Small Contact Force Research Articles

Evaluation of different robotic grippers for simultaneous multi-object grasping.

For certain tasks in logistics, especially bin picking and packing, humans resort to a strategy of grasping multiple objects simultaneously, thus reducing picking and transport time. In contrast, robotic systems mainly grasp only one object per picking action, which leads to inefficiencies that could be solved with a smarter gripping hardware and strategies. Development of new manipulators, robotic hands, hybrid or specialized grippers, can already consider such challenges for multi-object grasping in the design stages. This paper introduces different hardware solutions and tests possible grasp strategies for the simultaneous grasping of multiple objects (SGMO). The four hardware solutions presented here are: an under-actuated Constriction Gripper, Linear Scoop Gripper suitable for deform-able object grasping, Hybrid Compliant Gripper equipped with mini vacuum gripper on each fingertip, and a Two-finger Palm Hand with fingers optimized by simulation in pybullet for maximum in-hand manipulation workspace. Most of these hardware solutions are based on the DLR CLASH end-effector and have variable stiffness actuation, high impact robustness, small contact forces, and low-cost design. For the comparison of the capability to simultaneously grasp multiple objects and the capability to grasp a single delicate object in a cluttered environment, the manipulators are tested with four different objects in an extra designed benchmark. The results serve as guideline for future commercial applications of these strategies.

Development of Pneumatic Force-Controlled Actuator for Automatic Robot Polishing Complex Curved Plexiglass Parts

Due to the temperature-sensitive characteristic of plexiglass materials, it is necessary to maintain a constant small contact force to avoid surface burn damage when polishing complex curved plexiglass parts. To handle the issue, in this paper a pneumatic force-controlled actuator was developed to keep the normal contact force between the polishing tool and the workpiece constant during the robotic polishing process. The force-controlled actuator is configured with a double-acting cylinder as the driving element, and two electrical proportional valves are used to control the output force by adjusting the pressure difference between the two air chambers of the cylinder. In this case, a small contact force can be exactly achieved, and the cylinder can always work within the optimal pressure range. In order to judge the stability of the system and reduce the commissioning time of the force-controlled actuator, a mathematical model of the force-controlled actuator is established. Meanwhile, for eliminating the influence of the gravity of the polishing tool on the contact force control, a gravity compensation algorithm is also given according to the roll-pitch-yaw (RPY) angle calculation method. Since there are some nonlinear factors in the operation of the force-controlled actuator, a fuzzy proportion-integral-derivative (PID) control strategy is adopted without steady-state errors. Finally, the polishing experiment of a complex curved plexiglass part was carried out by using the robot automatic polishing system. The experimental results show that the contact force control effect of the force-controlled actuator meets the processing requirements, and the curved plexiglass part has good surface quality and optical performance after polishing.

Study on Dynamic Behavior of a Titanium Alloy Drill Pipe in Complex Wells

Traditional steel drill pipes can no longer meet the requirements of complex wells with ultradeep, ultrahigh pressure, and long horizontal section; hence, titanium alloy drill pipes are an ideal substitute. This paper explores how titanium drill pipes behave in complex wells. The extrusion and tensile test of titanium alloy pipe was first established. Then, based on the experimental data, the downhole mechanical behavior of a titanium alloy drill pipe was studied from the buckling, contact force, and operating friction with an actual complex well. Meanwhile, the mechanism of friction reduction is analyzed and discussed. The research achievements indicate that the strength of a titanium alloy drill pipe is equal to that of a steel pipe and has good plastic deformation capacity. The titanium alloy drill pipe is more prone to buckling during operation, but it has a smaller contact force, which can effectively reduce the operation friction. It was found that the influent of buckling on slide force was much less than of the gravity and stiffness by mechanism analysis. The research achievements can provide specific theoretical and practical references for the revelation of the mechanical behavior and functional performance of titanium alloy drill pipe in the field operation.

Volume Contraction in Shallow Sediments: Discrete Element Simulation

Displacements induced by mineral dissolution and subsurface volume contraction affect overlying soils. In this study, we examine the consequences of mass loss or volume contraction at shallow depths using a discrete element method. The goal of the study is to identify particle-scale and global effects as a function of the relative depth of a dissolving inclusion, initial soil density, and granular interlocking. There are successive arch formation and collapse events, and a porosity front propagates upwards as grains slide down to refill the space. Grains around and within the refilled cavity are loosely packed and have small contact forces that are sufficient to avert the buckling of granular arches that form around the dissolving zone. Denser packings and interlocking combine to exacerbate rotational frustration and lead to more pronounced force chains along granular arches, looser fill, and reduced surface settlement. In fact, surface settlement vanishes, and the sediment hides the localized dissolution when deep inclusions z/D ≥ 5 dissolve within dense sediments. While scaling relations limit the extrapolation of these numerical results to tunneling and mining applications, macroscale trends observed in the field resemble results gathered in this study.

Effect of particle shape on raceway size and pressure drop in a blast furnace: Experimental, numerical and theoretical analyses

Experiments and simulations are conducted to explore the changing laws of blast furnace (BF) raceway morphology and pressure drop with cylindrical particles. Experimental data show that there are five typical stages for the pressure drop during the raceway formation. The closer the aspect ratio (Ar) of the particle to 1, the bigger the raceway size and the wider the particle moving band will be. When the raceway is in stable stage, the pressure drop can be ascribed to the cooperative action of the bed height, inlet gas velocity and Ar. Numerical results reveal that the formation of large raceway for sphere-like particles is due to the small drag and contact forces. The contact forces in the prolate particle system are very large and thus result in a small raceway. Finally, the influence of particle shape is employed to improve a raceway size predictive correlation which can increase the average calculational accuracy by 3.4%.

Soft robotic surface enhances the grasping adaptability and reliability of pneumatic grippers

Soft grippers are an emerging field of robotics in recent years owing to their reduced control complexity, easy fabrication, and excellent compliance. However, the small contact area and force usually limit their grasping reliability. Herein, a soft gripper consisting of two soft robotic surfaces (SRSs) is proposed inspired by Venus flytrap, which can provide enveloping and pinching grasp modes. To guide the design of the gripper, an analytical model of the SRS is developed for both bending inward and outward. The experimental results show an excellent agreement with analytical results, which demonstrates the validity of the analytical model. Additionally, the grasping performance of the soft gripper is investigated experimentally. The results indicate that the grasp and pull-off force depend on the size and shape of objects and applied pressure. As the applied pressure is 60 kPa, the maximum grasp force and pull-off force reach 0.71 N and 8.15 N, respectively. Various grasping experiments are also conducted to study the effect of diverse objects, errors of object position, and SRS installation on the grasping capacity. Our findings demonstrate that our gripper can observably enhance the grasping adaptability and reliability due to the high passive compliance of the SRS allowing a large contact area between the gripper and objects.

Sensorless force estimation for industrial robots using disturbance observer and neural learning of friction approximation

Contact force estimation enables robots to physically interact with unknown environments and to work with human operators in a shared workspace. Most heavy-duty industrial robots without built-in force/torque sensors rely on the inverse dynamics for the sensorless force estimation. However, this scheme suffers from the serious model uncertainty induced by the nonnegligible noise in the estimation process. This paper proposes a sensorless scheme to estimate the unknown contact force induced by the physical interaction with robots. The model-based identification scheme is initially used to obtain dynamic parameters. Then, neural learning of friction approximation is designed to enhance estimation performance for robotic systems subject with the model uncertainty. The external force exerted on the robot is estimated by a disturbance observer which models the external disturbance. A momentum observer is modified to develop a disturbance Kalman filter-based approach for estimating the contact force. The neural network-based model uncertainty and measurement noise level are analysed to guarantee the robustness of the Kalman filter-based force observer. The proposed scheme is verified by the measurement data from a heavy-duty industrial robot with 6 degrees of freedom (KUKA AUGLIS six). The experimental results are used to demonstrate the estimation performance of the proposed approach by the comparison with the existing schemes. • A novel sensorless scheme is developed to estimate unknown contact force for robots. • NN learning of friction compensates model uncertainties with good approximation. • Disturbance Kalman filter observer is to achieve accurate contact force estimation • Observers and NN learning of friction show robust observation to small contact force.

An improved design of a seesaw-type MEMS switch for increased contact force

Microelectromechanical systems (MEMS) switches have a wide range of possible applications due to their promising working characteristics, but commercial success of these devices is limited by low reliability. In contrast with macroscopic electromechanical relays, MEMS switches typically develop a small contact force, which results in unstable and high contact resistance. In this paper we propose the design that ensures several times higher contact force in comparison with previously used structure. Enhancement of the force is achieved without increasing the footprint and operating voltage of the switch. The analytical calculation and finite element simulation of the working characteristics are performed. At the driving voltage of 100 V the modified switch provides the contact force as high as 130 µN, which is an order of magnitude higher than the force developed by the basic device.

Spacer fabric/flexible polyurethane foam composite sandwiches: Structural design and quasi-static compressive, bursting and dynamic impact performances

Nylon fabrics are used as the reinforcing upper layer. A warp-knitted spacer fabric that is saturated with flexible polyurethane foam is used as the lower layer. They are combined in order to form innovative impact energy absorbent composite sandwiches employing a two-step foam process. The quasi-static compressive strength, bursting strength, and dynamic shock cushioning characteristic of the composite sandwiches are evaluated, the latter of which is tested using a drop weight impact tester in order to simulate the impact resistance corresponding to complex environments. The experimental results suggest that a combination of warp-knitted spacer fabric increases the compressive strength and bursting strength of composite sandwiches by 107% and 60%, respectively. The shapes of impactors determine the contact force and dynamic impact time. The greater contact area results in the smaller contact force, indicating that the composite sandwiches have greater cushioning efficacy. Designed with a gradient combination, the flexible polyurethane foam strongly bonds the upper polyamide nonwoven fabric and the lower warp-knitted spacer fabric. The optimal compression resistance, a bursting strength of 1677 N, and a maximum energy absorption of 90% at a 15 J impact energy are presented. The innovative composite sandwiches have a promising perspective in packaging and individual impact protection in the future.

Soft Magnetic Powdery Sensor for Tactile Sensing.

Soft resistive tactile sensors are versatile devices with applications in next-generation flexible electronics. We developed a novel type of soft resistive tactile sensor called a soft magnetic powdery sensor (soft-MPS) and evaluated its response characteristics. The soft-MPS comprises ferromagnetic powder that is immobilized in a liquid resin such as polydimethylsiloxane (PDMS) after orienting in a magnetic field. On applying an external force to the sensor, the relative distance between particles changes, thereby affecting its resistance. Since the ferromagnetic powders are in contact from the initial state, they have the ability to detect small contact forces compared to conventional resistive sensors in which the conductive powder is dispersed in a flexible material. The sensor unit can be made in any shape by controlling the layout of the magnetic field. Soft-MPSs with different hardnesses that could detect small forces were fabricated. The soft-MPS could be applied to detect collisions in robot hands/arms or in ultra-sensitive touchscreen devices.

Analysis of the Melt Erosion Patterns at Rail-Armature Contact of Rail Launcher in Current Range of 10–20 kA/mm

Sliding electrical contact between rail and armature in rail launchers is characterized by high speed and large current. Melt erosion is caused by the current concentration on the contact surface of armature. This current melt erosion (CME) has been experimentally studied in the current range of 10–20 kA/mm with a 20-mm caliber lab-scale rail launcher, and payload separated method was used to keep the recovered armatures intact. Thus, the onset behavior and erosion patterns of CME can be successfully observed in such relatively low current densities. In this paper, we will further our study to analyze the patterns of CME with the increasing current densities under the effect of different initial mechanical contact force. Results show that CME all start at the side edge, and then spreading longitudinally and transversely with the increase of current. It is found that the starting position of CME appears near the trailing edge at small mechanical contact force and moves to the leading edge with larger mechanical contact force. At the same current magnitude, erosion amounts are much more under small mechanical contact force. Stationary mechanical contact pressures and current distribution of armature surface are simulated to investigate the effect of initial mechanical contact force on starting position of CME. Current distributes at the edges of actual contact zone and the maximum of current density appears at the side edges of armature. As the initial mechanical contact force increases, actual contact zone and current density move toward to the leading edge from trailing edge. It can be confirmed that the erosion starting position is at the side edge along actual contact zone and will move toward to the leading edge from trailing edge with the increase of initial mechanical contact force. Smaller initial mechanical contact force will cause more erosion at the same current magnitude because of larger current density. Furthermore, an average friction coefficient calculation method is proposed to evaluate the melt lubrication effect of CME. The result shows that the lubrication effect becomes better with larger current densities.

Effects of femoral component placement on the balancing of a total knee at surgery

Misalignment and soft-tissue imbalance in total knee arthroplasty (TKA) can cause discomfort, pain, inadequate motion and instability that may require revision surgery. Balancing can be defined as equal collateral ligament tensions or equal medial and lateral compartmental forces during the flexion range. Our goal was to study the effects on balancing of linear femoral component misplacements (proximal, distal, anterior, posterior); and different component rotations in mechanical alignment compared to kinematic alignment throughout the flexion path. A test rig was constructed such that the position of a standard femoral component could be adjusted to simulate the linear and rotational positions. With the knee in neutral reference values of the collateral tensions were adjusted to give anatomic contact force patterns, measured with an instrumented tibial trial. The deviations in the forces for each femoral component position were then determined. Compartmental forces were significantly influenced by 2 mm linear errors in the femoral component placement. However, the errors were least for a distal error, equivalent to undercutting the distal femur. The largest errors mainly increase the lateral condyle force, occurred for proximal and posterior component errors. There were only small contact force differences between kinematic and mechanical alignment. Based on these results, surgeons should avoid overcutting the distal femur and undercutting the posterior femur. However, the 2–3 degrees varus slope of the joint line as in kinematic alignment did not have much effect on balancing, so mechanical or kinematic alignment were equivalent.

Stick-to-sliding transition in contact-resonance atomic force microscopy

Contact-resonance atomic force microscopy (CR-AFM) has been used to measure the viscoelastic loss tangent of soft materials such as polymers. Usually, the damping is attributed to the dissipation in the contact volume due to internal friction and air damping of the cantilever. However, partial slip or even full sliding can exist in the contact zone under tangential loading, and thus, the accompanying energy dissipation must be taken into account when measuring the damping constant Qloc−1. Here, the stick-to-sliding transition of the tip-sample contact in CR-AFM was studied. Amplitude drops were observed in the resonance curves caused by such stick-to-sliding transitions. The results show that the stick-to-sliding transition arises under small contact forces and large excitation amplitudes. Extra energy loss from full sliding induces large contact damping. The critical lateral displacement needed for the stick-to-sliding transition varies linearly with the contact radius.

Study of low-velocity impact response of sandwich panels with shear-thickening gel cores

The low-velocity impact response of sandwich panels with shear-thickening gel cores was studied. The impact tests indicated that the sandwich panels with shear-thickening gel cores showed excellent properties of energy dissipation and stress distribution. In comparison to the similar sandwich panels with chloroprene rubber cores and ethylene-propylene-diene monomer cores, the shear-thickening gel cores led to the obviously smaller contact forces and the larger energy absorptions. Numerical modelling with finite element analysis was used to investigate the stress distribution of the sandwich panels with shear-thickening gel cores and the results agreed well with the experimental results. Because of the unique mechanical property of the shear-thickening gel, the concentrated stress on the front facesheets were distributed to larger areas on the back facesheets and the peak stresses were reduced greatly.

Small nanoparticles, surface geometry and contact forces.

In this molecular dynamics study, we examine the local surface geometric effects of the normal impact force between two approximately spherical nanoparticles that collide in a vacuum. Three types of surface geometries-(i) crystal facets, (ii) sharp edges, and (iii) amorphous surfaces of small nanoparticles with radii R<10 nm-are considered. The impact forces are compared with their macroscopic counterparts described by nonlinear contact forces based on Hertz contact mechanics. In our simulations, edge and amorphous surface contacts with weak surface energy reveal that the average impact forces are in excellent agreement with the Hertz contact force. On the other hand, facet collisions show a linearly increasing force with increasing compression. Our results suggest that the nearly spherical nanoparticles are likely to enable some nonlinear dynamic phenomena, such as breathers and solitary waves observed in granular materials, both originating from the nonlinear contact force.

Calculation and study of the failure of the connecting-rod small end based on thermoelastic plasticity

In this study, the mechanisms that cause bush loosening and rotation failure were studied on the connecting-rod small end of a high-power-density engine. Using the thermoelastic plasticity model and the thermomechanical coupling method, the stress field and the assembly contact force of the connecting-rod small end were predicted by considering four different loading conditions: first, the initial loading condition; second, the thermal loading condition; third, the mechanical loading condition; fourth, the unloading condition. This process showed the effects of the initial interference fit, the operating temperature and the wall thickness of the bush on the assembly contact force. It was found that, first, a large interference leads to a small assembly contact force after unloading, second, a higher operating temperature significantly reduces the assembly contact force and, third, the wall thickness of the bush has little influence on the assembly contact force. In conclusion, a high operating temperature is the main cause of bush loosening and rotation. This conclusion was validated by the experimental results. On the basis of this study, it is suggested that the operating temperature is limited in order to maintain the reliability of the connecting-rod small end.

Balance recovery during induced falls is impaired in people with knee osteoarthritis: Implications for falls prevention

Balance recovery during induced falls is impaired in people with knee osteoarthritis: Implications for falls prevention

An Impedance-Based Force Control Scheme to a Plate-to-Plate Nanoimprinter

Nanoimprint lithography (NIL) is a novel technique that utilizes direct deformation of the resist to fabricate micro/nanometer scale patterns. NIL outperforms traditional photolithography technology for its low cost, high productivity, and high repeatability. For a general plate-to-plate NIL, it is of central importance to precisely control the embossing force during the entire embossing process to tune the orientation of the imprint head and also to guarantee the uniformity and fidelity. Existing approaches used in NIL are position-based feedback controls. However, a position signal is believed to be indirect and is more likely to be influenced by the thickness of the photoresist layer. To address this problem, in this paper, we propose a two-stage force control scheme to explicitly control the embossing force based on the impedance control framework. The first stage is to parallelize the template surface with the substrate surface using the small initial contact force before embossing. Then, the second control stage is to regulate the large embossing force to the desired value. With the proposed force control approach, the embossing force can be explicitly controlled during the entire stamp process even without a prior knowledge of the environmental parameters. Experimental tests performed on a 500-nm grating scale further illustrate and verify the effectiveness of the proposed design.

Semi-contact measurements of three-dimensional surfaces utilizing a resonant uniaxial microprobe

Recent development trends in the area of tactile probing systems led to miniaturized highly accurate probes that allow measurements of small structures with a high aspect ratio. Additionally, small contact forces are aspired to achieve measurements without damage to the specimen. Thus, more and more challenges arise because of an increasing influence of scaling effects. Sticking is one crucial effect that generates measurement faults such as snap-back and false triggering. This work presents investigations on a highly accurate electrostatic probing system, which operates at resonant motion to minimize the influence of capillary forces. The unique feature of the uniaxial microprobe is the fully integrated design in a silicon-on-insulator substrate, which minimizes the number of coupling connections and improves the measurement accuracy.

Contact Force and Scanning Velocity during Active Roughness Perception

Haptic perception is bidirectionally related to exploratory movements, which means that exploration influences perception, but perception also influences exploration. We can optimize or change exploratory movements according to the perception and/or the task, consciously or unconsciously. This paper presents a psychophysical experiment on active roughness perception to investigate movement changes as the haptic task changes. Exerted normal force and scanning velocity are measured in different perceptual tasks (discrimination or identification) using rough and smooth stimuli. The results show that humans use a greater variation in contact force for the smooth stimuli than for the rough stimuli. Moreover, they use higher scanning velocities and shorter break times between stimuli in the discrimination task than in the identification task. Thus, in roughness perception humans spontaneously use different strategies that seem effective for the perceptual task and the stimuli. A control task, in which the participants just explore the stimuli without any perceptual objective, shows that humans use a smaller contact force and a lower scanning velocity for the rough stimuli than for the smooth stimuli. Possibly, these strategies are related to aversiveness while exploring stimuli.