A systematic overview on the subject of assembly sequencing is presented. Sequencing lies at the core of assembly planning, and variants include finding a feasible sequence--respecting the precedence constraints between the assembly operations--, or determining an optimal one according to one or several operational criteria. The different ways of representing the space of feasible assembly sequences are described, as well as the search and optimization algorithms that can be used. Geometry plays a fundamental role in devising the precedence constraints between assembly operations, and this is the subject of the second part of the survey, which treats also motion in contact in the context of the actual performance of assembly operations.

/pdf/survey-on-assembly-sequencing-a-combinatorial-and-3talnh0rtw.pdf

Survey on assembly sequencing: a combinatorial and geometrical perspective

This paper proposes an acceleration-based robust controller for the motion control problem, i.e., position and force control problems, of a novel series elastic actuator (SEA). A variable stiffness SEA is designed by using soft and hard springs in series so as to relax the fundamental performance limitation of conventional SEAs. Although the proposed SEA intrinsically has several superiorities in force control, its motion control problem, especially position control problem, is harder than conventional stiff and SEAs due to its special mechanical structure. It is shown that the performance of the novel SEA is limited when conventional motion control methods are used. The performance of the steady-state response is significantly improved by using disturbance observer (DOb), i.e., improving the robustness; however, it degrades the transient response by increasing the vibration at tip point. The vibration of the novel SEA and external disturbances are suppressed by using resonance ratio control (RRC) and arm DOb, respectively. The proposed method can be used in the motion control problem of conventional SEAs as well. The intrinsically safe mechanical structure and high-performance motion control system provide several benefits in industrial applications, e.g., robots can perform dexterous and versatile industrial tasks alongside people in a factory setting. The experimental results show viability of the proposals.

An Acceleration-Based Robust Motion Controller Design for a Novel Series Elastic Actuator

In this paper, we present an overview of robotic peg-in-hole assembly and analyze two main strategies: contact model-based and contact model-free strategies. More specifically, we first introduce the contact model control approaches, including contact state recognition and compliant control two steps. Additionally, we focus on a comprehensive analysis of the whole robotic assembly system. Second, without the contact state recognition process, we decompose the contact model-free learning algorithms into two main subfields: learning from demonstrations and learning from environments (mainly based on reinforcement learning). For each subfield, we survey the landmark studies and ongoing research to compare the different categories. We hope to strengthen the relation between these two research communities by revealing the underlying links. Ultimately, the remaining challenges and open questions in the field of robotic peg-in-hole assembly community is discussed. The promising directions and potential future work are also considered.

Compare Contact Model-based Control and Contact Model-free Learning: A Survey of Robotic Peg-in-hole Assembly Strategies

An introductory review of active compliant control

Series elastic actuators (SEAs) have several mechanical superiorities over conventional stiff and non-back-drivable actuators, e.g., lower reflected inertia at output, greater shock tolerance, low-cost force measurement, energy storage, safety, and so on. However, their applications generally suffer from performance limitations, particularly in position control, due to insufficient controller designs. This paper proposes a unified active disturbance rejection motion controller for the robust position and force control problems of SEAs by using differential flatness and disturbance observer. It can suppress not only matched but also mismatched disturbances. Robust state and control input references are systematically generated in terms of a fictitious design variable, namely differentially flat output, estimations of disturbances and their successive time derivatives. The proposed robust motion controller improves the performance of SEAs when they suffer from internal and external disturbances, such as friction, inertia variation and external load, in real implementations. Experimental results are given to validate the proposal.

/pdf/a-unified-robust-motion-controller-design-for-series-elastic-57j80agv53.pdf

A Unified Robust Motion Controller Design for Series Elastic Actuators

Whether they are asked to polish or assemble parts, clean the house or open doors, the future generation of robots will have to cope with contact tasks under uncertainty in a stable and safe manner. Obtaining a controlled contact motion under uncertainty is still a major challenge for the robotics community. At present most research groups focus on one of the subcomponents (i.e., modeling, planning, estimation or control) of the system, and no overall system is developed yet. This paper presents a literature survey of the state-of-the-art of the subcomponents and points to the need for effective integration of those components.

Gudrun de Gersem

Papers

Active compliant motion: a survey