Tolerance analysis for robotic pick-and-place operations

Robotic workcell design is a complex process, especially in case of flexible (e.g., bin-picking) workcells. The numerous requirements and the need for continuous system validation on multiple levels place a huge burden on the designers. There are a number of tools for analyzing the different aspects of robotic workcells, such as CAD software, system modelers, or grasp and path planners. However, the precision aspect of the robotic operation is often overlooked and tackled only as a matter of manipulator repeatability. This paper proposes a designer tool to assess the precision feasibility of robotic pick-and-place workcells from the operation point of view. This means that not only the manufacturing tolerances of the workpiece and the placing environment are considered, but the tolerance characteristics of the manipulation and metrology process (in case of flexible applications) as well. Correspondingly, the contribution of the paper is a novel tolerance modeling approach, where the tolerance stack-up is set up as a transformation chain of low-order kinematic pairs between the workpiece, manipulator, and other workcell components, based on manipulation, seizing, releasing, manufacturing, and metrology tolerances. Using this representation, the fulfillment of functional requirements (e.g., picking or placing precision) can be validated based on the tolerance range of corresponding chain members. By having a generalized underlying model, the proposed method covers generic industrial pick-and-place applications, including both conventional and flexible ones. The application of the method is presented in a semi-structured pick-and-place scenario.

Tolerance Modeling and Analysis Considering Form Defects for Spaceborne Array Antenna

Tolerance analysis is becoming increasingly important for tolerance design and optimization. When dealing with electromechanical products such as the array antenna, the form errors of the assembly must be considered. Traditional tolerance analysis with form errors relies on a double loop process, which is computationally expensive. A new tolerance analysis method is proposed in this paper, which can be achieved by a single loop process. First, a new tolerance modeling method considering form errors was proposed, it can represent the geometric error of a surface feature precisely. Then an effective sampling method was developed by introducing the variance separation method. An assembly simulation method was proposed to determine the final state of the whole assembly. Finally, the tolerance analysis was achieved based on the sufficient sample. The proposed analysis method was applied to an X-band spaceborne active-phased array antenna, numerical simulation results show the effectiveness of the method.

A Framework for Tolerance Modeling Based on Parametric Space Envelope

Geometric dimensioning and tolerancing (GD&T) tolerance standards are widely used in industries across the world. A mathematical model to formulate tolerance specifications to enable comprehensive tolerance analysis is highly desirable but difficult to build. Existing methods have limited success on this with form and profile tolerance modeling as a known challenge. In this paper, we propose a novel tolerance modeling framework and methodology based upon parametric space envelope, a purposely built variation tool constructed from base parametric curve. Under proposal, geometric variation (deviation as well as deformation) is modeled and linked to envelope boundary control points’ movement. This indirect tolerance modeling brings various benefits. It is versatile and can handle full set of tolerances specified under GD&T standards including form, profile, and runout tolerance. The proposal can deal with complex manufacturing part and is capable of providing modeling accuracy required by many applications. The proposed approach has added advantage of facilitating integration of various computer-aided systems to meet emerging industry demands on tolerancing in a new era of digital manufacturing. The proposed methodology is illustrated and verified with an industrial case example on a two-part assembly.