IK Biped, where to start?

Biped Robots

IK Biped, where to start?

Postby IKNovice » Wed Jun 11, 2008 7:09 pm

Hi ,
I'm new to the forum but not to bipeds.
I feel that I have significant holes in my knowledge concerning how I want to control bipeds, ie I am currently using a slightly modified Robonova, but I am unhappy with the control/programming and stability. I have used an industrial robot arm, and feel that without implementing Inverse Kinematics (IK) in the controller, bipeds will not achieve there full potential. Also without a proper Inertial Management Unit (IMU) the data from the gyros and accelerometers is too crude.
I notice that the book Introduction to Robotics: Mechanics and Control by John J. Craig is mentioned in the ic hexapod pages., before i buy this book, are there any more suitable - directly relevant books available in English similar to this Japanese book?:
MATLAB / Simulink model-based design and a two-legged robot simulation
三田 宇洋 (著), 高島 博 (著), 宅島 章夫 (著), 田中 明美 (著) Mita宇洋 , Hiroshi Takashima , Akio Takushima , Akemi Tanaka
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Re: IK Biped, where to start?

Postby paulp » Wed Jun 11, 2008 7:28 pm

Like yourself, I started with a Bi-Ped, KHR 1HV to be exact and I found similar problems.

I tried to implement I.K. through the serial link as its not possible to code onto the KHR controller board. I think you will find that is the same for Robonova.

This causes difficulties when it comes to balance control. By the time the imbalance has been detected and sent to a PC to be acted upon it has fallen over.

Other things to consider are Momentum and Inertia.

In Tripods and multi-pods these can be largely ignored but with humanoids they can't.

Servo quality comes into play as well. At some point during a gait, the whole weight of the robot has to be supported and manouvered by 1 servo, i.e. the Ankle and cheap servos fail quickly.

I did read an article on wikipaedia about this and you may need to implement what is called an "Inverse Pendulum". Supposedly difficult to do well.

Still its a challenge and one worth doing so Good Luck if you go ahead with it...
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Re: IK Biped, where to start?

Postby IKNovice » Wed Jun 11, 2008 8:03 pm

Hi paulp,
Sounds like you are lightyears ahead of me. Where did you learn about IK?
From what little I know about IK currently, it seems like it may just be the tip of the iceberg?
I am surprised there isn't a whole biped open source IK community on the web by now?

I tried to implement I.K. through the serial link as its not possible to code onto the KHR controller board. I think you will find that is the same for Robonova.


Do you intend to replace the on board controller with a faster one, or would reprogramming it help?
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Re: IK Biped, where to start?

Postby paulp » Wed Jun 11, 2008 9:46 pm

Not at all, I read enough to realise that the 3 DOF of a typical Hexapod / Octopod leg could be implemented with nothing more than Pythagorus and the Cosine Rule. Any more than 3 especially if one is a ball joint (on a biped the 2 servos on the top of the leg can be considered to act together as a ball joint) involves matrices and other solutions.

When I got that far I decided to dismantle my KHR and build a hexapod. If you look round the site, you will find my project. Lexapod.

It sounds a bit of a get-out but after reading articles where scientists are only now beginning to make break-throughs in Biped motion and they have Universities and Funding behind them. I just saw this as more achievable.

As to the controllers on Robonova and KHR. They are PROM and cannot be programmed. Essentially they are sequential controllers with a few minor features.

What Matt is proposing is a controller that can achieve more than Robonova or Kondo. I know he is offering it as a hexapod controller but it is essentially a dsPic on a board and is configured to be user programmable.

The dsPic processors are designed to be digital signal processors and therefore perfect for what you want to achieve. If you can program in C then it would seem an ideal starting point.

Again though, it is a much larger beast than the board in Robonova..

I did rip out the board from my KHR before I completely dismantled it and fitted a Lynxmotion SSC in there. Was a tight fit.

Image

Image



These videos show it in motion but they are nothing more than pre-programmed sequences
[youtube=eNxdDNBPvQU][/youtube]

[youtube=sf1QcBSvKUA][/youtube]

[youtube=S4Wq-m4ZrvU][/youtube]
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Re: IK Biped, where to start?

Postby Matt Denton » Wed Jun 11, 2008 10:57 pm

At some point I do intend to have a look at the control systems on Bi-peds. From what I have seen they could do with some finesse!

The p.Brain-ds24 could be an ideal starting point, and the reason I have built it in a modular format is so that a smaller PCB could be made to fit bi-peds, quads, arms etc.

The book I recommend on my website: Introduction To Robotics, Mechanics And Control' by John J. Craig I found to be very useful when learning IK. However, like everything, if you don't use it regularly, you tend to forget it! so getting the Ik for a bi-ped working will be another learning curve to undertake. :)
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Re: IK Biped, where to start?

Postby paulp » Wed Jun 11, 2008 11:05 pm

I still have all the fittings to rebuild my biped, so when you get that far, I will be following with interest.
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Re: IK Biped, where to start?

Postby Matt Denton » Wed Jun 11, 2008 11:42 pm

paulp wrote:I still have all the fittings to rebuild my biped, so when you get that far, I will be following with interest.


Cool! I think there is much room for improvement, even with the sequencing software!
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Re: IK Biped, where to start?

Postby paulp » Wed Jun 11, 2008 11:49 pm

The mechanics is a major issue as well. Standing at over 300mm high, the backlash in even the best of servos can cause swaying of several centimeters and as the batteries are usually chest mounted they form quite a top heavy teeter-totter.

The one video of the bot getting up off its back, when it finally straightens up draws nearly 7 amps from the battery. Once upright the servos have to dynamically brake the movement to stop it landing on its face. Another high-amperage gear stripping movement.
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Re: IK Biped, where to start?

Postby IKNovice » Fri Jun 13, 2008 9:42 pm

Thanks Guys, your comments have been very helpful.

paulp, lexapod is very 8-) ......I'm not sure I can force myself to 'disassemble" Robonova"...but experimenting with 3DOF sounds like a good idea to start with while I'm learning.......
Matt wrote:
At some point I do intend to have a look at the control systems on Bi-peds. From what I have seen they could do with some finesse!

Excellent, I look forward to it. In the mean time I will buy the book you suggest.
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Re: IK Biped, where to start?

Postby VDX » Thu Jun 19, 2008 12:39 pm

Hi,

... i stumbled herein from the RepRap-forum ( http://forums.reprap.org/index.php ) where Matt blogged his Hexapod-CNC and hope to find some synergetics for some of my other projects :)

I was sometime busy with simple robotics too - here ( http://www.ctbot.de/index.php?page=4&ac ... y&cat_id=4 ) you can find some minimalistic bipeds and hexapods with only two motors or servos (try downloading the Yeti-Video and the Soccer-bot-video).

Maybe, when i'm ready with my 3D-fabbing-tasks, i'll dive in IK-robots too?

Cheers, Viktor
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