TEAM MEMBERS
Musa Erdem Sevim
Irem Gürdal
Our dynamics term project is Theo Jansen mechanism and in this paper, we are going to explain the procedure of its manufacture, the analysis we fulfilled before and after the manufacturing part and the conclusion that we gathered from the comparison we made between these analyses. In order to be able to begin the project, we made some research about the bases and creation of our project.
Theo Jansen, the inventor of the mechanism, is a Dutch artist and kinetic sculptor. In 1990, he put forth his creation, which made him known by lots of people. He built a mechanism out of PVC that has the ability to move on its own. This mechanism is a collection of basic devices and it offers the kind of mobility that wheels may struggle with.Theo Jansen mechanism is composed of different linkages and it resembles the skeletons of the animals. In this mechanism, bars are arranged to convert the rotational motion to a specific walking pattern. The qualifications of the mechanism seem to fulfill the optimized needs sought by legged robotics researchers. The scales for these qualifications can be considered as the energy efficiency, low payload, bio-inspired locomotion etc.
Theo Jansen formulated the optimized lengths of the mechanism with combining Descartes’ coordinates and Lagrange’s equation of motion. Via this procedure, the niche mechanism was created. Various parameters were set before creation to actualize the perfection of the movement of the mechanism such as; inertia moment, mass, viscous friction coefficient, various lengths and angles. The movement of the four-legged mechanism can be actuated with only one motor or wind power; minimizing the consumption of the energy sources. The efficiency is considerably high.
Inspecting such features of the mechanism that we planned to create, we gained a clear opinion on how to plan the scheme of our project. After this planning and research period, we recognized that our motivations were out of focus as we were not aware of the importance of the kinematic equations that we had to solve and that this was the major part of the project. Therefore, we gave the put the emphasis on performing the necessary calculations before manufacturing the mechanism. Due to these calculations we manufactured the mechanism and continued with gathering the experimental values of the calculations we performed before. The details of this procedure will be clearly explicated through this paper.
Manufacturing
First of all, we tried to understand the working principle of the machine that we are going to design and manufacture. In the beginning, we watched some videos of Theo Jansen which are explanation of the main idea and the components of the mechanism prepared by him. He has lots of conferences and TV programs in which he explains the concept and the aim of these machines.
To produce the mechanism, we needed some lengths and information about the structure of the rods. There were some distinct ratios that Theo Jansen and his team discovered with the long computer aided calculations. Idea behind the exact lengths was the plot that one leg was doing on the ground. This path has to be as far as horizontal at the bottom level of the graph and it should make a smooth, long flow to give the perfect motion to the machine.
The graph that they found had a shape like a wing of an airplane. It had a straight line at the bottom and makes a soft curve on the top while the leg is coming forward to prepare to the next step. We determined those lengths from paper those we found on internet. We had a brain storm on the material that we are going to use on the manufacturing. There were three options; wood, Plexiglas and PVC pipes. We eliminated the wood because we need certain lengths and proper design to have better result. Then we run over the PVC. However we realized that we were going to demonstrate the mechanism on a small presentation field and we are not able to find thin pipes. Also the precision of the lengths would not be good enough for a mechanism which is going to be walk.
At the end we decided to use Plexiglas. It had some benefits such as we had a laser cutter on the machine shop, so our CAD designs will directly be implemented to the material. Also it is light and the probability of the occurrence of defects is less than any other production material.
After determining the material, we performed the CAD design according to the 5 mm thick Plexiglas pieces. The lengths of the rods were given with rates which were the best to show as paperwork. So we doubled the lengths to manufacture them easier because we were planning to use Plexiglas and with that material, we were not able to produce a strong skeleton of a mechanism under the thickness of 5 mm.
We used NX for CAD designs of the parts of mechanism. Creating the parts were easy but assembly and giving motion was harder.
After producing pieces on laser cutter, we found that the pins are moving freely inside of their holes on Plexiglas. To reduce this vibration, we wrap Teflon(n-CF4) to the pins. Reason of this error was the idea that we had: we thought we should make the holes on 6.1 mm diameter to leave free space to the pins (6mm diameter) turn freely. But while they are turning freely, they started to lose their function of carrying the weight of the machine.
We also used a motor that has two shafts on opposite directions. This saved us to use one motor for each leg pair. We firstly produce one mechanism with 4 legs, 1 motor and 1 battery but this was not able to carry itself and it had some balance problems. To reduce this problem we designed new pieces to place to the edges of the each leg but it did not solve the problem. Because this time apparatus take totally wrong position and they create a jam on the system. Although our designs on the PC were working, our real model did not and this is one of the problems that we may face in our daily lives as mechanical engineers.
One of the biggest reasons on this problem was nuts and pins that we cut with wire saw. They were not well shaped and they were not on exact lengths that we wanted, so they cause jam while another rod is passing through them. We used aluminum supports to fill some empty spaces on the long rods which are fixing the body and connecting legs to reduce these jams.
After correction of those mistakes, mechanism worked on the air and it seems like it is working properly but whenever we leave it on its legs, it loses its structure and joints took positions which they should not be found. This unbalance locks the system and legs falls to the ground.
We decided to double the number of legs to increase the balance. After adding those legs, balance problem removed but our extra apparatus on each leg where failing during the motion because they were not well designed and they were not tested on the computer. We removed those pieces and mechanism is now walking on its own legs.