(A) switchable stiffness hydrostatic transmission for safe physical human-robot interaction

Abstract

Collaborative robotic arms must be safe in any physical contact as well as have high control performance to be successfully introduced to our everyday lives. Hydrostatic transmission, using incompressible fluid to transmit the force, enables the manipulator to achieve high backdrivability, low mass and inertia, high stiffness, and high energy capacity. However, it could not provide inherent safety due to the incompressibility. Thus, it would be advantageous to combine series compliance on the hydrostatic transmission depending on the situation.

Here, we propose a switchable stiffness hydrostatic transmission implemented with an air spring and solenoid valve. Air spring is serially connected to hydraulic lines for achieving compliance. Its modes (i.e., stiff and compliant modes) can be switched by modulating water flow via the solenoid valve. Rolling diaphragms provide leak-free operation and low seal friction. We experimentally validated that the stiff mode is 9.63 times stiffer than the compliant mode at 100 kPa, and the compliant mode could mitigate the impact force to the level that a tangerine would not be crushed. Since the stiffness switching time is within 12 ms, the proposed mechanism is rapid enough for feedback applications with vision or tactile sensors.

We develop a two-link planar manipulator based on the proposed transmission as an application for a robot manipulator. Experimental results exhibit the possibility of the proposed mechanism to be applied for a human-friendly manipulator. The developed manipulator is identified as two decoupled 4th-order linear models by the sine wave of 1 Hz. Trajectory tracking experiments show that root mean square errors are 0.65 and 0.53 deg for stiff mode and 0.67 and 0.59 deg for compliant mode. Therefore, the developed system can ensure safety with less compromising control performance, owing to the simple and rapidly switchable stiffness mechanism.