Design of Regenerative Servo for Prosthetic Applications

Nika Zolfaghari, Jung Soo Kim, Fakhreddine Taleb
supervised by Dr. James Smith

This project is centered on the improvement of the technology and energy efficiency of prosthetics. Prosthetics help improve the quality of life of many disabled individuals. However, they are not without their setbacks. One disadvantage of current prosthetics is power deficiency. Since the prosthetic has to be mobile and therefore light weight, the required batteries cannot provide a sufficient amount of power for long durations of time due to their low energy density; they need to be recharged approximately every six hours. Our objective is to design a prosthetic limb with compact servo motors, which combine compliance and four-quadrant regenerative operation to minimize energy usage and, thus, maximizing battery life, in turn, optimizing the efficiency of the device.

In order to meet our goals, a study of the movement of an arm was conducted in the horizontal and vertical planes. To follow, a design had to be built in order to test these movements and achieve the end goal. The said design is composed of two servo motors, where the specific built-in controller for each motor was taken out in order to implement our own controller; although the design is limited to provide only two degrees of freedom, whereas an actual arm has more than six degrees of freedom, the model provides insight into how the motion is generated through motors and allows for the observation of regeneration within the motions. The design uses MicroRax to link the motors together in a composition similar to the bones of an adult human arm. Our first model was built to move along the horizontal plane since it is in this configuration where minimal gravitational forces are experienced. After sufficient observation and study of its behaviour, our model was designed to present its motion in the vertical plane; experiencing the maximum gravitational forces as a final testing phase.

The motion of arm is controlled and directed by a PID controller built on a microcontroller to drive an H-bridge based motor shield. First, the trajectory of motion was based on simple bang-bang control. As the project developed, the trajectory of the arm has evolved to be based on cycloidal motion to allow for smoother motion of the arm. In addition, to observe the current flow of the motor, and the voltage that is being supplied to the motor, two testing boxes were built, one for each motor where each contained a circuit to test voltage, and another to test current. When working towards designing a complex robotic device, such as a prosthetic arm, it becomes crucial that test simulations are run. By performing accurate simulations, the outcome and expected results can be viewed and tested before the actual hardware components are assembled. If proper simulations are not performed, valuable resources will be wasted; the cost of the project will drive up as testing will have to be performed on the hardware itself, which in turn will take up time resources that would otherwise be more effectively spent elsewhere. In the case of the prosthetic arm, before the hardware components were put together, the required forces and torques that will be acting on the arm to produce expected outcomes were first determined by the process of simulation in MapleSim 5.

The values of the simulated PD controller were then manipulated in the software layer and programmed onto the microcontroller in order to run the motor and achieve the desired regenerative outputs. Once the motor performs successfully, specific parameters such as voltage, current, potentiometer values, position, velocity, and acceleration are be determined, and a software analysis takes place between the simulations and experimental results to evaluate the success of the project.

Project targeted applications: