Kinematic Characterisation of Hexapods for Industry

International audiencePurpose-The aim of this paper is to propose two simple tools for the kinematic characterization of hexapods. The paper also aims to share the authors' experience with converting a popular commercial motion base (Stewart-Gough platform, hex-apod) to an industrial robot for use in heavy duty aerospace manufacturing processes. Design/methodology/approach-The complete workspace of a hexapod is a six-dimensional entity that is impossible to visualize. Thus, nearly all hexapod manufacturers simply state the extrema of each of the six dimensions, which is very misleading. As a compromise, we propose a special three-dimensional subset of the complete workspace, an approximation of which can be readily obtained using a CAD/CAM software suite, such as CATIA. While calibration techniques for serial robots are readily available, there is still no generally-agreed procedure for calibrating hexapods. We propose a simple calibration method that relies on the use of a laser tracker and requires no programming at all. Instead, the design parameters of the hexapod are directly and individually measured and the few computations involved are performed in a CAD/CAM software such as CATIA. Findings-The conventional octahedral hexapod design has a very limited workspace, though free of singularities. There are important deviations between the actual and the specified kinematic model in a commercial motion base. Practical implications-A commercial motion base can be used as a precision positioning device with its controller retrofit-ted with state-of-the-art motion control technology with accurate workspace and geometric characteristics. Originality/value-A novel geometric approach for obtaining meaningful measures of the workspace is proposed. A novel, systematic procedure for the calibration of a hexapod is outlined. Finally, experimental results are presented and discussed

Kinematic Calibration of Parallel Kinematic Machines on the Example of the Hexapod of Simple Design

The aim of using parallel kinematic motion systems as an alternative of conventional machine tools for precision machining has raised the demands made on the accuracy of identification of the geometric parameters that are necessary for the kinematic transformation of the motion variables. The accuracy of a parallel manipulator is not only dependent upon an accurate control of its actuators but also upon a good knowledge of its geometrical characteristics. As the platform's controller determines the length of the actuators according to the nominal model, the resulted pose of the platform is inaccurate. One way to enhance platform accuracy is by kinematic calibration, a process by which the actual kinematic parameters are identified and then implemented to modify the kinematic model used by the controller. The first and most general valuation criterion for the actual calibration approaches is the relative improvement of the motion accuracy, eclipsing the other aspects to pay for it. The calibration outlay has been underestimated or even neglected for a long time. The scientific value of the calibration procedure is not only in direct proportion to the achieved accuracy, but also to the calibration effort. These demands become particularly stringent in case of the calibration of hexapods of the so-called simple design. The objectives of the here proposed new calibration procedure are based on the deficits mentioned above under the special requirements due to the circumstances of the simple design-concept. The main goals of the procedure can be summarized in obtaining the basics for an automated kinematic calibration procedure which works efficiently, quickly, effectively and possibly low-cost, all-in-one economically applied to the parallel kinematic machines. The problem will be approached systematically and taking step by step the necessary conclu-sions and measurements through: Systematical analysis of the workspace to determine the optimal measuring procedure, measurements with automated data acquisition and evaluation, simulated measurements based on the kinematic model of the structure and identifying the kinematic parameters using efficient optimization algorithms. The presented calibration has been successfully implemented and tested on the hexapod of simple design `Felix' available at the IWM, TU Dresden. The obtained results encourage the application of the procedure to other hexapod structures

Hexapod Design For All-Sky Sidereal Tracking

In this paper we describe a hexapod-based telescope mount system intended to provide sidereal tracking for the Fly's Eye Camera project -- an upcoming moderate, 21"/pixel resolution all-sky survey. By exploiting such a kind of meter-sized telescope mount, we get a device which is both capable of compensating for the apparent rotation of the celestial sphere and the same design can be used independently from the actual geographical location. Our construction is the sole currently operating hexapod telescope mount performing dedicated optical imaging survey with a sub-arcsecond tracking precision.Comment: Accepted for publication in PASP, 10 page

Reconfiguration and tool path planning of hexapod machine tools

Hexapod machine tools have the potential to achieve increased accuracy, speed, acceleration and rigidity over conventional machines, and are regarded by many researchers as the machine tools of the next generation. However, their small and complex workspace often limits the range of tasks they can perform, and their parallel structure raises many new issues preventing the direct use of conventional tool path planning methods. This dissertation presents an investigation of new reconfiguration and tool path planning methods for enhancing the ability of hexapods to adapt to workspace changes and assisting them in being integrated into the current manufacturing environments. A reconfiguration method which includes the consideration of foot-placement space (FPS) determination and placement parameter identification has been developed. Based on the desired workspace of a hexapod and the motion range of its leg modules, the FPS of a hexapod machine is defined and a construction method of the FPS is presented. An implementation algorithm for the construction method is developed. The equations for identifying the position and orientation of the base joints for the hexapod at a new location are formulated. For the position identification problem, an algorithm based on Dialytic Elimination is derived. Through examples, it is shown that the FPS determination method can provide feasible locations for the feet of the legs to realize the required workspace. It is also shown that these identification equations can be solved through a numerical approach or through Dialytic Elimination using symbolic manipulation. Three dissimilarities between hexapods and five-axis machines are identified and studied to enhance the basic understanding of tool path planning for hexapods. The first significant difference is the existence of an extra degree of freedom (γ angle). The second dissimilarity is that a hexapod has a widely varying inverse Jacobian over the workspace. This leads to the result that a hexapod usually has a nonlinear path when following a straight-line segment over two sampled poses. These factors indicate that the traditional path planning methods should not be used for hexapods without modification. A kinematics-based tool path planning method for hexapod machine tools is proposed to guide the part placement and the determination of γ angle. The algorithms to search for the feasible part locations and γ sets are presented. Three local planning methods for the γ angle are described. It is demonstrated that the method is feasible and is effective in enhancing the performance of the hexapod machine. As the nonlinear error is computationally expensive to evaluate in real time, the measurement of total leg length error is proposed. This measure is proved to be effective in controlling the nonlinear error

Sensor Data Fusion for Body State Estimation in a Hexapod Robot With Dynamical Gaits

We report on a hybrid 12-dimensional full body state estimator for a hexapod robot executing a jogging gait in steady state on level terrain with regularly alternating ground contact and aerial phases of motion. We use a repeating sequence of continuous time dynamical models that are switched in and out of an extended Kalman filter to fuse measurements from a novel leg pose sensor and inertial sensors. Our inertial measurement unit supplements the traditionally paired three-axis rate gyro and three-axis accelerometer with a set of three additional three-axis accelerometer suites, thereby providing additional angular acceleration measurement, avoiding the need for localization of the accelerometer at the center of mass on the robot’s body, and simplifying installation and calibration. We implement this estimation procedure offline, using data extracted from numerous repeated runs of the hexapod robot RHex (bearing the appropriate sensor suite) and evaluate its performance with reference to a visual ground-truth measurement system, comparing as well the relative performance of different fusion approaches implemented via different model sequences

Test Frame Design for Characterization of Additive Manufacturing Compliant Materials

With the application of using surrogate models with General Purpose Graphics Processing (GPGPU) computing to meet the need for “real-time” characterization of nonlinear anisotropic material systems and the growing work of using multiaxial robotic test frames for material characterization, there has been a solution for a specific application towards additive manufacturing materials, specifically polymers. Traditional testing using uniaxial and biaxial test machines has proven insufficient in characterizing the material properties of additive manufacturing materials, therefore developing a need for a multiaxial testing machine for characterization that can dynamically excite strain states for a more in-depth look at the material properties. This design report presents the design of a multiaxial robotic test frame that incorporates a Stewart-Gough (SG) platform design to allow 6 degrees-of-freedom for multiple and combined loading applications. This solution is the next generation multiaxial machine focusing on additive manufacturing materials, specifically polymers. The problem statement is the following: Design and fabricate a multiaxial robotic test frame that can test additive manufacturing materials, focusing on polymers and some metals, in 6 degrees-of-freedom while improving on performance and cost over the CSM design

The Design of Parallel Kinematic Machine Tools Using Kinetostatic Performance Criteria

International audienceMost industrial machine tools have a serial kinematic architecture, which means that each axis has to carry the following one, including its actuators and joints. High Speed Machining highlights some drawbacks of such architectures: heavy moving parts require from the machine structure high stiffness to limit bending problems that lower the machine accuracy, and limit the dynamic performances of the feed axes. That is why PKMs attract more and more researchers and companies, because they are claimed to offer several advantages over their serial counterparts, like high structural rigidity and high dynamic capacities. Indeed, the parallel kinematic arrangement of the links provides higher stiffness and lower moving masses that reduce inertia effects. Thus, PKMs have better dynamic performances. However, the design of a parallel kinematic machine tool (PKMT) is a hard task that requires further research studies before wide industrial use can be expected. Many criteria need to be taken into account in the design of a PKMT. We pay special attention to the description of kinetostatic criteria that rely on the conditioning of the Jacobian matrix of the mechanism. The organisation of this paper is as follows: next section introduces general remarks about PKMs, then is explained why PKMs can be interesting alternative machine tool designs. Then are presented existing PKMTs. An application to the design of a small-scale machine tool prototype developed at IRCCyN is presented at the end of this paper

rcHex: A Radio-controlled Hexapod

rcHex is a radio-controlled hexapod with 18 degrees of freedom, capable of positional and rotational body adjustment as well as omnidirectional travel at variable speeds using three common gaits. Its general-purpose design accessible to hobbyists makes rcHex an platform for further development, whether it be experimentation in advanced robotic movement or retrofitting sensors to utilize technologies such as computer vision and artificial intelligence. This report explores some of the design intricacies of hexapod movement, including gait sequencing and the application of inverse kinematics to multi-jointed limbs

An Overview of Kinematic and Calibration Models Using Internal/External Sensors or Constraints to Improve the Behavior of Spatial Parallel Mechanisms

This paper presents an overview of the literature on kinematic and calibration models of parallel mechanisms, the influence of sensors in the mechanism accuracy and parallel mechanisms used as sensors. The most relevant classifications to obtain and solve kinematic models and to identify geometric and non-geometric parameters in the calibration of parallel robots are discussed, examining the advantages and disadvantages of each method, presenting new trends and identifying unsolved problems. This overview tries to answer and show the solutions developed by the most up-to-date research to some of the most frequent questions that appear in the modelling of a parallel mechanism, such as how to measure, the number of sensors and necessary configurations, the type and influence of errors or the number of necessary parameters