Force Control Mode Research Articles

Pedicle drilling force control of a robotic surgical system via spine-soft tissue coupling model and parameters optimization

Bone drilling is a crucial operation in spinal fusion surgery that requires precise control of the applied force to ensure surgical safety. This manuscript aims to enhance the force servo performance of the orthopedic robot during automatic bone drilling operations. Firstly, an analytical model is introduced to describe the spinal mobility of the spine-soft tissue coupling structure. Then, the model is calibrated using force data obtained from stress relaxation tests. Next, optimal force controller parameters are determined through drilling force control simulations based on the identified model. The dynamic performance and robustness of the closed-loop control system are analyzed to ensure safe drilling procedures. Finally, bone drilling experiments are conducted in a force control mode to verify the effectiveness of the proposed method. The step drilling force response's steady-state error is less than 0.15 N, the relative control error is less than 3 %, and there is no noticeable force overshoot. The amplitude of the sinusoidal force response decays to −3 dB when the target force frequency is up to 3.49 rad/s, indicating a wide control bandwidth. These results demonstrate that the proposed method can rapidly and safely provide an adequate force servo to carry out automatic bone drilling.

Design and Control of an Assistive Device for Elbow Effort-Compensation

In this article, we introduce an assistive device for the elbow joint that is easily wearable, lightweight, and cable driven using a series elastic actuation principle implemented by an endless-shape elastic bungee element. This type of elastic element is selected due to its intrinsic damping, and compliant features similar to human muscles, which provide mechanical filtering against dynamic uncertainties such as impulsive forces, and oscillated movements because of possible controller issues. Furthermore, a spool system is designed targeting to maximize the transmission of the generated elastic force to the wearer while avoiding multiple coiling for the cable wrap/release operation. The design parameters of the device are selected through design and optimization studies. The manufactured actuator's performance is validated on a rigid link under position and force control modes. To demonstrate the effectiveness of the elastic element, we conducted actuator validation tests with and without bungee cases. The results show that while the bungee-integrated setup can transfer the generated elastic force to the link without oscillation at different frequency movements (0.05–0.16 Hz), the bungee-excluded setup performs unstable movements under the same control gains, producing 50.74% more vibrations detected through fast Fourier transform analysis. In addition, to test the system under aggressive conditions, we applied impacts to it, and the results indicate that the damping ratio index of the bungee incorporated system is 56.14% more than that of without bungee case, demonstrating the intrinsic safe behavior of the mechanism. Finally, the device is assessed on six human subjects (different arm weights and dimensions) in a simulated industrial painting task with 5 min duration, with an average <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$22.3^\circ /\text{s}$</tex-math></inline-formula> elbow velocity under force control. The average effort reduction on biceps muscle among the subjects is measured 64.42% with respect to the without assistance test.

Compliant human–robot object transfer based on modular 3-axis force sensor for collaborative manufacturing

The primary motivation of this paper is to present a compliant and cost-effective solution for object transfer between human and robot. The application prospect of this study is robot–human collaboration in manufacturing. To achieve above goals, a novel modular 3-axis force sensor is proposed for the grasping system to achieve interactive force sensing. Compliant object transfer control strategy, which is composed of incremental force control mode and gravity balance control mode, is proposed for object transfer between human and robot. A prototype of underactuated grasping system which is mounted on the proposed modular 3-axis force sensor is fabricated to investigate the effectiveness of the proposed interactive control strategy. Experimental results reveal that the incremental force control mode is suitable for the lighter objects with a higher interactive sensitivity. For transferring heavier objects, the gravity balance control mode is more suitable. In gravity balance control mode, the human hand could achieve a quasi-static equilibrium with the object, and achieve a compliant transfer operation. Due to the above characteristic, the proposed control strategy has the potentials to enhance the object transfer compliance and safety in the human–robot object transfer process.

Control structures for rail vehicles using active steering technology

ABSTRACT In this study, an analytical method based on a linear single bogie model was used to investigate the mechanism of active steering for three control structures: wheelset yaw control (WYC), wheelset lateral control (WLC), and bogie yaw control (BYC). The results revealed that the WYC structure reduced both circumferential and axial wear, whereas the WLC and BYC structures mainly reduced circumferential wear, and BYC was ineffective when the primary lateral stiffness was low. Moreover, the impact of external forces on the control quantity was investigated by using a nonlinear full-vehicle model, with the wheelset unbalanced moment having the largest impact. Subsequently, the adaptation of the WYC and BYC structures to two control modes, namely, force and displacement control, was analysed. The results showed that for the WYC, the displacement control mode was favoured, whereas for the BYC, the force control mode was the sole alternative. Finally, the control efficiency was determined, with the WYC being recommended for low primary longitudinal stiffness and BYC for higher stiffness.

Multimode Control of a Parallel-Mobile-Coil System for Adaptable Large-Workspace Microrobotic Actuation

Recent studies on magnetic microrobotics for biomedical applications have resulted in varieties of magnetic actuation systems. However, the simultaneous actuation and tracking of magnetic microrobots in large workspace (e.g., the human-body size) remains challenging due to the fast decay of magnetic fields and the contradiction between resolution and field of view of a feedback instrument. Our previous work reported the concept of the parallel-mobile-coil system (PMCS), which comprises three electromagnetic coils that can move in large 3-D space for microrobotic actuation. Here, to make the PMCS have adaptability to different actuation scenarios or microrobot types, we propose three different control modes by utilizing the mobility of the coils. The torque control, force-assisted torque control, and force control modes are designed for pure torque actuation with energy conservation, torque actuation with force assistance, and pure force actuation, respectively. Comparative simulations and experiments validate the effectiveness of the three control modes. Moreover, to endow the system with <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">in-vivo</i> feedback capability, we implement Hall-effect sensors and demonstrate the 3-D large-workspace actuation based on magnetic localization.

Exploring friction stir welding for joining 316L steel pipes for industrial applications: Mechanical and metallurgical characterization of the joint and analysis of failure

Efficient welding of pipelines is critical for industries like power plants. Friction stir welding (FSW) has shown the ability to provide higher strength joints and is envisaged to be suitable for joining industrial pipelines as well. Hence, the present study assesses the application of FSW for joining 316L stainless steel industrial grade pipes. The welding is carried out by varying the process parameters, viz., spindle speed (rpm) and welding speed (mm/min), within a suitable range under force control mode. Microhardness and tensile tests have been conducted to evaluate the mechanical strength of the joints, whereas microstructure evaluation by optical microscopy (OM) and field emission scanning electron microscopy (FE-SEM) has been done in order to comprehend the mechanical performance of the weldments. It is found that both the selected welding process parameters (spindle speed and welding speed) have significant influence on the metallurgical and mechanical properties of the welded joints. High grain refinement due to dynamic recrystallization (DRX) in the stir zone results in enhanced hardness. At the optimal combination of parameters, i.e., 400 rpm spindle speed and 50 mm/min welding speed, the weld zone displays higher tensile strength than that of base metal. Also, at the same combination of parameters, stir zone of the welded samples displayed maximum hardness (255HV0.5kg). Some of the weldments displayed early failure during tensile tests. The present study also explains the cause of these failures. Optimum amount of heat generation during FSW process is key to achieving defect free good quality weldments.

A Practical Deceleration Control Method, Prototype Implementation and Test Verification for Rail Vehicles

Currently, the theoretical braking force control mode, characterized by actual deceleration as an unstable open-loop output, is the most widely used brake control mode in trains. To overcome the shortcomings of non-deceleration control modes, a deceleration control mode is proposed to realize the closed-loop control of train deceleration. First, a deceleration control algorithm based on parameter estimation was derived. Then, the deceleration control software logic was designed based on the existing braking system to meet the engineering requirements. Finally, the deceleration control algorithm was verified through a ground combination test bench with real brake control equipment and pneumatic brakes. The test results show that the deceleration control can make the actual braking deceleration of the train accurately track the target deceleration in the presence of disturbances, such as uncertain brake pad friction coefficients, line ramps, vehicle loads and braking force feedback errors, as well as their combined effects, and does not affect the original performance of the braking system. The average deceleration in the deceleration control mode is relatively stable, and the control error of instantaneous deceleration is smaller.

Friction stir welding of industrial grade AISI 316L and P91 steel pipes: A comparative investigation based on mechanical and metallurgical properties

Welding of pipes and tubes are important as the same are used for transportation of oil and gas in various industries and under demanding situations. Friction Stir Welding (FSW) has the potential to provide strong, efficient and cost effective joint for this type of applications. Carrying out circular FSW has its own share of challenges related to pipe fixture and curved geometry. The current study thus attempts to investigate the performance of FSW in case of industrial grade steel pipes made of AISI316L and P91 materials which are commonly used in powerplant related applications. FSW has been carried out in force control (FC) mode with 35 kN applied load. The effect of two parameters i.e. spindle speed (rpm) and welding speed (mm/min) on the weld characteristics of the two steel pipes have been investigated. Both the steel pipes displayed more than 100% joint efficiency at optimal combination of parameters. Stir zone of both the welded samples revealed higher hardness values than respective base metal (BM). However, this increase is significant in case of P91 steel. Thermo-mechanically affected zone (TMAZ) in case of P91 is wider compared to 316L as revealed by optical microscopy (OM) and field emission scanning electron microscopy (FE-SEM). The results indicate that FSW is able to produce high quality joints in both AISI316L and P91 steel pipes which will be useful for the industry.

Comparative study on the sealing performance of packer rubber based on elastic and hyperelastic analyses using various constitutive models

Evaluation of the sealing performance of the packer rubber varies according to specific simulation models. This paper aims at revealing the difference between elastic and hyperelastic analyses using the finite element mothed (FEM). The study extracts the hyperelastic parameters of the neo-Hookean, the Mooney-Rivlin, and the Yeoh models from a uniaxial tensile test. Then, the setting process of a mechanical packer is simulated by elastic and hyperelastic calculations. We compare the deformed configuration and the contact stress given by these models. Our results show that the Yeoh model produces the minimum residual sum of squares (RSS) among the hyperelastic models for hydrogenated nitrile butadiene rubber (HNBR). The mooney-Rivlin model has a negative parameter, making the calculation unstable. The linear elastic model fails to simulate the setting process, while the neo-Hookean model overestimates the contact stress. Despite the similar stress distribution, the nonlinear elastic model provides a 17.8% higher contact stress on average than the Yeoh model. A parametric study based on the Yeoh model points out that the sub-thickness of the packer rubber needs an elaborate design. Reducing the sub-thickness could increase the contact stress but decrease the seal length in the force control mode. From an engineering perspective, this study demonstrates that it needs to pay more attention when selecting an appropriate material model and a sound analysis method to evaluate the sealing performance of an oil packer.

Friction stir welding of AISI-316L steel pipes: Mechanical and metallurgical characterization of the joint

The present work attempts to develop a procedure for friction stir welding (FSW) of AISI-316L pipes. These pipes find applications across power, marine, food and chemical industries. Initially, welding is carried out in position control mode by setting parameters viz. spindle speed (RPM), transverse speed/feed rate (mm/min) and tool depth (mm) to get an idea about the welding process and to assess the effect of these parameters on the weldment. Based on the weld quality and the data recorded during the welding process, further welding is carried out in force control mode of the FSW setup. The welded joint at 300 rpm spindle speed and 35 kN force display greater strength (around 94%) compared to the base metal. The hardness of stir zone (SZ) as well as heat affected zone (HAZ) in welded samples was significantly higher compared to the base metal. The microstructural characterization revealed fine austenitic grain structures in SZ due to discontinuous dynamic recrystallization. In general, the weld is found to be consistent and the data obtained could be employed to produce high quality pipe welds.

Mechanical and metallurgical properties of friction stir welded austenitic steel pipes in position control mode

The current work involves friction stir welding of austenitic steel pipes under position control mode. AISI 316L pipes of thickness 6.75 mm has been joined by considering parameters viz. feed rate (mm/min), spindle speed (rpm), tool depth (mm). The welding was conducted at same rotational speed of 400 rpm but with two different welding speeds of 50 and 100 mm/min and with fixed plunge depth of 6 mm. The microstructural study has been done through Optical microscopy (OM) and Field Emission Scanning Electron Microscope (FE-SEM). Hardness of the weld zone is found to increase significantly compared to the base metal which is due to grain refinement. Tensile tests show that the weldments have a higher yield strength compared to the base metal. However, the ultimate tensile strength decreases due to the appearance of defects viz. microcracks. These defects may be primarily due to the lack of heat generation. The joints display an efficiency of almost 87%. The present study also provides an idea about the optimal range of vertical force which may yield good quality weldments. With this range of vertical force, welding is carried out under force control mode and found to achieve higher efficiency. Moreover, the present experiments provide idea about the process parameters and how they impact the characteristics of the weldments.

Development of multi-input multi-output virtual tuned mass damper controls for the vibration suppression of structures

This study is concerned with the development of multi-input multi-output control algorithms for the active vibration suppression of structures using accelerometer signals and force-type actuators. The concept of the single-input single-output virtual tuned mass damper control algorithm developed in the previous study was extended to cope with multiple natural modes of structure equipped with a limited number of sensors and actuators. Two control algorithms were developed based on the assumption of collocated control. One is the decentralized virtual tuned mass damper control that produces the actuator signal using only the accelerometer signal of that actuator position. The other is the centralized virtual tuned mass damper control that is designed in modal-space, and produces the modal control force using the modal coordinate. Both the theoretical and experimental results show that the proposed control algorithms are effective in suppressing multiple natural modes with a lesser number of sensors and actuators. However, the decentralized virtual tuned mass damper control can be designed and implemented more easily than the centralized virtual tuned mass damper control.

Integration of magnetic tweezers and traction force microscopy for the exploration of matrix rheology and keratinocyte mechanobiology: Model force- and displacement-controlled experiments

In this work, we present a new experimental methodology that integrates magnetic tweezers (MT) with substrate deformation tracking microscopy (DTM) and traction force microscopy (TFM). Two types of MT-DTM/TFM experiments are described: force-control mode and displacement-control mode experiments. In model bead-on-gel experiments for each mode, an MT device is used to apply a controlled force or displacement waveform to a fibronectin-coated superparamagnetic bead attached to a fibrillar type I collagen gel containing a layer of covalently attached red-fluorescent microspheres. Serial fast time-lapse differential interference contrast and epifluorescence image acquisition steps are used to capture displacements of the bead and microspheres, respectively, in response to the applied force or displacement. Due to the large number of acquired images and the dynamic nature of the experiment, new quantitative approaches are implemented to adapt TFM for the analysis of the data, including (i) a temporospatial correction algorithm for improved tracking of microsphere displacements, (ii) a method for the objective determination of L2 regularization parameters for computing incremental traction stress solutions, and (iii) an empirical means for identifying time intervals within the data that can be approximated by elastostatic conditions. We also illustrate how force and energy balances in a force-control mode bead-on-gel experiment can be used to estimate the elastic modulus of a collagen substrate. Finally, in a proof-of-concept, bead-on-cell demonstration, measurements of incremental cell–matrix traction stresses are used to observe how a force applied to a focal contact on the apical surface of a keratinocyte is transmitted to the collagen substrate below the cell.

Virtual semi-active damping learning control for robot manipulators interacting with unknown environment

Position controllers are used for free motion, whereas force controllers are used for constrained motion of robotic manipulators. The hybrid controller switches between position and force control modes depending on whether the manipulator is in contact with the environment. To improve production efficiency, the velocity of contact between the manipulator and environment is not set to zero. However, the high impact force due to the nonzero contact velocity might damage the environment surface or manipulators. In this article, we propose a virtual semi-active damping learning method to suppress force overshoot without decreasing the contact velocity. Virtual semi-active damping is adjusted according to the manipulator position in force control. The limited-memory BFGS method is used to obtain the ideal impedance model for the unknown environment. By minimizing the defined cost function, we get the desired interaction performance. The correctness and effectiveness of the proposed method are verified by conducting simulations and experiments.

Implementation of right angle friction stir welding (RAFSW) to assemble the side panels of truck box

Friction stir welding (FSW) has attracted many attentions in both academic and industrial sectors due to its promising characteristics compared with other conventional joining techniques. However, it still needs some improvements to become reliable and cost-effective for industrial applications. In this study, the aim is to develop a right angle friction stir welding (RAFSW) technique to assemble large side panels of truck boxes so that the technique be industrially reliable and cost-effective. For this purpose, Taguchi method and artificial neural network (ANN) modeling were used to design the experiments and optimize the welding process parameters. The efficient working window of welding process parameters was obtained at traverse speeds as high as 1700 mm/min. Then, the technique was developed to operate in force control mode using a spring load tool holder and a newly designed tool with a tapered entry. It is shown that the developed technique could tolerate high levels of joint fit-up disturbances like big gaps between the sheets before welding. Moreover, it was demonstrated that the developed technique could be operated using large, low-cost, CNC machining routers and with clamping only at the corners of panels. The use of weld tags prior to welding was beneficial in order to reduce clamping and distortion while maintaining a high strength level along the joints. NX-NASTRAN Finite Element software and various experiments including static and fatigue tests of the panels were used to design and evaluate the welded assembly. It was found that the developed RAFSW technique has the potential to be used for truck panel assembly.

Optimal Vibration Control of Membrane Structures with In-Plane Polyvinylidene Fluoride Actuators

Different kinds of membrane structures have been proposed for future space exploration and earth observation. However, due to the low stiffness, high flexibility, and low damping properties, membrane structures are likely to generate large-amplitude (compared to the thickness) vibrations, which may lead to the degradation of their working performance. In this work, the governing equations are established at first, taking into account the modal control force induced by the polyvinylidene fluoride (PVDF) actuator. The optimal vibration control of the membrane structure is explored subsequently. A square PVDF actuator is attached on the membrane to achieve the vibration suppression. The influence of actuator position and control gains on the vibration control performance are studied. The optimal criteria for actuator placement and energy allocation are developed. Several case studies are numerically simulated to demonstrate the validity of the proposed optimization criteria. The analytical results in this study can serve as guidelines for optimal vibration control of membrane structures. Additionally, the proposed optimization criteria can be applied to active control of different flexible structures.

Dynamics and Isotropic Control of Parallel Mechanisms for Vibration Isolation

Parallel mechanisms have been employed as architectures of high-precision vibration isolation systems. However, their performances in all degrees of freedom (DOFs) are nonidentical. The conventional solution to this problem is isotropic mechanism design, which is laborious and can hardly be achieved. This article proposes a novel concept; namely, isotropic control, to solve this problem. Dynamic equations of parallel mechanisms with base excitation are established and analyzed. An isotropic control framework is then synthesized in modal space. We derive an explicit relationship between modal control force and actuation force in joint space, enabling implementation of the isotropic controller. The multi-DOF system is transformed into multiidentical single-DOF systems. Under the framework of isotropic control, parallel mechanisms obtain an identical frequency response for all modes. An identical corner frequency, active damping, and rate of low-frequency transmissibility are achieved for all modes using a combining proportional, integral, and double integral compensator as a subcontroller. A 6-U P S parallel mechanism is presented as an example to demonstrate effectiveness of the new approach.

A new lower limb portable exoskeleton for gait assistance in neurological patients: a proof of concept study

BackgroundFew portable exoskeletons following the assist-as-needed concept have been developed for patients with neurological disorders. Thus, the main objectives of this proof-of-concept study were 1) to explore the safety and feasibility of an exoskeleton for gait rehabilitation in stroke and multiple sclerosis patients, 2) to test different algorithms for gait assistance and measure the resulting gait changes and 3) to evaluate the user’s perception of the device.MethodsA cross-sectional study was conducted. Five patients were recruited (4 patients with stroke and 1 with multiple sclerosis). A robotic, one-degree-of-freedom, portable lower limb exoskeleton known as the Marsi Active Knee (MAK) was designed. Three control modes (the Zero Force Control mode, Mode 1 and Mode 3) were implemented. Spatiotemporal gait parameters were measured by the 10-m walking test (10MWT), the Gait Assessment and Intervention Tool (G.A.I.T.) and Tinetti Performance Oriented Mobility Assessment (gait subscale) before and after the trials. A modified QUEST 2.0 questionnaire was administered to determine each participant’s opinion about the exoskeleton. The data acquired by the MAK sensors were normalized to a gait cycle, and adverse effects were recorded.ResultsThe MAK exoskeleton was used successfully without any adverse effects. Better outcomes were obtained in the 10MWT and G.A.I.T. when Mode 3 was applied compared with not wearing the device at all. In 2 participants, Mode 3 worsened the results. Additionally, Mode 3 seemed to improve the 10MWT and G.A.I.T. outcomes to a greater extent than Mode 1. The overall score for the user perception of the device was 2.8 ± 0.4 95% CI.ConclusionsThe MAK exoskeleton seems to afford positive preliminary results regarding safety, feasibility, and user acceptance. The efficacy of the MAK should be studied in future studies, and more advanced improvements in safety must be implemented.

Machine learning models applied to friction stir welding defect index using multiple joint configurations and alloys

Friction stir welding process has been studied extensively in the last decades since its early stage. Most of the research done so far is related to the process development including tool design, material weldability, post-weld mechanical behavior, and microstructural properties. More recently, in-line process monitoring and artificial intelligence algorithms are introduced into this process, but mainly to specific material configuration and joint thicknesses. This study will focus on the evaluation of different machine learning approaches including principle component analysis, K-nearest neighbor, multilayer perceptron, single vector machine, and random forest methods on a friction stir welding cell environment. The input variables provided from this cell environment are namely divided into two groups: one group refers to the application variables and the other group is related to the friction stir welding process variables. The application variables target the aluminum alloys, joint configuration, sheet thicknesses, initial mechanical properties, and their chemical composition. The friction stir welding process variables dictate the rotational speed, travel speed, forging force, longitudinal and transverse forces, torque, and specific energy. The output response to model from these machine learning algorithms is the defect index, which has been quantified using high-resolution immersed bath ultrasounds. This nondestructive evaluation technique has been described previously, which can detect defects ≥150 µm in thin sheets. The defect index has been classified into five classes, which is distinguished by the nature of defect, cold weld, or hot weld, as well as the width of the internal volumetric defect upon ultrasound C-scan result. The dataset, which is composed of around 500 various process conditions, has been generated over the last few years and the variables were taken exclusively in constant weld regime and in the force control mode using the output average values. This paper compares the best resulting machine learning methods applied on a friction stir welding cell basis, which is the K-nearest neighbor and multilayer perceptron algorithms. The K-nearest neighbor model reaches a deviation of 0.55 on the defect index in comparison with the experimental values, which is slightly better than the multilayer perceptron model, which obtains a score of 0.69. Over the initial 59 available model parameters, 10 and 15 of them were retained in the final algorithm using these techniques. The main predictors include the material thickness, base material ultimate tensile stress, rotational speed, travel speed, weld forces, and specific energy. The K-nearest neighbor model was able to provide a map of defect indices with regard to rotational speed and travel speed but was only possible when a higher density of data was found within the prediction area. A data density score was also included within the model to inform the end-user about the prediction reliability. The machine learning models are mainly about differentiating various cases rather than representing the physical phenomena as determined using the finite element analysis. That being said, in order to improve the prediction reliability as well as the machine learning models, the data twinning concept, which consists of generating simulated friction stir welding process conditions by finite element analysis, is briefly discussed.

Nonlinear vibration control effects of membrane structures with in-plane PVDF actuators: A parametric study

In recent years, with the increasing use of membrane structures in space applications, the problems of large-amplitude (compared to the thickness) vibrations arise; leading to the degradation of their working performance. It is challenging to control the large-amplitude vibrations of membranes due to the high flexibility, low damping, and strong geometrical nonlinearity. In this study, polyvinylidene fluoride (PVDF) actuators are considered to develop the active vibration control of a pre-tensioned Kapton membrane. The governing equations are established based on the geometrically nonlinear theory of plates, taking into account the modal control force induced by the actuators. To analyze the nonlinear dynamic characteristics of the membrane, the theoretical and approximate solutions of nonlinear frequencies of the membrane are obtained, and the discrepancies between the two solutions under different vibration amplitudes are discussed. Control performance for both free vibration and harmonic forced vibration of the membrane is numerically studied. A comprehensive parametric study is carried out to investigate the influences of different system parameters, including the pretension and the size of the membrane, and the position and number of the actuators, on the control performance. Analytical results indicate that the vibration of the membrane can be effectively suppressed with the appropriately laminated PVDF actuators. The results of this research can be extended to the active control of different gossamer space structures.