Constant-force Mechanism Research Articles

Topology Optimization Method for Designing Compliant Mechanism With Given Constant Force Range

Abstract This article presents a modified evolutionary topology optimization method for designing compliant constant force mechanisms (CFMs). CFM is defined as the mechanism that can generate constant force in the desired input displacement range, which is known as a constant force range. The force variation, i.e., fluctuation of output forces over the constant force range, is a critical parameter that reflects the stability of the output force. The key idea of the new method is that the design variables are increased or decreased for a certain small value instead of being changed between 0 (or xmin) and 1 in other evolutionary structural optimization (ESO) methods. As the CFMs have to experience a large deformation when it works, the influence of the nonlinearity needs to be considered. An additive hyperelasticity technique is utilized to alleviate the instability of the finite element analysis, which is introduced by the low-stiffness elements. The numerical examples show that the proposed design method can generate CFMs with desired constant force range and aspect ratio. The optimized CFM is manufactured by 3D printing, and the experimental result indicates that it can output an almost constant force (force variation ≤2%) in a large relative constant force range (56.7%).

Construction process of a novel multi-degree of freedom parallel passive compliant constant-force mechanism

Based on the configuration of the Stewart parallel mechanism (SPM), after analyzing the kinetostatic characteristic of a planar four-bar linkage with attached springs which works around the kinematic limb-singularity, a process of constructing a novel multi-degree of freedom (DOF) parallel passive compliant constant-force mechanism (CCFM) is presented. The kinematic limb-singularity of a rigid four-bar linkage with attached springs is obtained and then is applied to construct a constant-force compliant mechanism (CM) based on the pseudo-rigid-body-model by designing appropriate structure parameters to obtain appropriate equivalent spring stiffness of flexures. The CCFM is further used as the branches of the SPM and then a passive multi-DOF parallel mechanism (PM) is constructed. Here the method of replacing the original rigid branch of the PM with the CCFM is called the “Expanded Rigid-body Replacement Synthesis” (ERBRS). The multi-DOF parallel passive CCFM can generate a constant-force zone when makes a small pose transformation around a certain position, where each compliant branch is located at the constant-force zone. The proposed CCFM can be applied in practice such as the polishing machine which needs the constant contact-force between the polishing head and the workpiece. The proposed method of constructing a CCFM with one- or multi-degree of freedom mechanism has some important reference value on designing other types of CCFMs.

Design of Passive Constant-Force End-Effector for Robotic Polishing of Optical Reflective Mirrors

Polishing plays an indispensable role in optical processing, especially for large-aperture optical reflective mirrors with freeform surfaces. Robotic polishing requires effective control of the contact force between the robot and the mirror during processing. In order to maintain a constant contact force during polishing, traditional polishing robots rely on closed-loop control of air cylinders, whose performances heavily rely on high-fidelity force sensing and real-time control. This paper proposes to employ a compliant constant-force mechanism in the end-effector of a polishing robot to passively maintain a constant force between the robot and the mirror, thus eliminating the requirement for force sensing and closed-loop control. The compliant constant force mechanism utilizing the second bending mode of fixed-guided compliant beams is adopted and elaborated for the passive end-effector. An end-effector providing a constant contact force of 40 N is designed and prototyped. The polishing experiment shows that the passive constant-force end-effector provides stable contact force between the robot and the mirror with fluctuation within 3.43 N, and achieves RMS (Root Mean Square) lower than λ/10 (λ = 632.8 nm) of the polished surface of the large-aperture optical reflective mirror. It is concluded that the constant-force compliant mechanism provides a low-cost and reliable solution for force control in robotic polishing.

Design of compliant constant-output-force mechanisms using topology optimization

ABSTRACT Compliant constant-force mechanisms (CCFMs) are compliant mechanisms that provide nearly constant forces over prescribed deflection ranges. This study proposes a topology optimization method for designing hinge-free compliant constant-output-force mechanisms. Bifurcation cause the objective function to oscillate, leading to convergence failure. Therefore, a constraint for preventing bifurcation in constant-output-force mechanisms is proposed. Moreover, a minimum scale constraint method that extends the design domain is utilized in the proposed optimization algorithm. The relationship between the minimum length scale and the force variation in the optimized CCFM is obtained and discussed, and an optimized CCFM is fabricated and tested. The optimized CCFM exhibits a force variation of 0.7% over a constant-force stroke of 26% of the CCFM length and is thus competitive among recently proposed CCFMs.

Design and experimental study of an origami-inspired constant-force mechanism

Inspired by the strong nonlinearity of origami cartons, a novel origami-inspired constant-force mechanism (OriCFM) is systematically studied for obtaining a stable quasi-static constant force output. The proposed OriCFM is evolved by adding equivalent springs at horizontal and oblique creases of the rigid origami mechanism. Based on the geometry relationship, the mechanical model of the origami carton is established and its zero-stiffness is obtained. The influence of structural parameters and spring stiffness on the constant force characteristic of the OriCFM is analyzed in detail. By configuring different numbers of horizontal springs, various output characteristics could be observed in the prototype experiment. The case of 4 oblique springs and 2 horizontal springs exhibited partial constant-force output, and the constant-force range accounted for 48.54% of the total experimental stroke. Comparisons of theoretical results with experimental data were carried out to demonstrate the feasibility and effectiveness of the proposed origami-inspired mechanism. The outcomes of this work could expand the applications of origami mechanisms and promote the development of low-stiffness systems.

A Structural Optimization Framework to Design Compliant Constant Force Mechanisms With Large Energy Storage

Abstract In this study, we present a structural optimization framework to design constant force mechanisms (CFMs) with high energy storage capacity. In the framework, the constant force behavior with a zero preload is defined to be ideal, as this has the maximum energy storage given force and displacement limits. A graph-based topology selection, followed by shape optimization is conducted to select designs with energy storage most similar to the energy of the ideal constant force relation. The obtained CFM designs through this framework has a higher energy similarity index compared to typical designs from literature (0.95 versus 0.90). The constant force mechanisms developed through this study can be further applied in different robot/human–environment interfaces that benefit from both mitigating impact force and increasing energy storage.

Exploring and exploiting path based design optimization of a constant force mechanism

Compliant constant force mechanisms reduce wear and friction while providing precision in control, reduction in impact forces, and quasi zero stiffness. Current design techniques may result in high computational cost, significant stress concentrations, or design space inefficiencies. This study aims to give a low computational cost design strategy that more fully exploits the design space and reduces stress concentrations. A new design approach for these mechanisms is proposed that: (1) generates and describes paths from graph methods to avoid significant stress concentrations, (2) explores these paths to reduce the computational cost, and (3) more fully exploits the design space using design optimization. The optimized mechanism is then validated experimentally. The proposed method provides designs that exhibit a higher percentage of constant force-displacement (90% compared with 70.5%) compared with similar mechanisms of reduced friction in the literature. The design approach proposes five parent paths that can be optimized to design constant force mechanisms with arbitrary design requirements. This reduces the number of design variables and related computational costs required compared with similar methods in the literature.

Configuration design and experimental verification of a variable constant-force compliant mechanism

AbstractThe realizing of variable output constant force has received wide attention. To achieve a force regulation in an economic way, a configuration of the constant force mechanism (CFM) referring to positive and negative stiffness combination method is proposed in this paper. By adjusting preloading displacement applied on positive-stiffness structure of the CFM, the variable constant force output can be realized. The force–displacement expression of the CFM in the non-preloaded condition is deduced by the established analytical models. Furthermore, parametric sensitivity analysis with several architectural parameters are conducted for optimizing physical structures. Finally, the correctness of the proposed principle is verified by experimental studies. The observed experimental results show that the CFM under different preloading displacements can provide required output constant force, which is consistent with proposed hypothesis.

Modeling and design of a reconfigurable novel constant-force mechanism for assistive exoskeletons

Constant-force mechanisms can be used for gravity counterbalance and force regulation. While many constant-force mechanisms have been designed, most of them have a limited range of force adjustment, which brings very limited capability to maintain an equilibrium state when load changes. In this paper, a novel design of reconfigurable constant-force mechanism is proposed. The new design is inspired by a variable stiffness mechanism, which can work in a similar way as the hinged lever, but take a compact space. Moreover, the design allows to adjust the configuration parameters achieving a large range of the equilibrium state adjustment. Mathematical models are developed to simulate the equilibrium performance changed with configuration parameters. A prototype is constructed and validates the working principle of the design. A design case is included to show the application of the mechanism in a passive assistive upper-body exoskeleton.

Design of a new passive end-effector based on constant-force mechanism for robotic polishing

Polishing is an important final machining process in manufacturing. For robotic polishing, active compliance control is the most frequently used approach to control the contact force between the end-effector and workpiece. However, it usually exhibits the problem of force overshoot at the start of contact, with poor force accuracy normally larger than 1 N. This paper proposes the concept design of a novel end-effector based on constant-force mechanism for robotic polishing. This design is the first constant-force mechanism based robotic end-effector for use in polishing experiment. An industrial robot is used to position the end-effector and the end-effector regulates the contact force passively. The constant-force motion range acts as a buffer to counteract the excessive displacement caused by inertia. As a result, there is no force overshoot, producing the consistency for the workpiece’s surface quality. Moreover, the property of the constant force ensures more accurate contact force without using a complex controller. Design, modeling, and simulation study have been performed to demonstrate the proposed idea. A prototype is fabricated for experimental testing. The end-effector is adopted to polish rusty steel, and the recorded contact force signal during polishing demonstrates the effectiveness of the constant-force mechanism in counteracting the force overshoot and improving the force accuracy. Results indicate that the polished surface is extremely uniform with steady contact force regulated by the constant-force mechanism.

Design of a compliant adjustable constant-force gripper based on circular beams

Compliant grippers have been widely used in engineering systems to achieve precision operations. In this paper, a compliant adjustable constant-force gripper based on circular beams is developed for realizing the compatible and stable constant-force operation. In the developed gripper, a stiffness-combination constant-force mechanism (SCCFM) is applied as its basis. A pseudo-rigid-body PPRR model and particle swarm optimization algorithm are utilized for circular-beam modeling and identification of the SCCFM, respectively. A devised preload mechanism realizes the adjustment of the initial preloading state of the SCCFM. A series of experiments are conducted to demonstrate the effectiveness of the developed SCCFM and the gripper.

Experimental validation of a constant-force mechanism and analysis of its performance with a calibrated multibody model

The potential applications of constant-force mechanisms (CFMs) have increased in recent years with the development of new designs. This paper focuses on the experimental evaluation and the mathematical modelling of a CFM. The mechanism is based on the use of cams, rollers and springs, which allows for a large stroke and user-adjustable force value. The evaluation and validation of the mechanism is carried out by performing a series of tests examining the constancy of the input force, and where the influence of friction forces and the manufacturing process on the performance of the mechanism is revealed. A multibody model of the constant-force mechanism is developed and calibrated with the help of the experimental results. This model is based on the assumption of rolling without sliding between the cam and the roller, what makes the multibody model to include velocity constraints. The calibration of the model pursues the development of a mathematical tool to study the performance of the mechanism in conditions that are different from those of the experiments presented. The CFM model is used to perform a sensitivity analysis of some important design parameters of the mechanism.

A curved-beam based quasi-constant force mechanism supporting large range and force-sensitive robotic manipulation

Emerging applications of force-sensitive manipulation with the requirements on grasping safety and efficiency pose significant challenges on the design of compliant constant-force mechanisms with large operating ranges. In this paper, we propose a novel curved-beam based quasi-constant force mechanism fully compatible with the additive manufacturing technology, which is configurable with appropriate arc-elements to achieve quasi-constant force output with a large motion stroke. We develop an analytical model to describe the static deformation behavior of the proposed mechanism and analyze the performance sensitivity of its structural parameters, with which we also propose the optimization formulation and general guidelines for the design of curved-beam based compliant quasi-constant force mechanisms to meet force and displacement requirements. Finite element analysis and experiments are also conducted to comprehensively evaluate the proposed design, demonstrating significant performance improvement over existing results.

Design, analysis and evaluation of a self-lockable constant-force compliant gripper

Maintaining a constant gripping force is a challenging problem when manipulating fragile objects with variations of sizes and stiffness. In this paper we develop a novel constant-force compliant gripper with clamp state locking capability, which is composed of the constant force module, the multi-stable module, the adjustment module and the actuator module. In particular, the symmetric compliant slider-crank mechanism is employed to maintain a constant force between the jaw and object. The multi-stable module with behaviors of snap-through and negative stiffness is designed to balance the interaction force from the constant force mechanism (CFM). With this, the gripping state, including the gripping force and the jaw position, can be locked during the gripping operations. Theoretical model of the constant-force compliant gripper is also established, where the force-displacement relationships is analyzed and verified by finite-element simulations. Experimental studies on an additive manufactured prototype are further conducted, which effectively validates the constant gripping force and force self-locking performance of the developed gripper.

Constant Torque Mechanisms: A Survey

Abstract This work gives an overview of all types of constant torque mechanisms (CTMs) based on both function and structure. Based on their architecture, they can be divided into five distinct categories, which all have their specific behavior. It is also shown that these CTMs can be divided into two application types, namely, power assistance or torque stabilization. Constant force mechanisms (CFMs) are more prevalent in the literature; therefore, some discussion is proposed on how CFMs can be transformed into CTMs. It is also shown that some of these CTMs have a very high specific energy, which makes them potentially interesting for use as energy storage/power providers in novel fields like robotics.

Gravity Compensation for Mechanisms with Prismatic Joints

This paper is devoted to the design of gravity compensators for prismatic joints. The proposed compensator depends on the suspension of linear springs together with mechanical transmission mechanisms to achieve the constant application of force along the sliding span of the joint. The use of self-locking worm gears ensures the isolation of spring forces. A constant-force mechanism is proposed to generate counterbalance force along the motion span of the prismatic joint. The constant-force mechanism is coupled with a pin-slot mechanism to transform to adjust the spring tension to counterbalance the effect of rotation of the revolute joint. Two springs were used to counterbalance the gravity torque of the revolute joint. One of the springs has a moving pin-point that is passively adjusted in proportion with the moving mass of the prismatic joint. To derive the model of the compensator, a 2-DoF system which consists of a revolute and a prismatic joint is investigated. In contrast to previous work, the proposed compensator considers the combined motion of rotation and translation. The obtained results were tested in simulation based on the dynamic model of the derived system. The simulation shows the effectiveness of the proposed compensator as it significantly reduces the effort required by the actuators to support the manipulator against gravity. The derived compensator model provides the necessary constraints on the design parameters.

A Large Range Constant Force Microgripper With a Three-Stage Compliant Amplification Mechanism

This paper proposes a novel piezoelectric actuated constant force microgripper with large gripping range for high precision micro-manipulation. The microgripper consists of a three-stage compliant amplification mechanism, a guide mechanism, and a constant force mechanism, enabling large displacement output and stable force output. The analytical model of presented microgripper is established using the elastic beam theory. Numerical simulations provided by finite element analysis (FEA) are carried out to verify the analytical model, and to evaluate the force and displacement characteristics of the mechanism with different gripping objects. Experimental tests are further implemented to validate the performance of the mechanism, and the fabricated microgripper can grasp objects with a constant force of 0.3N in a large displacement interval ranging from 65um to 130um under a voltage application of 100-V. The results of simulation and experiment clearly demonstrate that the constant force microgripper has a large work stroke, allowing effectively gripping objects in various sizes and stiffnesses.

Sensorless Control for a Novel Electric Lift Cart with a Constant Driving Force Mechanism

Sensorless Control for a Novel Electric Lift Cart with a Constant Driving Force Mechanism

Design and Test of a Curved-Beam Based Compliant Gripper for Manipulations of Actively Deformable Objects

Sizes and stiffness variations of actively deformable objects pose significant challenges on the design of compliant constant-force gripper. This paper presents a curved-beam based constant force compliant gripper which is composed of the constant force module, the bistable module, the preloading module and the linear guide. A curved-beam constant force mechanism is designed to generate constant force output, the non-constant force motion range of which is further eliminated via curved-based bistable mechanism and preloading module. After a formulation to find the optimal gripper configuration, the design is verified through comparison with simulation results. Finally, a prototype of the proposed gripper is tested to demonstrate its grasping capacity.

FEA-based optimization and experimental verification of a typical flexure-based constant force module

To solve the problem of insufficient constant-force stroke of the compliant constant force mechanism (CFM), a FEA-based optimization method based on structural parameters is proposed in this paper. Firstly, a flexure-based constant force module (FBCFM) combined with a Z-shaped beam and a bi-stable beam is designed. By analyzing the force-displacement relationship of the two kind beams, the architectural parameters related to the constant force characteristic of the FBCFM are derived. Then, the initial constant-force stroke with 2.2 mm and output constant force with 13.8 N of the FBCFM are obtained by experimental studies. Furthermore, to explore the parametric effects for the designed FBCFM, parametric evaluation is conducted via MATLAB. Finally, the optimal solution set of architecture parameters is obtained with FEA-based optimization method. Through experimental verification, the constant-force stroke of the optimized FBCFM is 4.5 mm with constant force 8.5 N. The optimization results show the effectiveness and feasibility of the proposed FEA-based optimization method.