Need Help Designing Mechanism for Automatic Spring Return

[Thom Stoops]

I'm trying to design an entirely mechanical system for returning an extension spring to its extended state, without the assistance of electronics, pneumatics, hydraulics or any other form of auxiliary power. It's for a soft exosuit concept that simply enhances the speed and ease of walking or running for the user. Two cables are attached to the wearer's legs just below the knee, and as each foot hits the ground I need the extended spring to release, contract, and extend again within about a 1to 2 second timeframe. I have the springs housed in PVC tubing and mounted to a backpack frame, and what I need to know is how to return the extension springs to their extended state after being released without using any kind of auxiliary power, such as hydraulic pistons or linear actuators. (However, if there's legitimately no way to do this then I'll gladly introduce something else into the mechanism - I'm just trying to do this without those things.)

I'm sure I sound like an amateur to you folks, but I'm just a guy building dang in his basement. Any and all help is appreciated.

 

[Bystander]

The user stretches the spring (extra work for user), then the spring relaxes? Or, the stretched spring kicks the users leg out straight, decouples for the user to make the next move, and the spring recocks itself for free?

 

[Baluncore]

Springs will store energy and can then release it again. I think you need to analyse the energy flow in the whole system over a full cycle. You may find that the energy required is not available at that point in the cycle.

 

[Thom Stoops]

user stretches the spring (extra work for user), then the spring relaxes? Or, the stretched spring kicks the users leg out straight, decouples for the user to make the next move, and the spring recocks itself for free?

The spring is held in an extended state by a trigger for the duration of the user's step up to the point where they would push off the ground to propel themselves forward. At this point a switch is activated via a wire connected to the leg cable which releases the spring, pulling back the leg at the same time as they take the step. This multiplies the force with which their leg pushes off the ground, thus propelling them faster. It's all a question of how to return the spring back to the trigger for another cycle.

 

[Baluncore]

It's all a question of how to return the spring back to the trigger for another cycle.

Where does the energy for that return come from?

 

[Thom Stoops]

Where does the energy for that return come from?

I believe you are correct. I have analysed energy flow throughout the system and have found that there is no energy available at this point in the cycle, and that I'll have to use a secondary form of power to recock the springs. What would you recommend for this application? Preferably something light, cost-effective, and efficient in many climates. Much thanks.