Position Control Of Flexible Single-arm Robot Based On Visual Feedback

Flexible robots have advantages of simple structure,simple driving,high safety,high flexibility,low energy consumption and low cost,so they have high application value.At present,flexible robots are being widely used in the fields of medical speculum,light industry,small working space and humanmachine coordination.In these applications,flexible robots generally use open-loop position control to work.At present,existing position feedback solutions are costly,which severely limits the application range of flexible robots,so an economically viable position feedback solution for closed-loop position control of flexible robots is urgently needed to expand the application range of flexible robots.Funded by Festo,a flexible single-arm robot based on a parallel flexible rod structure is designed and manufactured in this paper and its kinematics model is analyzed.Main work of this paper is that a cooperative target is designed and a pose measurement technique based on monocular vision and cooperative target is proposed.Based on the position information from the pose measurement information,a position closed-loop control strategy of the flexible single-arm robot is designed.An experimental platform of the flexible single-arm robot for position closed-loop control is constructed.Specific work arrangements are as follows:(1)A flexible single-arm robot based on a parallel flexible rod structure is designed and manufactured in this paper.Ball screw molds and flexible rods are selected,then sheaths,rod guides,supporting joints and an end connecting disc are designed.Core components of the flexible single-arm robot are six parallel flexible rods,three of which are active flexible rods driven by ball screw modules,and the other three are driven flexible rods to strengthen the stiffness of the flexible single-arm robot,so this flexible single-arm robot has three independent degrees of freedom.(2)Based on constant curvature hypothesis,a kinematics model of the flexible single-arm robot is decomposed into two maps,first one of which is from the actuator space to the parameters space of the equivalent flexible rod arc of three active parallel flexible rods,and the second map is from the parameters space of the equivalent flexible rod arc to the end position space of the flexible single-arm robot,establishing kinematics models of the flexible single-arm robot including a forward kinematics model,an inverse kinematics model and a differential kinematics model.(3)Overall scheme and hardware design of pose measurement are proposed.Firstly,overall scheme of pose measurement is proposed,and through clever design a recognition pattern is proposed using the characteristics that center positioning accuracy is better than square vertex positioning accuracy and more accurate pose information can be provided in the limited recognition area.The identification pattern is then used to design a planar target and a stereo target,which can cope with position measurement requirements in different situations.Industrial camera,industrial lenses,and industrial light source are selected based on parameters such as working distance and field of view.Imaging model and calibration method for the camera are then derived,and the parameters of the camera are calibrated in MATLAB software.(4)Pose measurement algorithm and software design are proposed.Canny edge detection algorithm is improved and implemented,and ellipse center calculation process is proposed to realize fast robust detection of ellipse center coordinates.Calculation process of elliptical long axis is derived,according which ellipse feature is quickly matched.PNP model is used as a pose solving strategy to achieve rapid measurement of pose.In the C++ environment,combined with OpenCV3.4.0 and Daheng C++ SDK,Daheng Mercury industrial camera is used to write programs to achieve pose measurement.(5)Model-free position closed-loop control strategy is designed.It only needs to estimate the Jacobian matrix of the flexible single-arm robot online in real time,and it is easy to realize without understanding kinematics model of the flexible single-arm robot too much.(6)Experimental device is built to realize closed-loop position control of the flexible single-arm robot.In order to verify the accuracy of proposed pose measurement algorithm in measuring position,position measurement accuracy verification test bench is built firstly and experimental results show that proposed pose measurement scheme is accurate and reliable when measuring position.Based on obtained position measurement data,combined with the model-free control strategy,position closed-loop control experimental platform of the flexible single-arm robot is built.Research shows that modelfree control strategy based on position feedback is accurate and economically feasible.Repeating positioning accuracy in X and Y directions is less than ±0.5 mm,and Repeating positioning accuracy in Z direction is less than ±5 mm.