Forward kinematic calibration and gravity compensation for parallel-mechanism-based machine tools

Kinematic parameters’ calibration is a powerful method to improve the accuracy of the robot. This paper proposes an effective kinematic self-calibration method for dual-manipulators based on virtual constraints to estimate the actual kinematic parameters of the robots. This method only needs a camera mounted on one robot end-effector (EE) and a calibration target attached to another robot EE. First, a new calibration error model based on the straight line constraint is established to formulate the positions’ misalignment error with the kinematic parameters’ error. Then, the particle swarm optimization algorithm is developed to generate the optimal calibration poses of the robots under the constraints, which are used to ensure the poses feasible and the measurement errors acceptable. Finally, the kinematic parameter errors are identified with the Levenberg–Marquardt algorithm. The experiments of the kinematic parameters’ calibration with the dual-manipulators system are designed. The experimental results showed that the high positional accuracy of both robots can be achieved.

This study realizes face- and body-diagonal length tests using a double ball-bar (DBB) to measure squareness errors at the full working volume of machine tools (MTs). Each test consists of two steps: 1) length measurement at diagonal paths formed using machine commands, and 2) squareness error calculations. For each diagonal path, the machine receives sequential commands according to two nominal coordinates, and tool-tip positions are fixed on the workpiece table. Then, the distance between the two positions is measured using a DBB, which is not calibrated for initial length. The actual tool-tip coordinates are modeled using nominal coordinates and squareness errors. The relationship between the measured lengths and squareness errors is derived using the actual tool-tip coordinates to calculate the squareness errors. The two realized methods are applied to a MT, and verified by comparing the measurement lengths without and with compensation for the measured squareness error. The main advantages of the methods are 1) robust results, unaffected by set-up errors, 2) flexible measurements for various machine tools having different working volumes using a DBB with extension bars only, and 3) cost-effective measurements using the DBB, and the need for only a short time (~5 min) to complete measurements.

This paper addresses the design and implementation of an Android application for real-time precise motion control. To provide stable real-time performance, we implemented the application in two parts: Android service in the form of a daemon process, which periodically transfers a set of position commands for all motors through a real-time fieldbus, and Android UI application, which generates and delivers the set of position commands to the Android service. To support such a real-time motion control application, we use multi-core partitioning, which partitions the processor cores into a real-time partition to be used by the real-time motion control service and a non-real-time partition to be used by the Android application, and set up a shared buffer between them for communication. Our experiments show that we can obtain a motion control period of 2 ms with 99% task activation jitters less than for a configuration where each of the four threads controls two motors in a group.

Magnetic Fluid Seal for Linear Motion System with Gravity Compensator

Modeling and analysis of nonlinear guideway for double-ball bar (DBB) measurement and diagnosis

A robot's dynamic model depends on both the kinematic and mass-inertial parameters of a robot. Robot model identification therefore typically begins with kinematic identification; the mass-inertial parameters are then identified with the identified kinematic parameters used in the dynamic model. In this article we show that poorly identified kinematic parameters can lead to an uncorrectable bias in the dynamic model, leading to errors in the mass-inertial parameters that are many times that of kinematic parameter errors. We instead argue that a unified kinodynamic identification leads to more accurate identification of both the kinematic and mass-inertial parameters. A linearly weighted kinodynamic objective function is proposed, in which the weight can be interpreted from a maximum likelihood perspective as the relative accuracy of the kinematic sensors vis-à-vis the dynamic sensors. Recursive algorithms for computing exact analytic gradients of the kinodynamic objective function are newly derived, leading to robust and fast-converging identification algorithms. Extensive numerical and hardware experiments demonstrate the advantages of our unified kinodynamic identification procedure.

Positioning error compensation for parallel mechanism with two kinematic calibration methods

In most machining processes, large amounts of energy are needed to accomplish the machining operation. When this energy is transmitted through a structure that has minimal damping characteristics, such as a lathe or a milling machine, self sustained oscillations (chatter) can develop. When chatter develops, it can be viewed as a basic performance limitation of the machine tool. In order to suppress the chatter, a real-time controller using digital signal processing techniques has been implemented. This paper discusses a novel method for the real time computation of the transfer function of the machine tool-workpiece combination and illustrates how a real-time active chatter controller can be designed and integrated into existing machine tools to overcome this performance limitation.

In most machining processes, large amounts of energy are needed to accomplish the machining operation. When this energy is transmitted through a structure that has minimal damping characteristics, such as a lathe or a milling machine, self sustained oscillations (chatter) can develop. When chatter develops, it can be viewed as a basic performance limitation of the machine tool. In order to suppress the chatter, a real-time controller using digital signal processing techniques has been implemented. This paper discusses a novel way of computing the transfer function of the machine tool-work piece combination and illustrates how a real-time active chatter controller could be designed and integrated into existing machine tools to overcome this performance limitation. Currently, experimental verification of the analytical work is being pursued.

This paper proposes a novel measuring method which uses double ballbar (DBB) to inspect the kinematic errors of the rotary axes of five-axis machine tool. In this study, kinematic error mathematical model is firstly established based on the analysis of the rotary axes errors which originated from five-axis machine tools. In the simulation, working conditions considering different error origins are simulated to find the relationship between the DBB measuring patterns and the kinematic errors. In the measuring experiment, the machine rotary axes move simultaneously along a specified circular path while all the linear axes are kept stationary. The original DBB measuring data are processed to draw the measuring patterns in the polar plots which can be employed to observe and identify the kinematic errors. Rotary error compensation is implemented based on the function of external machine origin shift. Both the simulation and the experiment results show the convenience and effectiveness of the proposed measuring method as well as its operability as a calibration method of five-axis machine tools.

Laser non-contact precision measuring method of CNC machine tools under the high feed rate condition was put forward. The method combines the laser interferometer with the measurement principle of the double ball bar (DBB), and can measure the machine tool precision under the high feed rate condition. Two laser interferometers used as measuring the displacements and feed rate data along circular interpolation motion individually measured three group data in the XY, YZ and XZ work planes, and the space errors of the CNC can be acquired through solving the special mathematic model based on the measurement principle of DBB.

Real-time motion planning and control of multi-joint robot manipulators are studied using neural networks. The proposed neural network approach consists of two modules: one for real-time collision-free motion planning, the other for real-time fine control of the robot manipulators. The motion planning module is a biologically inspired, parallel connected, topologically organised neural network, where each neuron is characterised by a shunting equation, whose state space is the robot configuration joint space. This module is capable of planning a real-time optimal path for robot manipulators through the dynamic activity landscape of the neural network. The motion control module involves of a feedforward neural network together with a traditional PD feedback loop. It is capable of achieving real-time fine motion control of robot manipulators under significant uncertainties and without any prior knowledge of the robot dynamics. This module can quickly compensate sudden changes in the robot dynamics. The real-time fine control of robot manipulators is achieved through the online learning of the neural network. Both the motion planning and motion control modules are computationally efficient, and their global stabilities are proved using Lyapunov stability theory.

Advanced engineering ceramics have been widely used in modern industry due to their excellent physical and chemical properties. However, they are difficult to machine due to their high hardness and brittleness. End electrical discharge milling and mechanical grinding compound machining method is employed to machine SiC ceramic in this paper. The process is able to effectively machine a large surface area on SiC ceramic. Furthermore, the tool for the compound machining of SiC ceramic is designed based on the goal of the higher material removal rate, the lower tool wear, and the better machined surface quality. The results show that with the designed tool for the compound machining, the higher material removal rate, the lower tool wear, and the better machined surface quality can be obtained.

Assessment of upper limb function in young Friedreich ataxia patients compared to control subjects using a new three-dimensional kinematic protocol

Fixed beam gantry for large CNC boring and milling machine to bear the beams on the rail side apron, and other parts ram weight under weight and size of deformation produced a result of the assembly accuracy of less than standard, use three-dimensional modeling software UG and finite element analysis software ANSYS for dynamic beam gantry milling machine CNC beam three-dimensional modeling and modal analysis, based on weight and apron beams, the weight of ram and other components in relative deformation amplitude, and the gantry milling machine processing characteristics and overall design of the structural characteristics of machine tools, assembly of the beam is proposed to note some issues and some ways to solve the assembly size requirements, through finite element analysis of the deformation and comparing the actual measured value, the experiment proved data on-site assembly solution with a good theoretical support.