Haptic Hydrostatic Transmission
Our goal was to create a transmission and actuator system which would have applications in haptics and human-robot interaction.
When designing a robot to interact with humans it is advantageous to keep the mass of the arms as low as possible, to this end we designed a low friction, light weight fluid transmission system which has better torque density than an electric motor but suffers from less friction, and leaking, then traditional hydraulic or pneumatic systems.
The transmission system is based on rolling diaphragm actuators. The seal, or diaphragm, in these actuators inverts on itself and rolls up the piston shaft as the actuator moves. This has almost no friction, especially when compared to seals in traditional pistons which must slide along a moving surface. In addition, the rolling diaphragm seal is much less susceptible to leaks.
Because rolling diaphragm actuators can only apply force in one direction I prototyped a system where the actuators were mounted in antagonist pairs so the extension of one actuator would provide a restoring force to the other while simultaneously driving a shaft.
This setup provides a maximum torque of 2.7 N-m across a 75 degree rotation. Each antagonistic pair has a mass of 246 g when water is used as the internal fluid. Using air reduces the mass further.
I was interested in exploring the design of vacuum based rolling diaphragm actuators that operated by contracting. This is in contrast to standard diaphragm actuators which expand under a positive internal pressure.
The actuators I created can exert a limited force but, because they operate in tension, they are self-aligning and require no support structure. With a body of soft vinyl each unit weighs only 25 g. In addition, they can readily be stacked to make flexible muscles which have interesting applications in wearable robotics and haptics.
The transmission system was implemented in an interactive telepresence robot that provided haptic feedback to the user. The arms proved to be compliant, responsive, and were capable of doing tasks such as playing xylophone or moving a egg back and forth with a child.