The application of neural networks in fixture planning by pattern classification

The objective of this paper is to study the fixture planning of modular fixtures for cutting by means of a computer-aided fixture system (CAFS). The fixture planning presented here integrates the database of modular fixtures, neural networks, the heuristic algorithm of fixture position and the use of Advanced Modelling Extension Release 2.1 (AME) of AutoCAD R12 as the 3D graphic interface. The function suitable for the AME environment was also controlled to obtain detailed data of the geometry and topology of the workpiece to develop the fixture planning for modular fixtures. First of all, the concept of group technology (GT) was used to establish the coding database of the modular fixture element for use in the system. This paper presents fixture planning which combines the pattern recognition capability of the neural networks and the concept of GT to group the workpieces with different patterns but identical fixture modes into the same group. After network training, the fixture mode of the workpiece to be clamped can be inferred, and the selection of the fixture elements can be completed.     

Vision-assisted control for manipulation using virtual fixtures

We present the design and implementation of a vision-based system for cooperative manipulation at millimeter to micrometer scales. The system is based on an admittance control algorithm that implements a broad class of guidance modes called virtual fixtures. A virtual fixture, like a real fixture, limits the motion of a tool to a prescribed class or range of motions. We describe how both hard (unyielding) and soft (yielding) virtual fixtures can be implemented in this control framework. We then detail the construction of virtual fixtures for point positioning and curve following as well as extensions of these to tubes, cones, and sequences thereof. We also describe an implemented system using the JHU Steady Hand Robot. The system uses computer vision as a sensor for providing a reference trajectory, and the virtual fixture control algorithm then provides haptic feedback to implemented direct, shared manipulation. We provide extensive experimental results detailing both system performance and the effects of virtual fixtures on human speed and accuracy.     

Planning for Modular and Hybrid Fixtures

Fixturing encompasses the design and assembly of fixtures to locate and hold a workpiece during a manufacturing operation such as machining or assembly. We have implemented an automated design algorithm for a fixturing toolkit called the fixture vise; the fixture vise involves two fixture plates mounted on jaws of a vise and modular fixturing elements (pegs or flatted pegs). Generally, a fixture vise can handle appropriately sized prismatic workpieces in many different ways. The design algorithm runs in O(A) time, where A is the number of configurations achieving simultaneous contact between the modeled object and four fixture elements; since simultaneous contact is a necessary but not sufficient precondition for force closure, A provides an upper bound on the number of force closure fixture configurations. Received June 6, 1994; revised November 17, 1994, February 1, 1995, February 28, 1995, and July 11, 1995.     

Damage diagnosis and fixture classification using impedance-based sensors

Assembly fixtures are commonly used to locate rigid and compliant components for joining operations. Piezoelectric (PZT) impedance sensors can be used to individually monitor assembly fixtures and have been proven effective for structural health monitoring applications. The sensors rapidly record significant quantities of data, but must be bonded to a fixture in order to transmit an input signal and record the corresponding output signal. Thus, PZT impedance sensors become a permanent feature of an assembly fixture and may create unique systems defined by the assembly fixture and impedance sensor (AFIS). Previous research has shown success in detecting fixture damage using PZT impedance sensors. This paper extends previous fixture damage detection work to damage diagnosis through the use of data mining classifiers. Classifiers were used in three studies; the first was to show that classifiers can be trained to classify a healthy fixture, fixture damage, and multiple severities of fixture damage in an isolated AFIS. In the second study, classifier generalization was tested by simulating an unknown damage. Lastly, classifiers were used to study the uniqueness (i.e. fixture classification) of two AFIS, which could have implications related to the practical application of classifier models to any AFIS.     

An algorithm for the automatic generation of an assembly process for modular fixture parts

In metal mold production systems, rapid, high-quality, and low-cost production technologies are needed. In these systems, assembly tasks to make fixtures to immobilize the mold are required. The mold must be firmly fixed in order to resist the force generated by NC machine tools. In this article, we discuss the automatic generation of an assembly process for modular fixture parts in the systems. The assembly drawing of the parts can be obtained from STEP/AP203, which is a text file of CAD/CAF data. In these data, the shape and configuration of all assembled fixtures are included. The shape of each fixture is constructed with planar, cylindrical, and conical surfaces. From the data, the number of surfaces in contact with other surfaces is derived for each part, and valid contacts are judged for their degree of importance. A tree of contacts among all fixtures is derived. In the numerical examples, the assembly process is automatically generated by our proposed method. It is shown that a suitable assembly process is obtained.     

Quality prediction and compensation in multi-station machining processes using sensor-based fixtures

New fixture technologies, such as sensor-based fixtures, will significantly improve part quality through cutting-tool path compensations in multi-station machining processes (MMPs). Successful application of sensor-based fixtures depends on the development of new variation reduction methodologies to predict part quality in MMPs and detect the critical machining stations whose critical manufacturing variations can be estimated by installing a suitable sensor-based fixture. In this paper, a methodology is proposed to facilitate the implementation of sensor-based fixtures in MMPs. This methodology involves three key steps: (1) an identification of station-induced variations; (2) a sensor placement optimization method for designing sensor-based fixtures; and (3) a compensability analysis. A case study is conducted to demonstrate the effectiveness of the methodology.     

Case based reasoning method for computer aided welding fixture design

This paper presents a case-based reasoning (CBR) method for welding fixture design, a critical issue in the manufacturing of large and complicated equipment. However, previous fixture design research has mainly focused on machining fixtures rather than welding fixtures. In this paper, an approach of data abstraction for fixture design information representation is proposed, first to systemize and manage myriads of fixture related resources, e.g., past fixture design solutions, fixture units depository. Based on this approach, a multi-level CBR method for welding fixture design is then presented. This method could help designers, by referencing previous design cases, to make a conceptual fixturing solution quickly and, finally, finish the detailed solution of fixture design.     

A volumetric/iconic frequency domain representation for objectswith application for pose invariant face recognition

A novel method for representing 3D objects that unifies viewer and model centered object representations is presented. A unified 3D frequency-domain representation, called volumetric frequency representation (VFR), encapsulates both the spatial structure of the object and a continuum of its views in the same data structure. The frequency-domain image of an object viewed from any direction can be directly extracted employing an extension of the projection slice theorem, where each Fourier-transformed view is a planar slice of the volumetric frequency representation. The VFR is employed for pose-invariant recognition of complex objects, such as faces. The recognition and pose estimation is based on an efficient matching algorithm in a four-dimensional Fourier space. Experimental examples of pose estimation and recognition of faces in various poses are also presented     

Automated fixture configuration for rapid manufacturing planning

The wide adoption of agile manufacturing systems has necessitated the design and use of fixtures or work holding devices that have in-built flexibility to rapidly respond to part design changes. Despite the availability of reconfigurable fixtures, practical fixture configuration largely remains an experience driven manual activity to enable customization for varying workpiece geometry, and most automated solutions do not scale well to accommodate such variation. In this paper, we address the problem of rapidly synthesizing a realistic fixture that will guarantee stability and immobility of a specified polyhedral work-part. We propose that the problem of automated fixture layout may be approached in two distinct stages. First, we determine the spatial locations of clamping points on the work piece boundary using the principles of force and form closure, to ensure immobility of the fixtured part under external perturbation. In particular, we show that the candidate restraints mapped to the six dimensional vector space of wrenches (force–moment pairs) may be hashed in a straightforward manner to efficiently generate force closure configurations that restrain part movement against large external wrenches. When clamps are allowed to exert arbitrarily high reaction forces on the part, the spatial arrangement of the clamping locations ensures the part is in form closure. On generating force/form closure configurations, the chosen locations are matched against a user-specified library of reconfigurable clamps to synthesize a valid fixture layout comprising clamps that are accessible and collision free with each other and the part. Additionally, in the case of determining machining setups the clamps are chosen to avoid collisions with the moving cutting tool. We demonstrate fast algorithms to perform both location selection and fixture matching, and show several results that underscore the practical application of our solution in automated manufacturing process planning.     

Online task recognition and real-time adaptive assistance for computer-aided machine control

Segmentation and recognition of operator-generated motions are commonly facilitated to provide appropriate assistance during task execution in teleoperative and human-machine collaborative settings. The assistance is usually provided in a virtual fixture framework where the level of compliance can be altered online, thus improving the performance in terms of execution time and overall precision. However, the fixtures are typically inflexible, resulting in a degraded performance in cases of unexpected obstacles or incorrect fixture models. In this paper, we present a method for online task tracking and propose the use of adaptive virtual fixtures that can cope with the above problems. Here, rather than executing a predefined plan, the operator has the ability to avoid unforeseen obstacles and deviate from the model. To allow this, the probability of following a certain trajectory (subtask) is estimated and used to automatically adjusts the compliance, thus providing the online decision of how to fixture the movement.     

Fault detection and prognosis of assembly locating systems using piezoelectric transducers

Fixture faults have been identified as a principal root cause of defective products in assembly lines; however, there exists a lack of fast and accurate monitoring tools to detect fixture fault damage. Locating fixture damage causes a decrease in product quality and production throughput due to the extensive work required to detect and diagnosis a faulty fixture. In this paper, a unique algorithm is proposed for fixture fault monitoring based on the use of autoregressive models and previously developed piezoelectric impedance fixture sensors. The monitoring method allows for the detection of changes within a system without the need for healthy references. The new method also has the capability to quantify deterioration with respect to a calibrated value. Deterioration prognosis can then be facilitated for structural integrity predictions and maintenance purposes based on the quantified deterioration and forecasting algorithms. The proposed robust methodology is proven to be effective on an experimental setup for monitoring damage in locating fixtures. Fixture wear and failure are successfully detected by the methodology, and fixture structural integrity prognosis is initiated.     

An integrated fixture planning and analysis system for machining processes

Fixture planning is an important part of computer-aided process planning (CAPP), which is the link between design and manufacturing in a CIM environment. This paper presents a rational approach to computer-assisted fixture planning (CAFP), emphasizing integration of fixture planning with process planning, an issue that has not been adequately addressed until very recently. A systematic approach to fixture selection is outlined for planning of modular fixtures. A prototype CAPP-CAFP system has been developed at UCLA and linked to a commercial CAD system, namely, CADAM. Part design information can be extracted from the CAD model and multiple-view engineering drawings of a part stored in the CADAM system. Modular fixture elements can be selected automatically by the CAPP-CAFP system and the generated fixture layout can be displayed on the screen. Included also in the system is a fixture analysis module for verification and rationalization of a fixturing scheme. The force analysis module has a built-in local optimization routine that can determine the clamping forces of more reasonable magnitudes. The friction forces between the fixture and the workpiece can also be considered for simple cases.     

Stereo error detection, correction, and evaluation

An algorithm is presented for error detection and correction of disparity, as a process separate from stereo matching, with the contention that matching is not necessarily the best way to utilize all the physical constraints characteristic to stereopsis. As a result of the bias in stereo research towards matching, vision tasks like surface interpolation and object modeling have to accept erroneous data from the stereo matchers without the benefits of any intervening stage of error correction. An algorithm which identifies all errors in disparity data that can be detected on the basis of figural continuity and corrects them is presented. The algorithm can be used as a postprocessor to any edged-based stereo matching algorithm, and can additionally be used to automatically provide quantitative evaluations on the performance of matching algorithms of this class     

Efficient simplex computation for fixture layout design

Fixture, to hold an object by making multiple contacts, is a fundamental tool in manufacturing. A fixture layout is an arrangement of contacts on an object’s surface. Designing a fixture layout of an object can be reduced to computing the largest simplex and the resulting simplex is classified using the radius of the largest inscribed ball centered at the origin. We present three different algorithms to compute such a simplex: a simple randomized algorithm, an interchange algorithm, and a branch-and-bound algorithm. We also present methods to combine different algorithms to improve the performance and highlight their performance on complex 3D models consisting of thousands of triangles. Our randomized algorithm computes a feasible fixture layout in linear time and is well-suited for real-time applications. The interchange algorithm computes an optimal simplex in linear time such that no single vertex can be changed to enlarge the simplex, and it provides a practical method for complex 3D objects. The branch-and-bound algorithm computes the largest simplex by using lower and upper bounds on the radius of the inscribed ball, by which the computation time is reduced by a factor of five in comparison with the brute-force search.     

A geometric approach to automated fixture layout design

Fixtures are used in many manufacturing processes to hold objects. Fixture layout design is to arrange fixturing elements (fixels) on the object surface such that the object can be held in form-closure and totally immobilized. It is well known that 4/7 fixels are sufficient for immobilizing a 2D/3D object without rotational symmetry and their locations satisfy form-closure if and only if the convex hull of their primitive wrenches forms a 3D/6D simplex in the wrench space containing the origin as an interior point. This paper presents a method for finding form-closure locations of 4/7 fixels with enhanced immobilization capability. First, the Gilbert-Johnson-Keerthi distance algorithm and the Gram-Schmidt process are used to yield the fixel locations such that the simplex with vertices at their primitive wrenches is 3D/6D and contains the origin. Then, an interchange algorithm is developed for altering the fixel locations to meet form-closure and increase an immobilization capability index of fixture layouts. The meanings of this index in fixture localization accuracy and force balance capability are elucidated as well. Its value is proved to be equal to the minimum distance from the wrench origin to the facets of the wrench simplex in terms of a unit-invariant norm for wrench vectors. Without using any general optimization techniques, this method determines an optimal fixture layout very efficiently, so that it can be tried with various initial conditions to attain a result approaching the global optimum or with other good performance qualities.     

面向工业零件分拣系统的低纹理目标检测

目的 随着工业领域智能分拣业务的兴起,目标检测引起越来越多的关注。然而为了适应工业现场快速部署和应用的需求,算法只能在获得少量目标样本的情况下调整参数;另外工控机运算资源有限,工业零件表面光滑、缺乏显著的纹理信息,都不利于基于深度学习的目标检测方法。目前普遍认为Line2D可以很好地用于小样本情况的低纹理目标快速匹配,但Line2D不能正确匹配形状相同而颜色不同的两个零件。对此,提出一种更为鲁棒的低纹理目标快速匹配框架CL2D (color Line2D)。方法 首先使用梯度方向特征作为物体形状的描述在输入图像快速匹配,获取粗匹配结果;然后通过非极大值抑制和颜色直方图比对完成精细匹配。最后根据工业分拣的特点,由坐标变换完成对目标的抓取点定位。结果 为了对算法性能进行测试,本文根据工业分拣的实际环境,提出了YNU-BBD 2020(YNU-building blocks datasets 2020)数据集。在YNU-BBD 2020数据集上的测试结果表明,CL2D可以在CPU平台上以平均2.15 s/幅的速度处理高分辨率图像,在精度上相比于经典算法和深度学习算法,mAP (mean average precision)分别提升了10%和7%。结论 本文针对工业零件分拣系统的特点,提出了一种快速低纹理目标检测方法,能够在CPU平台上高效完成目标检测任务,并且相较于现有方法具有显著优势。     

Speed-accuracy characteristics of human-machine cooperative manipulation using virtual fixtures with variable admittance

This work explores the effect of virtual fixture admittance on the performance, defined by error and time, of task execution with a human-machine cooperative system. A desired path is obtained using computer vision, and virtual fixtures for assistance in planar path following were implemented on an admittance-controlled robot. The admittance controller uses a velocity gain, so that the speed of the robot is proportional to the force applied by the operator. The level of virtual fixture guidance is determined by the admittance ratio, which is the ratio of the admittance gain of the force components orthogonal to the path to the gain of the force components parallel to the path. In Experiment 1, we found a linear relationship between admittance ratio and performance. In Experiment 2, we examined the effect of admittance ratio on the performance of three tasks: path following, off-path targeting, and obstacle avoidance. An algorithm was developed to select an appropriate admittance ratio based on the nature of the task. Automatic admittance ratio tuning is recommended for next-generation virtual fixtures. Actual or potential applications of this research include surgery, assembly, and manipulation at the macro and micro scales.     

An Efficient Algorithm for Grasp Synthesis and Fixture Layout Design in Discrete Domain

This paper presents an efficient algorithm for grasp synthesis and fixture layout design in discrete domain. Given N candidate contact points on the surface of a 3-D object, the algorithm determines a minimal subset from the candidate points so that they construct a grasp or a fixture with the form-closure property. The proposed algorithm is implemented by solving a single linear program. It has been shown that a minimal subset of the contact points that achieves form-closure can be directly identified from the optimal basic solution vector of the linear program, provided that the solution vector is nondegenerate. The proposed algorithm has a straightforward geometric interpretation, which has also been described in the paper. The primary advantage of the proposed algorithm is its computational simplicity, of which the overall complexity is O(N). Numerical examples have been presented to verify the effectiveness and efficiency of the algorithm, which show that the computational time cost of the algorithm is much less than the existing techniques described in the literature     

Towards the design and development of a knowledge-based universal modular jigs and fixtures system

Jigs and fixtures are one of the important aspects of manufacturing. Parts may have different sets of fixturing requirements and call for different design strategies. Although there are numerous possibilities for fixture designs, a few basic configurations are clearly identifiable. Computer aided design (CAD) has done a little in assisting designers to design jigs and fixtures, making decisions of the best design selection, and providing designers with suggestions. The goal of this paper is to develop and document the design parameters and specifications utilized in jigs and fixtures design using universal modular jigs and fixtures design system (UMJFS). This is the first step to develop a knowledge-based Jigs and Fixture design and selection system. This application has the advantages of making the fixture design information completely modular and transparent, providing better match to the working conditions, reducing lead-time, and generally providing a significant enhancement of fixture productivity and economy. UMJFS has different standard and modular elements. This makes jigs and fixtures elements interchangeable and reusable. Designing a UMJFS then becomes a task of selecting and assembling the proper elements together.