Six-axis Force-torque Sensor Research Articles

Integrating force and vision feedback for flexible assembly system

The Industrial Assembly Challenge at the World Robot Summit 2020 was a competition showcasing the latest technologies in robot assembly tasks. Teams had two days to set up an autonomous assembly system and had to manipulate surprise parts on the competition day. The manufacturing industry has trended towards high-mix low-volume production, which often requires frequent tool changes and can be difficult to set up with industrial robots. Small- and medium-sized enterprises (SMEs) may struggle to use robots for a variety of products, particularly in assembly tasks requiring high accuracy and specialized tools. To improve assembly performance, we propose an assembly method using multiple types of sensors, including an RGB camera and six-axis force-torque sensor. This system allows for flexible assembly tasks without strict alignment, does not require jigs or dedicated hands for each object, and can be easily adapted to different production scenarios. In this paper, we present the motion strategy of our proposed assembly system that participated in the World Robot Summit 2020 and demonstrate how it was able to robustly accomplish tasks such as screw tightening and peg-in-hole by combining information from the camera with information from force-torque sensors.

Design, Manufacture, Test and Experiment of Six-Axis Force Torque Sensor for Chinese Experimental Module Manipulator.

A novel six-axis force/torque sensor (F/T sensor) for an Experimental Module Manipulator (EMM) in the Chinese Space Station (CSS) is developed in this paper. First, we designed the elastomer structure of the F/T sensor and used the analytical method and the finite element method to analyze the strain, in order to accomplish the strain gauges’ layout. Then, the electrical system was designed, which mainly realizes the acquisition of force/torque information, temperature and serial communication with the end effector (EE). Following this, we analyzed and designed the adaptability of the F/T sensor to the space environment. After this, the manufacturing process of the sensor was introduced in detail, and the F/T sensor was calibrated by a pulley weight system. Finally, the sensor was tested on the space environment adaptability of mechanical vibration and thermal vacuum on the ground. The test results show that the developed sensor has the ability to accurately measure three-dimensional force and three-dimensional moment information on orbit, which provides necessary conditions for the on-orbit fine operation of EMM.

Strategies of Pen Tip Path Estimation and of Workload Comparison for Handwriting Tasks

The magnitude of impulse and work at pen tip, which are related to the fingertip force, may be indicators of fatigue and pain during writing. Previous methods used specially prepared pens for testing which have different sizes and weights from those commercially available; however, they are not suitable for evaluating the operation and workload of ordinary handwriting tasks. This article proposes a data processing method of a six-axis force-torque sensor for determining the contact state of the pen tip and calculating the position and work of the pen tip without adding anything to a commercially available pen. The proposed method consists of kinematic calculation of the pen tip position as the observation and probabilistic determination of the contact state of the pen tip. The performance of the proposed method is presented by applying to the time-series data of force vector and moment vector during handwriting tasks of symbols. The data was measured by a low-resolution high-frequency 6-DoF force-torque sensor. Multiple handwriting tasks with the same or different load at one time were set. It was confirmed statistically whether the work amount with one pen tip can be evaluated appropriately if the work amount is the same or different. The experimental results demonstrate that the proposed method can compare the movement distance and work of the pen tip, depending on the severity of the writing task.

Lateral Extra-Articular Tenodesis Reduces ACL Graft Force Under Multiplanar Torques Simulationg Pivot Shit

Objectives:Utilization of lateral extra-articular tenodesis (LET) in conjunction with anterior cruciate ligament reconstruction (ACLR) has increased in recent years, however, the biomechanical impact of LET, when performed with contemporary techniques, on both load sharing between the ACL graft and the LET and on knee kinematics is not completely clear. The purpose of this study was to quantify the effect of LET performed with ACLR, in the presence of a compromised anterolateral tissues, on (1) forces carried by the ACL graft and the LET and (2) knee kinematics, during simulated pivot shift.Methods:manipulator equipped with a six-axis force-torque sensor. The robot applied multiplanar torques simulating two types of pivot shift (PS) subluxing the lateral compartment at 15° and 30° of knee flexion. The following loading combinations were applied: (PS1) 8 Nm of valgus and 4 Nm of internal rotation torques; (PS2) 100 N compression force, 8 Nm valgus torque, 2 Nm internal rotation torque, and 30 N anterior force. Anteroposterior (AP) translation in the lateral compartment of the knee was recorded in the following states: ACL intact, sectioned, reconstructed and, finally, after sectioning the anterolateral ligament (ALL) and kaplan fibers and performing a LET. ACLR was performed utilizing a bone-patellar tendon-bone autograft, via medial parapatellar arthrotomy. LET was performed using a modified lemaire technique with a metal staple femoral fixation at 60° of flexion in neutral rotation. Resultant forces carried by the ACL graft and LET at the peak applied load in all tested conditions were determined utilizing the principle of superposition and serial sectioning.Results:Under both simulated pivot shift types and at both flexion angles the ACL force decreased with the addition of a LET, with the least force reduction of 39% for PS2 at 15° (p=0.01) and the most force reduction of 80% for PS1 at 30° (p<0.001). While decreasing ACL force, the LET carried at least 43% of the force carried by the ACL graft when tested without LET for PS2 at 15° and 91% of the force carried by the ACL graft at most, for PS1 at 30° (Table 1). For both combinations of multiplananr torques and at both flexion angles, the anterior tibial translation in the lateral compartment decreased for the ACLR+LET knee compared to the intact knee (5.3mm and 7.6mm decrease, for PS1 15° and 30° respectively, p<0.001; 4.4mm p=0.005 and 7.6mm p<0.001, for PS2 15° and 30°, respectively). (Figure 2).Conclusion:During a simulated pivot shift, LET shields the ACL graft from loading. This effect was greatest at 30° of flexion with an 80% drop in ACL graft force. While some shielding of load from the ACL graft can be beneficial, a more significant reduction in the load of the ACL graft may potentially be detrimental to the graft remodeling, maturation and function. The optimal load sharing pattern for improved clinical outcomes is not well understood and merit further investigation. In addition, LET also decreases anterior tibial translation in the lateral compartment to less than that of the intact knee, which represents overconstraint of the lateral compartment. These findings may support the purported “protective” effect of LET on the ACL graft and its important role in stabilizing the lateral compartment in the setting of combined ACL and anterolateral structures deficiency. The influence of overconstraint of the lateral compartment with LET warrants further biomechanical and clinical evaluation.Table 1.Mean loads carried by the ACL graft before and after the addition of a LET and loads carried by the LET, under both simulated pivot shift types at 15 ° and 30° of flexionMean ACL Graft Loads (n=7)Mean LET Loads (n=7)Without LETWith LETDifference% Drop*LET LoadRelative Load** PS1 15° 58±13 N20±23 N29±18 N49%31±15 N53% PS1 30° 54±13 N11±7 N44±12 N80%49±17 N91% PS2 15° 68±15 N41±20 N26±19 N39%29±17 N43% PS2 30° 71±15 N21±20 N50±14 N70%49±14 N70%PS1=8 Nm of valgus and 4 Nm of internal rotation torquesPS2=100 N compression, 8 Nm valgus, 2 Nm internal rotation, and 30 N anterior force*the percent of ACL graft force drop**The percent of load carried by LET relative to the load carried by ACL graft without LETFigure 1.Figure 2.

Cabaud Memorial Award: Lateral Extra-articular Tenodesis Alters Regional Distribution of Contact Stress in the Lateral Knee Compartment under Simulated Pivoting Loads: An In Vitro Study

Objectives:Residual laxity after ACL reconstruction (ACLR) may adversely impact patient outcomes and function and has led to the increasing utilization of lateral extra-articular augmentation procedures in conjunction with ACLR and specifically, Lateral Extra-articular Tenodesis (LET). However, concerns of overconstraining the lateral compartment and subsequent increased lateral compartment contact stress and accelerated degenerative changes have been suggested with LET procedures. Therefore, the purpose of this work was to assess the impact of a LET on contact mechanics of the lateral compartment in response to multiplanar torques representing pivoting maneuvers.Methods:Nine cadaveric knees (4 male, 37.4 ± 11.6 years old) were mounted to a robotic manipulator equipped with a six-axis force-torque sensor. The robot applied multiplanar torques simulating two types of pivot shift (PS) maneuvers, subluxing the lateral compartment, at 30° of knee flexion. The following loading combinations were applied: (PS1) 8 Nm of valgus and 4 Nm of internal rotation torques; (PS2) 100 N compression force, 8 Nm valgus torque, 2 Nm internal rotation torque, and 30 N anterior force. Kinematics were recorded in the following states: ACL intact, sectioned, reconstructed and, finally, after sectioning the anterolateral ligament (ALL) and kaplan fibers and performing a LET. ACLR was performed utilizing a bone-patellar tendon-bone autograft, via medial parapatellar arthrotomy. LET was performed using a modified lemaire technique with a metal staple femoral fixation at 60° of flexion in neutral rotation. A contact stress transducer was then sutured to the tibial plateau beneath the menisci and the previously-determined kinematics were replayed, while recording the lateral compartment contact stress. At the peak applied loads, the following measures were determined in the lateral compartment: contact force, contact area, the anterior-posterior (AP) location of the center of contact stress (CCS), the mean contact stress, and the peak contact stress and its AP location. Statistical differences were assessed via one-way repeated measures ANOVA with Student-Neumen-Keuls post hoc test (p< 0.05).Results:Under combined valgus and internal rotation torques (PS1), the addition of a LET to ACLR increased lateral compartment contact force compared to the native knee by 51 ± 51 N (p = 0.035) on average. Contact area also increased by 60 ± 56 mm2 and 61 ± 58 mm2 relative to the ACL intact and ACL reconstructed knee (p ≤ 0.002), respectively. LET also shifted anteriorly the CCS by 4.6 ± 3.6 mm and 5.7 ± 3.1 mm on average relative to the ACL intact and ACL reconstructed knees (p < 0.001) (Fig. 1). No differences were detected for the mean and peak lateral compartment contact stress with the addition of LET compared to the ACL intact or ACL reconstructed knee (p> 0.854). The location of peak contact stress, however, shifted anteriorly compared to the ACL intact and ACL reconstructed knees by 6.2 ± 5.7 mm and 7.6 ± 5.4 mm (p < 0.001). (Fig.1). Similar results were observed under multiplanar torques with compression (PS2) for all outcome measures.Conclusion:In response to multiplanar torques representing pivoting maneuvers, the addition of a LET to ACLR, in the presence of compromised anterolateral tissues, did not increase lateral compartment contact stress. As contact force increased with the addition of LET, so did the contact area, likely mitigating changes in the level of contact stress. LET shifted the contact location anteriorly thereby altering regional loading of the lateral articular cartilage. Further study of the impact of changes in regional loading patterns in the lateral compartment on cartilage degeneration is warranted.Figure 1.

Three Dimensional Upper Limb Joint Kinetics of a Golf Swing with Measured Internal Grip Force.

The biomechanics of a golf swing have been of interest to golfers, instructors, and biomechanists. In addition to the complexity of the three-dimensional (3D) dynamics of multi-segments of body, the closed-chain body posture as a result of both hands holding a club together makes it difficult to fully analyze the 3D kinetics of a golf swing. To identify the hand-grip joint force and torque applied by each hand, we directly measured the 3D internal grip force of nine registered professional golfers using an instrumented grip. A six-axis force-torque sensor was connected to a custom-made axially separated grip, which was then connected to a driver shaft using a manufactured screw thread. Subjects participated in two sessions of data collection featuring five driver swings with both a regular and customized sensor-embedded grip, respectively. Internal grip force measurement and upper limb kinematics were used to calculate the joint force and torque of the nine-linkage closed-chain of the upper limb and club using 3D inverse dynamics. Direct measurement of internal grip forces revealed a threefold greater right-hand torque application compared to the left hand, and counterforce by both hands was also found. The joint force and torque of the left arm tended to precede that of the right arm, the majority of which had peaks around the impact and showed a larger magnitude than that of the left arm. Due to the practical challenge of measuring internal force, heuristic estimation methods based on club kinematics showed fair approximation. Our results suggest that measuring the internal forces of the closed-chain posture could identify redundant joint kinetics and further propose a heuristic approximation.

Elastic structure for a multi-axis forcetorque sensor

The multi-axis force–torque sensor is a key element of many accurate force and motion control systems, including intelligent robotic systems. This paper presents a novel elastic structure for multi-axis force–torque sensors that is easily manufacturable at low cost and has many detection points with high surface strains. By using the “design of experiment” method in the ANSYS software, we demonstrate that the designed elastic structure with four identical T-shapes has high surface strains and can provide various strain gauge configurations. The strain sensitivities of the elastic structure at predetermined points can be maximized manually by varying the thickness of the vertical and horizontal beams of the T-shaped components. The validity of the proposed elastic structure is demonstrated through the development of a low-cost six-axis force–torque sensor. Experimental results for the developed force–torque sensor with the proposed elastic structure demonstrate that the proposed structure is comparable to an expensive state-of-the-art commercial force–torque sensor.

Six-Axis Force Torque Sensor Model-Based In Situ Calibration Method and Its Impact in Floating-Based Robot Dynamic Performance

A crucial part of dynamic motions is the interaction with other objects or the environment. Floating base robots have yet to perform these motions repeatably and reliably. Force torque sensors are able to provide the full description of a contact. Despite that, their use beyond a simple threshold logic is not widespread in floating base robots. Force torque sensors might change performance when mounted, which is why in situ calibration methods can improve the performance of robots by ensuring better force torque measurements. The Model-Based in situ calibration method with temperature compensation has shown promising results in improving FT sensor measurements. There are two main goals for this paper. The first is to facilitate the use and understanding of the method by providing guidelines that show their usefulness through experimental results. Then the impact of having better FT measurements with no temperature drift are demonstrated by proving that the offset estimated with this method is still useful days and even a month from the time of estimation. The effect of this is showcased by comparing the sensor response with different offsets simultaneously during real robot experiments. Furthermore, quantitative results of the improvement in dynamic behaviors due to the in situ calibration are shown. Finally, we show how using better FT measurements as feedback in low and high level controllers can impact the performance of floating base robots during dynamic motions. Experiments were performed on the floating base robot iCub.

Robust and Inexpensive Six-Axis Force–Torque Sensors Using MEMS Barometers

Current commercial force–torque sensors are sensitive and accurate, but are also typically expensive and fragile. These features limit their use in cost-sensitive applications and unstructured environments such as people's homes. This paper presents a new design for an inexpensive and robust force–torque sensor that uses microelectromechanical system barometer transducers. The new design results in a six-axis force–torque sensor with an $R^2$ greater than 0.90 for F $_{x}$ and F $_{y}$ , and an $R^2$ greater than 0.98 for F $_{z}$ , M $_{x}$ , M $_{y}$ , and M $_{z}$ during compound loading. Furthermore, this sensor can be assembled in two days with off-the-shelf components for less than 20 USD.

ICub Whole-Body Control through Force Regulation on Rigid Non-Coplanar Contacts

This paper details the implementation on the humanoid robot iCub of state-of-the-art algorithms for whole-body control. We regulate the forces between the robot and its surrounding environment to stabilize a desired robot posture. We assume that the forces and torques are exerted on rigid contacts. The validity of this assumption is guaranteed by constraining the contact forces and torques, e.g. the contact forces must belong to the associated friction cones. The implementation of this control strategy requires to estimate the external forces acting on the robot, and the internal joint torques. We then detail algorithms to obtain these estimations when using a robot with an iCub-like sensor set, i.e. distributed six-axis force-torque sensors and whole-body tactile sensors. A general theory for identifying the robot inertial parameters is also presented. From an actuation standpoint, we show how to implement a joint torque control in the case of DC brushless motors. In addition, the coupling mechanism of the iCub torso is investigated. The soundness of the entire control architecture is validated in a real scenario involving the robot iCub balancing and making contacts at both arms.

Towards a Novel, Implantable Limb Lengthening Device1

Several surgical techniques and devices have been developed to help patients born with limb deformities, limb length inequalities, and extreme short stature. People with such ailments often experience pain, dysfunction, and joint degeneration. The primary method of treating such deformities is an osteotomy followed by callus distraction [1]. Commonplace lengthening devices are external fixators and intramedullary devices, but each has its drawbacks. Traditional external fixators, such as the Ilizarov device and Taylor spatial frame, are cumbersome, painful, and produce large residual scars [2]. Due to pin tract infection rates of 10–20%, lengthening with these methods requires careful surveillance [3]. Intramedullary lengthening devices can cause severe complications such as intramedullary infection [4]. Surgeons have recently experienced success with a motorized, intramedullary nail (Fitbone), but pediatric use of this device can be limited due to interference with open growth plates [5]. The investigators have designed an extramedullary device that retains the attractive qualities of an intramedullary nail, without the risk of deep infection or damage to growth plates. Additionally, the device can be equipped with a six-axis force-torque sensor capable of measuring forces and moments in real time.

Evaluation of Metallic Mold Surfaces Polished by an Industrial Robot with Stick Whetstones

Our ultimate goal is to develop an automatic polishing system that uses an industrial robot equipped with whetstones. The robot mimics the movements skilled workers make as they polish surfaces manually. This paper, the first step in the development of our system, presents experimental results to prove that our system completely removes the marks, which is the most time-consuming and crucial process for human workers, from the flat surfaces of metallic molds produced through the Electrical Discharge Machining (EDM) process. We build a polishing system that consists of a small 6-DOF industrial robot for material handling. Attached to its wrist is a six-axis force-torque sensor and it employs a whetstone holding mechanism. Before the robot system begins its polishing work, the normal force to be applied to the flat surface can be adjusted to a target value by altering the height of the hand. We adopt two polishing paths for the whetstone, reciprocating and zigzag. We then conduct experiments using the two paths, visually inspect the surface, and measure the surface roughness and shape. Based on the experimental results, we confirm that the system has completely removed the EDM mark layer. Although we find that the zigzag path leaves striped patterns on the polished surface, we are not able to determine their cause. Finally, we test three combinations of the two paths under different conditions to find the most suitable combination.

Six-axis force–torque sensor with a large range for biomechanical applications

A silicon six-axis force–torque sensor is designed and realized to be used for measurement of the power transfer between the human body and the environment. Capacitive read-out is used to detect all axial force components and all torque components simultaneously. Small electrode gaps in combination with mechanical amplification by the sensor structure result in a high sensitivity. The miniature sensor has a wide force range of up to 50 N in normal direction, 10 N in shear direction and 25 N mm of maximum torque around each axis.

MO-D-AUD B-04: Parameter Optimization for Brachytherapy Robotic Needle Insertion and Seed Deposition

Purpose: To investigate influence of different needle insertion and seed deposition techniques for robotic brachytherapy. To find optimal sets of low, normal and high translational and rotational velocities of the needle for decreasing insertion force, needle deflection and OR time, and increasing seed placement accuracy. Method and Materials: We have developed EUCLIDIAN — a fully automatic robotic prostate brachytherapy system. Robotic system parameters were optimized via preclinical experiments using two types of polyvinylchloride and tissue phantoms, cannula and stylet single-axis force sensors, and six-axis force-torque sensor. Cannula sensor measures the force on the cannula during insertion, withdraw, and axial force exerted by tissue at rest. Stylet sensor measures the force while seed is expelled from the cartridge, during seed travel through the cannula, and at the moment when seed is deposited into tissue. Position of the needle tip and consequently deposition depth into the phantom was measured using optical encoders on the cannula and stylet motors. Cannula and stylet translational velocity range was 5–120 mm/s, and cannula rotation range was 0–30 rev/s. Force patterns were analyzed based on the experimental data. Results: According to the criteria for minimizing insertion force and OR time while maximizing seed deposition precision, it was found that best performances were achieved when cannula and stylet normal speed was 70 ± 10 mm/s and optimal high speed was 100 ± 10 mm/s. Optimal cannula rotation speed range was 15–25 rev/s. In order to avoid seed jam in the cartridge, optimal speed for pushing seed out of the cartridge was 2–5 mm/s. Conclusion: Optimal parameters were programmed in the EUCLIDIAN configuration files. Seed deposition techniques have significant influence on reduction of insertion force, needle deflection and seed deposition accuracy. Future investigation will be on adaptive parameter tuning for specific clinical encounters. Acknowledgement: Supported by NCI-R01-CA091763.

Experimental Stiffness Measurement of WL-16RII Biped Walking Vehicle During Walking Operation

This paper describes an experimental method for evaluating the stiffness of a biped walking robot. A reliable procedure is proposed as based on a simplified version of Milli-CaTraSys (Milli Cassino Tracking System) with three wire encoders and two six-axis force-torque sensors. This measures both applied wrench and resulting compliant displacements. Experiments conducted on a prototype of the biped walking vehicle WL-16RII (Waseda Leg - No. 16 Refined II) under different dynamic conditions provided useful information for both design and control.

Three-Dimensional Touch Interface for Medical Education

We present the technical principle and evaluation of a multimodal virtual reality (VR) system for medical education, called a touch simulator. This touch simulator comes with an innovative three-dimensional (3-D) touch sensitive input device. The device comprises a six-axis force-torque sensor connected to a tangible object representing the shape of an anatomical structure. Information related to the point of contact is recorded by the sensor, processed, and audiovisually displayed. The touch simulator provides a high level of user-friendliness and fidelity compared to other purely graphically oriented simulation environments. In this paper, the touch simulator has been realized as an interactive neuroanatomical training simulator. The user can visualize and manipulate graphical information of the brain surface or different cross-sectional slices by a finger-touch on a brain-like shaped tangible object. We evaluated the system by theoretical derivations, experiments, and subjective questionnaires. In the theoretical analysis, we could show that the contact point estimation error mainly depends on the accuracy and the noise of the sensor, the amount and direction of the applied force, and the geometry of the tangible object. The theoretical results could be validated by experiments: applying a normal force of 10 N on a 120 mm x 120 mm x 120 mm cube causes a maximum error of 2.5 +/- 0.7 mm. This error becomes smaller when increasing the contact force. Based on the survey results, the touch simulator may be a useful tool for assisting medical schools in the visualization of brain image data and the study of neuroanatomy.

Measuring system for in vivo recording of force systems in orthodontic treatment–concept and analysis of accuracy

The aim of our developments is three-dimensional in vivo recording of those orthodontic force systems inducing tooth movements during treatment with fixed appliances. The concept presented here is the first to permit the forces and torques of these statically multiply undetermined systems to be recorded in vivo. For this purpose the force systems transmitted to the teeth from the archwire are isolated from the respective tooth by means of divisible special-design brackets and introduced into a 3D force–torque sensor via a gripping appliance. The sensor is fixed with a purpose-developed device relative to the patient’s dental arch. The patient’s head is positioned relative to the system by means of a bite fork as well as a forehead and chin support. Electrical measurement of the mechanical quantities is carried out by a six-axis force–torque sensor with semiconductor strain gauge elements, an electronical evaluator and a mobile measuring computer (PC). Extensive calibration of the sensor system has shown that the measuring uncertainty of the electrical measuring is less than 2%. Precise spatial fixing of bracket slot and archwire in the therapeutic position is crucial to the measuring accuracy of the system, as even minimum displacements affect the force system to be measured. Movements of the measuring system up to 0.04 mm result from a therapeutic force of 1.5 N. The results of extensive in vitro studies have already demonstrated that the system developed by us is suitable for the specified in vivo measuring function.