Hi everyone, here's an update on the recent work I have been doing on assembly of the quadruped robot design, and also some adventures using the Microsoft Robotics Developer Studio package. I also talk a bit about some experimentation with casting plastic parts using silicone moulds.


As you can see from the picture, the robot is getting near completely assembled now, the only parts remaining are the mounting of the two cameras on the head. No wiring up of servos has been done yet, and the battery pack hasn't yet been mounted (or purchased), but still it's looking more and more complete. The shoulder brackets you can see attaching each leg to the body are actually urethane plastic parts cast in a silicone rubber mould which I made using the original milled plastic version of the shoulder brackets. The milled HDPE version took me about 3 hours to machine, so I was looking for a faster way to make a few of these things. I have never done any plastic casting before, and this is a fairly tricky part to start with I think, as it needed a two part mould and had features like counter sunk holes which I chose to try to include in the casting. The brackets didn't come out as good as I'd hoped, with difficulties around the holes, bubbles in the casting and various imperfections all over and they needed a lot of cleanup, but they are functional.


As an interesting aside, since making these I discovered an interesting company on the net, Shapeways, who have an online 3d printing service where you can upload a 3d model, and get an almost instant quote to have it 3d printed in a choice of a few different materials. If you click on the Gallery and search for AX-12 you can see my parts... Price seems fairly reasonable, so I have actually got an order in for a set of shoulder brackets for the quadruped robot, as a concept test to see if the material is strong enough and whether this could be a good robot prototyping resource.

The feet for the quadruped are a simple design based on half a squash ball (would you believe!) which will give a uniform contact area with the ground during movement. I had originally intended to use some force sensors I have within the feet, however I have instead made a simple variety of the foot which has no sensor at present. During the construction of the feet I did have a good idea (I think) to use a low cost gas pressure sensor and make the squash ball hemisphere airtight - that way force from any direction on the ball will result in a pressure change and should be able to be sensed. That's an idea for the future at the moment.

I have been working recently on desktop software support for the quadruped, specifically Microsoft Robotics Developer Studio services to support the quadruped. Although I don't plan to limit the PC end software to MRDS alone, I did want to evaluate the use of Robotics Studio as a means of simulation (especially to develop some simple walking gaits initially) and also perhaps as a rapid software prototyping platform which might enable me to get some interesting behaviours going quite quickly using services developed by other people linked to the hardware of the quadruped robot via a custom MRDS service I would write. It is early days and still very much a work in progress, especially since C# is not my native tongue yet, but I got a physics simulation going and also a very simple walking motion (using the crawl gait).

Here's a video of the simulation environment running;



The crawl gait I have used is pretty unstable, and it doesn't take much to knock over the robot in the sim. However I intend to improve it by adding some inverse kinematics and a better method of controlling group moves of servos. I also need to add proper support for the differential drive service in MRDS which will enable simple steering and control of the robot using xbox wireless controllers and even the nintendo wiimote. Of course all this is just essentially to demonstrate the capabilities of the robot platform (and have a bit of fun with it) - eventually I would like to implement a brain like hierarchical memory network with the grand aim of teaching it to walk without classical control concepts.

Quad4 robot getting assembled

Posted by Robots Australia | 11:26 AM | | 2 comments »

Well, it's been a good 6 months since I received the laser cut acrylic panels for the quadruped robot and last posted progress on this blog. In that time I've got married, moved house, and starting fixing up both the old and new houses (still in progress)! Hopefully I can be forgiven for slow progress on the robot...

Finally I have machined a couple of the plastic parts I needed to assemble the robot and made a start on assembly. First I needed to move the milling machine from the old house (no small job) and get it set up.

These two images show the assembled progress versus the cad model of the whole assembly. I have made two of the custom plastic brackets for the AX-12 servos and have fitted them to the front left shoulder. There are two matching brackets required for each shoulder (click on these images for higher resolution).

I've also done the angle bending on the top acrylic cover and used velcro patches to hold it on. I was very happy with how the bends came out, though the job is a lot simpler than the bending needed for the head. I was also happy with the machining on the shoulder brackets, though it was very tricky to do, involving a lot of planning ahead of how to best grip the part in the vice so it was held firmly enough. Took about 3 hours each for the two I've done so far, and there's 8 needed in total...

Here is a close-up of the HDPE shoulder brackets. The two brackets needed for each shoulder are a mirror image of each other forming a left and right pair. While the standard Robotis brackets available in the Bioloid kits might do the job, these custom brackets allow for the minimum gap between the two servos, just enough for clearance as the shoulder swings outwards.

Just to check on how much weight he's gaining; the scales say 857 grams so far. The target weight for the whole robot including battery is around 2kg.

The next steps involve machining the other 6 shoulder brackets needed, thermoforming the head plate, drilling the holes needed in the shin parts, and making the feet. Because I have a fair bit on the go at the moment, it is just a matter of grabbing an hour here or there to progress the build, but at least it's getting exciting with real physical assembly taking place!

The nice people at Pololu have produced the laser cut acrylic panels which make up most of the body components of the Quadruped4 robot, and they arrived here in Melbourne yesterday.

I put together the base plate and side walls, and mounted on the robotics controller board, so I could get an idea of how it will all come together.

These two images show reality versus cad! There's a bit of catch up to do... But the slots in all the panels aligned well, and the PCB fitted in place correctly, as did the WiFi antenna tail.

I have a bit of precision acrylic bending to do to create the aesthetic bends in the top cover, and especially the head with its "Johnny 5" eyebrows.

I also have to purchase the rest of the AX-12 servos (I only have two at this point), and cnc cut the shoulder brackets which are custom designed offset brackets for the Ax-12's.