Let's Make Robots!

Theo Jansen legs - possible applications

Here is an open invote to all of you to join yet another Shoutbox debate. Oddbot and I are both thinking of ways to mount a vehicle, or creature, on Theo Jansen legs. (use the search box)

Most creatures living on the 'net have two sets of three legs. Each set on a side. In the front and in the back. Many creatures I saw even double up on the legs: six in front, six in the rear. All connected to the same crankshaft in the center of the creature.

See the original Transporter or  Animaris Rhinoceros

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At best a TJ-leg touches ground 50% of the crank's revolution (or 180 degrees of the cycle). The foot is air borne the other half of the time. Probably a bit longer. So for a set of legs to replace a wheel, it needs at least two legs. A stable vehicle needs three points of contact. So either give the creature four sets (Transporter) or plenty of legs on opposite sides of the vehicle.

Oddbot poses the question of steering the vehicle. Tank style steering (stopping one thread, powering the other) seems somewhat inelegant for this beautiful piece of art.

I plan to build a "shoe box with legs flush against the sides". But to be honest, steerability has not entered my design plans yet. Mostly because I plan to make it (eventually) a towed vehicle. Maybe I should drop Mr Jansen a line. My version probably will be much lighter to pull...

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Your local bearing supplier might have them. That's where I would get mine.

This is my current test setup as seen in my videos. Times three.

tj-sidelegs-pt10.png

Note that I am too lazy to draw each leg in its proper 120° phase difference. One side plate of my test chassis measures 21x30 cm (as does a sheet of A4 paper). That means that this contraption measures about 90 cm or three feet.

That will not do. Hopefully I will be able to condens the arrangement somewhat.

tj-sidelegs-pt10-condensed.png

But when this leg starts to move, it needs a lot of "elbow room". Maybe, I should connect two legs to one crank after all. Something like this.

tj-sidelegs-pt10-alternate.png

This drawing does not do the design much justice. The third leg has no twin and should be placed further to the left. But it is only meant to give a rough impression. Too bad InkScape does not do physical simulations. Or at least, I don't know how it might.

 

All three cranks and legs need to maintain an accurate phase difference of 120°. How will I achieve this? I am asking your help here folks. What do I need to shop for? I have two main alternatives. Gears (which I might fabricate myself with my jigsaw) or a timing belt (which I would have to buy).

tj-sidelegs-pt10-gears.png

I omitted the legs, much easier to draw and to see. Imagine the gears on in the inside of the chassis. They are driven by the red gear. The two smaller gears could of course be bigger. The five brown gears could be all the same size. Just as log as they all turn in the same direction at the same speed.

 

Here's the same drawing with a timing belt. Kinda.

tj-sidelegs-pt10-timingbelt.png

I have zero experience with these belts. But they seem like the strongest and most accurate solution to use. Would a setup like this work at all? Does the middle pulley have enough grip on the belt? Does size matter? Much?

 

What do I shop for? Automotive belts seem so heavy and rigid to me. And the teeth are so coarse. Aren't they? How about this one.

37140001.jpg (rapidonline)

It's 9.8 mm wide and can be as long as 840 mm. That means a max. axle-axle distance of 300 mm using two pulleys with 25 teeth and a diameter of 40 mm. That means that the third pulley needs to be driven by a second belt. Hmmmm. Not very KISSable.

About your belt setup, the middle pulley is likely to slip a tooth or several. Usually the makers specify that the belt needs to engage a certain radius of the cog, in order tonot slip. So it might be better to have 2 cogs coupls in the middle, with a belt from one to the front and another belt from the middle to the back.

Stock drive is a good source, Small Parts probably has a few, and perhaps WM Berg too.

they will keep me busy for while

Thank you too Jeremy and Russell.

I ordered a shipload of parts. From technobots I ordered timing pulleys and belts and shafts and collers and bearings.

pop-4620-110.jpgpop-4660-120.jpg

And to replace the one 9.6 V gearless motor I ordered two geared 12 V motors from robot-italy.

420120.jpg

And a Sharp thingey something 120. I'm gonna make me a proper start here bot, methinks. My Picaxe has been jobless for too many months.

 

Make that picaxe earn it's keep. I want to see those TJ legs racing around your house!

Oooh Aaaah.

 

Suggestions needed. How to fix the pulleys to the shaft? I need to drill 4 mm holes in them, so they will fit those shafts. How do I prevent them from rolling and sliding over the shaft? Drill and tap a hole for a set screw? Like the collars have them? Or is there an easier way?

tj-parts.jpg

tj-parts-close.jpg

I'm suprised that the pulley wheels didn't have a grub screw like the collars. Although not the easiest, drilling and tapping a thread, then fitting a set screw is the best way. Glue is unreliable under heavy load and once it's set, difficult to adjust.

With a 4mm shaft there are not many options.

Those parts look pretty nice, it's always cool to get new stuff in the mail. I'm very intrested to see what your bot will torn out to look like.

On a similar project I drilled and tapped the gear, and then ground a flat spot into the shaft so that the gear could not twist around the shaft, or move along it's length.

This is the place I always go to look for belts and gears, they have just about every kind you can imagine, with belts up to 72in+.

https://sdp-si.com/eStore/

The site leaves a little to be desired, but if you have the patience you can find about anything.

Also, if you decide to go the gear route and there not too big, I don't ming cutting a few on my laser for you:

http://4volt.com/Blog/archive/0001/01/01/laser-cut-gears-and-inkscape.aspx