Researchers at the National University of Singapore have developed a soft force sensor that converts physical pressure directly into fluid-driven robotic movement without electronic processing or an external power source. The device, known as the mechanical soft force sensor, or ME-SOFS, is designed to create a direct sensing-to-action loop. When force is applied, the sensor produces fluid flow that can drive connected actuators, removing the need for separate electronic sensors, signal-processing circuits and computer-controlled components.
The research was led by Benjamin Tee, a professor in the Department of Materials Science and Engineering, and Cecilia Laschi, a professor in the Department of Mechanical Engineering, at the university’s College of Design and Engineering. The findings were published in the peer-reviewed journal Science Advances.
Soft robots are constructed from flexible materials and are being developed for applications including minimally invasive surgery, human-machine interaction and underwater operations. Most existing systems depend on electronic components to detect forces, process signals and control movement. These components can add weight and complexity and may be vulnerable to water, heat, pressure and electromagnetic interference.
ME-SOFS consists of a three-dimensional-printed porous structure with a central pillar connected to five fluid-filled chambers. Four chambers are arranged horizontally, while the fifth is positioned vertically.
When pressure is applied, the pillar tilts and compresses the corresponding chambers. The displaced fluid travels through soft tubing to actuators elsewhere in the system. Because the chambers respond independently, the sensor can distinguish forces applied along horizontal, lateral and vertical axes.
The researchers also incorporated a passive mechanism that produces an electrical signal without requiring a powered circuit. Fluid displaced by an applied force moves small magnets past three-dimensional-printed metal arcs, generating voltage pulses through changes in magnetic flux. The number of pulses corresponds to the magnitude of the applied force.
The sensor’s sensitivity can be adjusted by modifying geometric features in its printing design, including hole diameter and the thickness and angle of the central structure. This allows versions of the device to be configured for different force ranges and applications.
The research team tested ME-SOFS in several robotic systems. One demonstration involved a soft glove containing five miniaturised sensors, each approximately the size of a green pea. The glove was printed as a single structure from one material without manual assembly.
When worn by a user, the glove measured grasping forces at each fingertip and was used to estimate the weight of held objects. The researchers identified prosthetics and human-machine interfaces as potential areas of application.
In another test, the sensor was connected to a soft haptic pad worn on a person’s fingertips. An operator with covered eyes used tactile feedback to control the grasp of a robotic arm. Force detected at the robot’s gripper was transmitted through fluid pressure to the pad, allowing the operator to feel changes in grip strength while handling objects including an egg, wooden blocks and a partially filled water bottle.
The force signals from a successful grasp were subsequently replayed to train the robot to reproduce the movement autonomously. The demonstration showed that the system could transmit tactile information in real time while also recording data for robotic training.
The researchers also used the mechanical sensing-to-action system to direct individual liquid droplets through a miniature fluid controller without software. Such fluid control could be applied in portable diagnostic equipment. A separate demonstration used the sensor to control flexible, hair-like structures that bent according to the direction and strength of an applied force.
Testing showed that ME-SOFS continued to operate in water heated to 90 degrees Celsius and under pressure comparable to a water depth of about 11 metres. Its open fluidic channels equalise with surrounding water pressure, allowing the sensor to measure applied forces rather than changes in ambient pressure.
Because the core sensing and actuation process does not use electronic components, the device is also resistant to electromagnetic interference.
The researchers plan to investigate further applications and explore changes in the system’s size and actuation force. They are also studying the additional information contained in the sensor’s dynamic signals, beyond the number of voltage pulses, to support more detailed force measurements in robotic interactions.
Potential uses identified by the team include medical training systems that reproduce force feedback, robotic assistance for elderly people and soft robots that need to react quickly when interacting with humans. In these applications, the direct mechanical connection between sensing and movement could allow a robot to respond to changes in force without waiting for a separate electronic control system.
Photo Credit: College of Design Engineering at NUS
