Topology Optimization for a Rigid‐Compliant Hybrid Constant Force Gripper

A compliant constant-force mechanism is a passive force regulation device that can generate a nearly constant output force over a range of input or output displacements while without the use of sensors and feedback control. In topology synthesis of a compliant constant-force mechanism for a given input displacement range, the constant output force can be achieved by maintaining the output displacement to be nearly a same value while the input displacement increases. In order to further control the desired output displacement for the compliant constant-force mechanism before contacting the object, this article introduces a new composite objective function that can consider both the output force (with contact) and the output displacement (without contact) of the synthesized compliant mechanism. The sensitivity for the proposed objective function with respect to the element density is derived while considering the effect of nonlinearity in the large deformation condition. The proposed topology optimization method is used to design an innovative constant-force compliant finger, and its prototype is manufactured by 3-D printing using a flexible thermoplastic elastomer. The experimental results show the developed constant-force compliant finger can provide a nearly constant output force of 41.9 N over the input displacement ranging from 15 to 30 mm while the maximum and average force variations within the constant-force range are 2.2% and 0.9%, respectively. In addition, the developed constant-force compliant finger is used to design a three-fingered constant-force compliant gripper that can be used in robotic grasping of fragile objects.

This paper presents the design of a novel flexure-based precision positioning stage with constant output force for biological cell micromanipulation. One uniqueness of the proposed design is that it produces a constant force without using a force controller. Only a motion control is needed to produce a constant output force, which significantly simplifies the system design process. The stage is driven by a piezoelectric actuator through a displacement amplifier. Analytical models of the displacement amplifier and the zero-stiffness structure are established and verified by conducting finite-element analysis simulations. The structure parameters are optimally designed to guarantee the requirement on output force, motion range, and physical size. A prototype stage is fabricated by 3-D printing process and a series of experiments is carried out. Experimental results show that the developed positioning stage delivers a near constant output force with slight fluctuation in the reachable constant-force motion range of $138~\mu \text{m}$ . The applications of the developed constant-force stage in biological cell manipulation have been demonstrated through experimental investigations. Note to Practitioners —A constant-force stage can produce a constant output force without using a force control. It is attractive for biological micromanipulation. This paper presents the design and testing of a novel constant-force flexure stage. The constant force indicates a zero stiffness for the mechanism. The stage mechanism is devised using modified leaf flexure (MLF) to achieve positive-stiffness structure. Bistable beams are used to design negative-stiffness structure by making use of their postbuckling characteristics. Two bistable beams and two MLFs are combined together to construct a zero-stiffness structure. A conventional stage is also fabricated for comparison study. The performance of the proposed constant-force stage has been verified by simulation and experimental studies. Results indicate that the developed stage system has great superiority over conventional one in terms of reducing driving force, increasing motion range, and reducing force fluctuation. Experimental demonstration of bio-micromanipulation has been presented to reveal its potential applications.

There is a huge demand of manipulating delicate micro objects in various fields such as biotechnology, micro electro mechanical systems (MEMS). However, due to the uncertainty in sizes and stiffness of those micro objects, it makes this task more challenging. Those systems have rigorous requirements for the regulation of the output force from the gripping tool. Apparently, the use of sensors and feedback systems makes the mechanism both complicated and cost intensive. This paper presents a novel design concept of a gripper which can manoeuvre objects in various sizes but it still can preserve a constant gripping force by integrating a constant force mechanism. No sophisticated sensors and control systems are required for the design thanks to the usage of compliant mechanism. The passive type integrated compliant constant force mechanism exhibits its characteristics from combined compression and bending of beam structures. The design methodology of this compliant constant-force gripper mechanism (CFGM) using genetic algorithm shape optimization is presented. Finite element analyses are used to characterize the constant-force behavior of the gripper under static loading. A design formulation is proposed to find the CFGM shape for a specified gripping range with constant output force of the mechanism. The benefits of the monolithic nature of the compliant mechanism would also extend the miniaturization possibility of the CFGM as a micro-gripper for MEMS applications.

This study presents an optimal design procedure, including topology and geometry optimization methods to design a compliant constant-force mechanism, which can generate a nearly constant output force over a range of input displacements. The proposed constant-force mechanism is a passive force regulation device that can be used in various applications such as precision manipulation and overload protection. The numerical optimization problem is treated as an error minimization problem between output and objective forces. Both material and geometric nonlinearities are considered in topology and geometry optimization steps. Although the element stiffness for void and gray elements after topology optimization are quite small comparing with solid elements, their existence also contributes to the output force characteristic of the synthesized mechanisms. As these low-stiffness elements are not easy to manufacture in physical prototype, a helical compression spring is introduced in the topology optimized constant-force mechanism to account for the effect of low-stiffness elements, and an additional geometry optimization step is utilized to identify the spring constant as well as to fine-tune the geometric parameters. The optimized constant-force mechanism is prototyped by three-dimensional printing using flexible thermoplastic elastomer. The experimental results show that the proposed design can generate a nearly constant output force in the input displacement range of 3-6 mm. The developed constant-force mechanism is installed on an electric gripper drive mounted on a robot arm for robotic picking and placing application. Test results show the constant-force gripper can be used in handling of size-varied fragile objects.

Design of an SMA-driven compliant constant-force gripper based on a modified chained pseudo-rigid-body model

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.

This chapter presents the design of a novel flexure-based compliant gripper with constant gripping force and compact structure size for cell micromanipulation applications. The gripper removes the use of force sensor and provides a near constant force output via its mechanical structure, which greatly simplifies the system design process. The compact size of the gripper is achieved by the serial connection of a bistable beam and a positive-stiffness beam. Moreover, a combined mechanism, which can alter the fixing angle of the two gripper jaws, is developed to enlarge the handling size. Analytical modeling and finite element analysis are conducted to predict the gripper performance. A prototype gripper is fabricated by 3D printer, and a series of experiments are carried out to verify its performance. Grasp testing of crab egg embryos has been carried out to demonstrate its effectiveness in biological micromanipulation application.

This paper introduces the design method of a novel constant force mechanism (CFM), which can provide specific constant force for large-stroke. The oblique trapezoidal mechanism with straight beam provides negative stiffness, and the oblique trapezoidal mechanism provides positive stiffness. The negative and positive stiffness mechanisms are paralleled to obtain the constant force mechanism. One uniqueness of the CFM lies in that it can produce a large-stroke stable constant output force through input displacement. The CFM has a simple structure and occupies little space. The mathematical model is established by using pseudo-rigid body model (PRB) for positive stiffness mechanism and chain beam constraint model (CBCM) for negative stiffness mechanism, and it is verified by conducting simulation study with finite-element analysis (FEA). The results show that the mechanism can provide a constant output force of about 15N in a motion range of 5 mm. The constant force mechanism can be used in micro-operation tasks requiring constant force to effectively avoid damage to objects caused by excessive output force.

This chapter proposes the design, modeling, and control of a compliant gripper with a passive-type constant-force mechanism. The constant force output is enabled by combining a positive-stiffness mechanism and a negative-stiffness mechanism. The negative stiffness is produced by a bistable fixed-guided beam with buckling behavior. The developed constant-force gripper allows the generation of constant force via its mechanical structure, which allows the elimination of force control. Analytical modeling and nonlinear finite element analysis (FEA) simulation study are carried out to evaluate the gripper performance. A prototype is developed for experimental study. To achieve a precise positioning of the gripper jaw, a discrete-time sliding mode control strategy is developed on the basis of a nonswitching-type reaching law. The effectiveness of the gripper system is validated by performing experimental studies on grasp-hold-release manipulation of micro-object.

This paper presents the design, simulation, and testing of a compliant gripper that can provide a constant gripping force to handle objects of various sizes. Maintaining a proper gripping force is challenging when manipulating delicate objects with uncertain sizes and stiffnesses. To avoid damage and provide a stable grip of an object, force feedback is often required so that the gripping force can be directly or indirectly regulated. Without using additional sensors and control, the proposed gripper passively maintains a constant prespecified contact force between fingertip and object. The gripper is designed to have a constant input force generated by a constant-force mechanism (CFM). Transmitted through a statically balanced (SB) mechanism, a constant gripping force is obtained at the fingertip. After a formulation to find the optimal gripper configuration, the design is verified through comparison with simulation results. Finally, a prototype of the constant-force gripper is demonstrated. The novel gripper is expected to serve as a reliable alternative for object manipulation.

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

This paper presents the design of a compliant constant force output gripper mechanism. The function of constant force output is achieved by using the negative stiffness effect of a buckled fixed-guided beam. One advantage is that it can eliminate the needs of complicated force-displacement combined control algorithm. Details of the nonlinear design process have been demonstrated. ANSYS APDL and MATLAB are used to solve this nonlinear problem. The structural design of the gripper is performed based on the theoretical model. An experimental study is carried out to verify the theoretical model. A force sensor and a displacement sensor are used to test the performance of the constant force output in the experiments. Results shows that the gripper can provide 1.1 N near constant force output in 200 µm range.

This paper proposes the design and simulation of a passive type of constant-force microgripper which is based on microelectromechanical systems (MEMS). The constant force is realized by making use of a combination of inclined bistable beams and straight leaf flexures. The constant output force is activated when the input displacement reaches in the range between 10 μm and 20 μm. Analytical model is built to facilitate the structure design of the gripper. Finite element analysis (FEA) simulation study is conducted to verify the theoretical model and to predict the performance of the microgripper. Owing to the large force and displacement, Z-shaped beams are selected as electrothermal actuator to drive the gripping arm. Results show that a gripping displacement of 39 μm can be achieved with an input voltage of 5 V.

Compliant constant-force mechanisms combine the effects of mechanical advantage and stored strain energy of flexible members to obtain constant output forces for a large range of input displacements. This paper extends and compliments previous work by accomplishing the following: i) dimensional synthesis is performed for a number of compliant constant-force mechanism configurations, ii) a simplified method of determining the magnitude of the constant output force is presented, and iii) experimental validation of the theory is addressed by reporting the results of testing three constant-force configurations. The results of i) and ii) are presented in a manner to be easily used by engineers designing such mechanisms. The results of iii) show that the mechanisms do follow a nearly constant force for a large input displacement, as predicted.

The evolution of constant-force mechanisms is propelled by a growing interest in being able to exert constant or near-constant force in various applications. Compliant mechanisms have recently received much attention in the design of constant-force mechanisms because of their several advantages, e.g. fewer parts, compact construct, natural energy storage, no backlash, among many others. There have been many research efforts in developing various techniques to design these mechanisms for applications in diverse fields. Several of these techniques require design optimization to generate a constant force over a desired range of motion. There is generally a lack of understanding of the mechanics of the generation of constant force. This paper presents the hypothesis that simple arrangements, such as a rigid link with a torsional spring, or compliant segments, under axial loading are capable of producing constant force. Three compliant segment types are considered herein: fixed-free, pinned-pinned, and fixed-guided beams under axial loading, to demonstrate that they can exert near-constant force, without the need for a design optimization. This paper further exemplifies that the proposed theory is the kernel to generating constant force by different mechanism configurations.