Indirect Force Control Research Articles

A Review of Electro-Mechanical Brake (EMB) System: Structure, Control and Application

With the development of automobile electrification and intelligence, the demand for electro-mechanical braking (EMB) systems is increasing rapidly. This paper reviews the development status of the EMB actuator on the basis of extensive patent and literature research. By analyzing the basic structure of the EMB actuator, this paper decomposes the actuator into five modules: service brake module, parking brake module, brake clearance compensation module, quick-return module and sensor module. On the basis of basic structure, the estimation algorithm for indirect clamping force control and the direct clamping force control algorithm of the actuator are summarized. In addition, the requirements of the EMB system for intelligent vehicles and its typical architecture are analyzed, and the preliminary application of the EMB system in intelligent driving is summarized.

Closed-Loop Kinematic and Indirect Force Control of a Cable-Driven Knee Exoskeleton: A Lyapunov-Based Switched Systems Approach

Closed-Loop Kinematic and Indirect Force Control of a Cable-Driven Knee Exoskeleton: A Lyapunov-Based Switched Systems Approach

Bounded Energy Collisions in Human–Robot Cooperative Transportation

In this article, we introduce the notion of bounded energy collision for physical human–robot collaboration to allow safe interaction with the environment, by keeping the collision impact within predetermined bounds. The proposed method is based on energy dissipation, implemented by a variable damping strategy along the direction of imminent collisions with the environment, and fully complies with any physical human–robot interaction strategy that combines an indirect force control (e.g., impedance control) with an estimation of the human motion intention. Finally, extensive experimental studies clarify and verify the proposed scheme.

Indirect Force Control of a Cable-Suspended Aerial Multi-Robot Manipulator

We present the control in physical interaction with the environment of a Cable-suspended Aerial Multi-Robot Manipulator (CS-AMRM) called the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Fly-Crane</i> , composed of three aerial vehicles towed to a platform by means of six cables. The control strategy enables the system to accurately and safely perform tasks involving expected or unexpected interactions between the platform and the environment, in the absence of dedicated force/torque sensors. A previously developed Inverse Kinematic Controller (IKC) is enhanced with an admittance framework, and contacts are estimated through a generalized momentum-based observer. To assess the validity of our approach, and to provide practical insights into the method, we perform extensive experimental tests, comprehending the admittance property shaping to modulate stiffness, damping, and virtual mass, as well as experiments in a more realistic scenario involving contacts between the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Fly-Crane</i> and the environment.

Unintentional Force Drifts as Consequences of Indirect Force Control with Spatial Referent Coordinates

We explored the phenomenon of unintentional force drifts in the absence of visual feedback. Based on the idea of direct force control with internal models and on the idea of indirect force control with referent coordinates to the involved muscle groups, contrasting predictions were drawn for changes in the drift magnitude when acting against external spring loads. Fifteen young subjects performed typical accurate force production tasks by pressing with the Index finger at 20% of maximal voluntary contraction (MVC) in isometric conditions and while acting against one of the three external springs with different stiffness. The visual feedback on the force was turned off after 5 s. At the end of each 20-s trial, the subjects relaxed and then tried to reproduce the final force level. The force drifts were significantly smaller in the spring conditions, particularly when acting against more compliant springs. The subjects were unaware of the force drifts and, during force matching, produced forces close to the initial force magnitude, which were not different across the conditions. There was a trend toward larger drifts during performance by the dominant hand. We view these observations as strong arguments in favor of the theory of control with spatial referent coordinates. In particular, force drifts were likely consequences of drifts of referent coordinates to both agonist and antagonist muscles. The lack of drift effects on both perception-to-report and perception-to-act fit the scheme of kinesthetic perception based on the interaction of efferent (referent coordinate) and afferent processes.

Workpiece force and position control for active and flexible fixtures in assembly

With strengthening focus on digitalization and flexibility, active and flexible fixtures can provide the right tools for manufacturing systems. However, fixturing solutions are often too individualized for processes and need a universal approach. Firstly, a system formulation of multiple workpieces interacting with each other is presented. Secondly, strategies for direct and indirect force control along with their characteristics have been developed. The stability of assembly of force-controlled components has been analyzed, and certain key characteristics have been presented. The position control under different signal types, including oscillatory, have been discussed. Finally, a set of experiments have been conducted for performance evaluation.

A generalized adaptive stiffness control scheme for robot manipulators with bounded inputs

AbstractIn general, indirect force control schemes (stiffness, impedance, etc.) assume that robot actuators can provide any torque value to achieve the goal of interaction control. This study attempts to regulate robot–environment interaction by generating bounded control signals and to avoid accurate knowledge of the parameters associated with gravitational effects and the stiffness of the environment. To achieve this aim, a generalized and saturating adaptive stiffness control scheme in task‐space is proposed. For the purpose of this work, the interaction or contact between the end‐effector of a robot manipulator and the environment is modeled as a vector of bounded spring‐like forces. The proposed control approach has a proportional‐derivative structure with static model‐based compensation of gravitational and interaction forces, which it achieves by including a regressor‐based adaptive term. As a theoretical basis to support the proposal, Lyapunov's stability analysis of the closed‐loop equilibrium vector is presented. Finally, the suitability of the proposed stiffness control scheme for interaction tasks is verified through simulations and experimental tests by using three‐degree‐of‐freedom robotic arms.

Indirect Force Control of a Cable-Driven Parallel Robot: Tension Estimation using Artificial Neural Network trained by Force Sensor Measurements.

In a cable-driven parallel robot (CDPR), force sensors are utilized at each winch motor to measure the cable tension in order to obtain the force distribution at the robot end-effector. However, because of the effects of friction in the pulleys and the unmodeled cable properties of the robot, the measured cable tensions are often inaccurate, which causes force-control difficulties. To overcome this issue, this paper presents an artificial neural network (ANN)-based indirect end-effector force-estimation method, and its application to CDPR force control. The pulley friction and other unmodeled effects are considered as black-box uncertainties, and the tension at the end-effector is estimated by compensating for these uncertainties using an ANN that is developed using the training datasets from CDPR experiments. The estimated cable tensions at the end-effector are used to design a P-controller to track the desired force. The performance of the proposed ANN model is verified through comparisons with the forces measured directly at the end-effector. Furthermore, cable force control is implemented based on the compensated tensions to evaluate the performance of the CDPR in wrench space. The experimental results show that the proposed friction-compensation method is suitable for application in CDPRs to control the cable force.

Design of force control system using tendon‐driven mechanism including linear springs and ultrasonic motor

AbstractActuators that have characteristics such as light weight, high thrust to weight ratio, and force controllability are desirable for applications such as finger robots and power assist suits. Recently, an indirect force control method using an ultrasonic motor (USM) and a spring has been attracting considerable attention because this method has the potential to satisfy all of the desirable characteristics, namely light weight, high thrust to weight ratio, and force controllability. The USM controls the length of the spring, and the tensional force of the spring is indirectly controlled by the length. As the resonance frequencies of indirect force control systems are low due to the springs, it is difficult to select a high feedback gain. This research therefore proposes a novel resonance ratio controller for indirect force control systems. The validity of the controller is verified by simulation and experiment. The overshoot ratio of the conventional controller is more than 10%, while that of the proposed controller is around 1%, when the rise time is 0.30 s.

Hierarchical Force and Positioning Task Specification for Indirect Force Controlled Robots

Indirect force control (IFC) architectures are a common approach for dealing with unknown environments. What all IFC schemes have in common is that the relation between the set point and the actual configuration of the robot is determined by a mechanical relationship (e.g., a mass–spring–damper system). In this paper, we propose a set-point generation method for IFC schemes, enabling intuitive specification of mixed force and positioning tasks on joint and Cartesian levels. In addition, the formulation of equality and inequality tasks is supported and a passivity-based stability proof is formulated using the concept of virtual energy storage. The resulting task programming interface is demonstrated on a 7-degree-of-freedom robot, running a joint space impedance controller. One sample task demonstrates the application of the developed approach and highlights the basic features.

Controller Design of Indirect Force Control System with Velocity-saturating Closed Loop Ultrasonic Motor Velocity Control System in Inner Loop

Controller Design of Indirect Force Control System with Velocity-saturating Closed Loop Ultrasonic Motor Velocity Control System in Inner Loop

Gemstone Grinding Process Improvement by using Impedance Force Control

Chula Automatic Faceting Machine has been developed by The Advance Manufacturing Research Lab, Chulalongkorn University to support Thailand Gems-Industry. The machine has high precision motion control by using position and force control. A contact stiffness model is used to estimate grinding force. Although polished gems from the Faceting Machine have uniform size and acceptable shape, the force of the grinding and polishing process cannot be maintain constant and has some fluctuation due to indirect force control. Therefor this research work propose a new controller for this process based on an impedance direct force control to improve the gemstone grinding performance during polishing process. The grinding force can be measured through motor current. The results show that the polished gems by using impedance direct force control can maintain uniform size as well as good shape and high quality surface.

Critical Damping Conditions for Third Order Muscle Models: Implications for Force Control

Experimental results presented in the literature suggest that humans use a position control strategy to indirectly control force rather than direct force control. Modeling the muscle-tendon system as a third-order linear model, we provide an explanation of why an indirect force control strategy is preferred. We analyzed a third-order muscle system and verified that it is required for a faithful representation of muscle-tendon mechanics, especially when investigating critical damping conditions. We provided numerical examples using biomechanical properties of muscles and tendons reported in the literature. We demonstrated that at maximum isotonic contraction, for muscle and tendon stiffness within physiologically compatible ranges, a third-order muscle-tendon system can be under-damped. Over-damping occurs for values of the damping coefficient included within a finite interval defined by two separate critical limits (such interval is a semi-infinite region in second-order models). An increase in damping beyond the larger critical value would lead the system to mechanical instability. We proved the existence of a theoretical threshold for the ratio between tendon and muscle stiffness above which critical damping can never be achieved; thus resulting in an oscillatory free response of the system, independently of the value of the damping. Under such condition, combined with high muscle activation, oscillation of the system can be compensated only by active control.

Grinding Force Control of Polishing Cubic Zirconia Gem by an Automatic Faceting Machine

This research work is to develop a controller that combines the position and indirect force control for the gemstone polishing process. The polished gemstones are attached to the end of drop-stick of the Chula Automatic Faceting Machine. The Chula Automatic Faceting Machine has 4-axis high precision motion control and the other 2-axis for spindle and crank motion. Many gemstones (number of gemstones will depends on the size of the gemstone) can be grinded and polished simultaneously with the result of uniform size and quality. In this research work, the controller is developed based on PID motion control. The reference position of the controller for the grinding process is obtained based on the compliance force control technique with known compliance model. This reference position will specified the pressing force for the grinding process indirectly. The controller developed in-house is used for testing with Cubic Zirconia gemstone which the hardness is 8.5 Mohs. The compliance model and appropriate parameters including the minimum and maximum boundary of polishing process can be obtained experimentally. Then all parameters obtained from the experiment can be used in the controller. The uniform size and quality of the multi-stone grinding and polishing process can be obtained with this controller.

Indirect force control development procedure

SUMMARYAddition of extra sensors, especially video cameras and force sensors, under control of appropriate software makes robotic manipulators working in factories suitable for a range of new applications. This paper presents a method of manipulator indirect force control development, in which the force set values are specified in the operational space and the manipulator is equipped with a force sensor in its wrist. Standard control development methods need the estimation of parameters of the detailed model of a manipulator and position servos, which is a complicated and time-consuming task. Hence, in this work a time-efficient hybrid procedure of controller development is proposed consisting of both analytical and experimental stages: proposal of an approximate continuous model of a manipulator, experimental determination and verification of its parameter values using the resonance phenomenon, continuous regulator development, and digitization of the regulator.

Optimal impedance control based on body inertia for a hydraulically driven hexapod robot walking on uneven and extremely soft terrain

AbstractThis paper presents the implementation of impedance control for a hydraulically driven hexapod robot named COMET‐IV, which can walk on uneven and extremely soft terrain. To achieve the dynamic behavior of the hexapod robot, changes in center of mass and body attitude must be taken into consideration during the walking periods. Indirect force control via impedance control is used to address these issues. Two different impedance control schemes are developed and implemented: single‐leg impedance control and center of mass‐‐based impedance control. In the case of single‐leg impedance control, we derive the necessary impedance and adjust parameters (mass, damping, and stiffness) according to the robot legs' configuration. For center of mass–based impedance control, we use the sum of the forces of the support legs as a control input (represented by the body's current center of mass) for the derived impedance control and adjust parameters based on the robot body's configuration. The virtual forces from the robot body's moment of inertia are adapted to achieve optimal control via a linear quadratic regulator method for the proposed indirect attitude control. In addition, a compliant switching mechanism is designed to ensure that the implementation of the controller is applicable to the tripod sequences of force‐based walking modules. Evaluation and verification tests were conducted in the laboratory and the actual field with uneven terrain and extremely soft surfaces. © 2011 Wiley Periodicals, Inc.

Destruction Avoidance for Dual Arm Construction Machinery Using Mechanical Impedance Adjustable Hydraulic Circuit

Dual arm work machine that consists of hydraulic excavator is expected for transportation of long objects such as steel stocks and destruction of concrete walls by cooperating dual arm. However, cooperation of two or more arms generates overload in a work object and arms. Because the arms form closed-loop through the work object, refuge of internal force disappears. In this paper, a nondestructive dual manipulation technique for reducing working load is designed using indirect force control that applies in a part of one arm. Operator's positioning accuracy in transportation of long object is measured by a front structure of the hydraulic excavator. From the results, permissible positioning error in nondestructive dual manipulation is clarified. Moreover, load of working object is reduced by indirect force control by using hydraulic circuit with accumulator. Target mechanical impedance is maintained by combining hydraulic circuit with adaptive control in hydraulic system that changes the load characteristic while working. Transportation work experiment of long object is conducted by using the lifter that corresponds to positioning control manipulator and the hydraulic excavator that applies the force control system. Working load has decreased to 20% of maximum working load of the front structure of the construction machinery. Usable range of nondestructive manipulation is clarified by a chart that shows relations between expected load decrease and target mechanical impedance characteristic decided with the work object. Therefore, nondestructive dual arm manipulation of the construction machinery can be achieved by using the mechanical impedance adjustment with combination of the hydraulic circuit and the adaptive control.

Force control experiments for industrial applications: a test case using an industrial deburring example

Purpose – The main objective of this paper is to report the development of an indirect force control strategy designed to operate with industrial robotic deburring applications. More specifically, the system reported here is developed to debur high‐quality knives that incorporate innovative design from well‐known authors (fashion designers). Therefore, these products are very difficult to manufacture and have quality requirements incompatible with human‐based deburring, since humans introduce too many unacceptable deviations as a consequence of their incapacity to maintain concentration for long periods of time.Design/methodology/approach – Since a good model of the environment is difficult to obtain, namely on industrial applications, a simple strategy was designed to obtain the relevant parameters leading to an acceptable performance. Consequently, the system implements an indirect force control strategy as a way to use actual robot controllers, explore the computing power of external personal computers...

Indirect and Direct Force Control of a Two-Phase Tubular Permanent Magnet Machine

Vector control and direct torque control are employed extensively in variable-speed ac drive systems, particularly for induction machine drives. This paper describes the application of both control techniques to a two-phase tubular permanent magnet machine supplied from a full H-bridge inverter, with space vector modulation being employed to derive variable-amplitude, variable-frequency, two-phase sinusoidal voltages. The direct force control (DFC) strategy controls the flux and thrust force by means of an active and reactive force estimator, utilizing the concept of active and reactive power. It is demonstrated that compared to vector control, DFC is more effective in compensating for the influence of the cogging force

Optimization and experimental verification for the antagonistic stiffness in redundantly actuated mechanisms: a five-bar example

A closed-chain mechanism having redundancy in force domain can produce an effective spring effect by proper internal load distribution. The so-called antagonistic stiffness is provided by redundant actuation in conjunction with nonlinear geometric constraints. The objective of this study is twofold. The first one is to propose a methodology for optimal kinematic design of antagonistic stiffness and the second one is to verify the phenomenon of the antagonistic stiffness through experimentation. In this work, a five-bar mechanism that can modulate antagonistic stiffness by using redundant actuators is employed as an exemplary system. The optimal structure of the five-bar mechanism that can maximize efficiency in generation of antagonistic stiffness is evaluated and analyzed. A measure of stiffness isotropy and the gradient property of the measure are employed as the performance indices in optimization. To deal with multi-criteria based design, a composite design index based on max–min principle of fuzzy theory is used as an objective function. Two optimization results are obtained. Furthermore, a five-bar mechanism has been developed for experimental verification. The phenomenon of antagonistic stiffness has been successfully demonstrated by free vibration and indirect force control experimentation.