Polymorph Caterpillar

By OddBot
@ March 1, 2009

• Actuators / output devices: 8x hextronic HXT 900 miniture servos
• Control method: Autonomous and IR remote
• CPU: Picaxe 28X-1
• Power source: 4x AAA NiMh 800mAH
• Programming language: Picaxe basic
• Sensors / input devices: Sharp IR GP2D12, IR reciever
• Target environment: Anywhere with easy to medium terrain

I'm trying to make a decent caterpillar style robot without a lot of mechanical complexity. I've had this idea ever since suggesting it to Thomas Countz who then made a carboard caterpillar.

This is the video of the caterpillar now that it's finished.

I've moved the processor to the centre to correct the

weight distribution problem.

The second video shows him in obstacle avoidance mode. No reverse yet!

The first video is my first attempt at moving straight ahead.

It isn't moving forward as much as I'd like.

Hopefully I will get it working properly this weekend.

To keep it simple the segments are made from a single polymorph bracket that joins two servos at 90 degrees. If you don't have polymorph then 3mm thick balsa wood sheet would be a good substitute. This is the pattern I used, the grey teardrop shape shows where the servohorn attaches.

After working out the shape of the bracket, I then printed it out with a laser printer and experimented with using that as an iron on transfer since both the toner and polymorph melt at a reasonably low temperature. I had the iron at its hottest temperature so the toner would melt as quick as possible. This meant I didn't have to keep it on the polymorph too long, just a few steady passes with the iron.

The sheet buckled slightly from the heat. I placed it between some books to keep it flat until it cooled. The paper stuck a little bit because I acidently melted the polymorph a bit but the pattern transfered well. By soaking in cold water and using a soft plastic scrubbing brush I removed the remaining paper.

I drilled 5mm holes where the servo horn screws into the servo and cut out the brackets with scissors and a sharp knife. I then bent the bracket as demonstrated in this walkthrough.

After mounting the servos and joining the segments together the design starts to make sense. The segments can pivot left and right for steering and flex upand down for locomotion.

This is his brain. The picaxe 20M has eight fixed outputs and eight fixed inputs. It's pins cannot be reconfigured and it has the same limited amount of memory that the 08M and 14M have. I may upgrade to a 28X1 later.

The rest of my servos finally arrived!

After mounting all the servos and battery holders I had a mess of wires. To tidy them up I used spiral binding. You can make your own by putting a plastic drinking straw through a pencil sharpener but it won't be as strong because drinking straws are made of thinner plastic.

After watching a video of a caterpillar walking I decided he needed feet.

I've posted a second video that shows the caterpillar in avoidance mode. He turns on the spot yet looks like a stretch limo doing a 30 point turn :) 

As pointed out by others, the head is too heavy. This is because of all the cables coonecting to the brain. I'm now going to rebuild him with the brain at the back.

I'll also upgrade to a 28X1 because I need more memory. This will allow other features to be added such as self correction if he rolls over.

I'm calling this project complete. I've changed to a picaxe 28X1 for the extra memory. With the processor mid-mounted I've eliminated 80cm of servo extension cable and the weight distribution problem. 

This is a 3D render of the caterpillar from DAGU electronics who are mass producing it.

I believe you will be able to buy it as just a body with servos allowing you to add your own sensors and processor. Later there will be picaxe and atmega8 versions available. I will make schematics and code available for those that want it.