Let's Make Robots!

TJ snow angels and pulley kebab


Today I decided on the arrangement of my Theo Jansen legs on the side of my future robot. I place one prototype on a piece of cardboard and traced the outlines with the legs in all possible positions. Like a robotic snow angel. This way I know what the "footprint" of the entire mechanism against the side of my robot will look like. And if one leg will hit a neighbour when flailing about.


Turns out that the belts I bought are just a little longer than the shortest possible distance. Center to center they will be spaced 28 cm apart. The board you see will be the flank of my creature and measures 80 cm. The rear and front legs will overshoot this board by just a tiny few cm.

Because I do not wish to run six shafts all the way starboard to port side, I came up with a "double hull" idea. Each flank will consist of two sheets of plywood, seperated by a piece of pine of the exact size required to house two pulleys. Here is a prototype of my pulley kebab.


From left to right you see: a bearing (buried in the plywood), brass washer, two aluminium timing pulleys, a brass washer, a bearing and a collet.

The legs will sit left in the picture, hugging the outer hull. In between the sheets of ply sit the pulleys and belts. Protruding on the inside is just short stub of the shaft. I think. Not decided yet.

Still to figure out: a way to make a crank that will sit firmly on a 4 mm steel shaft. And I have no idea how much torque it will endure. Somehow plywood feels inadequate.

[edit 24 april: trying out my new Theo Jansen banner]

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I have noticed that while 4volts TJ robot is very wide much like the original Theo Jansen creations yours is long and thin. Is this going to be the streched limo of TJ creatures? I suspect it will have trouble turning unless it is also going to be at least 60cm wide. It will end up being a huge bug!

Yeah 8-(

I figured that as soon as I drew a 1:1 scaled plan on my whiteboard. Tha base plate will be 80 cm long and 35 cm wide. Probably 47 cm if I feel ambitious (I need at least 37 cm inside to kneel down in it!).

The distance between feet is however what makes it maneuverable or not. I am estimating the length between front and rear feet approx. 57 cm. And the big difference with limousines is: wheels have constant contact with the ground, with a constant pressure/friction. Feet don't.

If the turning turns out a turn off, I will consider articulating my vehicle.


I think articulation is out because of your belt drive :(

Increasing the width will reduce the side on drag of the front and rear legs when turning on the spot as their path will be closer to the proscribed turning circle. (I don't know if that makes sense)

Quite possibly you need more legs so that there is one or more feet touching the ground at any time at any of your six drive locations. (especially if you want to sit in or on it).

This is starting to look like a centipede, perhaps it should articulate up/down and twist rather than side to side for turning? The ultimate off road vehicle :D

I recognise the need for width. And I understand the reasoning behind the proscribing turning circle. But I allow myself not to think to much about maneuverability for now. Just getting it to walk forward is enough for me right now.

More legs: quite possible indeed. For all kinds of reasons. But first I want to learn the constraints of this design. Will it carry a 2.5 kg SLA battery? Will my slow torqy motors do the job? How soon will I learn to work alu.

SOW: I got my flat bar 6x25 mm. 1500 mm long because it was a "leftover". Cutting it to desired length would have cost me less than the extra meter, but at 5 euro per meter I wasn't gonna be fussy.

Centipede: exiting thought. Up/down articulation would kinda negate the walking action, wouldn't it? Unless the whole body/leg arrangement would be reversed somewhat... Use the TJ linkage system to drive the individual body parts. The segments would touch the ground, not the feet. .... hmmm ... wild brain storm. Maybe for another day.

The idea of up/down/twist articulation would allow it to follow the terrain better like a good suspension system. To be honest, for your long thin design, full articulation would be best with individual drive motors for each segment. It would end up a cross between my 6WD platform and the caterpillar but with the coolest leg movements :-)

Try reinforcing your plywood with fibre glass. You can buy kits with fibre glass sheets and resin that you apply with a brush. Have your fibre glass sheet at 45 degrees to the grain of the wood for maximum strength.

I admit aluminium would be better than ply but if that is what you have then it will have to do.

Right. Right?

It's not so much the availability of materials. I live near a big city and an even bigger city (Internet). I can get my hands on 6 mm iron or alu. It's the required fabrication skills and tools. I do not wish to add to those collections any further, unless the function of the robot requires it.

If I can get metal sawing blades for my jig saw that would cut through 6 mm alu, I could simpy fabricate cranks the way I do right now in wood. Tapped hole for grub screw, flatten the shaft a bit. Fix.

4mm alumiium should do. Wood cutting blades should cut aluminium since it's a soft metal, just go slow and use some lubrication. Water soluble oil is fairly cheap. I've spent 4 years doing aluminium fabrication and we use wood blades in the bandsaw.
I will check out my local supplier to see what variations they sell. I prefer 6 mm because it leave more "flesh" for drilling into the side. I think I still have some blades that are advertised as suitable for soft metals. Like Aluminium.

Wow, this local shop (that is also this webshop) has a very wide assortment. Alu, brass, steel, copper you name it.

Dimensions are also varying greatly. Brass flat bar or "platstaf" 6 mm thick ranges from 10 to 80 mm wide. Alu 6 mm ranges from 15 to 100 mm wide. Plenty choices. Pay per kg.

Would Aluminium be easier for me to machine than brass?