Precise Force Control Research Articles

Motor inhibition and its contribution to recovery of dexterous hand use after stroke.

Recovery of dexterous hand use is critical for functional outcome after stroke. Grip force recordings can inform on maximal motor output and modulatory and inhibitory cerebral functions, but how these actually contribute to recovery of dexterous hand use is unclear. This cohort study used serially assessed measures of hand kinetics to test the hypothesis that behavioural measures of motor modulation and inhibition explain dexterity recovery beyond that explained by measures of motor output alone. We also investigated the structural and functional connectivity correlates of grip force control recovery.Eighty-nine adults (median age = 54 years, 26% females) with first-ever ischaemic or haemorrhagic stroke and persistent arm and hand paresis were assessed longitudinally, at 3 weeks, and at 3 and 6 months after stroke. Kinetic measures included: maximal grip force, accuracy of precision and power grip force control, and ability to release force abruptly. Dexterous hand use was assessed clinically with the Box and Block Test and motor impairment with the upper extremity Fugl-Meyer Assessment. Structural and functional MRI was used to assess weighted corticospinal tract lesion load, voxel-based lesion symptom mapping and interhemispheric resting-state functional connectivity.Fifty-three per cent of patients had severe initial motor impairment and a majority still had residual force control impairments at 6 months. Force release at 3 weeks explained 11% additional variance of Box and Block Test outcome at 6 months, above that explained by initial scores (67%). Other kinetic measures did not explain additional variance of recovery. The predictive value of force release remained significant when controlling for corticospinal tract lesion load and clinical measures. Corticospinal tract lesion load correlated with recovery in grip force control measures. Lesions involving the parietal operculum, insular cortex, putamen and fronto-striatal tracts were also related to poorer force modulation and release. Lesions to fronto-striatal tracts explained an additional 5% of variance in force release beyond the 43% explained by corticospinal injury alone. Interhemispheric functional connectivity did not relate to force control recovery.We conclude that not only voluntary force generation but also force release (reflecting motor inhibition) are important for recovery of dexterous hand use after stroke. Although corticospinal injury is a main determinant of recovery, lesions to integrative somatosensory areas and fronto-parietal white matter (involved in motor inhibition) explain additional variance in post-stroke force release recovery. Our findings indicate that post-stroke upper limb motor impairment profiling, which is essential for targeted treatment, should consider both voluntary grasp generation and inhibition.

Classification and simulation of process of linear change for grip force at different grip speeds by using supervised learning based on sEMG

Bionic prosthesis can help the disabled rebuild the athletic ability of their missing limbs. Biomimetic hand controlled by surface electromyography (sEMG) has been widely studied in recent years. However, a lot of researchers focus on the field of movement recognition, ignoring the precise control of grip speed and force in the design of the prosthesis. The study presents a new means to simulate the process of linear changes of grip force by using sEMG, which provides a simple and practicable way of precise control of the speed and strength of force for the myoelectric hand. We design a synchronous acquisition device for sEMG signal and grip force signal. A novel preprocessing methodology for the raw sEMG signal is introduced, which is adaptive filter based on the Normalized Least Mean Square (NLMS). Compared the traditional preprocessing method that uses a notch filter for removing power line interference (50 Hz), the presented method can effectively remove power line interference and its harmonic wave components, and it completely reserves useful information contained in the sEMG signal. The linear change process of grip force at five grip speeds are studied. A simple and efficient method for muscle contraction state detection is used. Six supervised classification algorithms are used to identify these grip patterns and achieve a highest recognition rate of over 99%. Five supervised regression algorithms are applied to predict the value of grip force and establish the mapping relationship between sEMG and force. The experimental results demonstate that the method can simulate the linear change process of grip force well, and it achieves a minimum standard deviation of 0.14272.

Simultaneous Positive and Negative Pressure Control Using Disturbance Observer Compensating Coupled Disturbance Dynamics

This letter introduces a simultaneous positive and negative pressure controller for a pneumatic pump. For precise position or force control of the pneumatic soft actuator, the pressure from the source has to be regulated. When the positive and negative pressures are regulated, coupled dynamics caused by the pressure change affect the controller performance. In other words, changes in the positive or negative pressure concurrently act as disturbances in the opposite pressure controller. Therefore, a disturbance observer is designed to compensate for the coupled pressure dynamics. A nominal model of the system is obtained based on the output-error model used in the MATLAB System Identification Toolbox. A simulation study is also performed to design a controller when the demand for the positive and negative pressures varies. Finally, a verification of the effectiveness of the controller is done through a series of experiments. In the step response test, the maximum pressure error of the positive and negative pressure was reduced from 0.159 to 0.152 kPa and from 0.3297 to 0.2334 kPa with the disturbance observer. Where positive and negative pressures change at the same time, the root mean square of the positive and negative pressure error decreased from 0.9701 to 0.4457 kPa and from 0.9848 to 0.4634 kPa applying the proposed pressure controller.

Leveraging Joint Mechanics Simplifies the Neural Control of Movement.

Behaviors we perform each day, such as manipulating an object or walking, require precise control of the interaction forces between our bodies and the environment. These forces are generated by muscle contractions, specified by the nervous system, and by joint mechanics, determined by the intrinsic properties of the musculoskeletal system. Depending on behavioral goals, joint mechanics might simplify or complicate control of movement by the nervous system. Whether humans can exploit joint mechanics to simplify neural control remains unclear. Here we evaluated if leveraging joint mechanics simplifies neural control by comparing performance in three tasks that required subjects to generate specified torques about the ankle during imposed sinusoidal movements; only one task required torques that could be generated by leveraging the intrinsic mechanics of the joint. The complexity of the neural control was assessed by subjects’ perceived difficulty and the resultant task performance. We developed a novel approach that used continuous estimates of ankle impedance, a quantitative description of the joint mechanics, and measures of muscle activity to determine the mechanical and neural contributions to the net ankle torque generated in each task. We found that the torque resulting from changes in neural control was reduced when ankle impedance was consistent with the task being performed. Subjects perceived this task to be easier than those that were not consistent with the impedance of the ankle and were able to perform it with the highest level of consistency across repeated trials. These results demonstrate that leveraging the mechanical properties of a joint can simplify task completion and improve performance.

Excess Force Reduction in Bilateral Control for Precise and Safe Operation

This paper proposes a constrained bilateral control method to maintain a safe force limit in a remote environment in an event where the operator applies excessive force on the master system. The disturbance observer compensates system disturbances, and the reaction force observer (RFOB) estimates the external force exerted on individual. The paper introduces a force regulator called excess force reduced RFOB (EFR RFOB) to reduce the excessive force applied by the operator to the safe force limit. The amount of excess force is determined by adding the master-side RFOB output and the replica-side safe force limit. Acceleration-based bilateral control is implemented in a virtual space with a master virtual force input, virtual position response, and replica real force-position responses. It realizes transparency in the virtual space. This paper derives the relation between master-side real and virtual space variables to facilitate reproducibility between the master operator and replica environment. The restrained master force-position responses in the virtual space realize precise and safe force control on the replica-side in real space. The proposed method is verified using experiments.

Precise and Smooth Contact Force Control for a Hybrid Mobile Robot Used in Polishing

Precise and Smooth Contact Force Control for a Hybrid Mobile Robot Used in Polishing

Precision force control of an underactuated stance leg exoskeleton for human performance augmentation

Lower limb exoskeleton which augments the human performance is a wearable human–machine integrated system used to assist people carrying heavy loads. Recently, underactuated lower limb exoskeleton systems with some passive joints become more and more attractive due to the advantages of smaller weight, lower system energy consumption and lower cost. However, because of the less of control inputs, the existed control methods of fully actuated exoskeletons cannot be extended to underactuated systems, which makes the robust controller design of underactuated lower limb exoskeletons becomes more challenged. This article focuses on the high-performance human–machine interaction force control design of underactuated lower limb exoskeletons with passive ankle joint. In order to solve the reduction of control inputs, the holonomic constraint from the wearer is considered, which help transform the dynamics of 3-degree-of-freedom underactuated exoskeleton in joint space into a 2-degree-of-freedom fully actuated system in Cartesian space. A two-level interaction force controller using adaptive robust control algorithm is proposed to effectively address the negative effect of various model uncertainties and external disturbances. In order to facilitate the control parameter selection, a gain tuning method is also presented. Comparative simulations are carried out, which indicate that the proposed two-level interaction force controller achieves smaller interaction force and better robust performance to various modeling errors and disturbances.

Manufacturing of 3D Helical Microswimmer by AFM Micromanipulation for Microfluidic Applications

Manufacturing of microsystems based on complex three-dimensional microstructures requires critical steps such as mass calibrations and integration process. However, such steps involve several manipulation steps which are not well controlled thus the fragile 3D microstructures could be damaged or destroyed. In this work, we have demonstrated an AFM based direct mass measurement and manufacturing of 3D helical microswimmer. The proposed method shows to be rapid, repeatable and minimally-destructive thanks to the precise force control and manipulation by AFM. The proposed micromanipulation steps consist of the pick-measure-integrate manipulation steps using van der Waals force-based attachment and the mass measurement by resonant frequency shift. For the testing structures, we fabricated the 6 different types of 3D helical microswimmers vertically fabricated on the conical shape microneedle supports for uniform surface metallization and facile detachment. With the mass measurement sensitivity of 25 Hz/pg and the direct integration to microfluidics, we successfully demonstrated the 3D propulsion and non-contact micromanipulation by 3D helical microswimmer in microfluidics.

Low-Level Force-Control of MR-Hydrostatic Actuators

Precise and high-fidelity force control is critical for new generations of robots that interact with humans and unknown environments. Mobile robots, such as wearable devices and legged robots, must also be lightweight to accomplish their function. Hydrostatic transmissions have been proposed as a promising strategy for meeting these two challenging requirements. In previous publications, it was shown that using magnetorheological (MR) actuators coupled with hydrostatic transmissions provides high power density and great open-loop human-robot interactions. Still, the open-loop force fidelity at low and high frequencies are decreased by the transmission's dynamics and by nonlinear friction. This letter compares control strategies for MR-hydrostatic actuator systems to increase its torque fidelity, defined as the bandwidth (measured vs desired torque reference) and transparency (minimizing the undesired forces reflected to the end effector when backdriving the robot). Four control approaches are developed and compared experimentally: (1) Open-loop control with friction compensation; (2) non-collocated pressure feedback; (3) collocated pressure feedback; (4) LQGI state feedback. A dither strategy is also implemented to smoothen ball screw friction. Results show that approaches (1), (2) and (3) can increase the performances but are facing compromises, while approach (4) can simultaneously improve all metrics. These results show the potential of using control schemes for improving the force control performance of robots using tethered architectures, addressing issues such as transmission dynamics and friction.

Immediate effect of mechanical lumbar traction in patients with chronic low back pain: A crossover, repeated measures, randomized controlled trial

BackgroundLumbar traction is a treatment method traditionally used for chronic low back pain (CLBP) in many countries. However, its clinical effectiveness has not been proven in medical practice. The purpose is to conduct a multi-center, crossover, randomized controlled trial (RCT) to prove the efficacy and safety of traction on CLBP patients, using equipment capable of precise traction force control and of reproducibility of the condition based on the previous biomechanical and pre-clinical studies. MethodsNinety-five patients with non-specific CLBP from 28 clinics and hospitals were randomly assigned to either the intermittent traction with vibration (ITV) first group (A: sequence ITV to ITO) or the intermittent traction only (ITO) first group (B: sequence ITO to ITV); the former was treated with repeated traction and vibration force added to preload. All patients were followed up weekly for 2 periods after study-initiation. The primary outcome measures were disability level including pain and quality of life (based on Japan Low back pain Evaluation Questionnaire; JLEQ), and JLEQ was measured repeatedly. Statistical analysis was performed using linear mixed model. ResultsComparing to pre-traction data, both traction modes significant improvement except the first intervention of ITO treatment. The differences in JLEQ scores over time showed significant improvements in the treatment to which vibrational force was added in contrast to the conventional traction treatment; Mean difference was significant to compare ITV treatment and ITO treatment (−1.75 (p = 0.001), 95% CI; −2.69 to −0.80). However, neither difference between the two sequences (p = 0.884) nor carryover effect (p = 0.527) was observed. ConclusionsAltogether, the results indicate that lumbar traction was able to improve the pain and functional status immediately in patients with CLBP. This study contributes to add some evidence of the efficacy of lumbar traction.

Ca2+ elevations disrupt interactions between intraflagellar transport and the flagella membrane in Chlamydomonas.

The movement of ciliary membrane proteins is directed by transient interactions with intraflagellar transport (IFT) trains. The green alga Chlamydomonas has adapted this process for gliding motility, using retrograde IFT motors to move adhesive glycoproteins in the flagella membrane. Ca2+ signalling contributes directly to the gliding process, although uncertainty remains over the mechanism through which it acts. Here, we show that flagella Ca2+ elevations initiate the movement of paused retrograde IFT trains, which accumulate at the distal end of adherent flagella, but do not influence other IFT processes. On highly adherent surfaces, flagella exhibit high-frequency Ca2+ elevations that prevent the accumulation of paused retrograde IFT trains. Flagella Ca2+ elevations disrupt the IFT-dependent movement of microspheres along the flagella membrane, suggesting that Ca2+ acts by directly disrupting an interaction between retrograde IFT trains and flagella membrane glycoproteins. By regulating the extent to which glycoproteins on the flagella surface interact with IFT motor proteins on the axoneme, this signalling mechanism allows precise control of traction force and gliding motility in adherent flagella.

Grasp Planning Pipeline for Robust Manipulation of 3D Deformable Objects with Industrial Robotic Hand + Arm Systems

In the grasping and manipulation of 3D deformable objects by robotic hands, the physical contact constraints between the fingers and the object have to be considered in order to validate the robustness of the task. Nevertheless, previous works rarely establish contact interaction models based on these constraints that enable the precise control of forces and deformations during the grasping process. This paper considers all steps of the grasping process of deformable objects in order to implement a complete grasp planning pipeline by computing the initial contact points (pregrasp strategy), and later, the contact forces and local deformations of the contact regions while the fingers close over the grasped object (grasp strategy). The deformable object behavior is modeled using a nonlinear isotropic mass-spring system, which is able to produce potential deformation. By combining both models (the contact interaction and the object deformation) in a simulation process, a new grasp planning method is proposed in order to guarantee the stability of the 3D grasped deformable object. Experimental grasping experiments of several 3D deformable objects with a Barrett hand (3-fingered) and a 6-DOF industrial robotic arm are executed. Not only will the final stable grasp configuration of the hand + object system be obtained, but an arm + hand approaching strategy (pregrasp) will also be computed.

Sliding Mode Contact Force Control of n-Dof Robotics by Force Estimation

Control of the force exerted on an object is important for boosting system performance in robotics manipulators. Any undesired applied force may leave remarkable effects on the system, with the potential to damage the object. In addition, measuring external force is another challenge associated with such cases. Proposing an appropriate force estimation algorithm is a solution to overcome this deficiency. In this research, a control strategy is proposed to control the external force applied on the n-dof robotics. To eliminate force measurement in the controller, a force estimation strategy based on a disturbance observer is employed. Subsequently, a sliding-mode based control is implemented to cope with the force estimation error. The closed-loop stability of the system in the presence of estimated force is analytically considered. The proposed algorithm was implemented on piezoelectric actuators as the experimental setup. The experimental results confirm that by employing the proposed control scheme, precise force control is achievable. The force estimation algorithm can also suitably estimate external force.

Overcoming the ceiling effects of experts' motor expertise through active haptic training.

One of the most challenging issues among experts is how to improve motor skills that have already been highly trained. Recent studies have proposed importance of both genetic predisposition and accumulated amount of practice for standing at the top of fields of sports and performing arts. In contrast to the two factors, what is unexplored is how one practices impacts on experts' expertise. Here, we show that training of active somatosensory function (active haptic training) enhances precise force control in the keystrokes and somatosensory functions specifically of expert pianists, but not of untrained individuals. By contrast, training that merely repeats the task with provision of error feedback, which is a typical training method, failed to improve the force control in the experts, but not in the untrained. These findings provide evidence that the limit of highly trained motor skills could be overcome by optimizing training methods.

Soft Magnetic Pillars to Secure an Upper Airway for the Relief of Obstructive Sleep Apnea

AbstractThe inherent mechanical properties of soft structures allow fragile objects to be safely manipulated without the need for precise force control. Soft structures and driving mechanisms are employed in applications ranging from gripping and locomotion to biomedical devices. Such soft structures can be especially beneficial for manipulating excessively relaxed live soft human tissue, to relieve discomfort or medical conditions. An example is obstructive sleep apnea (OSA), where soft tissue narrows or blocks the upper airway while sleeping, causing difficulty breathing. In this paper, an approach to relieve OSA using injectable soft magnetic pillars paired with a removable magnet holder that can be worn while sleeping to secure the upper airway is reported. The approach provides an alternative to currently available treatments that may cause inconveniences such as difficulty swallowing, protrusion through soft tissue, repetitive surgeries, or sleep disruption. Experimental results and computational studies are used to determine the necessary degree of pulling force and distance, which can be adjusted by controlling the number of implanted soft magnetic pillars and external magnetic field strength. Further experiments using cadavers demonstrate the opening of the upper airway confirming the feasibility of the approach.

Modeling and Control of a 2-DoF Meso-Scale Continuum Robotic Tool for Pediatric Neurosurgery

This article introduces the analysis and control of a meso-scale two degree-of-freedom robotic endoscopic tool body for minimally invasive surgeries. The design of the robotic tool uses two types of a tendon-driven joint known as a bending flexure joint that allows us to control each degree-of-freedom by minimizing interjoint coupling by design. Pure kinematic modeling and control for these robots may not provide precise control performance due to kinematic uncertainties arising from tendon elongation, tendon slacking, gear backlash, etc. We propose a static model for each of the joints of the robotic tool that avoids several of these problems. Depending on the direction of tendon tension application, the proximal joint displays considerable hysteresis due to the superelastic material characteristics and this is included in our static model. The statics of a highly compliant distal joint is also modeled and validated using finite element analysis and experimental data. Using these models, we develop a control system that comprises of a disturbance observer and the proposed static model to provide precise force control and compensate for joint hysteresis.

Design and Control of a Passive Compliant Piezo-Actuated Micro-Gripper With Hybrid Flexure Hinges

This article presents the design, control, and test of a piezo-based passive compliant micro-gripper capable of combining position and force control. The gripper's structure is compact due to a super-large amplification ratio of 52 that is achieved by a two-stage displacement amplifier. The position/force switching control strategy is utilized for the position and force regulation. Particularly, a dual feedforward controller combining Gaussian process regression based hysteresis compensator and input shape technique based vibration suppressor is employed to the position regulation, while a hybrid controller composed of dual feedforward and PID feedback is used for the precision force control. Compared to the conventional design, the system is simplified because displacement sensor is removed. In order to protect the micro-object from contact, the passive compliance design method is introduced to the gripper tip. An experimental study, executing grasp-hold-release operations of a gold wire, is carried out to validate the feasibility of the gripper system. The result confirms that the proposed scheme is feasible and the stable clamping operations can be achieved.

A Novel Two-Stage Constant Force Compliant Microgripper

Abstract The manipulating objects of the micron scale are easily damaged, hence the microgrippers, the key components in micro manipulating systems, demand precise force control, plus miniaturized size. Consequently, the constant force microgrippers, generally lack the ability to fit different sizes. To avoid the overload damage, apply multi-size microparts and simplify the control method, a novel two-stage compliant constant force microgripper is proposed in this paper. Based on the negative stiffness effect, this gripper is connected in parallel with a two-stage negative stiffness module and a positive stiffness module. Then, the elliptic integral method and the pseudo-rigid-body method are both employed to derive the kinetostatic and dynamic performances. Finally, the analytical results are validated. It is observed that two-stage constant forces of 1.33 N in 305.6 μm and 1.11 N in 330.8 μm are acquired.

Shape memory alloy tube actuators inherently enable internal fluidic cooling for a robotic finger under force control

This paper presents the design, control and evaluation of a novel robotic finger actuated by shape memory alloy (SMA) tubes which intrinsically afford an internal conduit for fluidic cooling. The SMA tubes are thermomechanically programmed to flex the robotic finger when Joule heated. A superelastic SMA plate provides a spring return motion to extend the finger when cooling liquid is pumped through the internal channel of the SMA tube actuators. The mechanical design and nonlinear force controller are presented for this unique robotic finger. Sinusoidal and step response experiments demonstrate excellent error minimization when operated below the bandwidth which was empirically determined to be 6 rad s−1. Disturbance rejection experiments are also performed to demonstrate the potential to minimize externally applied forces. This method of internal liquid cooling of Joule heated SMA tubes simultaneously increases the system bandwidth and expands the potential uses of SMA actuators for robotic applications. The results show that this novel robotic finger is capable of precise force control and has a high strength to weight ratio. The finger can apply a force of 4.35 N and has a mass of 30 g. Implementing this design into wearable prosthetic devices could enable lightweight, high strength applications previously not achievable.

A Monolithic Force Sensing Integrated Flexure Bonder Dedicated to Flip-Chip Active Soft-Landing Interconnection

A flip-chip bonding system has a high demand for both force sensing and control functions to ensure the high-quality chip interconnection. The motivation of this article is to combine the ability to enable a common flip-chip bonding system to run in the manner of active soft-landing (ASL) interconnection. The developed flexure bonder achieves these functions by designing a flexure force sensing and control mechanism integrated with strain gauge sensing and piezoelectric actuating functions. First, the design, modeling, and optimization of the flexure mechanism are presented. Theoretical analyses, including force sensing and control working principle demonstration, force-strain model derivation, and dynamics response modeling, are carried out. Besides, aiming at highly sensitive force control under high-dynamic working condition, the mechanism is optimized by the multiobjective genetic optimization algorithm. Then, this flexure bonder mechanism is analyzed and evaluated by finite-element analysis. Finally, a series of validation experiments, including force sensing calibration and performance tests, open- and closed-loop force controlling tests, ASL tests, and actual bonding tests, are successfully implemented. The results indicate that the operation accuracy of the developed system is improved up to ±1 N under 400-N range, and the overshoot of ASL is less than 2 N under 6000-N/s loading speed. All the results uniformly confirm that the proposed bonding system can achieve precise force control and satisfactory chip interconnection performance with the proposed ASL bonding strategy.