Sophon Somlor

Design and Characterization of a Three-Axis Hall Effect-Based Soft Skin Sensor

This paper presents an easy means to produce a 3-axis Hall effect–based skin sensor for robotic applications. It uses an off-the-shelf chip and is physically small and provides digital output. Furthermore, the sensor has a soft exterior for safe interactions with the environment; in particular it uses soft silicone with about an 8 mm thickness. Tests were performed to evaluate the drift due to temperature changes, and a compensation using the integral temperature sensor was implemented. Furthermore, the hysteresis and the crosstalk between the 3-axis measurements were evaluated. The sensor is able to detect minimal forces of about 1 gf. The sensor was calibrated and results with total forces up to 1450 gf in the normal and tangential directions of the sensor are presented. The test revealed that the sensor is able to measure the different components of the force vector.

Robust in-hand manipulation of variously sized and shaped objects

Moving objects within the hand is challenging, especially if the objects are of various shape and size. In this paper we use machine learning to learn in-hand manipulation of such various sized and shaped objects. The TWENDY-ONE hand is used, which has various properties that makes it well suited for in-hand manipulation: a high number of actuated joints, passive degrees of freedom and soft skin, six-axis force/torque (F/T) sensors in each fingertip, and distributed tactile sensors in the skin. A dataglove is used to gather training samples for teaching the required behavior. The object size information is extracted from the initial grasping posture. After training a neural network, the robot is able to manipulate objects of untrained sizes and shape. The results show the importance of size and tactile information. Compared to interpolation control, the adaptability for the initial posture gap could be greatly extended. Final results show that with deep learning the number of required training sets can be drastically reduced.

A novel tri-axial capacitive-type skin sensor

This paper introduces a novel tri-axial capacitive force sensor. The sensor can measure the force vector, is embedded in soft 7 mm-thick silicone skin, enables temperature sensitivity compensation and has digital output. To measure the force vector, tilted capacitive sensor elements are used which are facing in different directions to differentiate the tangential forces. The sensor is intended for distributed contact sensing in a robotic skin, but could be also used for other applications such as novel haptic user interfaces in wearable devices. A series of experiments was performed and showed good sensor characteristics. The concept of the tilted force transducers has been proven to have the capability of detecting the force vector acting on the local sensor surface. Graphical Abstract

A Concept for a robot arm with adjustable series clutch actuators and passive gravity compensation for enhanced safety

Passive compliance is useful for robotic arms to ensure their safety. Often springs are used, but they are problematic because they reduce the achievable accelerations and can lead to underdamped oscillations. Torque limiters enhance the safety, but usually the torque limit cannot be adjusted to a desired torque. Electronically adjustable torque limiters, also known as series clutch actuators, have several benefits, especially for robotic arms, but they also have severe limitations. This paper suggests incorporating series clutch actuators into a gravity compensated arm. Consequently, gravity should not limit the isotropically achievable force anymore and in the case of power outage the arm keeps its position. The benefits and limitations of a series clutch actuator in a gravity compensated arm are discussed, and a prototype of such an arm is presented. Commercially available magnetic friction clutches are used. Preliminary experiments demonstrate that the safety can be increased.

A prototype force sensing unit for a capacitive-type force-torque sensor

Force sensing is a crucial task for robots, especially when the end effectors such as fingers and hands need to interact with an unknown environment, for example in a humanoid robot. In order to sense such forces, a force/torque sensor is an essential component. Many available force/torque sensors are based on strain gauges, but other sensing principles are also possible. In this paper we describe steps towards a capacitive type based sensor. Several MEMS capacitive sensors are described in the literature; however very few larger sensors are available, as capacitive sensors usually have disadvantages such as severe hysteresis and temperature sensitivity. On the other hand, capacitive sensors have the advantage of the availability of small sized chips for sensor readout and digitization. We employ copper beryllium for the transducer, which has been modified from the ones described in the literature to be able to be used in a small sized, robust force/torque sensor. Therefore, as the first step toward the goal of building such a sensor, in this study we have created a prototype sensing unit and have tested its sensitivity. No viscoelastic materials are used for the sensing unit, which usually introduce severe hysteresis in capacitive sensors. We have achieved a high signal-to-noise ratio, high sensitivity and a range of 10 Newton.

A modular, distributed, soft, 3-axis sensor system for robot hands

Integrating distributed sensors in the skin of robot hands is challenging, as the space is limited. This paper presents a dense and small tactile sensor system that can be installed on robotic hands. In the current implementation, the system is constituted by modules that are 26mm long and 27mm wide and they have been successfully integrated on the internal side of each finger phalange of the commercially available Allegro Hand (except the fingertips). Each sensor module contains 16 tri-axial taxels; each taxel is able to measure the applied 3D force vector using a Hall effect sensor and a magnet. The sensor modules are 4mm high, including the printed circuit board (PCB) with the sensors and the soft silicone with the magnets. The back of the PCB is flat without any components mounted, which eases the integration. Each sensor has I2C digital output, and each sensor module is connected to four I2C buses, requiring only seven wires for each module. The tri-axial taxels are close to each other (4.7 mm from the center of one taxel to the next), but experiments proved that independent force vectors can be measured and that the crosstalk is limited.

Development of a hall-effect based skin sensor

In this paper we introduce a prototype of a novel hall-effect based skin sensor for robotic applications. It uses a small sized chip that provides 3-axis digital output in a compact package. Our purpose was to evaluate the feasibility of measuring 3-axis force while maintain a soft exterior for safe interactions. Silicone was used to produce the soft skin layer with about 8 mm thickness. An MLX90393 chip was installed at the bottom of layer, with a small magnet approximately 5mm above it to measure 3-axial magnetic field data. To evaluate the sensors performance, an experiment was conducted by measuring normal and shear force when applying total forces of 0.7-14N in the normal and tangential directions of the sensor. The test revealed that the sensor prototype was able to differentiate the components of the force vector, with limited crosstalk. A calibration was performed to convert the measurements of the magnetic field to force values.

SNR modeling and material dependency test of a low-cost and simple to fabricate 3D force sensor for soft robotics

This paper presents a low cost, easy to produce, small tactile sensor system, that can be embedded in a soft material and limited space. In the current implementation, we use a Hall-effect sensor and a magnet to measure the force. One sensor module can measure 3D force vector and temperature. This chip is planted inside a 55 × 55 × 8 mm of the silicon layer. The module has I2C digital output, requiring only four wires for each module. The experiment shows that the signal to noise ratio (SNR) for this module is relatively high, 21.4658 dB when 20g load is applied. The experiment also indicates that the sensor module measured loads differently depending on the type of material that is in contact.

Development of a capacitive-type 6-axis force-torque sensor

Force sensing is a crucial task for robots, especially when end effectors such as fingers and hands need to interact with unknown environments; to sense such forces, a force-torque (F/T) sensor is an essential component. In this paper, we propose a small-sized 6-axis F/T sensor with a novel arrangement of 12 transducers using the force transducer we have previously developed. The copper beryllium used in our sensor reduces hysteresis in each transducer. Additionally, the sensor provides digital output via I2C bus to reduce the susceptibility to noise, and reduce the number of required wires. Sensor characteristics such as its sensitivity, signal-to-noise ratio, linearity, and hysteresis are determined. More importantly, we showed that our sensor can detect and measure the 6-axis F/T.