On the calibration of multicomponent microforce sensors

A calibration method for multicomponent microforce sensors using Lorentz forces is proposed. The method permits generation of six force and moment components in three orthogonal directions on two-dimensional (2-D) structures that can be fabricated through existing silicon micromachining technology. Two multicomponent microforce sensor and calibration devices were developed and tested to verify the method     

Tactile data modeling and interpretation for stable grasping and manipulation

This paper is devoted to present the latest results on the exploitation of the force/tactile sensor developed by the authors in terms of modeling and interpretation of the data provided by the device. An analytical nonlinear model of the elastically deformable sensor is derived and validated, which allows to reconstruct the position and orientation of the surface in contact with a rigid object on the basis of the sensor signals. The reconstruction is performed via an Extended Kalman Filter able to counteract the measurement noise and to handle the nonlinearity of the model at the same time. The contact plane position and orientation information together with the contact force vector measured by the sensor are used to estimate the physical parameter most relevant to manipulation control purposes: the friction coefficient. A slippage control algorithm is presented which exploits the estimated friction and a novel slipping detection algorithm is proposed to cope with the unavoidable uncertainties of the real world and its effectiveness is experimentally proved in comparison with the existing techniques.     

Grip-force control of an elastic object by vision-based slip-margin feedback during the incipient slip

In this paper, a grip-force control of an elastic object is proposed based on a visual slip-margin feedback. When an elastic object is pressed and slid slightly on a rigid plate, a partial slip, called "incipient slip," occurs on the contact surface. The slip margin between an elastic object and a rigid plate is estimated based on the analytic solution of a Hertzian contact model. A one-degree-of-freedom gripper consisting of a camera and a force sensor is developed. The slip margin can be estimated from the tangential force measured by a force sensor, the deformation of the elastic object and the radius on the contact area both measured by a camera. In the proposed method, the friction coefficient is not explicitly needed. The "eccentricity" is used to estimate the displacement of the elastic object at the contact area with high accuracy. The grip force is controlled by a direct feedback of the estimated slip margin. The proof of the contact stability by the proposed control is analytically given. As a result, the slip margin is maintained at a desired value, without occurring the gross slip against a disturbance traction force to the object. The validity of the proposed method is confirmed by experiments.     

弓网电弧接触线温度分布仿真研究

为了分析弓网电弧对接触网导线温升的影响,以接触线为研究对象,建立了电弧作用下弓网系统滑动摩擦副的三维有限元模型,利用COMSOL Multiphysics仿真分析,得到不同类型的接触网导线在不同环境下的温度分布特征.结果表明:相同条件下,铜镁合金、纯铜、铜锡合金接触线的温升依次升高.接触线的截面积越大温升越高.同种接触导线分别处于正常、覆冰和潮湿状态时,接触线的温升依次升高.仿真结果对选择接触线以及降低弓网电弧对接触线的热损伤具有一定参考价值.     

An automatic incrementation technique for contact problems with friction

A numerical method for analysis of elastostatic contact problems with friction has been developed. This class of problems are load history dependent because of the irreversible nature of frictional forces. An automatic incrementation technique of the applied load has been developed and implemented in the algorithm. The method is a direct method based on an iterative procedure applied to a set of linear equations established with the finite element method. The size of the applied load increments, automatically chosen by the algorithm, is in general influenced both by the nature of the problem and of the discretization of the bodies involved. The frictional forces occurring in the slip zone of the contact area are treated as known tangential forces calculated from the normal forces in the previous iteration. This piecewise linear treatment of the frictional contact problem requires an innermost iteration loop over the applied tangential force.The tangential force must coincide with the coefficient of friction times the normal force obtained in the last iteration, otherwise a new tangential force has to be calculated and the system of equations must be solved for a new right hand side vector. The automatic incrementation technique is based on the fact that each iteration is a linear problem. A tentative load increment is used in the solution of a certain iteration. A linear scaling of this solution is performed afterwards. A load scale factor is calculated in each contact node pair where a change of contact condition will occur. The change in contact status corresponding to the node pair with the smallest load scale factor is the only change which is accomplished in a certain iteration. The uniqueness of this kind of contact problem with friction has not been mathematically proven for a general case.The method has been applied to a case of loading and unloading of an elastic halfspace by a rigid cylindrical stamp and compared to solutions by Spence and Turner.     

The Effect of Adhesion on the Static Friction Properties of Sidewall Contact Interfaces of Microelectromechanical Devices

Static friction between sidewall contact surfaces of polycrystalline silicon micromachines was investigated under different contact pressures, vacuum conditions, relative humidity levels, and temperatures. The static coefficient of friction exhibited a nonlinear dependence on the external contact pressure. A difference between in-contact and pull-out adhesion forces was observed due to the elastic recovery of the deformed asperities at the contact interface. The true static coefficient of friction was determined by considering the effects of the dominant adhesion forces (i.e., van der Waals and capillary forces) on the normal force applied at the sidewall contact interface. The roles of van der Waals and capillary forces in the sidewall friction behavior were analyzed in light of results for the interfacial shear strength and the adhesion force. The major benefits of the present friction micromachine and the developed experimental scheme are discussed in the context of static coefficient of friction and adhesion force results obtained under different environmental and loading conditions     

Experiments in contact control

This article describes the implementation, experimentation, and application of contact control schemes for a 7-DOF Robotics Research arm. The contact forces and torques are measured in the sensor frame by the 6-axis force/torque sensor mounted at the wrist, are compensated for gravity, and then are transformed to the tool frame in which the contact task is defined and executed. The contact control schemes are implemented on the existing robot Cartesian position control system at 400Hz, do not require force rate information, and are extremely simple and computationally fast. Three types of contact control schemes are presented: compliance control, force control, and dual-mode control. In the compliance control scheme, the contact force is fed back through a lag-plus-feedforward compliance controller so that the end-effector behaves like a spring with adjustable stiffness; thus the contact force can be controlled by the reference position command. In the force control scheme, a force setpoint is used as the command input and a proportional-plus-integral force controller is employed to ensure that the contact force tracks the force setpoint accurately. In the dual-mode control scheme, the end-effector approaches and impacts the reaction surface in compliance mode, and the control scheme is then switched automatically to force mode after the initial contact has been established. Experimental results are presented to demonstrate contact with hard and soft surfaces under the three proposed control schemes. The article is concluded with the application of the proposed schemes to perform a contact-based eddy-current inspection task. In this task, the robot first approaches the inspection surface in compliance control until it feels that it has touched the surface, and then automatically levels the end-effector on the surface. The robot control system then transitions to force control and applies the desired force on the surface while executing a scanning motion. At the completion of the inspection task, the robot first relaxes the applied force and then retracts from the surface. © 1996 John Wiley & Sons, Inc.     

Simulation of thermal flying height control slider with built-in contact sensor

This work investigates the piezoelectric contact sensor in the thermal flying height control (TFC) slider. A finite element model is built for the thermal flying height control slider with a piezoelectric contact sensor, which is used to detect the contact between the slider and disk. A constant force is applied at the maximum thermal protrusion point of air bearing surface. The simulation results show that the ZnO sensor with shear-mode is more sensitive to contact force than that with transverse-mode. The sensitivity of contact sensor can be increased by reducing the cross-sectional area of sensor, increasing the thickness of sensor, and choosing a short distance of sensor to air bearing surface. In addition, the thermal-stress effects from TFC heater on contact sensor are significantly large and the amplitude of thermal-stress inducing output voltage is orders larger than that induced by contact force. However, by optimizing the distance of sensor to ABS, it is possible to eliminate the thermal-stress effects. Finally, the response time of thermal-stress induced electrical voltage of contact sensor is about 0.3?ms.     

Development of a smart handheld surgical tool with tactile feedback

This paper presents a handheld surgical tool adapting a tactile feedback system. The tool consists of a 3-degree-of-freedom (DOF) force sensor and three tactile displays. The sensor is easily embedded in the tool by adopting the capacitive transduction principle. The sensor measures the direction and magnitude of the 3-DOF force applied to the tool tip. The fingertip grasping the tool is stimulated by the tactile display to transmit the contact force information measured by the sensor. The tactile display is actuated by employing a soft actuator technology based on a dielectric elastomer actuator such as a type of electroactive polymer actuator. In this work, a prototype of the tool is designed and fabricated. Its performance is experimentally validated.     

Sensor Fusion for Compliant Robot Motion Control

Force feedback is necessary for accurate force control in robotic manipulators, and thus far, wrist force/torque (F/T) sensors have been used. But an important problem arises when only these types of sensors are used. In a dynamic situation where the manipulator moves in either free or constrained space, the interaction forces and moments at the contact point and also the noncontact ones are measured by the mentioned sensor. In this paper, an estimator based on a sensor fusion strategy integrating the measurements of three different sensors (a wrist F/T sensor, an inertial sensor, and joint sensors) was developed to determine the contact force and torque exerted by the manipulator to its environment. The resulting observer helps to overcome some difficulties of uncertain world models and unknown environments since it reduces the high-frequency and low-frequency spectral contents, i.e., the low-frequency component due to inertia of a heavy tool mass and the high-frequency component due to impacts. The new improvement was experimentally validated in a force/position impedance control loop applied to a Staubli RX60 industrial robotic platform.     

水下金刚石绳锯机张力检测系统

介绍了串珠绳张力传感器的主要结构和测量原理,在此基础上,设计出一种新型动静态串珠绳张力检测系统.该系统实现了由应变计组成的电桥测量线路,以及传感器输出信号的处理,并实时传送至显示仪表.该系统能够准确、可靠地测试和显示串珠绳张力状态,有利于提高绳锯机的工作效率.该装置具有良好的防水密封性能和抗振动性能,保证了在水下安全稳定地工作.实际应用表明:该检测系统具有结构简单、操作方便、测量准确度达到0.1 %FS,适用范围为3 000 N以内,能满足工程现场使用要求.     

滑动电接触磨耗测控系统的研究

为了研究受电弓滑板和接触导线的摩擦磨损性能,研制了一台高性能滑动电接触磨耗试验机;其可以实现导线和滑板的均匀磨耗,可在一定范围内实现电流、载荷和速度等方面的单、多因素控制,实现对接触导线与滑板摩擦系数、滑板往复移动次数、接触导线与滑板磨耗量等的实时在线测量和数据储存,同时设计了基于LabVIEW的监测界面和数据处理系统。通过实验,分析了强电流对浸铜碳滑板摩擦磨损的影响,得出摩擦系数和磨耗率随电流的增大而增大的结论。     

Mechanical stability of the interface between a padded slider and magnetic recording disk

Contact state, friction, and meniscus load between a padded slider and a smooth disk are discrete; friction is determined by the number of pads in contact and the meniscus load acting at each pad. By inducing frictional changes through varying sliding direction, we force the slider from one stable state to another and infer which pads are in contact from the corresponding changes in friction. We find the stability of a given contact state is influenced by friction coefficient, pad location, meniscus load, applied mechanical loads and applied moments. A model is proposed to account for these effects.     

对未知环境的机器人力控自律跟踪及建模

在考虑摩擦力的情况下，利用传感器所感知的机器人和未知轮廓间的相互作用力确接触处轮廓的切矢和法矢，据此建立端点约束坐标系，在该坐标系中沿切矢进行位置控制并沿法矢进行力控制。实现机器人对未知轮廓的自律跟踪运动。由跟踪运动所确定每一点处切矢信息及该点位置信息构造未知轮廓几何模型。在机器人学开放研究实验室的ＰＵＭＡ５６２机器人上实现了上述自律运动并建立了环境模型。     

Effect of layer compliance on frictional behavior of soft robotic fingers

In this paper, the modeling and design for frictional loads applied by robotic fingertips through a soft contact interface are investigated. The dependence of the sustainable friction forces and moments due to the normal force applied on the contact area is studied. A model that considers the dependence of both the contact area and the coefficient of friction on the application of normal force is proposed. Specimens of simplified fingertips, made with different materials and shaped with different thickness of the covering soft layer, were tested in order to validate the proposed model via designed experiments. The results obtained have very important relevance to the design of robotic hands and fingers equipped with soft pads or skins, and can help in the choice of suitable features for the different layers of contact pads or skins.     

A method for producing a uniform contact stress distribution in composite bodies with interference

One of the most important problems in the design of press- and shrink-fitted assemblies is to avoid stress concentrations at the contact of two bodies. A method is presented whereby the contact-stresses can be made uniform by using variable interferences. In the proposed method the finite element technique is applied to determine the stresses of composite bodies, ignoring the effects of friction at the contact surfaces. A method of optimum structural design is used to obtain a uniform contact stress within the limit of the contraction force. In this method, interferences at contact nodal points are used as design variables. Some simple examples are presented and the effectiveness of the proposed approach is also demonstrated.     

光纤光栅的封装效果对其性能的影响

In this paper, the effects of packaging material and structure of fiber Bragg grating sensor performance are investigated. The effects of thermal expansion coefficient of different embedding materials on the temperature sensitivities of the FBG sensors are studied both theoretically and experimentally with good agreement, which provides a means for selection of FBG packaging material to achieve desired temperature sensitivity. We also demonstrate a 4-point bending structured FBG lateral force sensor that measures up to 242N force with well-preserved reflection spectrum, whereas for 3-point bending structure, multiple-peaks start to occur when applied force reaches 72N.     

Implementation of the Contensou–Erismann tangent forces model in the Hertz contact problem

In frame of the Hertz contact problem, an approximate model to compute resulting wrench of the dry friction tangent forces is built up. The wrench consists of the total friction force and the drilling friction torque. An approach under consideration develops in a natural way the contact model constructed earlier. The dry friction force and torque are integrated over the contact elliptic area. Generally, an analytic computation of the integrals mentioned leads to the cumbersome calculation, tens of terms, including rational functions depending in turn on complete elliptic integrals. To implement the elastic bodies contact interaction computer model fast enough, one builds up an approximate model in the way initially proposed by Contensou.     

Hand-eye system with three-dimensional vision and microgripper for handling flexible wire

A three-dimensional hand-eye system has been specially developed for handling, forming, and inserting flexible wires in electronic assemblies. The system consists of a vision system capable of measuring three-dimensional positions and a gripper capable of microforce control. The hand-eye system performs its tasks based on visual and force sensor data.The vision system uses a camera, a multiplanar laser light projector, and a video-rate image processor to measure the three-dimensional position of a flexible wire on a complex background. The projector consists of multiplanar light beams and a single-planar light beam and is robot-mountable. The single-planar light is used to simplify the matching of the multiplanar light beams and projection images. This is one of the most important features of our method. The measurement time is within 1.2 s for 20 positions on the wire, which is less than one robot cycle. The measurement accuracy is better than 0.2 mm in the plane for a 100×100 mm scene and is better than 1 mm perpendicular to the plane for a depth of 500 mm.The microgripper has 3-axis sensors capable of detecting three types of forces: grasping, pulling, and pushing. The dynamic range of the grasping force is 0.005 to 0.6 N, enabling high-resolution control. Two piezoelectric actuators are used in the microgripper fingers and a 3-mm stroke is achieved by amplifying the actuator's displacement using a two-stage lever.This system has been used for wire handling in magnetic disk slider manufacture, which is a difficult and delicate task. Measurement, grasping, and upper-stage insertion had a 100 percent success rate using autorecovery, making the hand-eye system feasible. This paper discusses design concepts, the range measurement principle, the system itself, and wire-handling applications.     

Contact force estimation and regulation in active pantographs: An algebraic observability approach

In this paper we extend our previous work concerning the regulation of the contact force in active train pantographs with wire‐actuators. In particular, we suggest a second‐order sliding mode based control scheme that avoids the direct measurement of the contact force, which is quite unpractical in real applications. The idea is to estimate the contact force using the measured displacements of the upper and lower pantograph frames. Unlike the contact force, the frames displacements can be measured accurately and reliably in practice. The estimation method, based on the algebraic observability theory, entails the use of real‐time sliding‐mode differentiators. It makes use explicitly of the mechanical characteristics of the upper frame only. The considered second‐order sliding mode control scheme also includes proper linear compensating cascade filters devoted to cope with the de‐stabilizing effects of the resonant wire actuator. We show that such a control scheme can be effectively combined with the given contact force estimation method. Copyright © 2010 John Wiley and Sons Asia Pte Ltd and Chinese Automatic Control Society