Design and calibration of spoke piezoelectric six-dimensional force/torque sensor for space manipulator

The large manipulator outside the space cabin is a multi-degree of freedom actuator for space operations.In order to realize the automatic control and flexible operation of the space manipulator,a novel spoke structure piezoelectric six-dimensional force/torque sensor with redundancy ability,high stiffness and good decoupling performance is innovatively proposed.Based on the deformation coordination relationship,the redundancy measurement mechanism is revealed.The mathematical models of the sensor with and without branch fault are established respectively.The finite element model is established to verify the feasibility of structure and redundancy measuring principle of the sensor.Depending on the theoretical analysis and simulation analysis,the prototype of the sensor is developed.Static and dynamic calibration experiments are carried out.The actual output voltage signal of the six-dimensional force/torque sensor is collected to establish the equation between the standard input applied load and the actual output voltage signal.Based on ant colony optimized BP algorithm,performance indexes of the sensor with and without branch fault are analyzed respectively.The experimental results show that the spoke piezoelectric sixdimensional force/torque sensor with the eight-point support structure has good accuracy and reliability.Meanwhile,it has strong decoupling characteristic that can effectively shield the coupling between dimensions.The nonlinear errors and maximum interference errors of decoupled data with and without branch faults are less than 1% and 2%,respectively.The natural frequency of the sixdimensional force sensor can reach 2856.45 Hz and has good dynamic characteristics.The research content lays a theoretical and experimental foundation for the design,development and application of the new six-dimensional force/torque sensors with redundancy.Meanwhile,it will significantly improve the research level in this field,and provide a strong guarantee for the smooth implementation of force feedback control of the space station manipulator project.

Flexible Tactile Electronic Skin Sensor with 3D Force Detection Based on Porous CNTs/PDMS Nanocomposites

Flexible tactile sensors have broad applications in human physiological monitoring,robotic operation and human-machine interaction.However,the research of wearable and flexible tactile sensors with high sensitivity,wide sensing range and ability to detect three-dimensional(3D)force is still very challenging.Herein,a flexible tactile electronic skin sensor based on carbon nanotubes(CNTs)/polydimethylsiloxane(PDMS)nanocomposites is presented for 3D contact force detection.The 3D forces were acquired from combination of four specially designed cells in a sensing element.Contributed from the double-sided rough porous structure and specific surface morphology of nanocomposites,the piezoresistive sensor possesses high sensitivity of 12.1 kPa?1 within the range of 600 Pa and 0.68 kPa?1 in the regime exceeding 1 kPa for normal pressure,as well as 59.9 N?1 in the scope of<0.05 N and>2.3 N?1 in the region of<0.6 N for tangential force with ultra-low response time of 3.1 ms.In addition,multi-functional detection in human body monitoring was employed with single sensing cell and the sensor array was integrated into a robotic arm for objects grasping control,indicating the capacities in intelligent robot applications.

Development and Calibration of a Hyperstatic Six-component Force/Torque Sensor

The six-component force/torque sensor has become one kind of the most important sensors with the ability of measuring all the external forces and torques. A novel hyperstatic six-component force/torque sensor based on the Stewart platform structure, which can be used for the force measurement of the robot wrist, is proposed, and its structural optimal design, finite element analysis and calibration experimentation is presented. The characteristic of the sensor structure is analyzed in comparison with the traditional Stewart platform-based sensor. The mathematical expression of the sensor＇s force mapping matrix is introduced. The condition number and generalized amplifying coefficient defined by singular values of force Jacobian matrix are used to evaluate the performances of isotropy and sensitivity of the sensor respectively. The optimal design of the sensor structure is performed with the objective of achieving high measurement sensitivity and good isotropy. The sensor prototype is fabricated, and the static and dynamic characteristics of the sensor are analyzed with finite element analysis software package ANSYS. The calibration device is manufactured, and the data acquisition and processing system is developed. The theoretical and experimental study of the static calibration of the sensor prototype is carried out. The results of simulation analysis and calibration experimentation prove the feasibility of the hyperstatic sensor structure, and the contents of this paper possess theoretical significance and engineering value for the further research and practical application of the six-component force sensor.

Force/Moment Isotropy of 8/4-4 Parallel Six-Axis Force Sensor Based on Performance Atlases

A six-axis force sensor with parallel 8/4-4 structure is introduced and its measurement principle is analyzed.Based on condition numbers of Jacobian matrix spectral norm of the sensor,the relationship between the force and moment isotropy and some structural parameters is deduced.Orthogonal test methods are used to determine the degree of primary and secondary factors that have significant effect on sensor characteristics.Furthermore,the relationship between each performance index and the structural parameters of the sensor is analyzed by the method of the atlas,which lays a foundation for structural optimization design of the force sensor.

ROBOT'S MOTION ERROR AND ONLINE COMPENSATION BASED ON FORCE SENSOR

Robot＇s dynamic motion error and on-line compensation based on multi-axis force sensor are dealt with.It is revealed that the reasons of the error are formed and the relations of the error are delivered.A motion equation of robot＇s termination with the error is established,and then,an error matrix and an error compensation matrix of the motion equation are also defined.An on-line error＇s compensation method is put forward to decrease the displacement error,which is a degree of millimeter,shown by the result of simulation of PUMA562 robot.

Optical Fiber Grating Sensor for Force Measurement of Anchor Cable

The development of the sensor suitable for measuring large load stress to the anchor cable becomes an important task in bridge construction and maintenance. Therefore, a new type of optical fiber sensor was developed in the laboratory - optical fiber grating sensor for force measurement of anchor cable （OFBFMAC）. No similar report about this kind of sensor has been found up to now in China and other countries. This sensor is proved to be an effective way of monitoring in processes of anchor cable installation, cable cutting, cable force regulation, etc, with the accurate and repeatable measuring results. Its successful application in the tie bar cable force safety monitoring for Wuhan Qingchuan bridge is a new exploration of optical fiber grating sensing technology in bridge tie bar monitoring system.

A novel fully-integrated miniature six-axis force/torque sensor

This paper presents a new designed miniature six DOF （degree of freedom） force/torque sensor. This sensor is fully integrated with a micro DSP （digital signal processor）, so all the signal conditioning, A/D, decoupling, digital-signals serial output are performed in the sensor. Some experimental results are presented to demonstrate the capability of the proposed design. Finally, a neural network was used for decoupling the interacting signals, compared with the conventional method using the inverse matrix, this new method is more accurate.

An intraoperative lumbar neurological force monitoring system with high-density flexible pressure sensor array

In the surgery of lumbar disc herniation(LDH),the nerve root retractor is used to pull the nerve root to prevent damage.The traditional medical nerve root retractor cannot quantify the force on the nerve root.In order to improve the nerve root retractor,this paper proposes an intraoperative lumbar neurological force monitoring system.The core module of this system is the improved nerve root retractor equipped with the high density flexible pressure sensor array.The high density microneedle array and multiple pressure detection units are used in the pressure sensor to realise sensitive pressure monitoring in a narrow surgical operation area.The sensing area is 4 mm×17 mm,including 6 detection units.The sensitivity of sensor is 67.30%/N in the range of 0-5 N.This system is used for in vitro animal experiments,which can continuously detect pressure.

Isotropic Optimization of a Stewart-Type Six-Component Force Sensor

This paper presents the isotropic optimization of a Stewart-type six-component force sensor. First,the static model of the sensor is built by the screw theory and the forward isotropy indexes and the inverse isotropy indexes are further presented. Second,a comprehensive evaluation function is established to evaluate the isotropic performance of the sensor. By compromising all the isotropy indexes and solving the extreme value of the function,the sensor optimization process is completed and an optimal solution of a set of sensor structure parameters is obtained. Finally,the design of the components and the assembly of the prototype are established by 3D modeling software Pro-E. The verification of the isotropic performance of the sensor is conducted by the finite element analysis software ANSYS. The results obtained by our research can provide useful reference to the isotropic performance evaluation and structure design of the stewart-type six-component force sensor.

Conducting Rubber Force Sensor: Transient Characteristics and Radiation Heating Effect

Compression force sensors are indispensable to tactile sensors in humanoid robots. We are investigating the application of low-cost electrically conducting rubber sheets to force sensors, of which the biggest problem is its poor reproducibility. We have found that the deposition of aluminum by a vacuum evaporation method shows such an excellent characteristic that the sensor can be used in a wide range under 10.33 N/cm2. In this article, we investigated time response of the sensors and also studied how the radiation heating during the vacuum evaporation process for Al deposition affected their sensing property. We found that the radiation heating induces deterioration from the point of view of standard deviation of the output voltage of the sensors at a transient region. We convince that a low-temperature Al deposition method should be developed to form electrodes on the electrical conducting rubber sensors.

Electrode for Force Sensor of Conductive Rubber

Tactile sensors are believed to be a key element in order to realize robotic fingers to catch a fragile object without damage. Force sensitive conductive rubber is a low-cost material and then attractive for the application to tactile sensors. We have studied the effect of electrodes attached to the rubber sheets. We have tried four kinds of electrodes: vacuum deposited Al, adhesive Cu tape, Al thin film sheet and silver paste. It can be concluded that vacuum deposited Al has the highest potential from the practical point of view;it has the widest dynamic range and good precision at the same time.

WRIST FORCE SENSOR'S DYNAMIC PERFORMANCE CALIBRATION BASED ON NEGATIVE STEP RESPONSE

Negative step response experimental method is used in wrist force sensor＇s dynamic performance calibration. The exciting manner of negative step response method is the same as wrist force sensor＇s load in working. This experimental method needn＇t special experiment equipments. Experiment＇s dynamic repeatability is good. So wrist force sensor＇s dynamic performance is suitable to be calibrated by negative step response method. A new correlation wavelet transfer method is studied. By wavelet transfer method, the signal is decomposed into two dimensional spaces of time-frequency. So the problem of negative step exciting energy concentrating in the low frequency band is solved. Correlation wavelet transfer doesn＇t require that wavelet primary function be orthogonal and needn＇t wavelet reconstruction. So analyzing efficiency is high. An experimental bench is designed and manufactured to load the wrist force sensor orthogonal excitation force/moment. A piezoelectric force sensor is used to setup soft trigger and calculate the value of negative step excitation. A wrist force sensor is calibrated. The pulse response function is calculated after negative step excitation and step response have been transformed to positive step excitation and step response. The pulse response function is transferred to frequency response function. The wrist force sensor＇s dynamic characteristics are identified by the frequency response function.

Dynamic analysis of double-layer and pre-stressed multi-limb six-axis force sensor

In order to adapt to the specific task, the six-axis dynamic contact force between end-effectors of intelligent robots and working condition needs to be perceived. Therefore, the dynamic property of six-axis force sensor which is installed on the end-effectors of intelligent robots will have influence on the veracity of detection and judgment to working environment contact force by intelligent robots directly. In this paper, dynamic analysis to double-layer and pre-stressed multi-limb six-axis force sensor is conducted. First, the structure of the sensor is introduced, and the limb number is confirmed by introducing the related definitions of convex analysis. Then, based on vibration of multiple-degree-of-freedom system, a mechanical vibration simplified model of double-layer and pre-stressed multiple limb six-axis force sensor is set up. After that, movement differential equations of sensor and the response of analytical expression are deduced, and the movement differential equations is solved. Finally, taking the double-layer and pre-stressed seven limb six-axis force sensor as an example, numerical calculation and simulation of deriving result is conducted, which verify the correctness and feasibility of the theoretical analysis.

A Sputtering Deposition of Al Enhances the Output Reproducibility in a Conducting Rubber Force Sensor

Compressive force sensors or pressure sensors are indispensable to tactile sensors in humanoid robots. It is investigated that low-cost electro-conducting rubber sheets are applied to the force sensor, of which the biggest problem is its poor reproducibility. It was found that the aluminum deposition by a vacuum evaporation method shows excellent characteristics but suffers deterioration by a radiation heating effect. The aluminum electrode was deposited by a sputtering method, known to have an advantage of a low-temperature method, and the reproducibility of the output was improved.

Stacked Structure Improves Output Reproducibility of Rubber Force Sensor

The electricity-conducting rubber force sensor is an attractive candidate as a low-cost material for tactile sensors. This article shows the evidence that the output reproducibility is largely improved when two identical sheets of the sensors are stacked. The stacked structure may reduce accidental error that is a fatal obstacle in an accurate control system.

Multi-dimensional force sensor for haptic interaction:a review

Haptic interaction plays an important role in the virtual reality technology,which let a person not only view the 3D virtual environment but also realistically touch the virtual environment.As a key part of haptic interaction,force feedback has become an essential function for the haptic interaction.Therefore,multi-dimensional force sensors are widely used in the fields of virtual reality and augmented reality.In this paper,some conventional multi-dimensional force sensors based on different measurement principles,such as resistive,capacitive,piezoelectric,are briefly introduced.Then the mechanical structures of the elastic body of multi-dimensional force sensors are reviewed.It is obvious that the performance of the multi-dimensional force sensor is mainly dependent upon the mechanical structure of elastic body.Furthermore,the calibration process of the force sensor is analyzed,and problems in calibration are discussed.Interdimensional coupling error is one of the main factors affecting the measurement precision of the multi-dimensional force sensors.Therefore,reducing or even eliminating dimensional coupling error becomes a fundamental requirement in the design of multi-dimensional force sensors,and the decoupling state-of-art of the multi-dimensional force sensors are introduced in this paper.At last,the trends and current challenges of multi-dimensional force sensing technology are proposed.

DISTRIBUTED OPTIC－FIBER FORCE SENSOR BASED ON MODE COUPLING EFFECTS IN HIGH－BIREFRINGENT FIBER

An optic-fiber sensor system for measuring distributed forces is presented. The sensor system chooses high-birefringent fiber as sensing e-iement. Based on coupling effects of polarization modes in high-birefringent fiber, the distribution of the force points along the sensing fiber can be measured by detecting mode couplings in the fiber which are caused by external force disturbance. The location and magnitude of the measured forces are determined by a heterodyne interferometry and the technique of optic path scanning. The spatial resolution of the sensor system is better than 0. 15m for a 50m testing fiber.

AN OPTICAL FIBRE SENSOR BASED CUTTING FORCE MEASURING DEVICE

A cutting force measuring device is developed which is composed of a common tool shank and an optical fibre sensor. The sensor is mounted at the rear of the shank and the deformation of the shank caused by cutting force is detected as the measurement of cutting force. The device possesses good adaptability to workshop conditions as well as satisfying static and dynamic performances.

The Anti-Vibrating-Impact Research of the Piezoelectric Resonator Force Sensor

Based on the analysis of one-dimension inertial accelerometer movement model,from which the resolution of the accelerometer inertial mass displacement equation was derived,the response of the sensor sensing element to vibration and impact of various frequencies was studied.The theoretical and experimental results show that a reasonable configuration among the sensing element inherent frequency,environmental exciting frequency and the damp factors of the sensor is the key to prevent the sensor from damage.The sensor has good anti-vibrating impact ability when the relative damp factor is 0.7,and the environmental interferential vibrating frequency is less than 0.35 times of the inherent frequency of the sensing element.

Miniature 6-axis force/torque sensor for force feedback in robot-assisted minimally invasive surgery

In order to restore force sensation to robot-assisted minimally invasive surgery(RMIS),design and performance evaluation of a miniature 6-axis force/torque sensor for force feedback is presented.Based on the resistive sensing method,a flexural-hinged Stewart platform is designed as the flexible structure,and a straightforward optimization method considering the force and sensitivity isotropy of the sensor is proposed to determine geometric parameters which are best suited for the given external loads.The accuracy of this method is preliminarily discussed by finite element methods(FEMs).The sensor prototype is fabricated with the development of the electronic system.Calibration and dynamic loading tests for this sensor prototype are carried out.The working ranges of this sensor prototype are 30 N and 300 N·mm,and resolutions are 0.08 N in radial directions,0.25 N in axial direction,and 2.4 N·mm in rotational directions.It also exhibits a good capability for a typical dynamic force sensing at a frequency close to the normal heart rate of an adult.The sensor is compatible with surgical instruments for force feedback in RMIS.