High Precision Force Control Research Articles

Force Tracking Control Method for Robotic Ultrasound Scanning System under Soft Uncertain Environment

Robotic ultrasound scanning has excellent potential to reduce physician workload, obtain higher-quality imaging, and reduce costs. However, the traditional admittance control strategy for robotics cannot meet the high-precision force control requirements for robots, which are critical for improving image quality and ensuring patient safety. In this study, an integral adaptive admittance control strategy is proposed for contact force control between an ultrasound probe and human skin to enhance the accuracy of force tracking. First, a robotic ultrasound scanning system is proposed, and the system’s overall workflow is introduced. Second, an adaptive admittance control strategy is designed to estimate the uncertain environmental information online, and the estimated parameters are used to modify the reference trajectory. On the basis of ensuring the stability of the system, an integral controller is then introduced to improve the steady-state response. Subsequently, the stability of the proposed strategy is analysed. In addition, a gravity compensation process is proposed to obtain the actual contact force. Finally, through a simulation analysis, the effectiveness of the strategy is discussed. Simultaneously, a series of experiments are carried out on the robotic ultrasound scanning system, and the results show that the strategy can successfully maintain a constant contact force under soft uncertain environments, which effectively improves the efficiency of scanning.

Micromechanical Loading Studies in Ex Vivo Cultured Embryonic Rat Bones Enabled by a Newly Developed Portable Loading Device.

Mechanical loading has been described as having the potential to affect bone growth. In order to experimentally study the potential clinical applications of mechanical loading as a novel treatment to locally modulate bone growth, there is a need to develop a portable mechanical loading device enabling studies in small bones. Existing devices are bulky and challenging to transfer within and between laboratories and animal facilities, and they do not offer user-friendly mechanical testing across both ex vivo cultured small bones and in vivo animal models. To address this, we developed a portable loading device comprised of a linear actuator fixed within a stainless-steel frame equipped with suitable structures and interfaces. The actuator, along with the supplied control system, can achieve high-precision force control within the desired force and frequency range, allowing various load application scenarios. To validate the functionality of this new device, proof-of-concept studies were performed in ex vivo cultured rat bones of varying sizes. First, very small fetal metatarsal bones were microdissected and exposed to 0.4 N loading applied at 0.77Hz for 30s. When bone lengths were measured after 5days in culture, loaded bones had grown less than unloaded controls (p < 0.05). Next, fetal rat femur bones were periodically exposed to 0.4 N loading at 0.77Hz while being cultured ex vivo for 12days. Interestingly, this loading regimen had the opposite effect on bone growth, i.e., loaded femur bones grew significantly more than unloaded controls (p < 0.001). These findings suggest that complex relationships between longitudinal bone growth and mechanical loading can be determined using this device. We conclude that our new portable mechanical loading device allows experimental studies in small bones of varying sizes, which may facilitate further preclinical studies exploring the potential clinical applications of mechanical loading.

Design and hybrid impedance control of a new compliant wrist rehabilitation device

Over a decade, wrist rehabilitation robots have been proved to be efficient in the rehabilitation of patients with stroke or wrist injuries. Wrist rehabilitation devices compensate for deficiencies in manual rehabilitation training and reduce the workload of physiotherapists with their repetitive and gradual training necessary to rehabilitate patients with wrist impairments properly. Two design approaches have been reported in literature for wrist rehabilitation robotic devices. In one design approach, the strategy of placing the actuators, either linear or rotary, on the forearm to minimise the weight and inertia has been proposed. This necessitates a use of sophisticated transmission systems. In the other design approach, rotary motors have been proposed to be placed directly on the wrist joints, which results in a high weight and inertia. In this paper, a new 2-DOF compliant wrist rehabilitation device that can achieve the flexion-extension and radial-ulnar rehabilitation movements compliantly is proposed. In the proposed design, two series elastic actuators (SEAs), each consists of a servomotor and a custom-designed series elastic unit, are proposed to be placed directly on the wrist joints. In order to minimise the inertia and the required torques, the device is counterweight balanced by optimising the centre of mass of the device in the SolidWorks model. Based on the device dynamics modelled by ADAMS software, hybrid position and force tracking impedance control with the incorporation of a disturbance observer controller for the rotary SEA is developed for position and force tracking through the different therapeutic exercises. The disturbance observer controller is incorporated for high precision force control and robustness against exogenous disturbances. Simulation results demonstrate high performance of the proposed wrist rehabilitation device in terms of precise position and force trajectories tracking and robustness against exogenous disturbance as well as patient safety during all therapeutic exercises.

High-Precision Robust Force Control of a Series Elastic Actuator

A series elastic actuator (SEA) is a promising actuation method in force control applications that intelligently interacts with environments. The SEA is characterized by a spring placed between a load and an actuator, which is an electric motor in most cases. Since the spring plays the role of a transducer between position (i.e., the spring deflection) and force, it is able to control the output force (torque) precisely by utilizing typical position control methods. However, the force control performance of the SEA is considered to have limitations due to its elasticity, and thus, to be inferior to rigid actuators in terms of bandwidth. This paper proposes that the force control performance of the SEA can be improved by exploiting the dynamic model of the SEA. To this end, the SEA is modeled and analyzed utilizing the two-mass dynamic model, which is a well-known and widely accepted model of the flexible system. The disturbance observer and feedforward controller are introduced as the model-based control algorithms for the SEA to achieve the high-precision force control. In addition to high-bandwidth force control, the proposed controller can address the robust stability and performance against model parameter variance and exogenous disturbances. For the analytic and quantitative assessment of the proposed force control system, the dynamic characteristics of an SEA under various control algorithms are analyzed, and the experimental results are provided for an actual SEA system in this paper.

High-Precision Force Control of Short-Stroke Reluctance Actuators with an Air Gap Observer

A short-stroke reluctance actuator linearization scheme that simultaneously achieves high linearity, high bandwidth, and low stiffness is demonstrated. These properties are required in high speed and high precision motion systems. They are achieved by combining various control schemes, namely flux feedforward and analog sensing coil feedback for high bandwidth, Hall probe feedback to stabilize the drift, and an air gap observer together with gain scheduling to reduce the remaining stiffness. Using the presented scheme, the attractive force of the actuator can be controlled with high precision without the need for a position or force sensor. Experiments indicate that a linearization error of $\mathrm{50}\, {\mathrm {mN}}$ for $\mathrm{\mathrm{seco}{\mathrm{nd}}}$ -order $\mathrm{200}\, {\mathrm {N}}$ force reference profiles is obtained. This translates into force predictability of $\mathrm{99.98}{\mathrm\%}$ . Furthermore, absolute actuator stiffness below $\mathrm{500}\, {\mathrm{N/m}}$ at force levels of $\mathrm{100}\, {\mathrm {N}}$ is achieved, which is comparable to more linear Lorentz actuators.

Anticogging: Torque ripple suppression, modeling, and parameter selection

Smooth motion is critical to some robotic applications such as haptics or those requiring high precision force control. These systems are often direct-drive, so any torque ripple in the motor output must be minimal. Unfortunately, low-torque ripple motors are expensive. Low cost brushless direct current motors are becoming more prevalent, especially from the hobby remote control community. These motors often have the required high-torque density; however, they also have significant torque ripple. This paper presents a low cost method for anticogging, the compensation of cogging torque. While other methods exist to compensate for current-based torque ripple (mutual or reluctance torque), none have addressed cogging torque, except by adding expensive force sensors. This paper presents two methods that use a position sensor (already present for servo motors) to map cogging torque to rotor position. The map is played back according to position reported from the sensor to cancel the cogging torque. The design and testing of a low cost haptic arm using anticogging shows validation; however, the approach is much broader, and can be applied to any precision force application. A model of torque ripple sources are included as a function of pulse-width modulation frequency to help choose the optimal pulse-width modulation frequency to minimize torque ripple. Test results on eleven different motors show a removal of up to 88% of torque ripple with no added cost in robotic servo applications, and in some cases having better performance than motors that are over nine times as expensive.

Functional Corticospinal Projections from Human Supplementary Motor Area Revealed by Corticomuscular Coherence during Precise Grip Force Control

The purpose of the present study was to investigate whether corticospinal projections from human supplementary motor area (SMA) are functional during precise force control with the precision grip (thumb-index opposition). Since beta band corticomuscular coherence (CMC) is well-accepted to reflect efferent corticospinal transmission, we analyzed the beta band CMC obtained with simultaneous recording of electroencephalographic (EEG) and electromyographic (EMG) signals. Subjects performed a bimanual precise visuomotor force tracking task by applying isometric low grip forces with their right hand precision grip on a custom device with strain gauges. Concurrently, they held the device with their left hand precision grip, producing similar grip forces but without any precision constraints, to relieve the right hand. Some subjects also participated in a unimanual control condition in which they performed the task with only the right hand precision grip while the device was held by a mechanical grip. We analyzed whole scalp topographies of beta band CMC between 64 EEG channels and 4 EMG intrinsic hand muscles, 2 for each hand. To compare the different topographies, we performed non-parametric statistical tests based on spatio-spectral clustering. For the right hand, we obtained significant beta band CMC over the contralateral M1 region as well as over the SMA region during static force contraction periods. For the left hand, however, beta band CMC was only found over the contralateral M1. By comparing unimanual and bimanual conditions for right hand muscles, no significant difference was found on beta band CMC over M1 and SMA. We conclude that the beta band CMC found over SMA for right hand muscles results from the precision constraints and not from the bimanual aspect of the task. The result of the present study strongly suggests that the corticospinal projections from human SMA become functional when high precision force control is required.

Series Viscoelastic Actuators Can Match Human Force Perception

Series elastic actuators (SEAs) are frequently used for force control in haptic interaction, because they decouple actuator inertia from the end effector by a compliant element. This element is usually a metal spring or beam, where the static force-deformation relationship offers a cheap force sensor. For high-precision force control, however, the remaining small inertia of this elastic element and of the end effector still limit the sensing performance and rendering transparency. Here, we extend the concept to deformable end effectors manufactured of viscoelastic materials. These materials offer the advantage of extremely low mass at high maximum deformation and applicable load. However, force and deformation are no longer statically related, and history of force and deformation has to be accounted for. We describe an observer-based solution, which allows drift-free force measurement with high accuracy and precision. Although the description of the viscoelastic behavior involves higher-order derivatives, the proposed observer does not require any numerical differentiation. This new integrated concept of sensing and actuation, called series viscoelastic actuator (SVA), is applied to our high-precision haptic device OSVALD, which is targeted at perception experiments that require sensing and rendering of forces in the range of the human tactile threshold. User-device interaction force is controlled using state-of-the-art control strategies of SEAs. Force estimation and force control performance are evaluated experimentally and prove to be compatible with the intended applications, showing that SVAs open up new possibilities for the use of series compliance and damping in high-precision haptic interfaces.

2A1-3F-G6 光駆動ナノマシンの最適設計と高精度力制御

我々は, 2光子サブミクロン造形法を用いて光駆動ナノマニピュレータを開発している。本報では, 2自由度化と, 光トラップ部位の形状最適化により, マニピュレータの駆動特性を向上させ, 細胞やタンパク質の操作に必要なfNオーダーの微小な力制御を実現した。