This project was initially inspired by learning how spider legs move. Relatedly, have you ever wondered why spiders curl up when they die? Let me explain...

For starters, think about how humans move our arms and legs. Each limb, simply put, is moved by two contradicting groups of muscles. To bend in one direction, one muscle group relaxes while the other contracts. To move the other way, the opposite happens. 

Spiders don't do that. At each leg joint, spiders have flexors that bend the leg inward. To extend a leg, the spider pushes fluid from its abdomen to specialized sacks inside the joints. As these sacks expand, they push against the leg and extend that segment. An engineering analogy would be an arm that uses hydraulics to extend and springs to contract.

See this in action and how Rice University is using spiders for actuators

What does this mean for the robot?

Loosely modeling real spiders, I plan to put torsion springs in each leg joint for my robot. They will help offset the robot's weight so the motor actuators won't work hard while standing. To extend the leg, the actuators will need to overcome the spring force; however, this moment doesn't need to be rapid or particularly strong. Conversely, having the spring pressure downward means that the actuators will get assistance while pushing down. This should help the spider move quickly and possibly jump. It will also let me use smaller motors with less torque.

I created a simple test platform to test the idea of springs supporting weight.