View Full Version : [Contest Entry] T-Hex, a Mech inspired 6x4 DOF hexapod

11-23-2009, 05:38 AM

Lately I've been working with a new hexapod project. This time I wanted to go a different way when it comes to design. I must say that all the talk and focus on the Mech warfare has inspired me too, therefore I choosed to try out a mech inspired body design. The legs are made of standard SES brackets and the only custom made parts are the two body panels and the spacers.

Part list (I'll add the SES parts and more details later):

24 x 645 MG Hitec servos for all legs
2 x 85 MG Hitec servos for head (optional
SSC32 V2 Servocontroller
BB2 Botboard for BAP
2x Turnigy LiPo 6v regulator
1x 5400 mAh LiPo
DIY RC remote (future plan to use XBEE)
RC receiver
The major reason for starting this project was that I wanted to make a hexapod with 4 DOF legs. The joint are called coxa, femur, tibia and tars. You may ask why I choosed to add an extra joint (the tars)? First of all this is more like an experiment and a proof of concept. A 3. link to the 2 link IK chain (femur + tibia) gives us an endless amount of IK solutions. So like a robot arm the tarsus act as an end-effector. The IK calculations are relative easy if the tarsus are defined to stay in a perpendicular position in relative to the ground. I'm using a variable called TarsToGroundAngle and as a reference I've set the perpendicular position to be TarsToGroundAngle = 0 deg (not 90 deg). So, if we wanted the tars to hold a constant perpendicular position to the ground the formula for finding the tarsus angle would simply be:

Tars = Femur - Tibia, or Tars = TarsToGroundAngle + Femur - Tibia
(The femur are calibrated with 0 deg in horisontal position and the tibia and tars are calibrated in a vertical position). This actual pose of the leg are illustrated here (screendump from PEP):


But using a constant TarsToGroundAngle wouldn't always be the best solution for a leg. I therefore made an algorithm/formula for calculating the variable TarsToGroundAngle. The input values are the feet positions (coordinates). Here are some pictures that demonstrates how the algorithm work: (pay attention to how the tarsus varies in relative to the ground).





I'll be the first to admit that adding the extra joint (tars) can be a vaste of weight, energy and cost. And its also a bit overkill when it comes to the IK part. But there are some advantages.. The leg are way more flexible and are able to walk over large obstacles without the need for extending the femur. Here is a picture of the SES based leg:


The maximum ground clearance is about 18 - 19 cm (about 7 - 7,4 inches). My idea is to use T-Hex for experimenting on terrain adaption. At this moment I've not added any feet sensors, future plans though.
Here are some pictures of the main body:


I'm using only one battery (LiPo) for both electronics and servos.

For those of you who are interested, here is a little tip of how I made the battery holder for T-Hex:
I used the little spring that are inside the Hitech plastic boxes:
http://forums.trossenrobotics.com/gallery/files/1/5/3/5/pb111420_thumb.jpg (http://forums.trossenrobotics.com/gallery/showimage.php?i=1955&c=3&userid=1535)
Then I cut off the upper half of the spring clamp like this:
http://forums.trossenrobotics.com/gallery/files/1/5/3/5/pb111421_thumb.jpg (http://forums.trossenrobotics.com/gallery/showimage.php?i=1956&c=3&userid=1535)
I made a custom spacer with a hole for holding the spring:
http://forums.trossenrobotics.com/gallery/files/1/5/3/5/pb111429_thumb.jpg (http://forums.trossenrobotics.com/gallery/showimage.php?i=1957&c=3&userid=1535)
The spacer has a hole on each end, here are the spring mounted on the spacer:
http://forums.trossenrobotics.com/gallery/files/1/5/3/5/pb111432_thumb.jpg (http://forums.trossenrobotics.com/gallery/showimage.php?i=1958&c=3&userid=1535)
And the spacers are mounted inside the body:
http://forums.trossenrobotics.com/gallery/files/1/5/3/5/pb111434_thumb.jpg (http://forums.trossenrobotics.com/gallery/showimage.php?i=1959&c=3&userid=1535)
Battery holder with the battery in place:
http://forums.trossenrobotics.com/gallery/files/1/5/3/5/pb111439_thumb.jpg (http://forums.trossenrobotics.com/gallery/showimage.php?i=1960&c=3&userid=1535)
Another view of how the spring holds the battery:
http://forums.trossenrobotics.com/gallery/files/1/5/3/5/pb111440_thumb.jpg (http://forums.trossenrobotics.com/gallery/showimage.php?i=1961&c=3&userid=1535)

As an option I also wanted to make a little head for some distance sensors. I'm using the new SES microbrackets for the head, and the head are mounted inside the body. The only modification needed to do is to remove one spacer in the front of the body. The head are mounted using a L-bracket that are screwed onto the hub that holds the right front leg. I drilled out some extra holes for holding the bracket, not really necessary but made it a bit stronger since the thicknes of the L-bracket and the body plate made the screw a bit short.

http://forums.trossenrobotics.com/gallery/files/1/5/3/5/pb111459_thumb.jpg (http://forums.trossenrobotics.com/gallery/showimage.php?i=1968&c=member&imageuser=1535)

Here are some pictures with the head mounted:



A closeup picture of the head:


Thats it for now. I'll post more info later.
Stay tuned... ;)


11-23-2009, 10:30 AM


Must, have, fully, assembled, pictures.

11-23-2009, 11:53 AM
That thing is gonna look cool :p

11-23-2009, 12:14 PM
I'm totally going to borrow this design. Incredible work as usual.


11-24-2009, 03:36 AM
Thanks for your comments!

I'm totally going to borrow this design. Incredible work as usual.

Thanks DB, feel free to borrow that idea. I believe there are great potential for improvements too.


The pictures I posted yesterday was almost two weeks old... So I've been busy lately to make T-Hex fully assembled. As usual the wiring part took several evening/nights. I'm mounting the electronics in a sandwich in vertical position:

http://forums.trossenrobotics.com/gallery/files/1/5/3/5/t-hex_800600_20_thumb.jpg (http://forums.trossenrobotics.com/gallery/showimage.php?i=1983&c=member&imageuser=1535)

This picture show the electronics mounted and I made a power switch panel ABS plastic. I'm thinking of mounting a XBee Explorer Regulated board beside the power switches. I'm using one switch for SSC32, BB2 and all servos.


This picture show the power wiring. Pay attention to that only one power source are used for both electronics and all 26 servos. The LiPo has a very high capacity and can deliver about 97 amps constant! and thats a factor for avoiding powerdrop when the all servos are under high load.

http://forums.trossenrobotics.com/gallery/files/1/5/3/5/pb151533_thumb.jpg (http://forums.trossenrobotics.com/gallery/showimage.php?i=1985&c=member&imageuser=1535)

http://forums.trossenrobotics.com/gallery/files/1/5/3/5/pb151534_thumb.jpg (http://forums.trossenrobotics.com/gallery/showimage.php?i=1986&c=member&imageuser=1535)

The new 5400 mAh Polyquest LiPo in place. This is a "relative" cheap LiPo compared to the one I use for A-Pod (5100 mAh). It can "only" deliver 18C instead of 25C, but as I mentioned above thats plenty of power for a robot.

http://forums.trossenrobotics.com/gallery/files/1/5/3/5/pb151535_thumb.jpg (http://forums.trossenrobotics.com/gallery/showimage.php?i=1987&c=member&imageuser=1535)

Finaly, here are some samples of my fully assembled T-Hex:
(All my pictures can be seen at the TRC Gallery (http://forums.trossenrobotics.com/gallery/browseimages.php?do=member&imageuser=1535))





Two pictures that demonstrate full body rotation:
About 33 deg roll rotation:

And about 38 deg pitch rotation:

T-Hex and his little sister Phoenix:

T-Hex, A-Pod and Phoenix (my hexapod family) :wink: :

Single leg pose:

I'll post a video when get time for it.
Thats all!


11-24-2009, 04:26 AM
awesome like your other two hexapods! keep us updated :D

11-24-2009, 08:54 AM
Nice work as usual Zenta! Beautiful pods!

11-24-2009, 10:52 AM
That thing is awsome!!!:robotsurprised:

11-24-2009, 12:06 PM
Looks great!!

Looking foward to some video.

11-24-2009, 02:49 PM
A video is worth many thousand words. I can wait to see at least a teaser video.
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11-25-2009, 09:36 PM
Beautiful work! I can't wait for the video!

11-26-2009, 09:14 AM
Thanks for your comments!

I'll see if I can post a little "teaser" video soon. Maybe this weekend.
There are still some work left on the code.


11-27-2009, 12:17 PM
Very nice project Zenta!

11-28-2009, 10:57 AM
Very nice project Zenta!
Thanks Sam!

I grabbed my mobile phone today and did a short video of T-Hex doing some movements. If you pay attention to how the tars joint move you'll get a little picture of how the IK algorithm works.
Sorry for the bad quality video, hopefully I'll post something better later.



11-28-2009, 05:50 PM
I love it :veryhappy: Another awesome creation by you! Let me me be the first to ask ,will you be selling at least the framework as a kit? I sure hope so !

11-28-2009, 08:14 PM
That's the best hexapod yet!

Does the software allow it to walk at different heights?

11-29-2009, 11:34 AM
Fantastic. Keep the video updates coming.


11-29-2009, 01:38 PM
That rocks!

12-01-2009, 03:19 AM
awesome, keep them vids comming!

12-01-2009, 12:57 PM
That's the best hexapod yet!

Does the software allow it to walk at different heights?

I respectfully disagree... A-Pod is still my #1 Favorite. Don?t get me wrong, This design rocks too. :veryhappy:
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12-01-2009, 02:57 PM
Thanks to you all for your nice comments, highly appreciated!

Does the software allow it to walk at different heights?
Yes it does.
The software I'm using is a modified version of the V2.0 phoenix program written by Xan (Jeroen Janssen) for the Basic Atom Pro.

12-01-2009, 04:07 PM
I respectfully disagree... A-Pod is still my #1 Favorite. Donít get me wrong, This design rocks too. :veryhappy:

A-Pod is cool, but not revolutionary. Zenta's unique application of a 4th tarsus DOF and resulting capabilities push this one over the top. :wink:

12-01-2009, 05:20 PM
I have to say that the movement seems really fast and fluid. Seems to walk faster with less of a jerky movement. Also the height can vary much more.

I'm still waiting the next video :rolleyes:

Take your time. :cool:

EDIT : Just wondering, are the 645's having trouble with the 4 DOF?

12-02-2009, 01:25 AM
Just wondering, are the 645's having trouble with the 4 DOF?

I'm not sure what you mean by that? Are you thinking of the load and current draw?

Yesterday I did some measuring of how much current T-Hex draw. All electronics, SSC32, BB2, RC receiver and servos are powered from one LiPo, and I measured the current drawn directly from the battery.

And this is the result I got:

Standing down with both body and all legs touching the ground: ~1 Amps
Standing with body lifted, all legs down: ~2 Amps
Current drawn while raising body upwards: ~4 Amps
Walking ripple gait: ~3,3 Amps
Walking tripod gait: 3,3 - 4 Amps
To be honest the result surprised me, I was expecting a much higher current draw. When doing many movements at one time (like fast body rotations) the current raised to over 5 amps. And adding more load/weight to the body also increased the current drawn very fast. I wasn't able to measure the current when doing very fast walking though.
Here you got a picture of T-Hex standing while measuring the total current draw:


The result did vary a bit though.


12-14-2009, 06:05 PM

Rough ballpark... how does that thing cost to make?

12-14-2009, 07:32 PM
OOOH...let me try this one!

~ $875 for the servos
~ $270 in brackets
~ $80 for the controller boards and electronics (switches and such)
~ $150 in wiring (servo extention wires and other stranded wires
~ $80 for the battery (that is just a guess...it is a lithium ion right?)
~ $40 in aluminum sheet metal...though its a dollar a pound at the recycle center.
Add in the cost to have the parts cut out if you don't have a CNC or capable hands...


Having the coolest bot on the block...priceless!

Am I close? I say closest person who doesn't go over gets to keep it. :p


12-15-2009, 03:43 AM
Am I close? I say closest person who doesn't go over gets to keep it. :p


I don't have the correct answer myself, but you are pretty close though.
I can't say I used as much as 150$ in wires and the battery is a LiPo at 55$ cost.
The DIY remote control I'm using was a gift from Jim.

I've just updated the DIY remote with a XBEE module for bidirectional communication:


Thanks to Kurt Eckhardt for all his work on the code.

I'm waiting for more XBEE modules + regualtors from Trossen.


01-05-2010, 10:08 AM
Zenta, I was considering a project very much like your remote. Do you have any build details?

01-05-2010, 10:46 AM
Zenta, I was considering a project very much like your remote. Do you have any build details?

The remote was built by Jim Frye (owner of Lynxmotion).
And the code was mainly written by Kurt Eckhart.

For more information about the DIY RC transmitter, read this thread on the Lynxmotion forum: http://www.lynxmotion.net/viewtopic.php?f=21&t=4399

And you can read about Kurt's DIY XBee project here:

Hopefully you'll find enough details there for making your own version.