Thought Leaders

The Development of a Magnetic Skin for Super-resolution Tactile Sensing

Thought LeadersYajing Shen, Ph.D.Associate Professor, Department of Biomedical Engineering

City University of Hong Kong
AZoSensors speaks with Dr. Yajing Shen, Associate Professor at CityU’s Department of Biomedical Engineering (BME). Dr. Shen is part of a team comprised of researchers from the City University of Hong Kong (CityU) and the University of Hong Kong (HKU), that developed a novel tactile sensor with several characteristics matching those of natural skin. Dr. Shen was one of the co-leaders of the study, the findings of which have been recently published in the scientific journal Science Robotics, titled “Soft magnetic skin for super-resolution tactile sensing with force self-decoupling.”

Can you give our readers more information about your recent research?

Over the past decades, robots have revolutionized modern life in many aspects, from industrial manufacturing to medical service, education and transportation. The prevalence trend of robotics in everyday life presents new challenges to the sensory systems of current robots. One of the great challenges for domestic robots' advancement is to develop artificial tactile sensing comparable to human skin.

Human skin can sense subtle changes of both normal and shear forces (self-decoupled), as well as perceive the stimulus detail finer than the average spacing between mechanoreceptors (super-resolved).

By contrast, the existing tactile sensors for robots are relatively inferior, lacking accurate force decoupling and proper spatial resolution at the same time. This has a negative impact on the robots’ performance in many tasks like dexterous grasping and manipulation. This research addressed the above challenge by developing a sinusoidally magnetized flexible magnetic film (with the thickness ~0.5 mm). We achieved the skin-comparable self-decoupling and super-resolution abilities for the first time, and successfully conducted several challenging tasks like stably grasping fragile objects under external disturbance and threading a needle via teleoperation.

What tasks can the sensor help users perform?

The sensor can enable the implementation of several challenging tasks in robotics, such as contact position tracking, reliable grasping with external disturbance and teleoperated needle threading.

What do you think are the benefits of combining sensors and robotics?

The advancement of soft artificial tactile sensors with skin-comparable characteristics could make domestic robots part of our everyday lives possible. The tactile feedback can enhance the safety of the human-robot interaction and enable robots to accomplish challenging daily tasks like dexterous grasping and manipulation that involve both normal and shear forces. Imagine there is a magnetic skin (or tactile sensor) covering a domestic robot (like a health-care robot), when the robot contacts a person or an object, the tactile sensor can sense the contact force immediately and provide the force feedback to the robot. If the contact force is larger than a pre-defined safety threshold, the robot will stop moving so that the person (or object) will not be injured (or damaged).

What insight does this research provide for tactile sensor design?

Compared to conventional force decoupling methods that are complex either in sensor structure or mathematical model, our method is simple in both aspects, thus providing a new design strategy for self-decoupled tactile sensing. Other researchers could develop more powerful tactile sensors using such a self-decoupled design strategy and develop more efficient tactile super-resolution algorithms built on our methods.

What applications could this sensor be used for in the future?

The proposed soft magnetic skin can be used in various applications in the robotics field, such as dexterous grasping and manipulation, texture recognition, smart prosthetics, human-robot interaction, etc.

Robotic hand with the new sensor completes challenging tasks

Can you tell us more about the R&D that was involved when developing the sensor?

The key component of the proposed sensor is the flexible magnetic film. During our research, we found that a sinusoidal magnetization pattern of the magnetic film could result in two naturally decoupled components under the film, in terms of the magnetic strength and the magnetic direction. Taking advantage of this special property, we developed the self-decoupled magnetic skin that can accurately measure normal and shear forces with a simple structure.

How were the characteristics of human skin taken into consideration when developing the use of the sensor? What benefits can these characteristics provide?

Human hands are amazingly skilled at recognizing texture and handling objects with different shapes and sizes. One of the main reasons behind such fine operation abilities is that human skin can sense subtle changes of both normal and shear forces (i.e., self-decoupled) and perceive stimuli with finer resolution than the average spacing between mechanoreceptors (i.e., super-resolved).

To develop robotic skin that is comparable to human skin, we achieved the above two characteristics simultaneously. To be specific, the force self-decoupling ability of the sensor can provide accurate force feedback to the robot, which enables the robot to accomplish challenging tasks that human hands are good at, such as dexterous grasping and manipulation, etc. At the same time, the tactile super-resolution can improve the spatial resolution of the robotic skin with the least number of sensing units (or wirings), which is very important in real-world robotic application, because crowded wirings (or too many sensing units) could result in high cost and a bulky sensor, as well as a large amount of time for signal transmitting.

Can you provide our readers with more information on what is meant by the term ‘decoupling’? Why is this such an important feature of the sensor?

An external force can be broken down into two components in terms of normal force (in normal direction) and shear force (in tangential direction). Force decoupling means accurately measuring the two force components (normal and shear) independently. It is important to decouple the external force because each force component has its own influence on the object, and it is necessary to know the accurate value of each force component in order to analyze or control the state (stationary or moving) of the object.

For example, when we hold an object (like a bottle), we know how tight we should grasp it so that it will not slip from our hand. In this example, we are adjusting our finger pose or grasping force (i.e., normal force) according to the weight of the bottle or shear force exerted on our skin; therefore, when we use a robot to conduct such a grasping task, it is also necessary to know the accurate (i.e., decoupled) values of the two force components, and this is why the force decoupling ability is important for a tactile sensor.

Gripper stably holds the bottle because of the force feedback from the tactile sensor

What’s next for the research team?

We will apply our tactile sensor in the fields of human-robot interaction, dexterous manipulation, texture recognition, smart prosthetics in the future.

What’s the next step in the development of the sensor?

We will develop tactile sensors to decouple the external forces/torques in more directions and develop AI-enhanced tactile super-resolution algorithms for different application scenarios.

Where can readers find more information?

More information can be found on our website and the full journal can be found here.

About Dr. Yajing Shen

Dr. Yajing Shen is currently working as an Associate Professor in the Dept. of Biomedical Engineering at City University of Hong Kong with his main research interest in small and bioinspired robotics. He is a Senior Member of IEEE, an Executive member of China Micro-nano Robotic Society, and the Associate Editor of IEEE Trans on Robotics. Dr. Yajing has received serval academic awards, including the Best Manipulation Paper Award in IEEE International Conference on Robotics and Automation (ICRA) in 2011, the IEEE Robotics and Automation Society Japan Chapter Young Award in 2011, the Early Career Awards of Hong Kong UGC in 2014, the Big-on-Small Award at MARSS 2018, and the “National Excellent Young Scientist Fund (Hong Kong & Macau)” for the topic “micro/nano robot” in 2019.

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Joan Nugent

Written by

Joan Nugent

Joan graduated from Manchester Metropolitan University with a 2:1 in Film and Media Studies. During her studies, she worked as a Student Notetaker and continued working at the University, after graduation, as a Scribe. Joan has previously worked as a Proofreader for a Market Research company. Joan has a passion for films and photography and in her spare time, she enjoys doing illustrations and practicing calligraphy.

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