The final design consists of two main components: the rotary actuator and fluid transmission system. The fluid system connects two identical actuators. One converts input rotary motion to linear motion, and the other takes the linear motion from the fluid transmission and converts it back to output rotary motion on a robotic arm joint.
A low-hystersis, low-backlash actuator that converts rotary motion from a motor to linear motion, moving fluid through hoses to a converter that creates rotary motion.
A hybrid hydraulic and pneumatic pressurization system that enables a pair of the transmissions to be used over a three meter span.
The rotary actuator is a cable-driven linear to rotary converter based on the shape and function of a violin bow. This bow central core is housed in a frame that provides structural support and mounting points for bearings and the rolling diaphragm actuators.
The core structure is the moving component of the rotary actuator, functioning similar to a violin bow. The core structure is pushed back and forth by rolling diaphragm actuators. As the core structure translates, the cables affixed to the walls wrap and unwrap along the grooves of the rotary pillar in the center, which rotates the rotary pillar. Vice versa, actuating the rotary pillar tugs on the walls, creating a translation that is transferred to the rolling diaphragm actuators.
Animation of the core structure and rotary pillar moving in tandem
The rotary pillar provides a helical capstan for cables to wrap around, reducing the termination force required to keep the cable from slipping. Capstan grooves consist of a rounded base corresponding to the cable's compressed diameter, with outwards sloping walls to accommodate the cables when they are uncompressed. The complex groove geometry is enabled by printing on a Formlabs Form 3 SLA printer.
On each end of the shaft, an encoder and potentiometer can be mounted for rotary position measurement. The stack includes a high-resolution optical encoder, enabling accurate feedback during operations. The stack also includes an absolute position-sensing potentiometer, enabling automated actuator zeroing and calibration.
The rolling diaphragm actuator is pressurized with either air or water by the fluid transmission system. When a pair of rolling diaphragm actuators are connected together, pushing with a piston on one actuator moves the other piston at the paired actuator, provided that a constant volume and pressure of fluid is kept. This paired motion allows for a mechanical transmission between the input and output rotary actuators.
In our design, additional bleed valves are added to aid in the removal of air when the diaphragms are filled with water. Furthermore, to support the plate frames, additional bracing structures are made part of the actuator housing.
The outer plates provide structure for the entire converter and offer mounting points for the diaphragm actuator housings and rotary bearings. To improve load transfer between the diaphragm actuators and plates, dowel pins are placed in precisely toleranced locating bores.
These plates were designed and simulated to be 3D printed using the Markforged Onyx Pro printer, which can place fiberglass inlays along loading axes to strengthen the plates. Even without this additional enhancement, the outer plates allow for safe operation of the converter at system pressures of up to 1,034 kPa.
The fluid transmission system is a hybrid system consisting of two fluid lines under equal pressure, connecting the rotary actuators together. The hydraulic line uses water to maintain an incompressible link between the actuators, and the pneumatic line pressurizes the entire hybrid system to ensure a stiff and reliable connection.
Water level control is very important, as it ensures the input and output actuators do not go out of phase from one another, which would result in incorrect and possibly unsafe robotic arm control. A potentiometer is included on each actuator to monitor if the rotary actuators start to move out of phase. In such a case, the solenoid valves can be triggered as part of the water refill sequence, until the potentiometer readings align again.
An electric proportional pressure regulator sets the entire system to any pressure to a maximum of 1000 kPa. The system generally is preloaded at 700-850 kPa. This creates a constant pressure on the rolling diaphragm actuators, which transfers a force across the bow wall structure to the opposing water-side rolling diaphragm actuator, leading to an equal pressure in the water line. Having the ability to change the system pressure is ideal, because the pressure of the transmission lines determines the responsiveness of the output actuator to the input actuator’s movements.
An Arduino microcontroller is the brain of the automatic pressurization system, controlling the pump, solenoid valves, pressure regulator, and reading inputs from the potentiometers. The entire setup and pressurization process is guided through a UI developed in the Arduino serial, which can be accessed on a computer connected through USB.
The automated system sets up new diaphragms, pressurizes the system for standard operation, phases the actuators to always be aligned, and shuts down the system safely. Each state guides the user through various system options and allows for user input when necessary.
Because the Arduino runs at 5V with very low currents, system electrical components were controlled by Arduino through various methods as shown in the image on the right.
Our Objectives
Enable a transmission distance of 3 meters between motors and CT/MRI area
Smooth motion, low backlash, low hysteresis
Provide at least 90º of rotational motion
Manufacturable through standard shop tools and 3D printing
Automated pressurization system
Our Prototype's Performance
Successful transmission over a distance of over 3.5 meters
Backlash amount less than 6 Arc-Minutes,
Lower friction compared to sponsor's previously used transmission
Rotational motion range of 300º
Actuators successfully manufactured and assembled within sponsor lab
Automated pressurization system successfully enables system initialization, water filling with air purging process, actuator calibration routine, and more
Using the learnings from the last physical iteration we were able to produce, a final iteration of the design was made, which improves upon critical areas of the hybrid-fluid actuator.
Larger carbon fiber tubes in the core structure for improved load capability
Balanced and strengthened core walls place the Center of Mass along the actuation axis, preventing unwanted torques to the core
Strengthened diaphragm actuator bushing sleeves
Tolerancing improvements for slot-in hex nuts and dowel pin depths
Larger clearance bores in the outer plates and bearing mounts to accommodate the central rotary pillar and future cable termination methods
The rotary pillar is centered on the outer plate, allowing for a symmetrical plate design and equal load distribution
The outer plate is designed to be waterjet or laser cut from an 1/8" 7075-T6 aluminum plate - dramatically improving load capability and maximum system operating pressure
Independent mounting legs, which can be designed for any application