Robust Slippage Degree Estimation Based on Reference Update of Vision-Based Tactile Sensor

Abstract

In this paper, we propose a new slippage degree estimation method of a vision-based tactile sensor. The advantage of the vision-based tactile sensor is simultaneous acquisition of the slippage degree, a multidimensional force and a moment. A CCD camera captures the sensor surface which has regularly arrayed dots. The slippage degree on the sensor surface is determined by the stick ratio obtained from the displacements of dots in the captured images. In our previous work [1], the stick ratio was successfully applied to avoid slippage or the shape deformation of grasped objects. Based on the adaptive selection of the reference image and the compensation of the dot displacement, the proposed method in this study extends use of the previously developed algorithm, to dynamically complex but general situations as follows. First, the contact surface deforms after the macroscopic slippage or the slippage direction is changed. Second, the contact surface rotates with an applied moment. Third, the captured image is locally zoomed with a significant change of a grip force. A heuristic weighted average method is also proposed to decrease each dot's variation in the captured image. Usefulness of the proposed method is confirmed through experimental results.

Acquisition of Contact Force and Slippage Using a Vision-Based Tactile Sensor with a Fluid-Type Touchpad for the Dexterous Handling of Robots

“…The contact region in this case is a circle. Xydas et al and Kao et al analyzed the stiffness and contact mechanics of soft fingers based on the relationship of the Hertzian contact model [22,23]. Their approach calculates the relationship between the radius of a contact region and normal force, which is given as follows:…”

“…We used the relative displacement of each dot to the object between the reference image and the current image. The reference image is a previously captured image, such as the initial image taken before testing, which is appropriately updated [22]. Figure 7 shows the displacement of the dots between the reference and current images.…”

“…The area ratio of the stick region to the total contact region is called the stick ratio ϕ, which is defined in [21,22] as follows:…”

“…We have proposed a vision-based tactile sensor that can sense multiple types of tactile information simultaneously [21,22].The sensor consists of a CCD camera, LED lights, a transparent acrylic plate and a transparent hemispherical elastic touchpad for contacting the object. Here, the touchpad is deformable and the structure of the sensor is simple.…”

“…This sensor measures normal force, tangential force and rotational moment based on preliminarily-constructed lookup tables, which relate the deformation of the elastic touchpad to force and moment [21]. This sensor can also estimate slippage between the touchpad and a contacted object [22].…”