ALPH - custom AX-12 Phoenix hexapod build thread

[_ADAM_]

Hi everyone,

Welcome to the build thread for my new custom Phoenix-based hexapod project named ALPH (or just Alph)!

The hexapod will use AX-12+ actuators and will be powered by a Raspberry Pi running custom Java code. Gait engine will be a port of "that" code base.

I scored these 18 AX-12+ from a used Comprehensive kit for a good price. They look basically brand new. They have none of the surface scratches you would see on assembled actuators. The two AX-12+ up top are for the "head" and were also scored used in a bundle deal a while back.



Here is a picture of the Phoenix parts, some brackets, and some 0.09" 5052 bar stock. FYI, as of this writing, the Lynxmotion Phoenix chassis kits are on sale for $12 and the femur pairs are on sale for $5! The chassis kit comes with a lot of extra parts such as aluminum standoffs, bearings, hardware, wiring. I'm using some other Lynxmotion parts that will be revealed later.



Here is a picture of the electronics, including Raspberry Pi 3, Samsung 16GB Micro SD, EzConnect HAT, USB2AX, Turnigy 5A UBEC, Turnigy 3000mah lipo, Arbotix power hub, DPDT 30 amp 12VDC switch, momentary switch, Deans Ultra connectors (will use about 8 pairs). Sensors/cameras not shown, TBD. There will also be a Bioloid 6-Port Cable Hub, need to order it. Also working on the wiring, but all the TTL wires will be wrapped as shown in the USB2AX.



Just getting started. As always, your thoughts, opinions, and random robotic musings are welcome.

[Brooks]

My suggestion is to add an Arbotix-M controller to the pile. Let it handle the high-speed details of 1Mb servo interfaces and gait interpolation, leaving your RPi freed for "higher level" functions. The processing load for driving servos and interpolation is pretty high, and I wholeheartedly agree with TR that the Arbotix is a nice solution for this. Think "Here's what I want to do" for the RPi, and "Now go do it!" for the Arbotix-M.

I will admit that I'm biased toward asymmetric dual-processor designs

I'm also thinking these days of combining autonomous exploration combined with gentle commands from "on high" (us!). So we can effectively say to our hexapods a) Normally go ahead and explore, and b) Please take a moment and go here, then return to your normal exploration mode. You'll want a radio modem in there - I like the XBee's.

Just my $.02 worth!

-- Brooks

[tician]

My suggestion is to add an Arbotix-M controller to the pile. Let it handle the high-speed details of 1Mb servo interfaces and gait interpolation, leaving your RPi freed for "higher level" functions. The processing load for driving servos and interpolation is pretty high, and I wholeheartedly agree with TR that the Arbotix is a nice solution for this. Think "Here's what I want to do" for the RPi, and "Now go do it!" for the Arbotix-M.

The USB2AX shown can do plenty of gait interpolation (and AX-psuedo-sync-read packets) in a much smaller footprint than the arbotix-M, and the gait itself consumes very little processing power on a RPi-3.

[KurtEck]

Sounds like fun.

It will be interesting to see how you adapt this kit that was setup for RC servos (HS-645MG). Also on how the Phoenix legs the segments are only connected on one side of the servos, so will be fun to see how that works.

If it does, maybe I need to rebuild mine with AX servos.

As for RPI3 to Arbotix-m instead of USB2AX - Again not much room here so I would go simple. Also at least with RC servos I tried to keep the weight pretty light on the Phoenix as again per the leg design. So depending on what else you are doing with it, another option is to JUST use arbotix-m (or Teensy)

Can not wait to see how all of the parts come together!

[_ADAM_]

My suggestion is to add an Arbotix-M controller to the pile.

I own an Arbotix-M, but no plans to add an arduino board to Alph. I'll talk more about my experimentation with synchronizing movement in my "prototyping" thread when I shift my focus back to that area. But I'm planning to run all the software on the Pi.

You'll want a radio modem in there - I like the XBee's.

I do want RC control. Current plan is to utilize the onboard Bluetooth with a PS3 controller, which I already own several of. Short range is fine for this little guy.

[_ADAM_]

Sounds like fun.

Yeah, this stuff is making me feel like a kid again.

It will be interesting to see how you adapt this kit that was setup for RC servos (HS-645MG). Also on how the Phoenix legs the segments are only connected on one side of the servos, so will be fun to see how that works.

The AX-12 actually works really well with the Lynxmotion stuff. I'm finding that some holes need to be widened a bit here and there to get things to fit and line up, but it's in the ballpark. I'll build some leg prototypes and post pictures to get feedback before I settle on a final design. I may do that tonight.

As for RPI3 to Arbotix-m instead of USB2AX - Again not much room here so I would go simple. Also at least with RC servos I tried to keep the weight pretty light on the Phoenix as again per the leg design. So depending on what else you are doing with it, another option is to JUST use arbotix-m (or Teensy)

Yeah simple is good, and space is a little tight. if I wanted more processing power, I'd upgrade to a Odroid XU4 before I would consider adding a slave board. Not saying there is anything wrong with that approach, it's just not for me.

[Zenta]

Good to see you're having fun with Phoenix! I've seen several AX based Phoenix hexapods so it should be good. I believe the tibia need some work to fit though.

Looking forward to follow your progress.

[Brooks]

Yeah, this stuff is making me feel like a kid again.

And in a nutshell, that's what we're doing - having fun! Pictures and videos, please!

[_ADAM_]

Good to see you're having fun with Phoenix!

Thanks, good to see you tune in to the thread.

I've seen several AX based Phoenix hexapods so it should be good. I believe the tibia need some work to fit though.

I'm glad you mentioned tibias because I'm making a decision what design to use. I did a little comparison to show the options:



From top to bottom, number in brackets is effective length in mm (roughly measured):

1. Lynxmotion 2.25" tubing [106].
2. Lynxmotion 3" tubing [126].
3. Inverted Phoenix [122].
4. Standard Phoenix [102].

I don't think I want to use the Phoenix tibias. I like the look, but I think the tubing leg will look right with the double femur setup I am using. Also, modifying the tibias would be a pain.

Now here is a picture of my first leg prototype, using the 3" tubing leg from above.



Here is a shot of it extended:



A couple key things I wanted to note about the leg:

1. The tibia is 1.65 times as long as the femur. I'm not sure if that's bad. I want the longer tibia because there is going to be a battery deck underneath, and I'd like the extra reach. I also like the way it looks.
2. I attached the femur actuator to the end of the coxa actuator instead of the side. The Phoenix uses the side, which is necessary due to the limitation of hobby servos and their brackets. However, my design does lengthen the coxa, and I'm not sure what effect that will have. And again, I like the way it looks this way.

If there are not major red flags with this design, I'll probably keep it like this. However, I would be open to using the shorter tibia and/or mounting the femur to the side of the coxa if necessary. Let me know what you think keeping in mind it doesn't have to be optimal, as long as it will not be problematic.

A few other minor things I wanted to point out about the hardware:

• I found some rubber feet that are a lot nicer than the tubing ends that Lynxmotion sells. They are heavy duty and have a nice dome shape on the bottom, plus they fit the tubes perfectly.
• I'm not a fan of the phillips head screws that come with the AX-12, so I used M2 6mm and 8mm socket head screws with M2 washers (and Loctite of course).
• I also used a stainless steel 4-40 screw on the tubing connection instead of the stock black oxide.
• For the tubing hub connector, I used phillips M2 instead of the 2-56 black oxide. I couldn't use the socket head there because it gets in the way of the tubing.
• The brackets were originally "gunmetal" anodized from CrustCrawler Robotics that I bought a long, long time ago. Obviously, I stripped the anodizing off (did this a long time ago as well). This was many years before Trossen offered the black aluminum brackets they sell now.
• I looked at using the Lynxmotion flanged bearings on the back of the femur and tibia actuators, but the 3mm thickness is too much for the bearing to be able to seat, so I used black plastic bushings . The bushings work fine, but the bearing would have been a bit cooler. It turns out the bearings work perfectly on the underside of the coxa since the chassis is only 1.5mm. More on that later.

One final thing... the tubing from Lynxmotion comes with a rather dull/ugly finish on it, so I polished one up a bit to see how it would turn out. Here is a before/after shot:



I think it's a big improvement.

That's all for now.

[Zenta]

I think you're good. No major red flags, especially if you are going to place the battery under the body. Good to see the tars is centered with the coxa. Your coxa design will also give you more free space inside the body. Nice rubber feet/tars, where did you get them?

A long coxa vs short femur has some con and pros.
Personally I prefer curved parts..

There are probably more to mention but my 5 year old is poking me..