An eight-legged tactile sensor to estimate coefficient of static friction

Abstract

It is well known that a tangential force larger than the maximum static friction force is required to initiate the sliding motion between two objects, which is governed by a material constant called the coefficient of static friction. Therefore, knowing the coefficient of static friction is of great importance for robot grippers which wish to maintain a stable and precise grip on an object during various manipulation tasks. Importantly, it is most useful if grippers can estimate the coefficient of static friction without having to explicitly explore the object first, such as lifting the object and reducing the grip force until it slips. A novel eight-legged sensor, based on simplified theoretical principles of friction is presented here to estimate the coefficient of static friction between a planar surface and the prototype sensor. Each of the sensor's eight legs are straight and rigid, and oriented at a specified angle with respect to the vertical, allowing it to estimate one of five ranges (5 = 8/2 + 1) that the coefficient of static friction can occupy. The coefficient of friction can be estimated by determining whether the legs have slipped or not when pressed against a surface. The coefficients of static friction between the sensor and five different materials were estimated and compared to a measurement from traditional methods. A least-squares linear fit of the sensor estimated coefficient showed good correlation with the reference coefficient with a gradient close to one and an r(2) value greater than 0.9.

Initial contact shapes the perception of friction

“…Interestingly, roboticists have leveraged these findings to estimate friction on initial contact from the gradient of the lateral traction field. This metric is used to control the force applied by robotic grippers to soft and fragile objects (19)(20)(21). In haptic rendering, it is possible to produce tactile sensations by releasing the accumulated stress using ultrasonic friction modulation (22).…”

Tactile Sensors for Friction Estimation and Incipient Slip Detection—Toward Dexterous Robotic Manipulation: A Review

“…The relationship between normal and shear strains was found to be dependent on μs and it was proposed that this could be exploited to estimate μs. Chen et al [103] [104] proposed an eight-legged tactile sensor that can estimate μs between a planar surface and the sensor itself (Fig. 16).…”

“…A relationship between strain and μs was found in [101]; however, the range of μs for which the sensor was operational was severely limited (μs < 0.5). Rather than providing an estimate of μs as a single value, the approach using rigid sensor legs [103,104] provides an estimated range of μs. Increasing the number of legs (meaning more friction angles are tested) would increase the precision of the μs estimate; however, at present, the approach applies only to objects gripped on a planar surface, so extension to curved surfaces would be necessary in general.…”

A Wearable Capacitive Friction Force Sensor for E-Skin

“…The traditional friction force detection generally adopts a complex structure. − The detection method is complicated, and the friction force cannot be detected in real time. − Particularly, the equipment cannot be integrated with E-skin due to its structural limitations. ,, In wearable electronics, to completely imitate the functionality of human skin, it also needs to detect friction force to hold an object without dropping it out from a hand. Friction detection as an electronic skin device has not yet been demonstrated.…”