One of the most important problems we had to solve was the obvious question of how do we even stick to a tree. Trees are a challenging environment with irregular structures and uneven surfaces.
Designing a mechanism that reliably works in this environment needs to deal with these difficulties.
We concluded that the robot screwing its “hands” into the bark of the tree is the best option as the design is simple and can hold a surprising amount of weight.
In our development process we developed a few iterations of our mechanism to better understand the requirements and possible downfalls of our design.
Weigth
With only 7mm screwing depth the end effector can already hold torques up to 10Nm. That’s equivalent to holding a 1m long stick with a 1kg weight attached to its end!
This means one of our three “hands” can hold our robot by itself statically.
Number of screws
Why did we decide to use two screws?
Generally the more screws we use the more weight we can hold. But these screws have to be spaced out so they can be connected with a motor. This limits the amount of screws we can reasonably add, especially because the curvature of the tree limits the amount of space we can take, as you can see in the image on a circle with a 15cm diameter.
Contact sensor
We need to know when the screws are in contact with the tree. Otherwise it could happen that we won’t drill into the tree with one or even both screws.
To solve this problem we added a contact sensor into our design. To the left you can see the rotating shaft highlighted. The contact sensor can’t rotate with the shaft as there are cables connected to it. Therefore we added an axial bearing that rotates on the one side and is stationary on the other.