Camera geometry determination based on circular's shape for peg-in-hole task

A simple, inexpensive system and effective in performing required tasks is the most preferable in industry. The peg-in-hole task is widely used in manufacturing process by using vision system and sensors. However, it requires complex algorithm and high Degree of Freedom (DOF) mechanism with fine movement. Hence, it will increase the cost. Currently, a forklift-like robot controlled by an operator using wired controllers is used to pick up one by one of the copper wire spools arranged side by side on the shelf to be taken to the inspection area. The holder and puller attached to the robot is used to pick up the spool. It is difficult for the operator to ensure the stem is properly inserted into the hole (peg-in-hole problem) because of the structure of the robot. However, the holder design is not universal and not applicable to other companies. The spool can only be grasped and pulled out from the front side and cannot be grasped using robot arm and gripper. In this study, a vision system is developed to solve the peg-in-hole problem by enabling the robot to autonomously perform the insertion and pick up the spool without using any sensors except a low-cost camera. A low-cost camera is used to capture images of copper wire spool in real-time video. Inspired by how human perceive an object orientation based on its shape, a system is developed to determine camera orientation based on the spool image condition and yaw angle from the center of the camera (CFOV) to CHS. The performance of the proposed system is analyzed based on detection rate analysis. This project is developed by using MATLAB software. The analysis is done in controlled environment with 50-110 cm distance range of camera to the spool. In addition, the camera orientation is analyzed between -20º to 20º yaw angle range. In order to ensure the puller will not scratch the spool, a mathematical equation is derived to calculate the puller tolerance. By using this, the system can estimate the spool position based on the camera orientation and distance calculation. Application of this system is simple and costeffective. A Modified Circular Hough Transform (MCHT) method is proposed and tested with existing method which is Circular Hough Transform (CHT) method to eliminate false circles and outliers. The results of the analysis showed detection success rate of 96% compared to the CHT method. It shows the MCHT method is better than CHT method. The proposed system is able to calculate the distance and camera orientation based on spool image condition with low error rate. Hence, it solves the peg-in-hole problem without using Force/Torque sensor. In conclusion, a total of 7 analysis consist of image pre-processing, image segmentation, object classification, comparison between CHT and MCHT, illumination measurement, distance calculation and yaw angle analysis were experimentally tested including the comparison with the existing method. The proposed system was able to achieve all the objectives

Robotic Assembly Control Reconfiguration Based on Transfer Reinforcement Learning for Objects with Different Geometric Features

Robotic force-based compliance control is a preferred approach to achieve high-precision assembly tasks. When the geometric features of assembly objects are asymmetric or irregular, reinforcement learning (RL) agents are gradually incorporated into the compliance controller to adapt to complex force-pose mapping which is hard to model analytically. Since force-pose mapping is strongly dependent on geometric features, a compliance controller is only optimal for current geometric features. To reduce the learning cost of assembly objects with different geometric features, this paper is devoted to answering how to reconfigure existing controllers for new assembly objects with different geometric features. In this paper, model-based parameters are first reconfigured based on the proposed Equivalent Theory of Compliance Law (ETCL). Then the RL agent is transferred based on the proposed Weighted Dimensional Policy Distillation (WDPD) method. The experiment results demonstrate that the control reconfiguration method costs less time and achieves better control performance, which confirms the validity of proposed methods

Suppress vibration on robotic polishing with impedance matching

Installing force-controlled end-effectors on the end of industrial robots has become the mainstream method for robot force control. Additionally, during the polishing process, contact force stability has an important impact on polishing quality. However, due to the difference between the robot structure and the force-controlled end-effector, in the polishing operation, direct force control will have impact during the transition from noncontact to contact between the tool and the workpiece. Although impedance control can solve this problem, industrial robots still produce vibrations with high inertia and low stiffness. Therefore, this research proposes an impedance matching control strategy based on traditional direct force control and impedance control methods to improve this problem. This method's primary purpose is to avoid force vibration in the contact phase and maintain force-tracking performance during the dynamic tracking phase. Simulation and experimental results show that this method can smoothly track the contact force and reduce vibration compared with traditional force control and impedance control

On disengaging a peg from a hole

In the field of manufacturing and remanufacturing, robots are employed in assembly tasks. Robotics researchers often use a cylindrical peg and a cylindrical hole as a model to understand high-precision insertion operations. During those operations, two main obstacles were identified, namely, jamming and wedging. Jamming occurs when the force is applied in the wrong direction and can be rectified easily by changing the direction. Wedging occurs when the peg appears to be stuck in the hole. The wedging of a peg is more complex than jamming, and it involves the deformation of the components. Many studies have been performed in the area of peg-hole assembly. Although researchers have mentioned the necessary conditions for wedging, the peg-hole jamming problem was the main focus. This thesis aims to better understand the peg-hole wedging problem to find methods to dislodge a wedged peg and to design a remote-centre-compliance (RCC) device to avoid the wedging and jamming of a peg that can be used in both assembly and disassembly. Using the definition and necessary conditions of peg-hole wedging, the systematic process of wedging a peg is analysed and illustrated. There are four steps to wedge a peg in a hole. First, the peg and hole must be in 2-point contact, and the two contact points must be within each other’s friction cone. A force or moment is then applied to deform the peg and hole, and the peg tilting angle increases. The force or moment is then released in the third step, and the peg tilting angle will be reduced by a small amount. Finally, when the peg tilting angle reduces, the reaction forces at the contact points will be collinear, and the peg is wedged. In the simulation and experiment in this research, the hole is divided into two sides, and a force-torque (FT) sensor is installed beneath each hole. The readings obtained from the sensors have shown that the hypothesis of the wedging process is correct, and when the peg is successfully wedged, the resultant force experienced by the FT sensors is balanced. The dislodging of a peg is also investigated in this thesis. To dislodge a wedged peg, intuitively, the peg is either shaken, twisted or knocked. Depending on the application, some would use a low force to dislodge the wedged peg to avoid damaging the components, while others would prefer a quicker disassembly process. In this investigation, the wedged peg is dislodged using different methods, such as applying a constant force and pulsating forces with different frequencies and magnitudes. The time needed to dislodge the peg is recorded to compare the effects of different combinations of parameters used. The result from the simulation shows that the peg can be dislodged at low impulses within a specific range of pulling force magnitudes. Adopting a pulsating force helps reduce the impulse required to dislodge the peg compared to using continuous force in the low magnitude region. However, in the lowest magnitude region, using a continuous force resulted in a lower impulse as the time for dislodging the peg was shorter compared to when a pulsating force was employed. Many techniques have been proposed and investigated to aid the peg-hole assembly process, and one of them is by using an RCC. At the University of Canterbury (New Zealand), researchers designed a passive compliant device, which was an inverted Gough-Whitehall-Stewart mechanism, to assist the peg-hole insertion process. This thesis analyses a modified version of that compliant device, where the legs do not meet in pairs at the platform but at points located remotely from it. This allows the device to have the features of an RCC mechanism, which has been proven by other researchers to be effective for precise peg-hole assembly tasks. This device is also suitable for both assembly and disassembly processes. Unlike the currently available RCC design, which can only withstand high compressive forces, the proposed compliant device can resist both compressive and tensile forces. The compliance matrix of the new design and the location at which it is diagonal are derived using small approximations, proving that the centre of compliance is situated away from the platform. The correctness of the small motion assumptions and the RCC properties of the new compliance device have been confirmed by performing the sensitivity analysis