Gripper Jaws Research Articles

Uncertainty-integrating, automated design of gripper jaws for robust grasping of electrical connectors

AbstractFor robust grasping of workpieces with complex surface geometries such as the plugs of electrical connectors (ECs), individually designed gripper jaws are commonly required. The manual design of the latter is time-consuming, iterative, expensive, and requires expert knowledge. Therefore, automating the design process offers the potential to increase efficiency and reduce costs. However, the plugs of ECs often involve interference contours which pose a high risk for a grasp to fail. Thus, this paper introduces an approach for the automated design of gripper jaws that considers complex surface geometries with interference contours. Thereby, expected geometric uncertainties in the relative pose between a plug and a gripper system can be parameterized by a user. The key steps of our approach are to automatically simulate collision volumes between the gripper jaws and the workpiece to map geometric uncertainties in software for computer-aided design (CAD) and to adapt the gripper jaws accordingly within minutes and without manual design. The results of the experimental validation using three different high-voltage plugs show that the presented approach is suitable for robust grasping of such workpieces. In summary, our work intends to contribute to a more efficient design process of gripper systems for workpieces with complex surface geometries such as plugs of ECs.

An SMA-based compliant adjustable constant force gripper for micro-assembly

Ensuring precise and consistent force regulation in a confined workspace is critical for high-quality micro-assembly operations. To enhance structural efficiency and reduce system complexity, this study leverages the variable stiffness properties of the shape memory alloy (SMA) and presents a novel compliant adjustable constant force microgripper. In this work, a passive variable stiffness chained pseudo-rigid-body model is established to address the coupling analysis problem arising from the mechanical and SMA’s in-homogeneous phase transformation properties, thereby facilitating the integrated design of the SMA and the mechanism. Exploiting the variable stiffness property of SMA, an improved stiffness-tunable two-beam-binding constant force mechanism is proposed for the gripper jaws, which makes adjustable constant force operation via current control can be achieved straightforwardly. Further, inspired by differential actuation mode, a compact SMA-based actuator comprising two parallel guide mechanisms and SMA drivers is developed to meet the requirement for the large working stroke of the gripper. Numerical simulations and experimental studies validate the design model’s efficiency and the proposed structures’ capabilities, and also prove the gripper’s constant operation force value can be regulated in the range of 0.152 to 0.381 N. With dimensions of 35×56.5×7 mm3 and a maximum stroke of 2.077mm, the gripper demonstrates fine compactness and potential for continuous, cross-scale micro-assembly.

Reorientation of a Part by Parallel Gripper Allowing Interference with the Gripper Jaw and Release in an Unstable Orientation

Reorientation of a Part by Parallel Gripper Allowing Interference with the Gripper Jaw and Release in an Unstable Orientation

Design of Gripper Jaws Applicable to Multiple Sizes and Types of Bowls for Upside-down Placement in a Dishwasher Rack

Design of Gripper Jaws Applicable to Multiple Sizes and Types of Bowls for Upside-down Placement in a Dishwasher Rack

Frictional properties of red pine cones harvested under different conditions

The climbing of trees to pick pinecones is a high-risk exercise. In this study, a mechanical gripper jaw was designed. Frictional characteristics between the pinecones and the mechanical gripper jaw during pinecone picking under different conditions were investigated using a workbench simulation, homemade inclined friction meter, and mass tester. Three-level orthogonal and one-factor tests were conducted. The relationship between the water content and friction properties and between the water content and hardness were investigated, and conclusions were drawn on how water content affected friction properties by influencing hardness. The results showed that the contact material greatly affected the friction properties. The pinecone water content was maintained between 24% and 28% to ensure that the coefficient of friction was maximized and that the pinecones were sufficiently hard to dislodge. Additionally, a prototype machine was used to perform pinecone-gripping experiments to validate the experimental and simulation results. Consequently, the results of this study provide a useful reference for the structural design of pinecone picking robots and the picking reason.

A Comparative Study of Different Fingertips on the Object Pulling Forces in Robotic Gripper Jaws

This paper presents a comparative study of the use of different fingertips in robotic gripper jaws with respect to measuring the pulling force of selected shaped objects from their grasp. The authors built a dedicated test stand and provided methodology to evaluate it. The authors’ innovative approach was to design accessory-controlled jaws for the base of the Robotiq 2F-140 gripper. For the study, rigid structures—flexible soft cushions filled with air and magnetorheological fluid (MRF)—were developed for the jaw. In this way, comparable measurement results were obtained in terms of the structure of the gripper set-up. The secondary purpose of the study was to demonstrate the potential of the soft cushions that are adaptable to the shape of a gripped object. As a result, an adaptive structure was obtained that allows object pulling forces that are comparable to rigid fingertips. In doing so, this does not damage the surface of any of the interacting components. The cushions were made of thermoplastic polyurethane (TPU) formed using 3D printing technology. The results obtained during the implementation of this research may be beneficial for comparing gripper capabilities; thus, they can contribute to advances in smart devices and many industrial fields, including robotics and bioengineering.

ROBOTIC GRIPPER ACTUATED USING THE SHAPE MEMORY ALLOY ACTUATORS

The article deals with the design of a gripper for a manipulator. The use of a shape memory alloy actuator, which is excited by heating by means of an electric current, is proposed for driving the gripper jaws. Variant solutions of the gripper kinematics and the final design of the gripper arrangement are solved. A gripper kinematics simulation is created to find a suitable gripper geometry.

Robust design optimization by spline dimensional decomposition

This article highlights new spline-empowered computational methods for solving robust design optimization (RDO) problems in complex mechanical systems. The methods are predicated on a spline dimensional decomposition (SDD) of a high-dimensional, discontinuous, or nonsmooth stochastic response for statistical moment analysis, a novel fusion of SDD and score functions for calculating the second-moment sensitivities with respect to the design variables, and standard gradient-based optimization algorithms. New closed-form formulae are derived for the design sensitivities that are concurrently determined along with the moments. The methods depend on how the statistical moment and sensitivity analyses are engaged with an optimization algorithm, engendering direct and multi-point single-step design processes. Numerical results reveal that the proposed methods deliver accurate and computationally efficient optimal solutions to RDO problems, including an industrial-scale shape design of a robotic gripper jaw.

ANALYSIS OF INCREASING THE FRICTION FORCE OF THE ROBOT JAWS BY ADDING 3D PRINTED FLEXIBLE INSERTS

3D printing technology plays a key role in the production of prototypes and final functional parts. The ability to produce almost any shape using this technology in combination with lightweight materials is often used to minimise the weight of the designed components. However, for some applications, such as robot gripper jaws, conventional most commonly used materials, such as PLA, may be unsuitable due to their low coefficient of friction on the material of the manipulated object, which in some cases may cause the object to slip in the robot jaws. This article describes a technical problem from practice, where a manipulated object made of steel material slipped in the printed PLA jaws of the robot during its working cycle. This work is devoted to increasing the friction force of the robot jaws by adding 3D printed soft inserts. Two insert surface shapes made of two flexible materials TPU 30D and TPE 88 are tested. The increase in friction force is measured on a measuring device with an industrial robot and a force measuring sensor. The most suitable type of inserts and material is then tested on a collaborative robot at its required working cycle. The results of this experiment are intended to help designers as a source of information or inspiration in designing similar applications.

Development of a 3D Printed Double-Acting Linear Pneumatic Actuator for the Tendon Gripping.

The lack of standardization in tissue testing procedures results in a variety of custom-made devices. In the case of the determination of the mechanical properties of tendons, it is sometimes necessary to adapt the existing laboratory equipment for conducting experiments when specific commercial equipment is not applicable to solve issues such as proper gripping to prevent tendon slipping and rupturing, gripping control and manoeuvrability in case of tendon submerging and without contamination of the testing liquid. This paper presents the systematic development, design, and fabrication using 3D printing technology and the application of the double-acting linear pneumatic actuator to overcome such issues. It is designed to do its work submerged in the Ringers’ solution while gripping the tendon along with the clamps. The pneumatic foot valve unit of the Shimadzu AGS-X tensile testing machine controls the actuator thus preventing Ringers’ solution to be contaminated by the machine operator during specimen set-up. The actuator has a length of 60 mm, a bore of 50 mm, and a stroke length of 20 mm. It is designed to operate with an inlet pressure of up to 0.8 MPa. It comprises the cylinder body with the integrated thread, the piston, the piston head, and the gripper jaw. Fused deposition modeling (FDM) has been used as the 3D printing technique, along with polylactic acid (PLA) as the material for 3D printing. The 3D printed double-acting linear pneumatic actuator was developed into an operating prototype. This study could open new frontiers in the field of tissue testing and the development of similar specialized devices for medical purposes.

6DLS: Modeling Nonplanar Frictional Surface Contacts for Grasping Using 6-D Limit Surfaces

Robot grasping with deformable gripper jaws results in nonplanar surface contacts if the jaws deform to the nonplanar local geometry of an object. The frictional force and torque that can be transmitted through a nonplanar surface contact are both 3-D, resulting in a 6-D frictional wrench (6DFW). Applying traditional planar contact models to such contacts leads to overconservative results as the models do not consider the nonplanar surface geometry and only compute a 3-D subset of the 6DFW. To address this issue, we derive the 6DFW for nonplanar surfaces by combining concepts of differential geometry and Coulomb friction. We also propose two 6-D limit surface (6DLS) models, generalized from well-known 3-D LS (3DLS) models, which describe the friction-motion constraints for a contact. We evaluate the 6DLS models by fitting them to the 6DFW samples obtained from six parametric surfaces and 2932 meshed contacts from finite element method simulations of 24 rigid objects. We further present an algorithm to predict multicontact grasp success by building a grasp wrench space with the 6DLS model of each contact. To evaluate the algorithm, we collected 1035 physical grasps of ten 3-D-printed objects with a KUKA robot and a deformable parallel-jaw gripper. In our experiments, the algorithm achieves 66.8% precision, a metric inversely related to false positive predictions, and 76.9% recall, a metric inversely related to false negative predictions. The 6DLS models increase recall by up to 26.1% over 3DLS models with similar precision. <xref ref-type="fn" rid="fn1" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">¹</xref>

Self-tuning hybrid fuzzy sliding surface control for pneumatic servo system positioning

This paper presents a new robust control strategy developed for the pneumatic servo system (PSS) by hybridizing two types of fuzzy logic control (FLC) rules as a self-tuner to the integral sliding mode control (ISMC), namely self-tuning hybrid fuzzy sliding surface control (SH-FSSC) controller. A sliding surface consisting of two switched fuzzification rules, relying on the tuning threshold value of the position error tracking, was designed to consider both the position and the force feedback of the pneumatic proportional valve with a double-acting cylinder (PPVDC) system. The approach is to acquire multiple features not only on tracking error but also faster transient response with finite-time convergence, chatter elimination, and robustness against uncertainty. The proposed control strategy was verified and validated by conducting experiments with the actual PPVDC unit linked to the tip of the robot’s tri-finger pneumatic grippers (TPG) platform. The experimental works were accomplished using two types of input trajectories: multi-steps and sinusoidal input trajectories. On the other hand, an additional external payload as a disturbance to the test rig has also been added at the end of the pneumatic gripper jaw, intended to evaluate the proposed controller’s robustness performance. The advantage of the proposed method was validated by significantly eliminating oscillation for each transient response, maintaining high tracking performance, and minimizing hysteresis effects. The oscillation was suppressed with minimal overshoot, and the proposed method was achieved without a significant loss of performance.

Experimental Research on Behaviour of 3D Printed Gripper Soft Jaws

In this paper, the problem of the behaviour of soft jaws that can be used to replace the steel jaws of grippers is studied. One of the advantages of additive manufacturing is the printing of fully functional parts. Choice of material is often related to the part strength. The mechanical properties of 3D printed parts should meet the service loading and, also, must be comparable with parts produced by traditional manufacturing techniques - machined parts or injection moulding. From the specialized literature information regarding the test results for effect of various printing parameters on part strength are available made in laboratory conditions and for standard test sample. For ABS materials various values for Young module are presented varying from 1.5 GPa to 2.15 GPa, for 100% infill rate and various modified parameters such as raster orientation. In order to study the behaviour of soft gripper jaws several part were printing and the resistance to bending was tested, by simulating the way a gripper works. An experimental stand was built using a force transducer and a displacement transducer to measure the deformation of the jaw, obtained by 3D printing, under load. The mechanical elastic hysteresis loop during an experimental loading/unloading was plotted and the amount of mechanical energy lost during a cycle, dissipated because the internal friction, was determined. Finite element analysis method was applied to make a comparison with the experimental results. In the finite element analysis, several simulations were considered, varying Young s modulus for the tested material.

Design, Fabrication and Vision Based Operational Analysis of Novel Shape Memory Alloy Micro Grippers

Object grip, grasp, hold and place type of tasks incorporate specific engineering design concepts. Considering such principals, eight gripper mechanisms were designed and produced which can be classified as micro grippers due to their small workspace, precise grasping and holding abilities. Common characteristics of all the designs are that, without using any electric motor, the open–close operation is by virtue of shape-memory alloys. Each gripper is designed for different purposes with different operational principles of which four were initially open and the other four were initially closed configurations. Parametric studies were performed by changing the duty cycle and frequency of pulse width modulation using a signal generator followed by measurement of gripper jaw clearance, open–close period, average open–close velocities, gripping force, holding and lifting abilities of the designed gripper mechanisms. Jaw displacement and velocity parameters were computed by image processing of video records captured during the operation of the gripper mechanisms.

Macro-to-micro positioning and auto focusing for fully automated single cell microinjection

The automation methods and technologies of the single cell micro-injection reported in the literature have the common assumption that both the cell and the microtools has already been positioned within the microscopic field of view and well-focused. However, moving the microtools and biological cells from the macro field of view (macro-FOV) into the micro field of view (micro-FOV), and then further moving down into the culture medium and focusing were conducted manually and proved to be time consuming. In this work, we present algorithms and methods to automate this process. An electrothermal microgripper is used for picking and holding a zebrafish embryo instead of traditional micropipette. In order to position the microgripper into the micro-FOV, an extra macro camera is employed such that the microgripper jaws are under the macro-FOV. The micro-FOV is searched by moving the microgripper jaws in a serpentine path and zigzag path, respectively, and the grid-line identification algorithm is proposed to recognize the microgripper jaws that appear in the micro-FOV. Then, a contact detection algorithm is used to determine whether the gripper jaws are in the culture medium or not. Finally, eight algorisms are evaluated and compared to select the algorism with the best performance for auto-focusing the microgripper jaws in the culture medium. Up to 100 experiments are conducted to validate the proposed method for the macro-to-micro positioning and auto focusing of the microgripper jaws with the success rate 100% and 90%, respectively.

Design, analysis, and experimental investigation of a single-stage and low parasitic motion piezoelectric actuated microgripper

The demand for high precision micro/nano manipulation is increasing for advanced manufacturing technology. The precise motion of the microgripper jaw is required to achieve high performance micromanipulation tasks. The parasitic motion of the microgripper reduces placement accuracy during pick and place tasks. This paper presents an asymmetric design of piezoelectric actuated microgripper. It investigates key characteristics including parasitic motion, output displacement, and displacement amplification ratio. The microgripper is designed with a single-stage displacement amplification mechanism to form a compact layout. The design of the microgripper integrates the bridge-type mechanism and the parallelogram mechanisms. The bridge-type mechanism amplifies the piezoelectric actuator output and the parallelogram mechanisms offer linear motion of the gripper jaw. The analytical modeling and finite element analysis were conducted to evaluate the characteristics of the microgripper. The design parameters of the microgripper were optimized through several finite element analysis. Further, experimental studies were conducted to verify the characteristics of the microgripper. The parasitic motion of the jaw was obtained as less than 0.18% of the microgripper jaw motion in the x-direction. The mechanism of the microgripper also achieves a high positioning accuracy. Further, a high displacement amplification ratio and large output displacement can be achieved.

Design and Fabrication of a Minimally Invasive Surgical Device with Customized Shape Memory Alloy Spring Actuator

A shape memory alloy (SMA)-based spring actuated gripper (SAG) was designed and developed for facilitating minimally invasive surgeries (MIS). Specifically the research consists of design and development of a SMA spring which acts as the actuator for the gripper. A novel mechanism was also developed to transfer the actuator force to the gripper jaws. Further, the actuator force was characterized by a testing apparatus which uses a full-bridge load cell as the force sensing element. The activation temperature of the SMA actuator was obtained by performing differential scanning calorimetry (DSC) tests on the samples.

An Accurate Force Regulation Mechanism for High-Speed Handling of Fragile Objects Using Pneumatic Grippers

Controlling the gripping force on fragile objects has been a challenging task for industrial grippers. The solution often requires an electric gripper with embedded force sensors and control feedback. This approach is costly and leads to extra gripper complexity. To avoid damage while handling fragile objects, this paper presents a novel force regulation mechanism (FRM) to be installed on pneumatic grippers. Without using additional sensors and control, the FRM can passively produce an adjustable contact force between the gripper jaws and objects of various sizes. Together with pneumatic grippers that have a higher gripping speed, lower cost, and simpler structure, this approach offers a more attractive solution than the use of electric grippers. In this paper, the design and analysis of the FRM are presented. A prototype of the FRM is illustrated to demonstrate the effectiveness and accuracy of force regulation. This novel mechanism is expected to serve as a reliable alternative for fragile object manipulation. Note to Practitioners —Pneumatic grippers are popular due to their low cost and simple structure. However, it is difficult to control the gripping force to handle fragile objects without damage. The FRM has a small size and can be easily installed on pneumatic grippers to provide fast force regulation. The gripping force depends on axial preload of the FRM rather than air pressure of the gripper. Axial preload is passively regulated using a screw or spacer. No additional source of power or sensor is required. The FRM has a large force adjustment range and can grip objects of various sizes using the same force. Applications include the automation in electronics, food, agricultural, and medical industries.

Diffusion of Carbon in Steel Grippers Due to Cyclic Contact Stress

The stress-assisted diffusion of carbon atoms due to high-stress cyclic loading of serrations on a case-hardened steel gripper, leading to the redistribution of carbon near the surface of the serrations is explored in this paper. An elastic plastic finite element model coupled with simulation of stress-assisted diffusion of carbon is employed for the study. Kinematic hardening rule is used to estimate the dissipated plastic strain energy due to the contact stress and these results are transferred to the nodes of a very fine square grid. Potential gradient due to dissipated plastic strain energy coupled with modified Fick’s law are employed to simulate the stress-assisted diffusion of carbon near the surface of a serration on the gripper jaw. Measures for quantifying the redistribution of carbon are introduced, and the effect of the number of cycles and loading condition on the redistribution of carbon in steel are explored.

Design and Control of a Novel Compliant Constant-Force Gripper Based on Buckled Fixed-Guided Beams

This paper presents the design and control of a novel gripper device with a passive type of compliant constant-force mechanism. The function of constant-force output is achieved by using the combination of a positive-stiffness and a negative-stiffness mechanism. The negative stiffness is generated by a bistable buckled fixed-guided beam. Conventionally, when gripping a target-object using a high-stiffness gripper, both the displacement control and force control are required to prevent the damage of the object. The uniqueness of the developed constant-force gripper is that it can eliminate the dependence on force control while keeping the force constant via its mechanical structure. The gripper performance is evaluated by the established analytical model and nonlinear finite-element analysis, and validated through experimental study on a fabricated prototype. To achieve a precise position output of the gripper jaw, a discrete-time variable structure control strategy based on a nonswitching type of reaching law is realized. The effectiveness of the gripper system is verified by conducting experimental studies on grasp-hold-release operations of a micro copper wire.