A robust tactile sensor matrix for intelligent grasping of objects using robotic grippers

Abstract

Intelligent power grasp of robotic gripper system has been instrumental in metal-working industries as well as handling of nuclear material in recent past. Slip-resistant, robust gripping of disc-shaped material during remote manipulation is a challenge in material handling. In a nuclear fuel manufacturing lab, sensitive fuel materials (objects) are handled in the form of a disc (melt) that is pre-cast. Depending on the process treatment, the metal discs are categorized as delicate, hard or soft. The metal discs that were handled by the robotic system and used in the experiment have a diameter of 60mm, a thickness of 15mm, and a weight of 2.3 kg., It is crucial to take care of the paradigms like grip force, profile and perception of slide for job planning and management of a robot in a hazardous radioactive environment. A multi-sensory tactile array is designed indigenously and fabricated by utilising commercial sensors attached to the prototype gripper so as to make it an intelligent instrumented-jaw gripper. The custom-built tactile sensory system is made up of a) force sensing resistors (FSRs) in a specific array, b) a matrix of FSR in the centre, c) PVDF (Polyvinylidene Fluoride) sensors in the perimeter and d) infrared (IR) sensors placed in a predetermined architecture. The mounting of the customized gripper sensory system includes a 3x2-array of FSRs and other sensors, each having a spatial resolution of 5 mm and a sensing area of $15,000 mm^{2}$. While PVDF sensor is employed for dynamic contact point estimation, texture recognition is accomplished using high frequency response. An integrated form of proximity and ambient light sensor is utilised (mimicing distance sensor) to detect the presence of an object and improve gripper alignment during grasping. This sensor-instrumented processing was achieved through microcontroller-based programming and allied electronics, such as scanning the array & storing the data. LabView® was used to record the digital signals and analysis of time-spanned raw data. Calibration was carried out to ensure that the acquired data was repeatable and reliable. The paradigms of indigenous development of the integrated sensory system for the gripper as well as its calibration, dynamic features and response characteristics are discussed in this paper. The tactile matrix has a force range of 30N, a resolution of 0.3N and a sensitivity of 0.5V/N., The FSR sensors produced a nonlinear relationship at various force ranges and the hysteresis therein was found to vary with load range and frequency. At lower frequencies of 0.2 and 0.8 Hz, cycle drift was significant, with a maximum inaccuracy in force value of 0.5 N. The force measuring inaccuracy of normal force was less than 5% at sensitive places. Based on the repeated experimentation, the developed matrix sensor was entrusted to have nearly 95% accuracy rate in recognising different textures. The developed tactile sensor matrix was found to be robust and it has good reproducibility in parameters like contact force measurement, contact point estimate, slip detection and texture recognition, according to the experimental results. To not withstanding benefits the developed tactile sensor system was also comprehended with few on-job limitations, signaling run-time error conditions.