COW arm

For this year's Winter Camp, Space is the theme. Therefore, after upgrading my electric wheelchair robot base this spring with |a new motor controller, I decided that it should also have a robotic arm, making it useful as a Mars rover - esque robot platform. The wheelchair base weights a hefty 100+ pounds, and I want this arm to be actually useful (ie, able to lift more than a tissue box). I have named it the Cow arm, as it is designed to be beefier than those crummy kit arms that use servos.

Upper and Lower Arm

I designed this arm after seeing this Hackaday.io project which uses linear actuators to provide forces along the length of the arm, much like the muscles in our own arm. Many arm designs I have seen use very high torque gearmotors in the arm joints to provide torque on the arm, but I don't have the ability to make one and they are astronomically expensive. Meanwhile, these linear actuators cost just 10s of dollars. I picked up a couple for testing, and if they work, they should take care of the largest force sections of the arm straight away.

First design of the arm in GeoGebra. The blue lines are the linear actuators, which I can change the length of with the sliders. The red lines represent the arm segments themselves. The green line is just a distance measurement. Also note the grey vectors placed to help estimate what the max load of the arm will be.

I also began to design the geometry of the arm; for this I have used a free program called GeoGebra, which I had remembered from a class I took. This program is essentially the electronic version of a paper, compass and straightedge. More importantly, objects can be changed - for example, I can change the diameter of a circle - and the rest of the lines will instantly update themselves to the new position. I can even attach the size of a circle to a slider, and drag it around to see what changing the size does in real time. It doesn't have all the constraint features of a CAD package, but the dynamic updating ability is really powerful.

I constructed the basic arm design I wanted to use, and then adjusted the joints of the linear actuators of the arm around until I got the range of the arm about how I wanted. Also, GeoGebra supports vectors, so by using the maximum force listed by the linear actuators and some vector projection, I could estimate the max force I might expect to pick up at the end of the arm. Obviously, this varies depending on how far the arm is extended, just like our own arms. Neglecting the weight of the arm itself, I was quite impressed to realize the arm might be able to pick up 10-30 pounds quite easily.