Quadruped robots are versatile locomotion platforms capable of a variety of manipulation tasks. For each operating environment, the quadruped's choice of kinematic design, actuators, and end-effectors requires tuning and optimization. This research introduces geometrically enhanced designs for limb mechanisms and magnetic variable stiffness end-effectors for a ship-board quadruped platform, SORREL.
The choice of kinematics in quadrupeds is highly dependent on the gait and certain leg designs might be more prone to waste power. Traditional robots, design their kinematics based on the actuator being a single unit consisting of a motor and a gearbox, overlooking the effects of their relative positions on the limb kinematics. A new method was developed for designing the leg kinematics by decoupling the design of the gearbox and the motor. This method gives more freedom in the design of the kinematics and allows for a wider range of configurations best fit for the required task. The quadruped leg design presented in this work has both actuators collocated at the hip in a parallel configuration; however, the gearbox of the knee motor is relocated from the hip to the knee joint. This seemingly simple modification significantly reduces the mechanical antagonism and motor losses and increases the stiffness of the leg.
Quadrupeds perform manipulation and grasping through task-specific actuators. These actuators need to interact with the environment safely while maintaining accurate force control capabilities. More specifically, SORELL needs to be equipped with gripper modules to deal with varying tasks such as typing on keyboards, precision manipulation of electronics, and general pick and place. Most grippers used in manipulators are based on stiff positioned-controlled actuators which provide accuracy but require bulky and expensive Force/Torque (FT) sensors for force control. Additionally, traditional grippers struggle with delicate handling tasks and pose safety risks due to their inherent high stiffness. By employing compliant actuators, namely nonlinear series elastic and variable stiffness actuators, robotic platforms can circumvent such limitations while maintaining the accuracy and stability of the end-effector.
As such, a family of actuators based on magnetic and electromagnetic forces was developed and tested to improve the safety of manipulation and grant accurate force control. The actuators presented use series elastic (SEA) and variable stiffness (VSA) concepts. Two actuators use passive nonlinear springs with constant stiffness profiles based on permanent magnet arrangements. These passive nonlinear SEAs, designed for lightweight and low-cost grippers, can achieve both stiff and soft behavior and have a higher capacity for energy storage than linear springs. With accurate spring modeling, these nonlinear SEAs can change the force control problem into a position control problem avoiding the use of FT sensors. Since passive SEAs use springs with fixed stiffness curves, their stiffness cannot be controlled independently. Moreover, changing the stiffness online is desirable in some grasping tasks that require adjustment in grip strength and compliance. Hence, two actuators that have active stiffness variation (VSA) capabilities through mechanical and electromagnetic means were developed. These actuators, while more complex and costly, give a wider range of stiffness and the ability to tune their stiffness curves. These actuators are studied and categorized for the intended manipulation tasks encompassing those required on the SORELL platform. Through these designs, multiple unique features were studied and observed such as water-cooled electromagnetic coils, eddy current damping for shock absorption, and the use of negative stiffness for fast explosive motion of gripper fingers.
