How can I implement a spring-back mechanism for robotic fingers in a servo-controlled hand?

Question

I'm working on designing a servo-controlled robotic hand, which I'm creating out of cardboard (Arduino based). The front of my finger has a fishing line attached to it, and the finger is pulled down by a pulley, which is turned by a servo motor. To fully close the finger, the servo motor is turned 180 degrees, which moves the pulley, and snaps the finger closed (180 degree position).

The problem I am facing is that once pulled down, the finger obviously won't automatically snap back up to restart to the 0 degree position (fully opened). I have used tried many strategies to try and create tension at the back of each finger, but none of them are effectively snapping the finger back to 0 degrees from any other intermediate position (1-180 degrees).

Attempts so far...

1. Elastic band - Placed at the back of the finger to provide tension, but I found that they snap easily and can also lose their elasticity very quickly. I've used a variety of elastics, yet none of them seem to work quite right.
2. Wider elastic bands - Placed at the back of the finger as well. These were really good since they were supportive, but the finger didn't function the way I wanted to. It had a lot of tension (which is alright), but in a lot of the cases, something would break. It wasn't very reliable.
3. DUAL fishing line with TWO pulleys - This was much a much more promising approach, where I had two fishing lines, one on either side of the finger (front and back). The front fishing line would be connected to a smaller pulley, while the back line would be connected to the larger one. Both pulleys would be connected side by side and controlled by the same servo. They were placed in such a way that when the front line was pulled, the back line would be loosened and vice versa, allowing the finger to move to a closed and open position, respectively. This seemed like the most reliable solution, but it wasn't able to fully close the finger (move to 180 degrees). If there's any solution to this, please let me know.

Below, I have provided a few images of my hand to provide you with more information to help me solve this issue. Please note: In the pictures I have shown SG90 servos, but I also have higher torque MG996R servos, which is what I am using now for this project.

Thank you for all your help. Please let me know if there is any additional information that is required.

Answer 1

Too bad the dual-pulley method wasn't quite a solution. As long as you are using springs, the servos will have to fight them. Even so, here are a couple ideas.

• To pull the backs of the fingers, instead of elastic bands, you could use helical tension springs. Perhaps mounted on the arm, connected to the back of the finger with fishing line. Depending on friction and other forces, even a ball-point pen clicker spring might work. Springs will fully extend the fingers better if they are under some tension even at full extension.
• To make the fingers want to straighten out by themselves, maybe try springs right on the backs of the fingers. You might be lucky enough to find bendy flat springs with the right strength. Or maybe twisty helical (like clothes clips) at each joint. Or maybe helical, but bending the axis instead of stretching or twisting.

BTW, it looks like you are doing remarkably successful work with corrugated cardboard. If your budget and supplies admit, maybe also try foamcore, although that needs a knife and not scissors.

I love the project. Good luck!