot standing and moving upright. Now it is back to software, to get the robot to finally do some interesting moves!

New Bioloid backpack

Adding to the endless stream of distractions from the main programming, I decided that the current mess of wiring and components on the back of the Bioloid needed tidying up properly. So, I have made a 3D model for a backpack, and I’m waiting for it to be 3D printed at Shapeways. The dimensions are similar to those of the original case which held the CM-5. The two large holes on the sides are for slotting in the new 2200 mAh battery, as it was too big to fit within the backpack.

Representing the robot in ROS

Building a virtual robot

In this post, I’ll be describing the first stage in getting a working representation of the, as of yet unnamed robot, into the ROS world. I will also be briefly exploring the MoveIt! library, as this might be a useful tool for the future.

The virtual representation of the Bioloid will be built using CAD models of all the individual servos and support brackets. The robot will be loaded up in RViz, where its links an joints will be manipulated. Inverse kinematics will hopefully be calculated by the MoveIt! package later on. But first, a definition is needed of how the joints and links are all connected, and how to move between their frames of reference. The individual CAD components will be oriented correctly onto these frames.

On a side note, I had the chance to photograph the Bioloid in its current state, and also had a Raspberry Pi 2 delivered!

After some background reading, I found out that the best way to create this representation in ROS is by using a standardised XML format file called the Unified Robot Description Format (URDF).


CAD models

In the past I had made a start on drawing a CAD model of the Robotis servo, but since then Robotis has released CAD drawings of all their robot kits online here (old link).

I tidied up the .stp files using FreeCAD, by removing some placeholder parts or sub-parts which would be of no use (e.g. screws, gears and electronics on the inside of the servos and CM-5). I then converted the files into .dae (Collada) format, and imported them into Blender to add some colour textures. For some reason, saving again as .dae in Blender shrunk the model dimensions by 100. I haven’t worried too much about the actual component size at the moment, as only their relative scales have to be correct for now. There is also a bug in RViz which replaces the ambient color of Collada materials by light grey if at least one component of the specified ambient color is 0. To fix this, I manually edited the file and replaced all 0’s with 0.1 in the “ambient” tags. Below are some of the main components, as rendered in Blender.


The Unified Robot Description Format

The URDF file itself is written in a plain markup language. As the definition of various links is written more conveniently with some basic maths, and because many bits of code will inevitably be repeated or recycled (e.g. the mirroring of the left-hand side based on the right-hand side), ROS has a macro language called xacro, which makes it much easier to create and maintain URDF files. A URDF is generated from a xacro file using:

rosrun xacro xacro.py model.xacro > model.urdf

Creating the file for the Bioloid took a fair while, as I created all the translations by eye without knowing the actual distance measurements between the various links, but rather by relying on the CAD components as each one was placed in the chain. I started with the right side of the model, first with the easier arms, then with the legs.

The validity of the URDF file can be checked with teh check_urdf tool. Another great tool for visualising the final result is urdf_to_graphiz, which generates a diagram of the joint and link tree. The tree of my model is shown below. Each joint (blue circles) is positioned with respect to its parent link (black rectangles), and the following/child link is positioned has the same origin as the joint, as shown here. The xyz and rpy labels next to the arrows show the translation and rotation required to get from parent frame to child frame, or in other words, it is the representation of the child’s frame with respect to the parent’s frame. You will also notice the addition of the IMU link, as well as an additional camera link, although this is just a placeholder for a possible camera in the future, and at the moment won’t be used.

Graphviz diagram of URDF file

Graphviz diagram of URDF file


The robot in RViz

The current state of the robot is shown in the screenshots below. The robot is displayed in RViz with the help of the ROS joint_state_publisher and robot_state_publisher. It is fully articulated and the individual joints can be moved with the help of the GUI which joint_state_publisher provides. The joint states will later be published from the joint values read by the real Bioloid servos. In addition to the kinematic model, I created bounding boxes around the components for the collision boundaries, which are shown in red. This was after the fact I realised that without them, the MoveIt! plugin would use the full CAD geometry in its collision detection routines, which made it almost grind to a halt!


Integration with MoveIt!

I have only played around with MoveIt! briefly so far, but the results seem very promising. The library has a useful graphical setup assistant, which essentially enhances the URDF with a Semantic Robot Description Format (SRDF) file. The URDF only contains information about how the joints and links are arranged, as well as some other information such as joint limits, and the visual and collision data. The SRDF doesn’t replace the URDF, but exists separately and contains other information, such as further self-collision checking, auxiliary joints, groups of joints, links and kinematic chains, end effectors and poses. So far I haven’t found any need to edit the SRDF directly, as it can be generated and edited by the setup assistant.

The assistant generates a new ROS package with various templates for path planning and visualisation, which is done via an RViz plugin. So far I have managed to interact with the virtual Bioloid’s arms and legs, in a similar way shown in this PR2 robot tutorial. The aim will be to later on create some poses and walking gaits which I can try out on the real robot, but that is all for now!


Useful links

URDF:

MoveIt!:

RViz scaling and ambient colour issues:

 

Robot Revival

The Bioloid story

Many moons ago, I purchased my first humanoid robot, an 18-servo Bioloid Comprehensive Kit. At the time, humanoid robotics for hobbyists was at its early stages, and I chose the Bioloid after much deliberation and comparison with its then main competitors, the Hitec Robonova and Kondo KHR-2HV. I went for the Bioloid mainly because of the generous parts list, which doesn’t limit the design to just a humanoid robot, as well as the powerful AX-12+ Dynamixel servos. These have a number of advantages over the more traditional simple servos, such as multiple feedback options (position, temperature, load voltage, input voltage), powerful torque, upgradeable firmware, internal PID control, continuous rotation option, a comms bus that enables the servos to be daisy-chained … and the list goes on!

After building some of the standard variants trying out the demos, attempting a custom walker, and playing around with Embedded C on its CM-5 controller board, I never got around to actually doing the kit any real justice. I have finally decided to explore the potential of this impressive robot, and make all that money worthwhile!

This post begins one of hopefully many, in which I will detail my progress with the Bioloid robot.


Initial hardware ideas

The general plan for hardware is to extend the base platform with various components, avoiding the need for custom electronic boards as far as possible, as I want my main focus to be on software.
The Bioloid’s servos are powered and controlled by the CM-5 controller, which has an AVR ATMega128 at its core. I have played around with downloading custom Embedded C to the CM-5, but in terms of what I have in mind, it is much more convenient to be able to control the servos directly from a PC. The standard solution is the USB2Dynamixel, however much of this chunky adaptor is taken up by an unnecessary serial port, so I went for a functionally identical alternative, a USB2AX by a company called Xevelabs. The PC/laptop control will hopefully be replaced by a Raspberry Pi 2 Model B (on back order!) for a more mobile solution. I have not thought about mobile power yet!

Despite the impressive servos, the stock Bioloid offers little in terms of sensors. A provided AX-S1 sensor module has IR sensors/receiver, a microphone and a buzzer, all built in to a single package, which resembles on of the servos, and acts as the Bioloid’s head. Although updated controllers by Robotis have emerged over the years, the original CM-5 had no way of directly integrating sensors to it, and was limited to the AX-S1.

Since a bunch of servos without any real-world feedback does not really make a robot, I am going to add a number of sensors to the base robot. The current plan is to use a MinIMU-9 v3 for tilt/orientation sensing, and a number of Interlink FSR 400 Short force sensing resistors on the feet. Very conveniently, the undersides of the Bioloid’s feet have indents in their four corners which perfectly match the shape of the FSRs! A Pololu A-Star 32U4 Mini SV (essentially an Arduino board) will perform the data collection and pass it over to the PC via serial-to-USB.

That is as far as my current considerations go in terms of hardware. At some point I will look into vision, which may be as simple as a normal webcam. I originally thought that an Xbox Kinect would be ruled out because of size, but apparently it can be done!


Initial software ideas

I plan on using ROS (Robot Operating System) as the main development platform, with code in C++. As well as playing around with ROS in the past, its popularity in a large number of robotics projects and large number of libraries makes it a very development platform. Also, I recently discovered the MoveIt! package, which would be great to try out for the Bioloid’s walking gait.

For simplicity, the A-Star will be programmed using the Arduino IDE. I was pleasantly surprised that I wouldn’t have to write any serial comms code to get the sensor data into the ROS environment, as a serial library for the Arduino already exists. ROS is already looking like a good choice!

The A-Star will initially just serve ROS messages to the PC. It may potentially perform other functions if it has the processing power to spare, but for now there is no need. A ROS service running on the PC will be needed to interface with the Dynamixel servos, instructing the servos to move, reading their feedback and publishing the robot’s joint states to various other ROS nodes.

My next post will focus on the new sensor hardware. Until then, please let me know your thoughts and suggestions, all feedback is welcome!