We have developed a wearable device that utilizes knee joint movement of a normal walking gait in order to generate electrical energy. The device was designed so it will require minimum effort from the user and can be set as a base for an Exoskeleton, we believe that a design of an efficient harvester can be set as a base to an efficient exoskeleton.

At level walking approximately 80% of the net knee joint work is negative. In phases of negative work, the muscles act as breaks and dissipate energy. The goal of the harvesting device is to replace part of the muscle work and generate electrical energy while doing it. This energy could then be stored in a battery and used further for portable devices such as GPS, laptops, etc. The harvesting device could be useful in remote places where electrical energy is needed, replacing the need for carrying batteries.

An analytical models of the device was developed;​ the model includes the generator harvesting torque as well as dynamical equations of the system, a model of possible harvest power and a model of the human metabolic consumption. Using the models, we solved an optimization problem in order to determine the optimal gear ratio, optimal generator, and optimal breaking profile that will cause a minimum effort to the user with maximum harvesting power.

In order to harvest energy only while the muscles are breaking with the desired optimal profile, we used a micro-controller that reads the knee joint angle to determine the proper harvesting timing with the appropriate instantaneous breaking torque level. Experimental results gave us some insights about the optimal harvest profile were was able to produce 6.3 Watts with a minimal COHt of 2.2. The experiments we held also helped us to define a better control system for the harvester and exiting new way of calculating the torque in the leg joints.

Our main contributions are:​

Optimization

To enable optimization we develop and defined the following:​ 1) Developed a conceptual design;​ 2) Defined the optimization cost function as the Total Cost of Harvesting (COHt). This parameter includes the changes in metabolic rate due to the device mass, location and the harvested energy to enable calculating COHt three models where developed (3,4 and 5);​ 3) a model to predicate the change in metabolic power as a function of the mass, it location and the device torque. 4) A model describing the dynamics of electromechanical systems and predicts the device torque profile;​ 5) A model for the electrical power that is generated.

The device

In addtion to the optimztion more step where prforemed 6) Built the harvesting device and the harvesting controller;​ 8) Designed a harvesting algorithm that decides when to preform energy harvest and it what level;​ 9)Validated our models by conducting experimentsw;​ 10) developed closed loop referance and plant model for the harvesting control;​ 11) Defined the equations for real time joints torque measurments using IMUs and error estimation of it.